MARINE ECOLOGY PROGRESS SERIESMar Ecol Prog Ser

Vol. 378: 71–80, 2009doi: 10.3354/meps07885

Published March 12

INTRODUCTION

The distribution of seagrasses along coastlines isdetermined not only by water depth (Duarte 1991) andlight availability (Dennison et al. 1993), but also bytemperature, salinity, wave exposure (Fonseca & Bell1998), current velocity and sediment characteristics(Koch 2001). While temperature and salinity determinewhich species will be present in an area (den Hartog1970), waves and currents determine the presence orabsence of seagrasses in areas where sufficient lightis available (Koch 2001). Sediment composition, i.e.amount of sand or mud, affects sediment geochemistryand microbial nutrient dynamics such as nitrogen fixa-tion, which in turn can also affect seagrass growth

(Capone 1982, Short 1987, Murray et al. 1992, Perry &Dennison 1999). Sediment organic content plays amajor role in microbial nutrient dynamics. For exam-ple, organic rich sediments appear to have highernumbers of diazotrophs (nitrogen fixing bacteria) thanorganic poor sediments (O’Neil & Capone 1989). Sea-grasses themselves also influence microbial nutrientdynamics by exuding organic compounds in the rhizo-sphere, increasing heterotrophic bacterial nitrogenfixation, which may in turn provide more nitrogen forseagrass growth (Perry & Dennison 1999).

Although extensive research has been done onthe effect of light on seagrasses and, to a lesser extent,on the effect of temperature, salinity, currents andwaves on seagrasses, fewer studies have investigated

© Inter-Research 2009 · www.int-res.com*Email: caroline.wicks@noaa.gov

Effects of sediment organic content and hydrodynamic conditions on the growth

and distribution of Zostera marina

E. Caroline Wicks1, 2,*, Evamaria W. Koch2, Judy M. O’Neil2, Kahla Elliston2, 3

1NOAA-UMCES Partnership, NCBO-Cooperative Oxford Laboratory, 904 South Morris Street, Oxford, Maryland 21654, USA2Horn Point Laboratory, University of Maryland Center for Environmental Science, 2020 Horns Point Road, Cambridge,

Maryland 21613, USA3Biology Department, 3258 Texas A & M University, College Station, Texas 78843, USA

ABSTRACT: The hypothesis that sediment organic content is limiting growth and distribution of theseagrass Zostera marina was tested in Chincoteague Bay, Maryland, and in a controlled mesocosmexperiment. In the field, Z. marina was usually absent from areas with sediment organic content> 4%, especially compared with areas with sediment organic content < 4%. In contrast, in a meso-cosm experiment, Z. marina thrived in organic rich (4 to 6%) sediment, developing long leaves anddisproportionately short roots. Such plants have high drag and low anchoring capacity. As a result,Z. marina plants grown in organic rich sediment are more likely to be dislodged than are plantsgrown in organic poor sand. We hypothesize that when organic rich sediments are found in hydrody-namically active areas, a mismatch occurs between plant morphology and the physical environment,leading to the loss of seagrasses due to uprooting. Therefore, sediment organic content limitations inseagrass habitats need to be evaluated within the local hydrodynamic settings. Fine organic sedimentmay be less limiting to seagrasses in quiescent waters while sand with low organic content may berequired for seagrass survival in hydrodynamically active areas.

KEY WORDS: Seagrass · Zostera marina · Eelgrass · Sediment organic content · Salt marsh ·Sediments · Morphology · Nitrogen fixation

