Benthic use of phytoplankton blooms: Uptake, burial and biodiversity effects in a species-poor system

Agnes M. L. Karlson

Department of Systems Ecology

Stockholm University

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Doctoral thesis in Marine Ecology, 2010 Agnes Karlson, agnes@ecology.su.se Department of Systems Ecology SE-106 91 Stockholm Sweden © Agnes Karlson, Stockholm 2010 ISBN 978-91-7155-991-3 Printed by US-AB, Stockholm, Sweden Cover illustration by Ilmari Hakala, 2009

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ABSTRACT

Animals living in marine sediments, the second largest habitat on earth, play a major role in global biogeochemical cycling. By feeding on organic matter from settled phytoplankton blooms they produce food for higher trophic levels and remineralize nutrients that can fuel primary production. In the Baltic Sea, anthropogenic stresses, such as eutrophication and introductions of invasive species, have altered phytoplankton dynamics and benthic communities. This thesis discusses the effects of different types of phytoplankton on the deposit-feeding community and the importance of benthic biodiversity for fate of the phytoplankton bloom-derived organic matter. Deposit-feeders survived and fed on settled cyanobacterial bloom material and in doing so accumulated the cyanobacterial toxin nodularin. Their growth after feeding on cyanobacteria was much slower than on a diet of spring bloom diatoms. The results show that settling blooms of cyanobacteria are used as food without obvious toxic effects, although they do not sustain appreciable growth of the fauna. Since all tested species accumulated the cyanotoxin, negative effects higher up in the food web can not be ruled out. Both species composition and richness of deposit-feeding macrofauna influenced how much of the phytoplankton bloom material that was incorporated in fauna or retained in the sediment. The mechanism behind the positive effect of species richness was mainly niche differentiation among functionally different species, resulting in a more efficient utilization of resources at greater biodiversity. This was observed even after addition of an invasive polychaete species. Hence, species loss can be expected to affect benthic productivity negatively. In conclusion, efficiency in organic matter processing depends both on pelagic phytoplankton quality and benthic community composition and species richness. Key words: biodiversity, ecosystem functioning, benthic-pelagic coupling, niche, resource partitioning, competition, eutrophication, cyanobacterial blooms, diatoms, invasive species, Baltic Sea

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Contents LIST OF PAPERS 5 INTRODUCTION AND AIMS 6 BACKGROUND 7

Benthic-pelagic coupling – processes fundamental for ecosystem functioning 7 Links between Biodiversity and Ecosystem functioning 7

Mechanisms relating biodiversity to ecosystem functioning Current challenges in research on biodiversity and ecosystem functioning

Importance of food quality in ecosystem functioning 10 Deposit-feeders and food quality STUDY SYSTEM: THE BALTIC SEA 12

Anthropogenic influences 12 Phytoplankton blooms in the Baltic Sea 12

Nutritional aspects of blooms Baltic soft-bottom benthos 14 EXPERIMENTAL APPROACH 16

Isotope tracers in food-web studies 16 Stable isotopes Assessing growth and nutritional value to animals 17 RESULTS AND DISCUSSION 18

Importance of phytoplankton quality for benthic fauna 18 Cyanobacteria as a food source Changes in phytoplankton dynamics affect benthic production? Biodiversity effects on processing of phytoplankton bloom material 20 Mechanisms behind biodiversity effects on measured processes Long term changes in benthos: implications for ecosystem functioning Recent changes in benthic biodiversity: implications for ecosystem functioning Generalizations of biodiversity results to other systems CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH 26 ACKNOWLEDGEMENTS 27 LITERATURE CITED 28

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LIST OF PAPERS This thesis is based on the following five papers, referred to in the text by their roman numerals. Published papers are reprinted with permission of the publishers. I Karlson AML, Nascimento FJA, Elmgren R (2008). Incorporation and burial of

carbon from settling cyanobacterial blooms by deposit-feeding macrofauna. Limnology and Oceanography 53(6): 2754-2758

II Karlson AML and Mozuraitis R. Deposit-feeders accumulate the cyanobacterial toxin

nodularin. (Manuscript) III Nascimento FJA, Karlson AML, Näslund J, Gorokhova E (2009). Settling

cyanobacterial blooms do not improve growth conditions for soft bottom meiofauna. Journal of Experimental Marine Biology and Ecology 368: 138-146.

IV Karlson AML, Nascimento FJA, Näslund J, Elmgren R. In press. Higher diversity of

deposit-feeding macrofauna enhances phytodetritus processing. Ecology V Karlson AML, Blomgren Rydén S, Näslund J. Effects of a polychaete invader on

ecosystem functioning in a species-poor soft bottom. (Manuscript).

My contributions to the papers: (I, V) –Major part in experimental design and execution, responsible for all analyses and data processing, writing. (II) – Major part in experimental design and execution, preparation of samples for mass spectrometry, data processing, writing. (III) – Participated in experimental design and execution and contributed in writing (IV) –Participated in experimental design and execution, responsible for all analyses and data processing, major part in writing.

Additional publications by the author of this thesis:

Karlson AML, Almqvist G, Skora KE, Appelberg M (2007). Indications of competition between non-indigenous round goby and native flounder in the Baltic Sea. ICES Journal of Marine Science 64: 479-486 Nascimento FJA, Karlson AML, Elmgren R (2008). Settling blooms of filamentous cyanobacteria as food for meiofauna assemblages. Limnology and Oceanography 53 (6): 2636-2643 Karlson AML and Viitasalo-Frösen S (2009). Assimilation of 14C-labelled zooplankton benthic eggs by macrobenthos. Journal of Plankton Research 31 (4): 459-463

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INTRODUCTION AND AIMS

The study of trophic interactions is central to ecology. Determining the factors regulating trophic transfer of energy and nutrients is essential for understanding ecosystem productivity. Marine sediment ecosystems cover about 70% of the planet’s surface and sediment-living organisms play a major role in global biogeochemical cycling and hence are key components in ecosystem functioning. By feeding on organic matter from settled phytoplankton blooms they produce food for higher trophic levels, including fish. Faunal feeding activities also result in burial of bloom material into the sediment, where break-down of organic matter (mineralization) is slower, so that the bloom material can serve as food for the animals over a longer period of time. The overall aim of this thesis is to study the role of sediment-living animals in these two fundamental ecosystem processes connecting the bottom of the sea (benthos) with the open water (pelagic ecosystem). The simple ecosystem of the Baltic Sea is a good model system for detailed studies on the role of biotic factors for these processes, also because it experiences large scale changes in both phytoplankton dynamics and benthic biodiversity. Little is known of how increased blooms of toxic cyanobacterial blooms and invasive species affect marine ecosystem productivity. I experimentally studied how phytoplankton bloom material that reaches the sediment is processed by benthic fauna. More specifically, I have focused on the importance of phytoplankton quality for growth and nutrition of deposit-feeding macrofauna (I-V) and how macrofaunal species composition and species richness affect incorporation and burial of the bloom material (IV and V).

Figure 1. Simplified illustration of benthic-pelagic coupling from a food-web perspective. This thesis discusses mainly incorporation and burial of bloom material whereas mineralization has been measured only indirectly. Processes in grey are given only to present the larger picture into which the studied processes fit.

SedimentationTrophic transfer

PHOTIC ZONE

APHOTIC ZONE

Mineralization

INO

RGAN

IC N

UTRI

ENTS

Burial

Primary production

Faunal incorporation

Microbial loop

SedimentationTrophic transfer

PHOTIC ZONE

APHOTIC ZONE

Mineralization

INO

RGAN

IC N

UTRI

ENTS

Burial

Primary production

Faunal incorporation

Microbial loop

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BACKGROUND

Benthic-pelagic coupling – processes fundamental for ecosystem functioning.

