Respiration in eutrophic lakes: the contribution of bacterioplankton and bacterial growth yield

Journal of Plankton Research Vol.10 no.3 pp.515-531, 1988

Respiration in eutrophic lakes: the contribution of bacterioplanktonand bacterial growth yield

Steen Schwaerter, Morten S0ndergaard1, Bo Riemann2 and Lars M0ller Jensen3

Botanical Institute, University ofAarhus, 68 Nordlandsvej, DK-8240 Risskov,2 Freshwater Biological Laboratory, University of Copenhagen, 51Helsing0rsgade, DK-34OO Hiller0d and3 The International Agency for 14CDetermination, 15 Agern Alle, DK-2970 H0rsholm, Denmark1 To whom correspondence should be addressed. Present address: Institute ofBiology and Chemistry, University of Roskilde, P. Box 260, DK-4000 Roskilde,Denmark

Abstract. The contribution of bacterioplankton to total plankton respiration was measured in twoeutrophic Danish lakes and in experimental enclosures treated with planktivorous fish and nutrients.Bacterial respiration was calculated from measured oxygen uptake rates in particles passing a 1.0-u.mpore size filter, the rates were then corrected for the size distribution of glucose uptake. Duringsummer the respiration of the planktonic bacteria contributed ~50% of the community respiration inthe two lakes. Prolific phytoplankton growth induced by biomanipulation and nutrient additioncreated situations where the contribution of the bacteria decreased to 20%. High bacterialcontributions to community respiration were found when the phytoplankton biomass decreased.Simultaneous measurements of bacterial respiration and production (by means of [3H]thymidineincorporation) allowed an estimation of bacterial growth yield, which ranged from 9 to 66%. In thetwo lakes the growth yield was constant with a mean of 29 ± 5% (±SD, RQ = 1). The variability ofthe growth yield was larger in the enclosures. The wide range (9-66%) was mainly caused by changesin bacterial net production without concomitant changes in respiration. The discussion includes anevaluation of the oxygen uptake method in size fractionated samples and the availability of labileorganic substrates as a factor controlling bacterial growth yield.

Introduction

With the introduction of new and improved methods to measure biomass andproduction of bacterioplankton (Hobbie et al., 1977; Hagstrom et al., 1979;Fuhrman and Azam, 1980, 1982), it has been recognized that bacteria areimportant biological components of pelagic environments.

Estimates of bacterial gross production are in the range of 20-80% of thephytoplankton production in lakes (Jordan and Likens, 1980; Riemann, 1983;Bell and Kuparinen, 1984; Brock and Clyne, 1984; Giide etal., 1985; Riemannand S0ndergaard, I986a,b) and 24-60% in marine environments (Fuhrman andAzam, 1980, 1982; Larsson and Hagstrom, 1982; Lancelot and Billen, 1984;Admiral et al., 1985; Ducklow and Hill, 1985). Most of these investigators haveused the [3H]thymidine method (TTI) to measure bacterial net production(Fuhrman and Azam, 1980) and assumed a bacterial growth yield of 40-60%.

Overall bacterial carbon demand and biomass production in aquatic environ-ments are currently a matter of intensive research and discussion. Growth yieldsin the range of 10-40% have been suggested (Newell et al., 1981; Bell andKuparinen, 1984; Bauerfeind, 1985; Bj0rnsen, 1986; Laanbroek and Verplanke,1986a,b), and factors used to convert bacterial biovolume to biomass and

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thymidine incorporation rates to cell production can vary considerably (Fuhr-man and Azam, 1982; Bell et al., 1983; Riemann, 1984; Bratbak, 1985).

The contribution of bacteria to overall plankton metabolism can also beassessed by measuring their direct oxygen consumption (Williams, 1981a; Belland Kuparinen, 1984; Laanbroek and Verplanke, 1986a,b). The oxygen methodprovides a tool to evaluate the accuracy of more direct methods of measuringbacterial production. The oxygen consumption approach also has the potentialto allow an estimation of the bacterial growth yield in the field, if bacterial netproduction is measured simultaneously. Combining the TTI method and oxygenconsumption in size-fractionated water samples, Bell and Kuparinen (1984)found an average bacterial growth yield of 25% (range 20-27%) in mesotrophicLake Erken, Sweden, and Laanbroek and Verplanke (1986b) estimated with asimilar approach a growth yield of 6-26% in a Dutch estuary.