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Mar Ecol Prog Ser 378: 71–80, 2009

whether sediment characteristics limit the growth anddistribution of seagrasses. The studies that have beenperformed have focused on sediment porewater nutri-ents (Short 1983, Murray et al. 1992) or hydrogensulfide (Carlson et al. 1994, Brueechert & Pratt 1996).Porewater nutrient concentrations are closely relatedto sediment organic content (Berner 1977). Organicpoor sediments, such as sand, are considered to benutrient limiting to seagrasses due to low levels ofammonium and phosphate (Holmer et al. 2001). Con-versely, organic rich sediments, such as mud, typicallyhave high amounts of ammonium and phosphate(Berner 1977). Hydrogen sulfide can be toxic to sea-grasses, but usually only when another environmentalstressor (e.g. low light, high temperature) is presentand sulfide levels are higher than normally found inhealthy seagrass beds (6 mM, Koch & Erskine 2001). Incontrast to porewater nutrients and hydrogen sulfide,sediment composition (organic content and grain size)has not been studied extensively. Such data are impor-tant for understanding the synergistic effects of sedi-ment type, geochemistry and dynamics on seagrassgrowth and distribution.

The motivation for this study was the observation ofa variety of sediment types from old marsh peat (aresult of marsh retreat) to sand (from eroding dunes)in subtidal seagrass habitats in Chincoteague Bay,Maryland, USA, and the fact that the sediments adja-cent to retreating marshes appear to be unvegetated.Therefore, our hypothesis stated that the organiccontent of subtidal sediments adjacent to retreatingmarshes (i.e. old marsh peat as the substrate in sea-grass habitats) limits seagrass growth. This hypothesiswas tested using a combination of in situ observationsand controlled experiments.

MATERIALS AND METHODS

Study site. Chincoteague Bay is a lagoonal system,characterized by shallow depths (<6 m), restrictedflushing and limited freshwater input (Wazniak & Hall2005). Mill’s Island in Chincoteague Bay (Fig. 1) wasselected as our study site based on the presence of sea-grasses, specifically Zostera marina, as well as a vari-ety of sediment types in a relatively small area. This isthe result of substantial marsh retreat (0.59 m yr–1) overthe past century (Wicks 2005) leading to marsh lossand erosion of a local dune (Fig. 1). The present studytook place at the southeast portion of the island, alonga beach extending from southwest to northeast. Thesubstrate in the seagrass habitat (i.e. <1 m waterdepth) along the southeast shoreline was dominatedby old marsh peat where seagrasses were absent, anda sand layer (1 mm to >10 cm) over old marsh peat

where seagrasses were present (Fig. 1). Old marshpeat is the term we use to describe highly compacted,clay sediment that has been buried by marsh processes(accretion) over decades. There is little or no refractorymarsh material (not quantified) remaining in thissediment. Due to sea level rise, marsh plants are dyingin this area (Stevenson et al. 2002) making the marshmore vulnerable to erosion. As the marsh erodes due towave action and more frequent flooding, the shorelineretreats exposing marsh peat, which becomes the sub-tidal seagrass habitat. The old marsh peat is coveredwith sand in some locations. The source of sand in thestudy area is an eroding dune within the marsh system(Fig. 1), a common process in the area (Rosen 1980).The sand from the eroding dune is carried by currents,deposited over the subtidal old marsh peat andreworked by waves. The astronomical tidal range atthe study site is less than 30 cm (i.e. microtidal) and thesalinity ranges from 18 to 35 (Wazniak & Hall 2005).

Light availability. To assure that shoreline erosionwas not increasing turbidity near shore to the pointthat light was limiting to seagrasses, the spatial vari-ability of light availability was determined on July 6,2004. Light measurements (LICOR LI 193 sphericalunderwater sensor) and GPS coordinates (eTrex, Gar-min International; accuracy = ±3 m) were taken at 5points along each of 10 transects that ran perpendicu-lar to the shoreline. This ensured that the entire areawhere seagrasses could be growing along the south-east side of Mill’s Island was covered. Using theLambert-Beer equation—Kd = –ln(I0/Iz)/z, where I0 isthe light just below the surface, Iz is the light at depth(z) and z is the difference in depth between I0 and Iz —the light attenuation coefficient (Kd) was calculatedbased on light measurements at 0.2 m (I0) and 1.4 m (Iz)below the surface.