In aquatic ecosystems, a considerable proportion of the primary production from phytoplankton blooms sink to the sediment (Wetzel 1995; Lignell et al 1993). Most benthic communities below the photic zone are entirely dependent on such imported organic matter (Elmgren 1978; Graf 1992). The growth and population dynamics of detritus-feeding animals living in sediments, so called deposit-feeders, are tightly coupled to sedimentation of organic material from the pelagic zone (e.g. Elmgren 1978; Graf 1992; Goedkoop and Johnson 1996; Lehtonen and Andersin 1998). Deposit-feeders play a key role in deep benthic ecosystems by converting detritus to secondary production available for higher trophic levels (Snelgrove et al. 1997). Also, the feeding, respiration and burrowing activities by the animals, collectively called bioturbation, have a major impact on the biogeochemical cycling of elements that limit primary production and hence on whole-scale ecosystem productivity (Meysman et al. 2006). The break-down of the deposited phytodetritus results in regeneration of nutrients in inorganic form (mineralization) that are subsequently made available for phytoplankton through bioturbation and mixing of water masses (Nixon 1981; Blackburn 1988; Graf 1992). The impact by deposit-feeders on mineralization can be both direct, through the metabolism of the animals and indirect, by stimulating bacterial activity. For instance, bioturbation results in enhanced transport and mixing of solids and solutes due to the increased area of sediment-water interface and improved oxygenation of deeper anoxic sediments, which enhance microbial activity and increase the mineralization rate (Aller and Aller 1998; Kristensen 2000; Mermillod-Blondin and Rosenberg 2006). Furthermore, bioturbation may result in burial of the settled phytoplankton into deeper sediment layers, where anoxia slows the rate of decomposition (Kristensen et al. 1995; Blair et al. 1996; Andersen 1996; van de Bund et al. 2001). In temperate water, the input of phytoplankton bloom material to sediments is a seasonal phenomenon and buried phytodetritus may therefore fuel the soft bottom communities for many months (Rudnick and Oviatt 1986; Andersen and Kristensen 1992; Josefson and Hansen 2003). Finally, bioturbation activities may influence population structure of the pelagic organisms with inactive benthic life-stages, e.g. resting cells of phytoplankton or dormant eggs of zooplankton, through facilitating or inhibiting their emergence from the sediment (Hairston et al. 2000; Ståhl-delBanco and Hansson 2002; Viitasalo et al. 2007).

Links between Biodiversity and Ecosystem functioning

Until recently, the assumption has been that biodiversity is the dependent variable responding to changes in ecosystem processes or functions (e.g. primary productivity or organic matter input) (Naeem 2002). However, due to rapid anthropogenically driven extinction rates and spread of non-indigenous species there are concerns that altered biodiversity will affect the “ecological services” that ecosystems provide to humanity (Costanza et al. 1997; Chapin et al. 2000; Hooper et al. 2005). The main biodiversity aspect studied in this thesis is species

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richness, however biodiversity in its broadest sense refers to genetic, taxonomic and ecological diversity over all spatial and temporal scales. Three main hypotheses that relate the responses of ecosystem processes to reductions in species richness have been proposed (e.g. Lawton 1994). First, the linear, “rivet” hypothesis suggests that all species contribute to ecosystem function (e.g. Lawton 1994). Second, the “redundancy” hypothesis suggests that ecosystems can lose many species with no consequences for their performance, as long as the major functional groups are still present, i.e. it is not the number of species per se which is important but the functional traits of the species (Walker 1992; Lawton 1994). Third, the “idiosyncratic” hypothesis states that species diversity affects ecosystem processes, but not in a predictable direction, because the roles of individual species are complex and context-dependent (Lawton 1994; Naeem et al. 1995). Clearly, the type of relationship observed depends on which measures of biodiversity (e.g. species richness within one or across several trophic levels) and on which function or functions are being considered.

Mechanisms relating biodiversity to ecosystem functioning

Accumulating theoretical and empirical evidence suggests a common positive relationship between biodiversity and ecosystem functioning (e.g. Loreau et al. 2001; Hooper et al. 2005; Cardinale et al. 2006). However, understanding the mechanisms that underpin this presumed causal relationship remain an important challenge when trying to predict the environmental consequences of species loss or addition (Loreau and Hector 2001; Naeem 2002; Fox et al. 2005). Three mechanisms that have been proposed to explain a positive relationship between species richness and ecosystem functioning are: (i) niche/trait differentiation, (ii) facilitation and (iii) the selection or dominance effect. The niche is often represented as an n-dimensional space, consisting of environmental conditions, resource levels and densities of other organisms, together allowing for the survival, growth and reproduction of species (Hutchinson 1957 in Leibold 1995). In this thesis, the niche concept is discussed mainly in relation to food resource partitioning. (i) Niche differentiation results from reduced interspecific competition through e.g. food resource partitioning. If species use different resources, more of the total available resources can be used and if these resources limit growth, then increasing richness should lead to greater productivity in an ecosystem (e.g. Tilman et al. 1997; Griffin et al. 2008). (ii) Facilitation occurs when coexisting species positively affect a process through direct interaction (e.g. Cardinale et al 2002; Bulleri et al. 2008). (iii) The dominance or selection effect occurs through selective processes, such as interspecific competition, which cause dominant species to outperform others in multi-species treatments (e.g. Cardinale et al. 2006; Jiang et al. 2008). Both niche differentiation and facilitation can increase the performance of communities above that expected from the performance of individual species, and are collectively referred to as complementarity. Dominance and complementarity, are both ecologically interesting mechanisms that can be confused with an experimental artifact called the “sampling effect”

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(Huston 1997), which is the increased probability with increased species richness of including a species with a particular trait in species-rich treatments. Analysis and interpretation of biodiversity experiments have progressed from simple comparisons of i.e. primary production across treatments to examining changes in relative performance (relative yield) among component species within and between treatments. Loreau and Hector (2001) used an equation to partition the net diversity effect (the observed deviation from the value predicted from individual species monoculture performance) into complementarity and selection components. Positive complementarity occurs when species function better than expected when combined, independently of their traits and not at the expense of other species. Dominance is analogous to natural selection in evolution (Price 1970) and occurs when species with particular traits dominate at the expense of other species. Fox (2005) further refined the equation, to take into account that species may have nested niches (trait-dependent complementarity), with species with wide niches attaining high relative yield in multi-species treatments, but not at the expense of species with narrow niches. See Box 1 for details. Box 1. Definitions and ecological interpretations (bold style) of diversity components calculated according to Fox (2005). The outcome for ecosystem functioning are also shown (bold style). TIC=Trait-independent complementaritry, TDC=trait-dependent complementarity, D=dominance.

Positive value

Negative value

TIC

Species increase their performance / yield without affecting other species. Facilitation or niche differentiation → Over-yielding

Species decrease their performance / yield at the expense of other species. Competition or inhibition → Under-yielding

TDC

Species with high monoculture yield increase their performance without affecting other species Nested niches → Over-yielding

Species with low monoculture yield increase their performance without affecting other species Nested niches → Under-yielding

D

Species with high monoculture yield dominate mixtures at the expense of other species No niche differentiation → Over-yielding

Species with low monoculture yield dominate mixtures at the expense of other species No niche differentiation → Under-yielding

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Current challenges in research on biodiversity and ecosystem functioning

As stated above, a prerequisite for differentiating among the possible mechanisms behind observed biodiversity-ecosystem functioning relationship is to determine the species-specific contribution to the measured process in mixed species assemblages. However, this is only possible when measuring non-cumulative processes (Rafaelli et al. 2003). Another problem that complicates interpretation of experimental results is that the effects of species loss have often been simulated by randomly or otherwise artificially reducing the species assemblages of naturally diverse ecosystems (see review by Loreau et al. 2001). Randomized species removal may result in the loss of a whole functional group present in the original community, sometimes leading to highly artificial set-ups with results difficult to translate to natural ecosystems (Bracken et al. 2008). Also, it is difficult to make inferences about the functioning of natural systems if only a single process has been measured, since different species can maximize different processes. Hence an approach measuring multiple processes is recommended (Gamfeldt et al. 2008). Moreover, integration with food web ecology is desirable since both bottom-up and top-down processes strongly affect secondary production and cycling of nutrients (Duffy et al. 2007; Woodward 2009). In other words, an important current challenge is to understand how trophic structure affects the relationship between biodiversity and ecosystem functioning.