Direct measurements of bacterial respiration also make it possible to evaluatethe fate of phytoplankton production, if the contributions by zooplankton, algaeand bacteria can be differentiated. In an analysis of carbon loss in lakes,Forsberg (1985) demonstrated that the majority of carbon fixed by photosyn-thesis in most cases was consumed in metabolic processes (respiration). Theimportance of respiratory loss is often overlooked or neglected in studies ofpelagic metabolism.

The purpose of this investigation was to study the contribution of thebacterioplankton to overall plankton respiration in eutrophic lakes. Simul-taneous measurements of oxygen consumption and bacterial production allowedestimates of their growth yield and thus the flow of carbon through thebacterioplankton.

Materials and methods

Study sites and water sampling

The investigation was carried out in two eutrophic Danish lakes, Lake Hylkeand Frederiksborg Slotss0 and in experimental enclosures placed in the lakesduring periods in 1983 and 1984.

Lake Hylke is situated ~20 km south of the city of Aarhus in Jutland. Thesurface area is 3.15 km2 and maximum and mean depths are 16.3 m and 7.1 mrespectively. No permanent summer stratification develops. Annual phyto-plankton production is ~400 g C m~2 (Emmery and Sandby, 1983) and bloom-forming cyanobacteria (Microcystis spp.) dominate the summer plankton.Sampling was during the period May-September 1983.

Frederiksborg Slotss0 is a small (0.21 km2) and shallow lake (maximum depth8 m, mean depth 3.1 m) located in the center of the city Hiller0d, 30 km northof Copenhagen. Cyanobacteria and chlorococalean green algae dominate thesummer plankton. Annual phytoplankton production is —560 g C m~2 (Ander-sen and Jacobsen, 1979). Sampling was carried out during June 1984.

During May and September 1983 and June 1984 intensive measurements wereundertaken in the two lakes as well as in large (7 m3) plastic enclosures situ-ated in the lakes. The lake water in the enclosures was manipulated with

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Respiration in eutrophic lakes

planktivorous fish and nutrients. The design of the enclosures and their use tostudy pelagic carbon metabolism have recently been published by Riemann andS0ndergaard (1986a).

All samplings were carried out within 1 h after sunrise and the experimentswere initiated within 1-2 h. Transport of water was in darkened thermoboxes.

Biological activity

Oxygen uptake in untreated and prefiltered (1.0-u.m pore size, 47-mmNuclepore filter, vacuum <5 cm Hg) water samples was measured in duplicate indarkened glass bottles (125 and 25 ml). The bottles were incubated for 24 h at insitu temperatures (±0.2°C) in a shaking water-bath. Changes in oxygen weremeasured with the Winkler method using an amperometric endpoint detection(250 mV, platinum-calomel electrodes). The precision of the method required adifference between samples of 33 yug O2 I"1 to be significant at the 95%confidence level.

Oxygen consumption in the untreated samples represents community respir-ation (RCom)> a nd oxygen consumption in the 1.0-|xm filtrates were assumed onlyto include bacterial respiration. The number of pigment-containing particles(picophytoplankton) in the filtrates was examined by epifluorescencemicroscopy. Subsamples of the filtrates prepared for respiration measurementswere fixed in glutaraldehyde, filtered onto dark 0.2-jjim Nuclepore filters andexamined within 2 days. No autofluorescent particles were found in any of >200samples. We conclude that the oxygen consumption in the filtrates (Ri) was dueto that part of the bacterial population which passed a 1.0-|xm filter. A fewheterotrophic picoflagellates might have passed the 1.0-^m filters, but weassume that they had no measurable effect on oxygen consumption.