Field surveys. Field surveys and aerial photographsshowed the seagrass bed to be narrow and to followthe shoreline. In August 2004, sediment samples (cores5 cm in diameter) were taken inside and outside theseagrass bed to determine the thickness of the sandoverlaying the old marsh peat, grain size and organiccontent. The sand and old marsh peat layers in thesecores were separated based on differences in grainsize and sediment color. The 2 different layers werethen put in separate, labeled plastic bags and taken tothe laboratory for characterization of grain size andorganic content (Erftemeijer & Koch 2001). Dry andwet sieving (USA Standard Testing Sieve, AmericanSociety for Testing and Materials [ASTM] E-11 Specifi-cation, W. S. Tyler, –1 through ≥ 5 Φ units) were used tocharacterize coarse (≤ 3 Φ) and fine (>3 Φ) grain size,respectively. Organic content analysis consisted ofcombustion at 450°C for 4 h (Barnstead ThermolyeFurnace 30400).

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Wicks et al.: Sediment organic content and Zostera growth

In June 2005, the waters off the southeast shorelineof Mill’s Island were surveyed for seagrass distributionalong the 80 cm depth contour (maximum depth of dis-tribution is 110 cm). Fifteen points along the 80 cmdepth contour that represented the range of sediment(sand, old marsh peat and sand overlaying old marshpeat) and seagrass shoot density (vegetated andunvegetated) combinations found at Mill’s Island werechosen. At each point, GPS coordinates (eTrex), waterdepth (meter stick), seagrass species and shoot density(25 × 25 cm quadrat) were measured. Sediment sam-ples were collected for analysis of thickness of the sandlayer overlaying the old marsh peat and organic con-tent, using the same methods as in 2004.

To determine whether plant biomass was affected bysediment organic content in situ, seagrass sampleswere taken at the same 15 points along the 80 cm

depth contour where the sediment samples were alsotaken. At each location, 3 samples of seagrasses werecollected with a 5 cm diameter core within 50 cm ofwhere sediment samples were taken. All plant mater-ial was rinsed in seawater in a sieve prior to bagging toremove any sediment attached to the roots and rhi-zomes. Samples were taken back to the laboratory andrefrigerated (6°C) until samples were separated intoaboveground and belowground biomass, placed in adrying oven (50°C), dried to constant weight andweighed to determine biomass.

Due to the stratified nature of the sediment colonizedby seagrasses at this site, an organic content valuefor the top 15 cm of sediment (maximum rhizospheredepth) was estimated for each sampling location. Thisvalue was then related to seagrass biomass. The depthof the rhizosphere was determined by measuring all

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Fig. 1. Location of the study site at Mill’s Island (38° 03’ N, 75° 18’ W) in Chincoteague Bay on the eastern shore of Maryland, andlocation of the Zostera marina bed studied (dashed outline). Edge of the seagrass bed was determined by visually assessing a2003 aerial photograph (courtesy of the Virginia Institute of Marine Science) for a change in color between the bed and bare sand.

Shoreline retreat at this site has occurred at a rate of 0.59 m yr–1 for the past century

Mar Ecol Prog Ser 378: 71–80, 2009

root lengths (n = 231) of 92 Zostera marina plants col-lected at 10 random sites within the seagrass bed. Asrhizomes of Z. marina are normally buried, the maxi-mum root length underestimates the rhizospheredepth. The equation used to determine the weightedestimate of sediment organic content (OCest) in theseagrass rhizosphere was:

OCest = (Fs × OCs) + (Fomp × OComp) (1)

where Fs was the fraction (unitless) of the top 15 cm ofsediment that was sand, OCs was the percentageorganic content (%) of the sand, Fomp was the fractionof the top 15 cm of sediment that was old marsh peatand OComp was the percentage organic content of theold marsh peat. Therefore, as the OCest increased,actual samples changed from sand only to sand over-laying old marsh peat to old marsh peat only.