Importance of food quality in ecosystem functioning

The flux of organic matter and nutrients in a food web is affected not only by the quantity but also the quality (elemental and biochemical content) of the food (Sterner and Elser 2002). Rates of ingestion, digestion, absorption and assimilation and resulting growth all depend critically on food quality. Research on the relationship between food quality and secondary production has so far concentrated on zooplankton (Brett and Müller-Navarra 1997; Sterner and Elser 2002; Vrede et al. 2002). Food availability and manageability affect ingestion efficiency, e.g. filamentous cyanobacteria may clog zooplankton feeding appendages (Holm et al. 1983). Differences in cell wall structure and in the level of decomposition of ingested food affects digestibility, and the chemical composition of the food affects the efficiency with which it is absorbed and assimilated. In particular, nitrogen (N) and phosphorus (P) have been considered important in this context (Elser and Sterner 2002). Large differences in elemental composition between the food source and the consumer have implications both for the growth of the consumer and for nutrient recycling in food webs (Sterner and Hessen 1994; Elser and Urabe 1999). The elemental limitation approach may, however, rather reflect limitation of one or more specific organic compounds (Müller-Navarra 1995). In recent years, the importance of essential long-chain poly-unsaturated fatty acids and sterols for invertebrate growth and reproduction has been demonstrated (Brett and Müller-Navarra 1997; von Elert et al. 2003). P content is sometimes a good predictor of the content of polyunsaturated fatty acids in phytoplankton material (Ahlgren et al. 1997), suggesting that it is important to take both approaches into account when the evaluating the nutritional quality of food. In addition, other

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factors, such as toxicity, are also important (Lampert 1981). Coping with toxin related stress may constrain the energy available for growth (Dahl et al. 2006).

Deposit-feeders and food quality

A particular challenge for deposit-feeders arises from the low levels of organic matter available to them in the sediment, forcing them to ingest large volumes of sediment in order to meet their nutritional requirements. They are often bulk-feeders, but can also be selective in terms of the organic-rich fraction of the sediment, feeding depth and particle size (Lopez and Levinton 1987). Especially nitrogen seems often to be limiting in benthic systems below the photic zone (Tenore 1988). Organic matter is mineralized at different rates depending on its chemical composition and on oxygen conditions in the sediment (Kristensen et al. 1995; Middelburg and Levin 2009). High quality organic matter may be decomposed rapidly in oxidized sediment, whereas organic matter which is more refractory can function as food for a longer time, thus contributing to long-term productivity despite its lower quality. Some sub-surface feeders show a preference for feeding on old organic matter rather then fresh phytodetritus (Rudnick and Oviatt 1986; Widbom and Frithsen 1995; Byren et al. 2006). Goedkoop et al. (2007) found that a sediment-dwelling freshwater midge could synthesize essential polyunsaturated fatty acids from low levels of shorter homologues, an example of an adaptation to a poor-quality food source. Temporary meiofauna and meiobenthos, such as bivalve spat and benthic eggs of zooplankton, may also serve as supplemental food sources (Elmgren et al. 1986; Karlson and Viitasalo-Frösen 2009). Bacteria play dual roles as organic matter decomposers by remineralizing organic carbon (into CO2) and as biomass producers (by incorporating carbon from the environment). Since they usually constitute a small fraction of the diet of macrofaunal deposit-feeders (Goedkoop and Johnson 1994), bacteria are still considered mainly a sink for organic matter in the sediment (van Oevelen et al. 2006). However, bacterial and meiofaunal conditioning of refractory organic matter may be an indispensable processing step in transforming organic matter into a form more readily digestible for macrofauna (Tenore et al. 1977). Bacterial activity can further facilitate trophic transfer through degradation of harmful natural toxins in organic matter (e.g. Mazur-Marzec et al. 2009). In conclusion, facilitative interactions among taxa can have a positive effect on trophic transfer and mineralization efficiency.

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STUDY SYSTEM: THE BALTIC SEA The Baltic Sea is a large, semi-enclosed, shallow brackish water system. Due to its recent geologic origin and low salinity, species diversity is low, with very few endemic species. Inflows of saline water from the North Sea through the Danish straits in combination with freshwater run-off result in a salinity gradient, ranging from about 20 in the south to 2 in the northern Bothnian Bay. The saltwater intrusions depend on specific weather conditions over the North Atlantic (Schinke and Matthäus 1998) and occur irregularly. The saltwater inflows create a strong permanent halocline at 60-80 m depth in the Baltic proper, which reduces vertical exchange between surface and bottom water. Under the halocline, oxygen is depleted, in the deeper parts generally to the point of anoxia.

Anthropogenic influences

More than 85 million people live within the drainage basin of the Baltic Sea and the ecosystem is under severe anthropogenic stress, through over-fishing, eutrophication and hazardous chemicals (Elmgren 2001). Increased nutrient load from anthropogenic activities during the last half century (Larsson et al. 1985) has resulted in increases in phytoplankton production and subsequent sedimentation of organic material (Jonsson and Carman 1994). This has resulted in anoxic bottoms becoming more widespread in the Baltic (Karlson et al. 2002). Since most multicellular organisms need oxygen, eutrophication negatively affects benthic fauna abundance, distribution, and diversity. Anoxic bottoms also influence cycling of nitrogen and phosphorus, which may result in accelerated eutrophication (Middelburg and Levin 2009). Another aspect of the anthropogenic influence is the largely unintentional spread of exotic species. In the last few decades more than 100 non-indigenous species have been recorded in the Baltic Sea, and about 70 of these have established reproducing populations (Olenin and Leppäkoski 1999; Leppäkoski et al 2002). The invasions of exotic species have significantly changed communities (Zettler et al. 1995; Perus and Bonsdorff 2004) and food web structure (Gorokhova et al. 2005) in the Baltic Sea.

Phytoplankton blooms in the Baltic Sea

As in other boreal waters, production is strongly pulsed and a succession of phytoplankton blooms takes place (Andersson et al. 1996; Höglander et al. 2004). The two major bloom events are the spring bloom, dominated by diatoms, and the summer bloom, dominated by filamentous, nitrogen-fixing cyanobacteria. The sedimentation of the spring bloom is high (Elmgren 1978; Lignell et al. 1993), since diatoms sink rapidly and zooplankton abundance at this time of the year is not high enough to graze down the algal biomass. Cyanobacterial blooms have been reported to have increased in frequency, biomass and duration in recent decades, probably due to increased phosphorus concentration as a consequence of eutrophication (Finni et al. 2001; Poutanen and Nikkilä 2001). The settling of cyanobacteria to the sediment have previously been considered negligible, but Gustafsson et

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al. (2004) recently showed that sediment trap collections may underestimate true rates of summer sedimentation several-fold in the Baltic Sea. Presence of pigment, toxin and isotope signatures specific for cyanobacteria have been demonstrated in Baltic proper sediments (Bianchi et al. 2000; Mazur-Marzec et al. 2007; Voss et al. 2005), suggesting that the cyanobacterial input to sediments may previously have been underestimated.