The respiration of the total bacterial population (Rb) was calculated from themeasured Rt values by means of a size fractionation of the heterotrophicactivity. The fraction of the total heterotrophic activity passing a 1.0-|xm filterand retained on a 0.2-u.m filter was measured by [14C]glucose uptake(S0ndergaard et al., 1985) and the R^ values were corrected accordingly. Thecalculation requires that the respiration as a percentage of uptake is similar forthe bacteria retained by a 1- m filter and those which passed through. This wastested on five occasions in Frederiksborg Slotss0 using a 14C-labelled amino acidmixture and adopting the methods of Hobbie and Crawford (1969) and Kato andSakamoto (1983). We found no significant differences between the twofractions, although the respiration in the <1.0-^m fraction was slightly lower(46 ± 6%, n = 10) than the total community respiration (53 ± 6%, n = 10).

An important prerequisite of the oxygen consumption method is a linearuptake within the incubation time (Williams, 1981a). This was tested severaltimes and linearity with time was always found in the <1.0-(xm fraction. Oxygenuptake in the untreated samples was linear with time in four of five time-coursetests. The one exception showing a curvilinear response was measured in asample collected at midday in Frederiksborg Slotss0. Thus, the algae hadpreviously been exposed to high irradiance, which is known to enhance algal

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respiration for a short period. Since all data presented here are from samplestaken around sunrise, we assume linearity in oxygen uptake.

A comparison of bacterial carbon production and respiration measured asoxygen consumption can only be valid if the respiratory quotient is known. Weused both a stoichiometric ratio (Bolter, 1982) and a value of 0.85. The latterwas chosen to account for more reduced substrates than simple carbohydrates(Sepers, 1981; Bell and Kuparinen, 1984).

Bacterial secondary production was measured by means of [3H]thymidineincorporation into DNA (Fuhrman and Azam, 1980; Riemann, 1984). Bacterialcell production was calculated from rates of thymidine incorporation using aconversion factor of 1.7 x 1018 cells per mol thymidine incorporated (Fuhrmanand Azam, 1982; Bell et al., 1983), an average cell volume of 0.159 n-m3

(Pedros Alio and Brock, 1982) and a carbon content of 121 fg C jim"3 (Watsonetal, 1977).

Results

Lake Hylke

One set of data from May contained only measured community respiration(i?com) and a bacterial respiration (R-rn) calculated from values of bacterialproduction (Riemann and S0ndergaard, 1986a) assuming a growth yield of 0.3(mean value from Lake Hylke, see later) and a stoichiometric respiratoryquotient. Both community and bacterial respiration in the lake itself fluctuated

Lake Hylke

Fig. 1. Respiratory oxygen consumption rates by the plankton community (/?«,„,) and thebacterioplankton (R-m) in Lake Hylke and two experimental enclosures, May-June 1983. R-mcalculated from bacterial production rates (Riemann and S0ndergaard, 1986a), a growth efficiencyof 0.3 and a respiratory quotient of 1.

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moderately with time without any specific trends. In the enclosures bothvariables increased at the end of the experiment (Figure 1). Although thequantitative development of the phytoplankton differed markedly (S0ndergaardet al., 1984; Riemann and S0ndergaard, 1986a), the bacteria responded almostsimilarly in the two enclosures, which is reflected in the fl-m values (Figure 1).At the end of the period Rcom almost doubled in the enclosure with fish andtripled in the enclosure with added fish and nutrients.

The contribution of bacterioplankton to the community respiration wasvariable among the different treatments. In the lake the contribution wasconstant at 40-50%, whereas it decreased to 24% in the fish + nutrientenclosure, which showed a continuous growth of algae. In the fish-onlyenclosure the bacterial contribution peaked at 76%, when the algal biomassdeclined at the end of the period.

During July and August, the Rcom and Rb were both larger than in May, butdecreased again in September (Figure 2). In three cases, the calculated bacterialrespiration exceeded the community respiration. The Rl values are included inFigure 2 to give a minimum estimate of the bacterial respiration. Forcomparison, the Ri values measured in July and August are also presented.