Sediment organic content experiment. An outdoormesocosm (3.07 m long × 0.66 m wide × 0.60 m high)with extensive aeration (using air stones) for carbondioxide supply and water movement was used to de-termine the response of Zostera marina to differentsediment organic contents. Use of a single mesocosmcontaining all organic content treatments ensured thatnutrient concentrations in the water column wereequal for all organic content treatments independentof nutrients possibly being released from the sedi-ments into the water column. To obtain different sedi-ment organic contents (0.1, 0.5, 1.2, 4.4 and 5.9%), dif-ferent types of sediments were mixed (Table 1). Thedegree of compaction of the experimental sediment(not quantified, only observed by touch) was differentfor all treatments. The 4.4% organic content treatment(100% old marsh peat) was the most compacted as itwas left in its natural state for the experiment. Threereplicates of each treatment were used, totaling 15compartments, each 25 cm long × 19 cm wide × 10.5 cmdeep.

The sediments were placed in an indoor annularflume in December 2004 to allow for equilibration ofgeochemical gradients for 3 mo under continuous waterflow (10 cm s–1 at sediment surface) and 20°C. Duringthis period the water (filtered Choptank River water,salinity = 10 to 15) was changed weekly.

The compartments were then moved to the outdoormesocosm in April 2005 for the start of the experiment.The mesocosm was covered with 2 layers of neutraldensity screening to prevent high water temperaturesand to minimize epiphytic growth. Two air pumps(Optima, no. 807) provided carbon dioxide and watermovement. There were no additional currents orwaves generated in the mesocosm; therefore, condi-tions were characteristic of a sheltered site. Ambientestuarine (Choptank River) water was combined withCrystal Sea Marinemix (Marine Enterprise Interna-tional) to raise the salinity to that of the collection site(salinity = 25) and a 50% water change occurredweekly. Zostera marina seedlings (single shoots) fromChincoteague Bay were planted in the compartments(4 plants per compartment) and were allowed to growfor 8 wk. At the end of the experiment, all plant mater-ial was processed the same way as were field samples.

Additionally, at the end of the experiment sedimentsamples (1 per compartment) were taken for determi-nation of nitrogen fixation, which was investigated inthe 5 organic content treatments using the acetylenereduction assay to determine the rate of nitrogenaseactivity (Burns & Hardy 1975, Capone 1982). Acetylene(C2H2), generated by reacting calcium carbide withwater, was added to gas-tight flasks containing sedi-ment samples at a volume equal to approximately 10 to15% of the gas phase volume. Acetylene is preferen-tially reduced to ethylene (C2H4) by nitrogenase (theenzyme responsible for nitrogen fixation). Ethylene isreadily detectable using gas chromatography-flameionisation detection (GC-FID). To determine the ethy-lene formation over time, 0.10 ml of gas from the head-space volume of the experimental flasks was with-drawn using a gas-tight syringe and analyzed on aShimadzu 8A gas chromatograph (GC) with a Hay-Sep A column equipped with an FID. The gas phase ofeach sample was measured over a 24 h time period toassess the rate of production of ethylene. Sample val-ues were compared with a standard of known concen-tration of ethylene (100 ppm). Samples were run underboth aerobic and anaerobic conditions to test for differ-ent nitrogenase activities by different diazotrophicbacterial populations, which can range from micro-aerophilic (low oxygen) to strictly anaerobic. Sedimenttreatments were amended with 30 µM glucose as alabile organic source to simulate seagrass exudationand resulting nitrogen fixation, and organic carboncontent was determined on amended samples. Anaer-