Nutritional aspects of blooms

Diatoms are generally considered to be good food for both zooplankton and benthic deposit-feeders due to their high content of lipids and polyunsaturated fatty acids, which are made available to the benthos due to efficient sedimentation (Andersson et al. 1993; Ahlgren et al. 1997). Sedimentation of spring bloom material results in rapid growth of benthic fauna, both in the Baltic (Elmgren 1978; Cederwall 1979; Lehtonen and Andersin 1998, Ólafsson and Elmgren 1997) and in lakes (Johnson and Wiederholm 1992). Cyanobacteria, on the other hand, have commonly been considered a low quality food source due to their potential toxicity and low content of polyunsaturated fatty acids (e.g. DeMott and Müller-Navarra 1997; Engström et al. 2000).

Figure 2. Surface accumulations of filamentous, nitrogen-fixing cyanobacteria, dominated by the toxic species Nodularia spumigena, in the Baltic proper, July 8, 2005 (courtesy of M. Kahru). In the Baltic Sea, the most common toxin is nodularin, which is produced by the filamentous nitrogen-fixing cyanobacterium Nodularia spumigena (Sivonen et al. 1989). Nodularin is a potent hepatotoxin, and surface accumulations of toxic cyanobacteria (Figure 2) have resulted in deaths of domestic animals and cause health concerns most summers (Edler et al. 1985). N.

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spumigena and nodularin have been shown to have negative effects on survival and reproduction of pelagic and littoral invertebrates and increased blooms of toxic cyanobacteria are suspected to have effects at the ecosystem level (Karjalainen et al. 2007). Several field studies have confirmed that bioaccumulation of nodularin takes place in Baltic zooplankton (Karjalainen et al. 2006), filter-feeders such as blue mussels and clams, and fish (e.g. Sipiä et al. 2001; Kankaanpää et al. 2001). Since it is generally assumed that cyanobacterial toxins are more harmful to vertebrates, including fish, than to other aquatic organisms (Kiviranta et al. 1991), nodularin may pose a health risk via transfer and accumulation in the food web. In addition, toxins other than nodularin are produced by cyanobacteria, for example the newly discovered neurotoxin beta-methylamino-L-alanine (Cox et al. 2005), which is suspected to reach humans through food chain transfer (Jonasson et al. 2008). Baltic soft-bottom benthos In the Baltic proper the soft-bottom community below the photic zone is dominated by only 4-7 macrofaunal species (Elmgren 1978) and about 50-60 meiofauna species (Elmgren 1976). Only three common species of native macrofauna are classified as deposit-feeders: the amphipods Monoporeia affinis and Pontoporeia femorata and the facultatively suspension-feeding bivalve Macoma balthica (Figure 3). The amphipods are morphologically similar but have different origin and ecological characteristics. M. affinis is a freshwater glacial relict while P. femorata is of marine origin. M. affinis is more active, has higher fecundity (Cederwall 1979) and is found closer to the sediment surface than P. femorata (Hill and Elmgren 1987). Both amphipods have a 2-year life cycle at sub-thermocline depths and are commonly the most abundant species, often occurring in densities of over 2000 ind m-2

(Ankar and Elmgren 1976; Cederwall 1999). They are night-active and mate during October to December, after which the males die. Females die after releasing juveniles around the time the spring bloom settles (Sundelin and Eriksson 1998). M. balthica often dominates the macrobenthic biomass in the Baltic (Ankar and Elmgren 1976, Laine 2003). It has a longer life cycle, and can live up to 27 years in deep waters (Segerstråle 1960). It switches feeding mode between suspension- and deposit-feeding, depending on food availability (Ólafsson 1986). Spawning takes place in late spring (Ankar 1980), and the planktonic larvae settle six to eight weeks later. All the native deposit-feeding species are important as food for both demersal and pelagic fish (Aneer 1975; Ehrenberg et al. 2005). They are also prey for three invertebrate predators: the isopod Saduria entomon, the polychaete Harmothoe sarsi and the priapulid Halicryptus spinulosus. In recent years, three species in the non-indigenous suspension- and deposit-feeding polychaete genus Marenzelleria spp. have invaded the Baltic Sea (Olenin and Leppäkoski 1999; Blank et al. 2008). Marenzelleria arctia is the dominant Marenzelleria species in muddy sediment bottoms at somewhat larger depths, sometimes co-existing with M. neglecta, while M. viridis is commonly found in shallow, sandy habitats (Blank et al. 2008). All species in this genus are probably facultative suspension-deposit-feeders and

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burrow much deeper in the sediment than the native Baltic species (Dauer et al. 1981; Zettler et al. 1995). Meiofauna, defined as benthic animals with a body size between 40 and 1000 µm, are omnivores and deposit-feeders. Generally, meiofauna have short generation times and often continuous reproduction, but their modes of feeding is highly specific (Giere 1993). Due to their high abundance (often over 106 individuals m-2 of sediment surface, corresponding to about 1 g dw of biomass), they play important roles in the benthic food web and for mineralization rates of organic matter (Goedkoop and Johnson 1996). In Baltic sediments, the most important groups are nematodes (constituting about 50% of the total meiofaunal abundance), followed by ostracods and harpacticoid copepods (Ankar and Elmgren 1976; Ólafsson and Elmgren 1997). Figure 3. Map showing the Baltic Sea and its major sub-basins and the gradient in species richness of common deposit-feeding macrofauna below the summer thermocline depth (from left: Monoporeia affinis, Pontoporeia femorata, Macoma balthica and Marenzelleria spp.). Dashed lines indicate addition of the invasive species.

Gulf ofBothnia

Bothnian Sea

Baltic proper

Askö

Gulf ofBothnia

Bothnian Sea

Baltic proper

Askö

Gulf ofBothnia

Bothnian Sea

Baltic proper

Askö

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EXPERIMENTAL APPROACH In this thesis the effect of phytoplankton on the growth of benthic fauna and the influence of the fauna on the fate of the phytodetritus have been studied in controlled laboratory experiments, where we simulated settling spring or summer blooms in microcosms with sediment, in which we had full control of the species assemblages. Fauna, sediment and phytoplankton bloom material were collected from the Askö area in the north-western Baltic proper and the experiments were carried out at the Askö Laboratory (Figure 3) in the summers of 2005 and 2006 and in spring 2008.

Isotope tracers in food-web studies

Isotope tracer techniques provide powerful tools for analyses of energy and material flow pathways and food web structures (Peterson 1999). Natural elements exist as both stable and unstable (radioactive) isotopes, i.e. atoms with the same number of protons but differing in the number of neutrons. The radioactive carbon isotope 14C is commonly used to study breakdown and uptake of settling organic matter (Lopez and Crenshaw 1982; Andersen and Kristensen 1992; Goedkoop et al. 1997; Byrén et al. 2002). Radioactive isotopes disintegrate with time, but the half-life of 14C is long (5570 years) and its specific activity low, making it suitable for experimental work. When added in carbonate form, 14C can be assimilated by algae or cyanobacteria during photosynthesis. By isotope labeling of phytoplankton (e.g. van de Bund et al. 2001; Byrén et al. 2006) it is possible to follow the fate of the added material. Labeled material taken up by animals and radioactivity in sediment can then be measured with a scintillation counter. The basic principle of this technique is to dissolve the radioactive sample in a liquid chemical medium capable of converting the kinetic energy of nuclear emissions into light energy.