The observed changes of Rcom in July and August were mainly due to oxygenconsumption by phytoplankton and zooplankton, as i?com ranged from 25 to150 u.g O2 P

1 h"1 compared with an Rb range of 22 to 44 jig O2 P1 h"1 (Figure

2). The number and biovolume of the major phytoplankton taxa (Figure 3)changed considerably during this period. The peak in Rcom on 25 July (Figure 2)coincided with a sharp peak in the biovolume of Rhodomonas minuta andCryptomonas ovata. Their combined biovolume was 14 mm3 P 1 on 25 Julycompared with 1 and 1.5 mm3 P 1 on 21 and 29 July respectively. A linearregression analysis between Rcom - Rb (an estimate of algal respiration) and thealgal volume showed the 'phytoflagellates' to be the only group significantlycorrelated (r = 0.94). Although the cyanobacteria contributed most of thebiovolume in August (Figure 3), their effect on community respiration wasminor. The density and activity of the zooplankton were not recorded.

10September

Fig. 2. Community (Rcom) and bacterial (Rb) respiration rates in Lake Hylke, July-September 1983.Respiration rates in the size fraction <1.0 (jim (R^) are also included.

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20

Fig. 3. The relative distribution of dominant algal groups based on biovolume. Lake Hylke,July-August 1983. Dominant species and genera included: greens, Pediastrum and 'small greenspheres'; cyanobacteria, Microcystis spp; diatoms, Slephanodiscus astrea; 'flagellates', Cryptomonasovata, Rhodomonas minuta.

During September Rcom was low (30-60 (xg O2 1 J h *) compared with thelevel in August (~100 |xg O2 T

1 h"1). No significant relationship between algalbiomass and /?com ~ Ri was found for the September situation; neither in thelake nor in four enclosures subjected to different treatments with fish andnutrients. The values of Rcom and bacterial production in the enclosures weresimilar to those in the lake.

The relative contribution of the bacterioplankton to overall planktonmetabolism in Lake Hylke is summarized in Figure 4. For the periodMay-September the bacterial contribution averaged 50 ± 18% (x ± SD).

The measurements in Lake Hylke during July, August and September alloweda comparison of bacterial production measured by the TTI method and bacterialoxygen consumption. Oxygen consumption was converted to carbon units usinga respiratory quotient of 1.

The comparison showed that bacterial production and respiration weresignificantly correlated (r = 0.85, y = 2.65* - 0.38) (Figure 5). The Septemberdata on bacterial production and respiration estimated by R\ values gave acoefficient of 0.85 and the equation: y = I Ax + 3.71. Bacterial productionversus Rb gave r = 0.81 and y = 3x + 7.87. The large positive y-interceptsindicate a significant bacterial respiration without any biomass production,which is highly unlikely. This result supports the idea of an erroneous sizefractionation during that period.

Frederiksborg Slotss0

The plankton metabolism in Frederiksborg Slotss0 was measured in the openlake and in three experimental enclosures. The relative distribution of oxygenconsumption among bacteria, algae + zooplankton <140 u.m and zooplankton>140 n-m is shown in Figure 6 and the actual RCOm and ^b values are summarized

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10 20

August

10 20

Septembei

Fig. 4. The relative contribution of bacterioplankton (Rb) to community respiration in Lake Hylke,May-September 1983. Respiration in the size fraction <1.0 jj.m (RJ is used as a minimum estimatein September.

T - 20

10 15 20

September

Fig. 5. Comparison of bacterial production rates measured by means of [3H]thymidine incorporation(TTI) and bacterial respiration rates (Rb and Ru see text for explanation of symbols). Therespiration rates were recalculated to carbon units using a respiratory quotient of 1. Lake Hylke,July-September 1983.

in Figure 7. The data on zooplankton respiration were provided by S.Bossel-mann (unpublished results).

The contribution of smaller zooplankton (<140 u,m) to respiration wasprobably low as predicted by the significant (r = 0.88, P < 0.05) linearrelationship between chlorophyll and /?«,„, - Rb. The contribution of themacrozooplankton to R^m was generally <10% and only increased significantly

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40

0

_ 80

f 40

Fredenksborg Slotssp

D zooplankton > K0 urn

0 algae* zooptakton<140ym

D bacteria

Fish + nutrients10 15

June

Fig. 6. The relative distribution of community respiration among larger zooplankton (>140 u.m),phytoplankton + zooplankton <140 u.m and bacterioplankton in Frederiksborg Slotss0 and threeexperimental enclosures, June 1984.