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Table 1. Sources of sediment used to obtain a range of sedi-ment organic content treatments for a mesocosm experiment.Beach dune sand obtained from Assateague State Park, MD;marsh dune sand obtained from Mill’s Island, ChincoteagueBay, MD; old marsh peat obtained from subtidal area adjacentto Mill’s Island, Chincoteague Bay, MD; and marsh sedimentobtained from Horn Point Marsh, Cambridge, MD. Sediment

was a mixture of decomposed marsh vegetation and soil

Treatment (% organic Sediment Sediment content ± SE) source 1 source 2

0.1 ± 0.0 Beach dune sand –0.5 ± 0.0 Marsh dune sand Old marsh peat1.2 ± 0.0 Marsh dune sand Old marsh peat4.4 ± 0.2 Old marsh peat –5.9 ± 0.0 Old marsh peat Marsh sediment

Wicks et al.: Sediment organic content and Zostera growth

obic samples were gassed for 3 to 4 min with a nitro-gen-helium gas mixture to purge oxygen from thehead space of the flask. The assays were maintained atconstant ambient temperature (~25°C) in an environ-mental growth chamber. Benthic nitrogenase activitywas normalized to sediment dry weight.

Statistical analysis. Data from the sediment organiccontent experiment were analyzed using ANOVA (α =0.05) in SAS 9.1. Aboveground biomass was testedacross treatments, with biomass being the dependentvariable and sediment organic content the indepen-dent variable. Homogeneity of variance was checkedusing Levene’s test (α = 0.05). Graphical representa-tion of the data and the Shapiro-Wilk’s test for normal-ity showed all parameters to be non-normal and werelog transformed. Transformed data were normally dis-tributed and homogeneity of variances for all data wasmet. If significant differences were found using 1-wayANOVA, then factors were tested using the leastsquares method (α = 0.05).

RESULTS

Light availability

Light attenuation coefficients off Mill’s Island rangedfrom 1.2 to 3.5 m–1 with an average of 2.1 ± 0.1 m–1

(mean ± SE). For locations where total depth ≤1.0 m,i.e. in seagrass habitats, the average light attenuationcoefficient was 1.9 ± 0.1 m–1 while for locations wheretotal depth >1.0 m, the average light attenuation co-efficient was 2.1 ± 0.1 m–1. Light levels ranged be-tween 100 and 1870 µmol m–2 s–1 at 1 and 0.2 m depth,respectively. The spatial light pattern did not showhigher turbidity near shore but did show localizedareas with higher turbidity (possibly a result of eddies).

Field surveys

The sand off Mill’s Island was dominated by fine sand(125 to 250 µm, Φ = 3) and the old marsh peat was char-acterized by silt plus clay (<63 µm, Φ ≥ 5, Fig. 2a). Sam-ples that consisted of sand overlaying old marsh peat hada thin (~1 cm) mixed layer between both sediment types(represented by SE values in Fig. 2a). Sediment organiccontent increased with silt plus clay content (Fig. 2b).

The seagrass bed consisted exclusively of Zosteramarina although other seagrass beds in the area oftenhave a fringe of Ruppia maritima along the shallowedge of the bed. The relationship between Z. marinashoot density and sediment organic content showeda threshold response with a 4% organic content criti-cal limit (Fig. 3a). Alternatively, a linear correlation

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Fig. 2. (a) Grain size distribution of sand and old marsh peat atMill’s Island. Error bars = SE. (b) Sediment organic content as

a function of the silt plus clay fraction

Fig. 3. Zostera marina. (a) Shoot density as a function of sedi-ment organic content in the rhizosphere (top 15 cm) at Mill’sIsland. Dotted line represents possible threshold responsewhile dashed line represents the best linear fit. (b) Shoot den-

sity grouped into categories of sediment type

Mar Ecol Prog Ser 378: 71–80, 2009

through the data showed a negative function with alow r2 value (r2 = 0.38, Fig. 3a). Most sites (n = 9) thathad low organic content (sand and sand overlaying oldmarsh peat) were vegetated, with average shootdensities of 427 ± 61 shoots m–2 (Fig. 3b). Sites that hadhigh organic content (old marsh peat only) had lowershoot densities or were unvegetated (average of 25 ±15 shoots m–2, n = 3, Fig. 3b). Average abovegroundand belowground biomass decreased linearly with in-creasing sediment organic content (r2 = 0.39 and 0.51,respectively, Fig. 4).