Stable isotopes

Nitrogen and carbon have two stable isotopes each, 14N and 15N, and 12C and 13C, respectively. The heavy isotopes are less common than the lighter ones in biological material. In naturally occurring carbon only ~1% is 13C and in natural nitrogen only ~0.4% is 15N. The small difference in atomic mass between the isotopes can be measured with high accuracy by mass spectrometry and the ratio (R) between the heavy and the light stable isotopes (15N:14N and 13C :12C) in a sample is expressed as the deviation in ‰ from an international reference standard. This deviation is denoted δ and defined by the equation (Peterson and Fry 1987): δ (‰) =[(Rsample- Rstandard)/Rstandard] × 1000 The standards are VPDB (a limestone) for carbon, and atmospheric nitrogen gas for nitrogen. Many natural processes affect the relative abundance of stable isotopes in predictable ways (DeNiro and Epstein 1978). Therefore, natural isotopes are commonly used to determine diet and origin of animals. The interpretation of data obtained from stable isotope analyses is

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rather complex, since this technique relies on a set of assumptions. To allow more precise estimates of the diet and the transfer of organic matter than possible by using stable isotopes at natural abundance, the isotopic composition of phytoplankton can be modified as for radiolabelling, e.g. by culturing them in medium where some or all C and N are replaced with the heavy isotope (e.g. Moodley et al. 2002). To quantify the contribution of different food sources with clearly different isotope values (e.g. sediment vs isotope enriched bloom material) to a sample (animal or sediment), we used a simple linear 2-source mixing model (Fry 2006).

Assessing growth and nutritional value to animals

Methods for determining growth rate often rely on repeated measurements of individuals, which is time-consuming and difficult, especially in species that are sensitive to handling. However, biochemical indicators that reflect growth are a promising alternative (Buckley et al. 1999). The RNA:DNA ratio is a sensitive method for estimating the instantaneous rate of somatic growth. It is based on the assumption that the amount of deoxyribonucleic acid (DNA) is quasi-constant, while the amount of ribosomal ribonucleic acid (rRNA) changes in proportion to protein synthesis, or growth. Several studies have found that RNA:DNA ratios of e.g. zooplankton respond rapidly to changes in quantity and quality of food (Vrede et al. 2002; Wagner et al. 2001; Gorokhova 2003). RNA and DNA are quantified using a dye that exhibits fluorescence when binding to nucleic acids. After a first reading, a nuclease, RNAse, is added to the sample to break down RNA, leaving only DNA, which can be quantified after a second reading. Hence, RNA content is the difference between the two detections. The cyanotoxin nodularin was quantified in animals using liquid chromatography followed by mass spectrometry (Karlsson et al. 2003). Samples were first purified by adding the extract of the animal to a solid-phase extraction column of such properties that the compound of interest (nodularin) is retained in the column. Thereafter nodularin is eluted with a solvent to which it has a higher affinity for than the material in the column. The purified sample can then be identified and quantified using coupled high pressure liquid chromatography-mass spectrometry (HPLC-ESI-MS). The principle consists of ionizing chemical compounds to generate charged molecules or molecule fragments which are measured and quantified by an electron multiplicator using their mass-to-charge ratios.

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RESULTS AND DISCUSSION

Importance of phytoplankton quality for benthic fauna

This subsection discusses the results of papers I-III in which different aspects of incorporation and growth of deposit-feeders fed cyanobacteria were studied. Since there were no previous studies on the effects of cyanobacteria on deposit-feeders, our first question was whether they survived and fed on settled cyanobacterial bloom material (I). Thereafter we studied if they accumulate toxins (II) and how animal growth after feeding on cyanobacteria compares to growth on a spring bloom of diatoms (III).

Cyanobacteria as a food source

Due to their morphology, nutritional inadequacy and frequent toxic content, cyanobacteria are generally considered a poor food for invertebrates (Holm 1983; DeMott and Müller-Navarra 1997; Engström et al. 2000). However, most studies so far have used pure laboratory cultures of cyanobacteria fed to laboratory cultures of zooplankton in short term experiments (de Bernardi and Giussani 1990). Studies using natural cyanobacterial bloom material have shown less negative and sometimes even positive effects on pelagic organisms, e.g. by constituting a supplemental food source, because they are associated with a rich micro-heterotrophic community (Engström-Öst et al. 2002). In support of this, the results of paper I-III demonstrate that settling natural cyanobacterial bloom material can be utilized as food by deposit-feeders with no cost in mortality (see also Nascimento et al. 2008). Compared to spring bloom material (van de Bund et al. 2001, Byrén et al. 2006), incorporation of both non-toxic and toxic species of cyanobacteria was, however, relatively low and did not result in growth of amphipods (I). In paper II, where the supply of cyanobacteria was higher and duration of the experiment longer, incorporation was accordingly higher, but still none of the macrofaunal species increased significantly in biomass. Lack of growth can perhaps be attributed to an energy demanding process of coping with cyanotoxins (i.e. nodularin), as has been proposed for zooplankton and fish (Koslowski-Suzuki et al. 2003; Pääkkönen et al. 2008). All macrofaunal species tested accumulated nodularin (II), indicating that the fauna fed directly on cyanobacteria, rather than on aged material, which had already been detoxified by the bacterial community (Torunska et al. 2007; Mazur-Marzec et al. 2009). However, the observed lack of significant growth may also be related to the poor nutritional quality of cyanobacteria, rather than to their toxicity. Paper III showed that diatoms supported growth of the meiobenthos but toxic cyanobacteria did not. Even though the cyanobacterial sample had relatively high concentrations of aminoacids and proportions of polyunsaturated fatty acids, the amount of bioavailable aminoacids and the proportion of the essential fatty acid eicosapentaenoic acid, which is believed to determine efficiency in trophic transfer between primary producers and consumers (Müller-Navarra et al. 2000), were much lower in cyanobacteria than in diatoms from the spring bloom. In conclusion, settling blooms of cyanobacteria are not directly harmful to the benthos, but are not a nutritionally fully adequate food source.

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Changes in phytoplankton dynamics affect benthic production?

In the Baltic Sea, the spring bloom of diatoms is considered the most important input of high quality organic matter to the sub-thermocline soft-bottom community (e.g. Elmgren 1978; Uitto and Sarvala 1991). After the spring bloom has settled to the bottom, the main benthic growth period of the year ensues (Cederwall 1979; Lehtonen and Andersin 1998, Ólafsson and Elmgren 1997), as shown experimentally for meiofauna in paper III. Also, while no significant growth was recorded for any of the macrofaunal species in sediments with cyanobacterial input, significant growth was recorded when they were fed diatoms (paper V). Both studied amphipod species (M. affinis and P. femorata) time the release of their young to coincide with the spring bloom (Sundelin and Eriksson 1998) and the bivalve M. balthica invests in gonadal growth during this time (Ankar 1980). This suggest that time of the year strongly influences the species-specific performance regarding incorporation and that the cyanobacterial input may supply the soft-bottom community with a much needed energy source at a time when the high quality spring bloom input to the sediment has been exhausted. There is however still uncertainty concerning the quantity of the cyanobacteria that reaches the benthos, and detailed studies on this topic are needed. In recent years, the biomass of the spring bloom in the Baltic proper and Gulf of Finland has decreased, while the summer bloom of cyanobacteria has increased (Wasmund et al. 1998; Raateoja et al. 2005). These large scale changes have been related to hydrographic changes and to eutrophication (Suikkanen et al. 2007). Lehtimäki et al. (1997) suggested that especially N. spumigena may benefit from a future temperature increase in concert with increasing phosphorus concentrations. Monitoring data (Swedish National Monitoring Program) show a decrease of winter concentrations of dissolved inorganic nitrogen (N) since the early 90s and a recent increase of dissolved inorganic phosphorus (P), resulting in a lower N:P ratio, which favors cyanobacterial growth (Stal et al. 2003). Since native deposit-feeders seem to rely on nutrients found mainly in the spring bloom input for growth (III), a shift in phytoplankton composition towards dominance by cyanobacteria may affect benthic productivity negatively. In contrast, the invasive polychaete M. arctia grew almost as rapidly in sediments without diatoms (paper V) and fed on toxic cyanobacteria without cost in mortality (pers. observation), suggesting that it may gain a competitive advantage over the native species if the spring bloom input is reduced. Cyanobacterial blooms have sometimes been described as part of a vicious circle (Vahtera et al. 2007), since more P as a consequences of eutrophication enhances summer blooms, which then increase the N content of the Baltic Sea through N fixation (Larsson et al. 2001). Addition of N to the system further stimulates phytoplankton growth, with increased sedimentation causing more anoxic, P-leaking sediments as a consequence. The fact that increased cyanobacterial blooms not only enhance eutrophication but also increase bioaccumulation of cyanotoxins (that are potentially harmful to humans) adds a further dimension to the vicious circle (II). There are indications that trophic transfer of nodularin might be more effective in the benthic than in the pelagic food chain, since higher