100

100

50

FREDERIKSBORG SLOTSSfl

Lake

Fish

10

June

Control

Fish * nutrients

10

June

Fig. 7. Respiratory oxygen consumption by the plankton community (/?<*,„,) and the bacterioplank-ton (Rb) in Frederiksberg SlotssO and three experimental enclosures, June 1984.

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80

_ 60

g 20

I

Frederiksborg Slotssrf

19 June

< 1pm < 3pm < 8p m < 20pm < lOOym R ^Plankton c s ze fractions

Fig. 8. The distribution of respiratory activity with particle size. Frederiksborg Slotss0, June 19,1984.

in the control enclosure without fish (Figure 6). A more detailed fractionation ofa lake water sample carried out on 19 June showed that the respiration was mainlycontributed by organisms in three size classes: <1.0 n-m, 3-8 u.m and20-100 u,m (Figure 8).

In the enclosures with fish the contribution of the phytoplankton to Rcom

increased from ~40% in the beginning of the experiment to ~70% at the peak,whereas the bacterioplankton reached a low level of 20% (Figure 6). Theenclosure without fish predation on the macrozooplankton was characterized bya decreasing algal population (Riemann and S0ndergaard, 1986a) and thecontribution by macrozooplankton and bacteria peaked at 20 and 70%respectively. In the lake the contribution by each group of organisms varied;35-70% for the bacteria, 2-12% for the macrozooplankton and 26-58% for thephytoplankton + smaller zooplankton including flagellates. The average oxygenconsumption by the bacterioplankton was 55% in the lake, 50% in the control,33% with fish and 43% with fish + nutrients. The general level was similar tothat found in Lake Hylke.

Bacterial growth yield

The bacterial growth efficiencies (net production/net production + respiration)were calculated by assuming both a constant respiratory quotient and that[3H]thymidine incorporation represents bacterial net production. Bacterialgrowth efficiencies ranged from 5 to 66% and differed among the three sets ofdata (Table I).

The results from Lake Hylke in September were biased by the problems withthe calculations of the total bacterial respiration. Thus, growth yield based onboth Rx and Rb are presented (Table I). Furthermore, we have included acalculation with subtraction of the positive _y-intercepts from the linearregression of [3H]thymidine incorporation versus Rt and Rb. The reasoning forthe latter approach is that a large positive intercept predicts bacterial respirationwithout production which is unlikely, and that the linearity predicts a closerelationship between production and respiration. The corrected growth yields of

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Table I.

Locality

Calculated bacterial growth yields (% of gross carbon

Respiratory quotient1

uptake)

n

0.85

Lake Hylke, July-August 29 ± 4 (25-35) 32 6

Lake Hylke, SeptemberRx 15 + 4 (10-22) 16 5Rb 8 ± 5 (5-15) 9 5K,a 20 + 4 (20-44) 33 5Rb° 17 ± 8 (11-32) 19 5

Frederiksborg SlottsO 37 ± 15 (9-66) 40 24

Mean ± SD, ranges in parentheses."Calculated with subtraction of positive ^-intercepts, see text.

17 and 30% are in close agreement with the other data (Table I), although thisby itself is no proof of a correct interpretation.

The overall wide range of growth yields from 9 to 66% were due to the largeand fast changes of the bacterioplankton in the enclosures in FrederiksborgSlotss0 (Figure 9). In the open lake the range was minor (28-34%), which wasalso the case in Lake Hylke.

Low and high growth efficiencies were mainly controlled by large (a factor 6),short-term (3 days) changes in bacterial net production (Riemann andS0ndergaard, 1986a) and not by the respiration rates, which only changed by afactor 1.6 within 3 days (Figure 7).

Discussion

The size-fractionation method

The accuracy of the measurements of bacterial respiration is dependent onseveral assumptions inherent to the size-fractionation procedure and otherpotential sources of error. Three of the most important are summarized in TableII along with a short statement of empirical and/or theoretical support for themethod used in this investigation.

We found linear oxygen uptake in the <1.0-p,m fraction over 24 h in five offive experiments and in four of five experiments with untreated samples (see alsoMaterials and methods). The use of glucose to assess the size distribution ofbacterial activity cannot directly be used to evaluate the distribution of oxygenuptake. However, no obvious alternative is available and most Rb values werewithin the range in the literature (see below).