Sediment organic content experiment

In general, all growth parameters in the mesocosmexperiment (aboveground biomass, r2 = 0.84; below-ground biomass, r2 = 0.91; leaf length, r2 = 0.84; rootlength, r2 = 0.51) tended to increase with sedimentorganic content up to the highest treatment (5.9%,Fig. 5). This observation was opposite to field trends.The ratio of leaf length to root length increased withincreasing sediment organic content (Fig. 6). Rates ofnitrogenase activity peaked in the 4.4% organic con-tent treatment under aerobic conditions and in the1.2% treatment under anaerobic conditions, but de-creased at higher organic content (5.9%) under bothconditions (Fig. 7).

Statistical analysis

There were significant differences (p < 0.0001) be-tween sediment organic content treatments for above-ground and belowground biomass (Table 2). The

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Fig. 4. Zostera marina. Average aboveground and below-ground biomass as a function of sediment organic content inthe rhizosphere (top 15 cm) at Mill’s Island. Error bars = SE;dw = dry weight. Note that y-axis is positive to both sides

of zero

Fig. 5. Zostera marina. (a) Average aboveground and below-ground biomass of plants grown in sediments with organiccontent between 0.1 and 5.9% in a mesocosm experiment.(b) Average leaf and root length of plants grown in sedimentswith different organic contents in a mesocosm experiment.Error bars = SE (not shown for root length due to small SE);dw = dry weight. Note that y-axis is positive to both sides

of zero

Fig. 6. Zostera marina. The ratio of leaf length to root length ofplants grown in sediments with different organic contents in a

mesocosm experiment

Wicks et al.: Sediment organic content and Zostera growth

aboveground biomass separated into 2 groups: (1) the0.1% and 0.5% treatments, which were not signifi-cantly different from each other but were significantlydifferent from the 3 higher organic content treatments,and (2) the 3 higher organic treatments, which werenot significantly different from each other (p = 0.05).The belowground biomass separated into 3 groups:(1) the 0.1% and 0.5% organic content treatment,

which were not significantly different from each other,(2) the belowground biomass in the 0.5% and 1.2%organic content treatments, which were not signifi-cantly different from each other, and (3) the below-ground biomass in the 1.2, 4.4 and 5.9% organic con-tent treatments, which were not significantly differentfrom each other (p = 0.05). See Table 2 for all pairwisecomparisons.

DISCUSSION

Despite the highly erosional shoreline at Mill’s Islandin Chincoteague Bay, light was not limiting to sea-grasses at the study site. Turbidity was not highestnear shore where seagrasses are located. Instead, tur-bid water seemed to be transported offshore by eddies.The presence of a healthy seagrass bed was furtherevidence that sufficient light was available to supportseagrass growth.

Although initially it may seem that light was not alimiting factor in the area and that sediments must belimiting seagrass distribution, one must consider whatcame first, the seagrasses or the sand. Seagrasses arewell known for their capacity to trap particles (Fonseca& Fisher 1986) due to the reduction of current velocityand attenuation of wave energy (Fonseca & Cahalan1992) by the vegetation. It follows that perhaps pre-existing seagrasses led to the deposition of sand in thevegetated area. This hypothesis is unlikely as Zosteramarina disappeared from Chincoteague Bay in the1930s due to wasting disease (Koch & Orth 2003).Recovery started to accelerate in the 1980s, but bedswere restricted to the eastern shore of the bay. Only in1996 did seagrasses appear on the western side of thebay where the study site is located. If all the sand cur-rently found overlaying the old marsh peat in the sea-grass bed (more than 50 to 100 cm in some locations)was deposited in the last 10 yr, seagrasses would havebeen buried by depositional rates of 5 to 10 cm yr–1 ormore. Even small levels of burial (25% of photosyn-thetic tissue) can be detrimental to Z. marina (Mills &Fonseca 2003). Therefore, it is unlikely that the sandcurrently found in the bed studied is a result of trap-ping by seagrass leaves. This is also supported by thefact that seagrass density in the area significantlydeclines during the winter season but the sandremains. Instead, it is likely that the accumulation ofsand allowed the seagrasses to colonize the area.