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concentrations have been detected e.g. in flounders and blue mussels than in pelagic fish and zooplankton (see Karjalainen et al. 2007). Nodularin is considered more toxic to vertebrates than to invertebrates (Kiviranta et al. 1991), and fish exposed to nodularin, either directly or by feeding on nodularin-containing prey, have shown evidence of oxidative stress and liver damage (Persson et al. 2009; Vuorinen et al. 2009) as well as reduction in feeding activity (Karjalainen et al. 2005). Even though feeding on cyanobacteria seems not to harm the benthos, it may still lower their value as prey. In conclusion, the cyanobacterial input may provide maintenance energy during a critical time of the year, but is inferior to the spring bloom of diatoms in supporting growth of the benthic fauna. Hence, increased cyanobacterial blooms can not fully compensate for reduced diatom blooms. In addition, feeding on cyanobacteria could also result in a benthic fauna that is harmful as food for fish due to bioaccumulation of toxins, with potential for transfer even to humans.

Biodiversity effects on processing of phytoplankton bloom material

In this subsection I discuss the results of papers IV and V, where the amounts of added phytoplankton taken up by fauna and remaining in the sediment were compared among communities of benthic deposit-feeding macrofauna, differing in species composition and richness. The studies were performed at different times of the year (summer and spring) and used different types of phytoplankton (cyanobacteria and diatoms). Paper IV focused on the species composition and richness of native species whereas paper V also included an invasive species in the experimental design. These studies benefit from their methodological approach. Direct measurements of incorporation and burial and indirect calculations of mineralization losses have led to improved understanding of benthic-pelagic coupling, since these three processes together determine the fate of settled phytoplankton. An example of the links between the different measured processes is that burial increases retention of phytodetritus in the sediment, which supports continued incorporation in the longer term. Incorporation provides information on species-specific contributions to multi-species treatments, a prerequisite for partitioning of a diversity effect into diversity components (trait-independent complementarity, trait-dependent complementarity and dominance; Fox 2005). Furthermore, the ability to relate the result to the behaviour of the individual species and their distribution in the sediment allows a greater level of insight into the mechanisms underlying the relationships between biodiversity and ecosystem functioning than is typical of such studies.

Mechanisms behind biodiversity effects on measured processes

The surface deposit-feeders generally had high incorporation values (IV, V) while the sub-surface feeder showed lower values but instead had stronger effects on burial (IV, V) and on the oxidized layer (V) than the other species. Macoma balthica was the species that showed the most contrasting effects in the two experiments, with low incorporation of cyanobacteria

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but high incorporation of diatoms. This difference may either be related to differences in feeding activity over the year or to a particular sensitivity to cyanobacteria as a food source (Lehtonen et al. 2003). Of the studied species, Marenzelleria arctia differed from the native species primarily through its ability for rapid growth, also from sediment without diatom input. The species-specific differences in performance resulted in species composition always being significant for the outcomes of the studied processes. However, species richness had a significant positive effect on incorporation of cyanobacterial N. In the experiment with diatoms, only composition was significant for the corresponding treatments, but species richness had a weak but significant positive effect on N incorporation and burial of N when the M. arctia treatments were also included in the test. Hence, the addition of the invasive species enhanced processing of bloom material and therefore both studies suggest that a reduction in deposit-feeding species richness would decrease the effectiveness of benthic processing of phytoplankton bloom material. Reasons for the positive effects of species richness on incorporation and burial could be differences among the species in feeding depth, resulting in niche differentiation. M. balthica feeds mainly on the surface of the sediment and on suspended material, Monoporeia affinis in the upper centimeter and Pontoporeia femorata deeper down. Accordingly, by comparing the relative yield among component species within and among treatments, it was clear that positive trait-independent complementarity was the main cause for over-yielding in both studies. The dominance effect was small or even negative, in the latter case counteracting positive complementarity components (paper V). Hence, weak net biodiversity effects could result from contrasting effects of ecologically interesting mechanisms. The results from the partitioning are strengthened by previous studies on the interactions among these species. The amphipods M. affinis and P. femorata provide an especially interesting case, since both field and lab studies show that they segregate by depth and feed on different resources when in sympatry (Hill and Elmgren 1987; Byrén et al. 2006), providing a clear ecological explanation to the observed complementarity (IV, V). In contrast, the community composed of the two native surface-feeding species M. affinis and M. balthica, showed negative complementarity in spring time (V), indicating competition over the diatom input. Interestingly, these species seldom co-occur in high abundances on a local scale (Segerstråle 1973). By partitioning the net diversity effects in paper V it was demonstrated that, on average, communities including the invasive species showed larger complementarity than communities composed of only native species. This comparison is however confounded by the fact that in the substitutive design used, species richness inevitably increases when adding a new species. Positive complementarity in communities including the invasive species still suggests facilitation and low niche overlap among the species. According to invasion theory, ”empty niches” are likely to occur in species-poor ecosystems, since the level of interspecific

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competition is expected to be lower with fewer species (Elton 1958). However, incorporation of bloom material only represents one specific component of niche dynamics. Utilization of old organic matter also influences growth, as shown in paper V, where the sub-surface feeding P. femorata and the invasive species M. arctia had almost as high biomass increase in treatments without bloom input to the sediment. Successful invaders are often thought to possess traits making them functionally different from the native species (Shea and Chesson 2002). The ability of M. arctia to grow rapidly from sediment without spring bloom input (V) could be an example of such a trait, resulting in positive complementarity through resource partitioning or, if directly translated into higher fitness and abundance, leading to competitive suppression of competitors in a longer perspective (Byers 2000).