An exception to the validity of the calculations of Rb from the measured Rivalues was found in Lake Hylke in September, where three of five Rb valueswere larger than Rcom (Figure 2). The lack of any direct relationship between thealgal biomass and Rcom and the rather similar pattern of decreasing values of/?„,„, Rb and Ri (Figure 2) suggest bacterial respiration to be the single mostimportant component of the oxygen consumption in this period characterized bya senescent and declining cyanobacterial bloom.

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10 15

June

20

Fig. 9. Bacterioplankton growth yield in Lake Hylke, Frederiksborg Slotss0 and experimentalenclosures in Frederiksborg Slotss0. The respiratory quotient was assumed to be 1.

Table II. A summary of potential sources of error and assumptions used in the calculation ofbacterial respiration and evidence to support the used method

Potential error/assumptions Evidence/support

Long enclosure times do not change the rate ofoxygen uptake and oxygen uptake is linear withtime

Correction of/?, to Rb with the size distributionof glucose uptake

Quantitative separation of bacteria and otherorganisms by a 1.0-u.m pore size filter

Empirical evidence of linearity. /?, constantwith time in five tests (in agreement withWilliams, 1981a)

Derenbach and Williams (1974)Riemann (1978)Cole etal. (1982)S0ndergaard el al. (1985)com > Rb except in Lake Hylke in

September

No picoalgae found (see also Caron et al.,1985)Any oxygen uptake by other organisms inthe 1.0-u.m filtrate assumed negligible

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The overestimation of Rb might have been due to major differences in therespiration rates of the attached (>1.0 \xm) and the free (<1.0 .m) bacteria.However, the results from Frederiksborg Slotss0 showed no significantdifferences in respiration percentages. Similar results were found by Kato andSakamoto (1983). A more likely explanation might be the observed largenumber of bacteria embedded in the mucilage of the cyanobacterial colonies,which dominated the phytoplankton.

Our conclusion is that the applied oxygen consumption method combinedwith particle size fractionation can be used to measure bacterioplanktonrespiration. Shortcomings occur during periods with low activity, where thesensitivity of the oxygen method is inadequate and when a large proportion ofthe bacterial population is embedded in or closely associated with cyanobacterialcolonies. The latter situation occurs at the end of a summer bloom.

Bacterial contribution to community respiration

Most of the literature concerning the contribution of different trophic levels tothe total plankton community respiration shows that larger zooplankton are ofminor importance in most environments. The highest annual contribution of~33% was estimated by Cole (1985) for oligotrophic Mirror Lake, while 13%has been estimated for eutrophic Frederiksborg Slotss0 (Andersen andJacobsen, 1979). Low (3-8%) to negligible values have been recorded in coastalenvironments (Williams, 1981a) and during the spring in mesotrophic LakeErken (Bell and Kuparinen, 1984). However, large seasonal variations are to beexpected with short-lasting peaks of zooplankton, e.g. during spring clear-waterphases (Lampert et al., 1986). Phytoplankton and microheterotrophs includingbacteria appear to dominate the respiration in most plankton communities.

Recent measurements of bacterial net production and estimated grossproduction indicate that 20-80% of the primary production enters the detritalfood web (e.g. Larsson and Hagstrom, 1982; Brock and Clyne, 1984; Ducklowand Hill, 1985; Riemann and S0ndergaard, 1986b). Thus, it can be predictedthat a significant part of the plankton respiration is caused by the bacterioplank-ton. Direct measurements—mostly in coastal environments—have confirmedsuch a prediction. About 50% of the community respiration in some coastalwaters and experimental enclosures was caused by bacteria (Williams, 1981a). Asimilar value was found during summer in shallow tidal water bodies byLaanbroek and Verplanke (1986b). Higher values approaching 90-100% wereestimated in stratified shelf waters in the English Channel (Holligan et al., 1984)and measured during winter by Laanbroek and Verplanke (1986b). These highvalues are probably overestimates as also discussed by the authors. The latterwas measured in <3.0-jim filtrates without correction for the respiration ofpicophytoplankton and microflagellates, and the former used the coefficientslisted by Williams (1981b) to calculate respiration from biomass data.