While our field results suggest that excessive organicmatter can be detrimental to Zostera marina, our meso-cosm results suggest the opposite. Our mesocosm ex-periment shows that a lack of sufficient organic mattercan be detrimental to seagrass growth, as seen by thereduced biomass found in Z. marina grown in sedi-

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Fig. 7. Nitrogen fixation (nmol C2H4 [g dry weight sediment]–1

h–1) in sediments with different organic contents in the meso-cosm experiment. Samples were analyzed under both aerobic

and anaerobic conditions

Table 2. One-way ANOVA (α = 0.05) and pairwise com-parisons for aboveground and belowground biomass in themesocosm experiment. Samples sharing the same letter arenot significantly different at p = 0.05. (ns = not significant)

Sources of df Mean F pvariation square

AbovegroundOrganic matter 4 0.3537 19.68 <0.0001Residuals 10 0.0153Total 14Transformation ln(x +1)Levene’s test 0.2996 (ns)

BelowgroundOrganic matter 4 0.1298 13.50 0.0005Residuals 10 0.0096Total 14Transformation ln(x +1)Levene’s test 0.1917 (ns)

Pairwise comparisonRank of the means 1 2 3 4 5

SOC (%) 0.1 0.5 1.2 4.4 5.9

log biomassAboveground –0.6071 –0.7550 –1.2002 –1.2306 –1.4155

–––––––––a––––––– –––––––––––––b––––––––––––

Belowground –0.5455 –0.5668 –0.8140 –0.8952 –1.0226–––––––––a––––––– –––––––––––– c ––––––––––––

––––––––b––––––––

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ments with ≤0.5% organic content. These results agreewith other studies in which sandy sediments werenutrient limiting (Short 1987). In contrast, the highestbiomass in the field was found in sediments with <1%organic content and seagrasses were absent from sed-iments with organic content >4%. This inconsistencysuggests that sediment organic content is not the onlyfactor limiting seagrasses in situ and that one or moreadditional factors need to be considered when evaluat-ing the effect of sediment organic content on seagrassgrowth and distribution. Hydrogen sulfide can be de-trimental to seagrass growth; however, hydrogen sul-fide concentrations were below toxic levels in the field(<1000 µM in the rhizosphere, Wicks 2005).

Previous research, as well as the present study,clearly show that sediment organic content has a majoreffect on seagrass morphology. Thalassia testudinumgrown in low porewater ammonium (~30 µM) sedi-ments (i.e. low organic content) had significantly shor-ter and narrower leaves than T. testudinum grown inhigh porewater ammonium (~100 µM) sediments (i.e.high organic content, Lee & Dunton 2000). Addition-ally, when plants grown in low organic content sedi-ments were fertilized, they increased in abovegroundbiomass, but not belowground biomass, resulting in asignificant difference in the ratio of aboveground tobelowground biomass between fertilized and unfertil-ized plots (Lee & Dunton 2000). Zostera marina grown

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Fig. 8. Zostera marina. Conceptual diagram describing the relationship of sediment organic content and nitrogen fixation with seagrass biomass and length in low and high energy environments