Long term changes in benthos: implications for ecosystem functioning

Long term monitoring data from both the Askö area (Figure 4) and other parts of the northern Baltic proper (Perus and Bonsdorff 2004; Laine et al. 2007) show that over the last 20 years, the bivalve M. balthica has increased in abundance and biomass while the two amphipods M. affinis and P. femorata, have decreased. M. balthica is more tolerant of low oxygen concentrations than the amphipods (Modig and Ólafsson 1998) and considering the large-scale changes in benthic communities due to eutrophication, this shift in community composition can be expected to have effects at the ecosystem level. Firstly, M. affinis has a high per capita uptake and metabolism (Cederwall 1979; van de Bund et al. 2001, IV, V) and constitutes an important trophic link to pelagic fish, such as the commercially important herring (Aneer 1975). Secondly, the bioturbation activities of the sub-surface feeding amphipod P. femorata proved to be especially important for retention of bloom material in the sediment as a large proportion was buried deeper down in the sediment where mineralization rates are slow (IV, V). Thirdly, the amphipods incorporated more C relative to N from the bloom material, while the opposite is true for M. balthica (IV, V). These species-specific differences in elemental requirements can have consequences for cycling of N. In paper IV, the M. balthica monoculture showed increased mineralization of both C and N relative to multi-species treatments, perhaps due to the observed high oxygen penetration depth in the sediment in this treatment. Hence, increased abundance of M. balthica could result in more rapid mineralization of bloom material, with less food supplied to sub-surface feeders and lower resource utilization. A reduction of species richness would lead to lower food web productivity in the long run, since the most species-rich community was also the most efficient in both incorporation and retention of cyanobacterial material (IV). Rapid mineralization in the presence of M. balthica is in accordance with a study in which this species stimulated mineralization more than M. affinis (Karlson et al. 2005). Furthermore, Viitasalo-Frösen et al. (2009) demonstrated that M. balthica (and M. cf. arctia) increased fluxes of ammonium and phosphate out of the sediment while M. affinis had the opposite effect. Such internal feed-backs on nutrient cycling as consequences of an altered benthic community may accelerate eutrophication.

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However, the results from paper V show that, given the right circumstances, M. balthica can be highly efficient in incorporation of bloom organic matter. Not only did M. balthica show as high per capita incorporation of diatom C and N as M. affinis, it even managed to grow when fed sediment without spring-bloom input, whereas M. affinis did not. This demonstrates that the type or quality of phytoplankton can affect the link between biodiversity and ecosystem functioning.

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Figure 4. Abundance of macrofaunal deposit-feeders in the Askö region from a 37-year time series (Swedish National Marine Monitoring Program; Cederwall, pers comm.). Data are averages from 4 stations at 20-40 meters depth. See text for details.

Recent changes in benthic biodiversity: implications for ecosystem functioning

On a regional scale, the biodiversity of deposit-feeders in the Baltic Sea has increased in recent years due to the invasion of the genus Marenzelleria spp. The first individuals were found in the Askö area in 2002 but the abundance at the National Marine Environmental Monitoring stations at 20-40 m depth remained low until 2006, after which it increased considerably (H. Cederwall, Dept. Systems Ecology, SU, pers. comm.). The Marenzelleria spp. invasion has in some areas been considered responsible for the decline of M. affinis (Zettler et al. 2002). This seems unlikely in the Askö area since the abundance of M. affinis has been decreasing here since the late 1970s (Figure 4). Still, Marenzelleria spp. abundances can locally reach abundances of 30,000 individuals m-2 (Zettler et al. 2002; pers obs.) and might therefore hinder a potential recovery of the M. affinis population. The invasions of the Marenzelleria spp. in the Baltic ecosystem have so far not been studied from a trophic perspective. This is complicated by the three Marenzelleria spp. potentially differing in their resource requirements. M. viridis occurs primarily in shallow sandy bottoms while M. arctia is dominant at larger depths in muddy bottoms, where it sometimes co-occurs with M. neglecta (Blank et al. 2008). Several studies demonstrating competition for space or

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resources (Neideman et al. 2002, Kotta and Ólafsson 2003) and studying the effects of Marenzelleria on contaminant fate (Hedman et al. 2008) and nutrient fluxes (Norling et al. 2007) have used animals collected from a shallow sandy habitat dominated by M. viridis or M. neglecta in microcosms intended to simulate deeper sediments, where the naturally dominant Marenzelleria species is M. arctia (for exceptions of studies concerning nutrient fluxes, see Hietanen et al. 2007 and Viitasalo-Frösen et al. 2009). The results of paper V show that M. arctia did not have a negative effect on utilization of bloom material by the native species. Eriksson-Wiklund et al. (2008) likewise found no evidence of negative interactions between M. cf. arctia and M. affinis and concluded that the reason for a dramatic decrease in M. affinis abundance in the Bothnian Bay during 1999-2003 was lower food quality (reduced spring bloom and increased terrestrial runoff), rather than the invasion of M. arctia. The ability of M. arctia to grow rapidly on sediment without spring bloom input has likely been important for the successful invasion of this species, but can perhaps increase competition over old organic matter with the native species during the part of the year when spring-bloom material is not available. Ehrenberg (2008) found that the sand goby, the main vertebrate predator on sub-thermocline bottoms in the Baltic proper (Ehrenberg et al. 2005), feeds on Marenzelleria spp., and so the addition of this species could increase trophic transfer of both sediment organic matter and freshly deposited bloom material. However, it is still possible that Marenzelleria spp. escapes predation better than the native species or is a less preferred food item, which would give the opposite result. Overall, the long-term net effect on ecosystem functioning, by the addition of Marenzelleria spp. to the benthic community, is difficult to evaluate. Bioturbation by Marenzelleria spp. can result in high P fluxes from the sediment (Hietanen et al. 2007; Viitasalo-Frösen et al. 2009) and in release of sequestered contaminants (Granberg et al. 2008). Increased P-fluxes could lower the N/P ratio, thus favouring cyanobacterial blooms (Stal et al. 2003), and remobilization of toxins could potentially slow the decreasing trends of contaminants recorded in Baltic biota (Bignert et al. 1998), both undesirable outcomes.

Generalizations of biodiversity results to other systems

These two studies (IV, V) are the first to test the effects of altered deposit-feeding biodiversity on incorporation and burial of organic matter from phytoplankton blooms. However, in addition to exploring relationships between biodiversity and ecosystem functioning in a poorly studied system, our studies also provide links to food web ecology (through consumer-resource interactions), evolutionary ecology (through the discussion on different life history traits in respect to feeding activity and elemental requirements) and invasion biology (through discussion on niche differentiation and competition). Bridging research fields is necessary for deeper insights concerning relationships between biodiversity and ecosystem functioning (Duffy et al. 2007; Schmitz et al. 2008; Woodward 2009). The Baltic Sea, with its naturally low biodiversity and clear gradient in species richness, proved to be a good system for testing hypotheses on the link between biodiversity and

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ecosystem functioning (Elmgren and Hill 1997). The results can, in a unique way, be extrapolated to field situations, since the species combinations used in experimental set-ups represent real species combinations found in the field. Considering the large-scale changes in benthic biodiversity observed in the study area, and the wider Baltic in general, inferences to natural systems are both facilitated and strengthened. The results from this species-poor system are likely widely applicable to other systems where detritivores drive nutrient and energy cycling, like sediments of lakes and seas, and perhaps even terrestrial soils, and especially to stressed environments with reduced species richness like sediments subject to periodic hypoxia and anoxia. On the other hand, it is difficult to make extrapolations to very species-rich systems, where each functional group is represented by many species. Since the macrofaunal species studied in this thesis seemed to maximize different processes, they can not be considered redundant, but must be considered to belong to different functional groups. The redundancy hypothesis can therefore not be tested in this system. Raffaelli et al. (2003) found that increased species richness of benthic macrofauna belonging to different functional groups had a significant effect on nutrient fluxes from shallow sediments while increased species richness within the same functional group had no effect. Taken together, the results of papers IV and V tend to support the idiosyncratic hypothesis, as also found in several other studies of detritivores in soil and soft bottom ecosystems (Mikola and Setälä 1998; Emmerson et al. 2001; Hiddink et al. 2009), where the effect of species on ecosystem processes are unpredictable and context-dependent. From a management point of view it would imply that management should strive to retain all species, since we cannot predict the outcome of ecosystem processes from the monoculture performances of the relevant species and since species richness tends to improve ecosystem functioning.