Wicks et al.: Sediment organic content and Zostera growth

in mesocosms in Alaska was larger in muddy sedi-ments (i.e. higher organic content) than in sandy sedi-ments, but there were fewer root hairs (Short 1983).This finding suggests that plenty of nutrients wereavailable so the plants did not need more root hairs totake up nutrients. Z. marina grown in our mesocosmfollowed similar patterns developing short leaves whensediment organic content was low and long leaveswhen sediment organic content was high (Fig. 8). Thisincrease in aboveground biomass as a function of sed-iment organic content is directly linked to sedimentnutrient availability, e.g. porewater nitrogen and phos-phorus (Wicks 2005). It may also be linked to nitrogenfixation by root-associated bacteria, as nitrogen fixa-tion under aerobic conditions increased from low tohigh organic sediments (Fig. 7). When compared withleaf length, root length did not show a proportional in-crease (Fig. 5b), leading to plants with disproportion-ately long leaves and short roots in organic rich sedi-ments (Fig. 6). In contrast, in sediments with organiccontent <1% leaf and root length were similar.

The short leaves and long roots of Z. marina grownin low organic content sediments are likely to lead tolow drag exerted on the leaves and a high anchoringcapacity of the roots. In contrast, the morphology ofZ. marina grown in sediments with high organic con-tent (long leaves and short roots) leads to a poor an-choring capacity and high drag being exerted on theleaves. Therefore, we hypothesize that, due to sedi-ment-induced morphology, Z. marina growing in sandis more likely to withstand hydrodynamic forces suchas currents and waves than are plants growing inmuddy sediments (Fig. 8).

Fine organic sediments are usually found in rela-tively quiescent hydrodynamic conditions, while coar-ser, lower organic sediments are characteristic at siteswith strong currents and/or waves. Under these condi-tions, plants will always develop appropriate mor-phologies: high-drag plants with low anchoring capac-ity under quiescent conditions (sediment with highorganic content) and low-drag plants with high an-choring capacity under hydrodynamically active con-ditions (sediment with low organic content). This hasbeen shown for Zostera noltii (Peralta et al. 2005). Incontrast, at our study site organic rich sediments arefound in an area with sufficiently strong waves toerode the shore. Any plants able to grow in these sedi-ments develop a morphology that may be unsuitable(high drag and low anchoring capacity) for the localhydrodynamic conditions. It follows that they are morelikely to be uprooted. Dislodgement may also explainwhy seagrass shoot density is inversely proportionalto sediment organic content and why seagrasses areabsent from sediments with more than 4% organiccontent in situ (Fig. 3a). This agrees with the 5%

threshold suggested in Koch (2001) and sediment or-ganic content observed in other studies. For example,at a site near Fyn, Denmark, Z. marina is found grow-ing in sediments with an organic content of 9.93%(Holmer & Laursen 2002), but a more typical organiccontent for organic rich sediments in Z. marina beds isaround 3% (Holmer & Laursen 2002, van Katwijk &Wijgergangs 2004). These studies did not report hydro-dynamic conditions at the study sites.

In summary, sediment organic content affects Zos-tera marina growth, but its importance in seagrass dis-tribution may only be fully understood when localhydrodynamic conditions are also taken into consider-ation. This hypothesis needs to be investigated further.Best growing conditions in the field and in the meso-cosm experiment occurred when sediment organiccontent was between 0.5% and approximately 4%.This was also the range over which nitrogen fixation inthe sediment was highest. Organic contents of <0.5%are likelyto be nutrient limiting toZ. marina while sedi-ment organic contents of >4% are not limiting per se.They only appear to be limiting when organic rich (4 to6%) sediments are found in hydrodynamically activeareas, leading to a mismatch between plant morpho-logy and the physical environment causing seagrassesto be uprooted (Fig. 8).

Acknowledgements. We thank W. Severn for his help withfield and laboratory research. This research was supported inpart by Horn Point Laboratory, University of Maryland Centerfor Environmental Science (UMCES), the National Oceanicand Atmospheric Administration and the Maryland SeaGrant, National Science Foundation sponsored ResearchExperience for Undergraduates (REU) Program. This paper isUMCES contribution no. 4234.

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Editorial responsibility: Otto Kinne, Oldendorf/Luhe, Germany

Submitted: November 3, 2007; Accepted: December 10, 2008Proofs received from author(s): March 2, 2009