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CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH This thesis shows that both the type of phytoplankton bloom and the species composition and richness of the benthic community affect the efficiency of resource utilization for settled phytoplankton bloom material. The native Baltic deposit-feeding macrofauna seem to depend largely on diatoms for growth even though the summer bloom of cyanobacteria can provide some additional and perhaps crucial maintenance energy before the winter starvation. Furthermore, it seems that the spring bloom input supports coexistence between native species and the invasive polychaete Marenzelleria arctia, since the latter was not a strong competitor for diatom bloom material. Therefore, reduced spring blooms might favour the invader. The type of phytoplankton may also affect the nutritional value of the benthic fauna for their predators - even though feeding on toxic cyanobacteria may have no immediate negative effect on the benthic invertebrates, their vertebrate predators may be more sensitive. The nutritional value of benthos as prey for higher trophic levels is an important future study area. The species-poor sediment of the Baltic Sea proved to be a good system for studying the link between biodiversity and ecosystem functioning, and for elucidating the ecological mechanisms behind the successful invasion of an exotic species, which is now an important benthic element in many areas of the Baltic Sea. Species-rich communities had higher incorporation of bloom material than expected, due to complementarity (niche differentiation and interspecific facilitation) among these functionally different species. The effect of species composition vs species richness on incorporation depended on the type of phytoplankton and also season, perhaps due to differences in life-history strategies among the species. However, incorporation of bloom material represents only one specific component of niche dynamics, and animal growth depends also on e.g. competition for old organic matter in the sediment. The invasive species showed resource partitioning with the native species when bloom input was available, but competition can be expected to be more intense during the rest of the year. Taken together, the data supports the idiosyncratic model of the relationship between biodiversity and ecosystem functioning, meaning that species performances and hence outcomes of ecosystem processes are context-dependent. Future studies should aim at simultaneously manipulating quality of phytoplankton and species richness over several trophic levels (i.e. including predators), to study how these factors jointly impact ecosystem functioning (Cardinale et al. 2009). In conclusion, we have provided evidence that altered benthic biodiversity leads to demonstrable changes in food web structure and ecosystem functioning.

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ACKNOWLEDGEMENTS Studying deep sediment is a cold and dirty job… But also fascinating since it is a transdisciplinary research field. I have had the pleasure to work with a number of skilled people. First and foremost, I would like to thank my supervisor Ragnar Elmgren, who has always been ready to give constructive criticism, improve any text in an elegant way and given me a lot of scientific and financial freedom! My closest colleagues and good friends, Francisco Nascimento and Johan Näslund: thanks for all the happy memories of planning and terminating experiments and discussions over small and big things. Thanks Johan for all the last minute help! Thanks also to my other coauthors for good collaboration: Elena Gorokhova for always being “spot on” the weak points in a manuscript, Raimondas Motsuraitis for mastering the HPLC-ESI-MS and Sara Blomgren Rydén for weighing more tin cups than any other master student. Askö laboratory staff and the Chemistry lab personnel are acknowledged for good collaboration. I would like to thank Clare Bradshaw for reading several of my papers and Jonas Gunnarsson for being so enthusiastic. Several happy assistants have helped sieving endless amounts of sediment and picking animals: Erland Karlson, Elke Morgner, Hanna Axemar and Ola Svensson. Thanks also to all the PhD students at the department and other departments (including former colleagues - you know who you are!) for all the fun during and after work. Thanks to Antonia Sandman for help with figures and file formats and Lisa Almesjö for the Baltic Sea map. Special thanks go to my former and present room mates Sigrid Ehrenberg, Francisco Nascimento and Peter Sylwander for being patient with my messy desk(s). I would also like to thank Satu Viitasalo and Sanna Suikkanen for good collaboration at Tvärminne in 2007, even though that work did not make it into this thesis, and to my mother Anna-Karin Borg-Karlson and the group at Ecologic Chemistry for help with nodularin analyses. Apart from pure science it has been great fun teaching with Mikael Tedengren at Askö and Tjärnö, and it would of course not have been as fun without all the Bio-geo students! Mamma och Pappa, tack för alla middagar och för all hjälp med huset! Finally, I would like to thank Doug Jones for enthusiastically reading my manuscripts and the summary, and for all the adventures we have had so far and for the ones to come! The work within this thesis was funded by: Swedish Scientific Research Council and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (Formas) (grants to Ragnar Elmgren), and to me from Stockholm Marine Research Center and Lars Hiertas Minnesfond.

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variations in food quality for pelagic and benthic invertebrates in Lake Erken - the role of fatty acids. Freshwater Biology 38: 555-570

Aller, R. C. and Aller, J. Y. 1998. The effect of biogenic irrigation intensity and solute exchange on diagenetic reaction rates in marine sediments. J. Mar. Res 56: 905-936 Andersen, F. Ø. 1996. Fate of organic carbon added as diatom cells to oxic and anoxic marine sediment microcosms. Mar. Ecol. Prog. Ser. 134: 225-233 Andersen, F. Ø. and Kristensen, E. 1992, The importance of benthic macrofauna in

decomposition of microalage in a coastal marine sediment. Limnol. Oceanogr. 37: 1392-1403

Andersson, A., Selstam E., Hagström Å. 1993. Vertical transport of lipid in seawater. Mar. Ecol. Prog. Ser. 98: 149-155 Andersson, A., Hajdu, S., Haecky, P., Kuparinen, J., Wikner J. 1996. Succession and growth

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Ankar, S. 1980. Growth and production of Macoma balthica (L.) in a northern Baltic soft bottom. Ophelia (Suppl. 1): 31-48 Ankar, S., Elmgren, R. 1976. The benthic macro- and meiofauna of the Askö-Landsort area,

northern Baltic proper – a stratified random sampling survey, Askö laboratory, no 11,University of Stockholm Sweden

Bianchi, T. S., Engelhaupt, E., Westman, P., Andrén, T., Rolff, C., Elmgren, R. 2000. Cyanobacterial blooms in the Baltic Sea: natural or human-induced? Limnol. Oceanogr. 45: 716–726.

Bignert A., Olsson M., Persson W., Jensen S., Zakrisson S., Litzen, K., Eriksson, U., Haggberg, L., Alsberg T. 1998. Temporal trends of organochlorines in Northern Europe, 1967-1995. Relation to global fractionation, leakage from sediments and international measures. Environmental Pollution 99: 177-198

Blackburn T. H. 1988. Benthic mineralization and bacterial production. In Blackburn, T. H., Sorensen, J. (eds) Nitrogen cycling in coastal marine environments. John Wiley and sons, New York, pp. 190-206

Blair, N. E., Levin, L. A., DeMaster, D. J., Plaia, G. 1996. The short-term fate of fresh algal carbon in continental slope sediments. Limnol. Oceanogr. 41: 1208-1219 Blank, M., Laine A. O., Jürss, K., Bastrop, R. 2008. Molecular species identifcation key based

on PCR/RFLP for discrimination of three polychaete sibling species of the genus Marenzelleria, and their current distribution in the Baltic Sea. Helgol. Mar. Res. 62:129– 141.

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Bracken, M. E. S., Friberg, S. E., Gonzalez-Dorantes C. A., Williams, S. L. 2008. Functional consequences of realistic biodiversity changes in a marine ecosystem. Proceedings of the National Academy of Science 105: 924-928

Brett, M. T., Müller-Navarra, D. C. 1997. The role of highly unsaturated fatty acids in aquatic food web processes. Freshwater Biology 38: 483-499

Buckley, L., Caldarone, E., Ong, T. L. 1999. RNA-DNA ratio and other nucleic acid-based indicators for growth and condition of marine fishes. Hydrobiologia 401: 265–277.

Bulleri, F., Bruno, J. F., Benedetti-Cecchi, L. 2008. Beyond competition: Incorporating positive interactions between species to predict ecosystem invasibility. PLOS Biology 6: 1136-1140

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