Measurements of bacterioplankton respiration in lakes are even more scarcethan in saline environments. To our knowledge the only direct measurementspublished are those by Bell and Kuparinen (1984) from mesotrophic LakeErken. Their results showed bacteria to be responsible for an average of 30% of

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the community respiration during the spring (calculated from their Figure 2 andtheir assumption of 75% bacterial contribution to the respiration in the <3.0-n,mfiltrates). Cole (1985) estimated a similar annual contribution based on a carbonbudget for oligotrophic Mirror Lake.

The present results from two eutrophic lakes and a number of experimentalenclosures are of a magnitude similar to those reported from coastal areas. Thecontribution of bacterioplankton to overall plankton metabolism during thesummer period averaged 50-55% in both lakes. Large short-term variationswere especially prominent in Lake Hylke. The relative distribution of thecontribution from different trophic levels to overall respiration could be alteredby biomanipulation.

The variation in Rb from 25 to 80% of RCOm within 4 days in Lake Hylke inJuly was caused by sudden changes in the phytoplankton respiration. Thereasons for these changes are not clear, but could be related to patchiness in thealgal distribution.

The manipulation with zooplankton mediated by exclusion or addition of fishand the addition of nutrients both affected the relative distribution and theactual rates of respiration in the enclosures in Frederiksborg Slotss0. Bacterialcontribution peaked at 60-70%, when the algal biomass was low in the controlenclosure. Conversely, minimum values of ~20% were found in the periods withhigh algal biomass (enclosures with added fish). From the actual respirationrates it is apparent that the relative changes were more dependent on thephytoplankton component than on actual changes in bacterial respiration rates.A dependency of the relative distribution of oxygen consumption on thephytoplankton also emerged as a trend from the enclosure experiments in LakeHylke in May. However, these tendencies are weakened by the lack of directmeasurements and a limited number of samples.

As an overall generalization, ~50% of the metabolic loss of carbon in theplankton of our eutrophic lakes was caused by bacteria during summer.

Bacterial growth yield

The impact of bacterial metabolism on carbon cycling and flow in the pelagiczone is not only a function of bacterial respiration, but also of the production ofnew biomass. Most evaluations concerning the importance of bacteria have useddirect production measurements and the respiration has been estimated byapplying a theoretical growth yield—mostly from 0.4 to 0.6—derived fromradiotracer studies (e.g. Williams, 1981b; Larsson and Hagstrom, 1982; Hobbieand Cole, 1984; Riemann and S0ndergaard, 1986a).

The range of conversion efficiencies encountered in this study (9-66%) almostcovered the total range from <15 to 90% predicted by Linley and Newell (1984)in their substrate quality hypothesis. The samples from Lake Hylke averaged agrowth yield of 30% (range 20-40%) during July-September. The Septemberdata were based on i?t values. In Frederiksborg Slotss0 the average was 30%,with a range of 20-34%. The very low (9%) and high (66%) growth yields wereall measured in the enclosures, where rather dramatic changes in phytoplanktonbiomass and activity took place (Riemann and S0ndergaard, 1986a).

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Bacterial growth yield has been examined in several recent studies. By meansof various methods and using different types of natural substrates, recent growthyield values range from 6 to 40% (Newell et al., 1981; Bell and Kuparinen, 1984;Bauerfeind, 1985; Bj0rnsen, 1986; Laanbroek and Verplanke, 1986a,b).Consequently, it has been suggested that previous assumptions concerningoverall bacterial carbon demand were underestimated.

One explanation for the wide range of published conversion efficiencies wasformulated in a substrate quality hypothesis by Linley and Newell (1984), whosuggested overall bacterial growth yield to be directly linked with the quality ofthe substrates (i.e. the C/N ratio and molecular complexity) and the supply ofinorganic nutrients (especially nitrogen). Small molecules released by phyto-plankton should be converted to biomass more efficiently than detrital material.Thus, a general conversion factor is not valid, and dynamic in situ conditionsshould theoretically give large variations in growth yield caused by changes insubstrate supply and nutrient availability.

The changes in growth yield with time in the enclosures compared with thealmost constant yield in the lakes were probably created by pulses of labileorganic matter released by the phytoplankton during prolific growth and fromcell lysis during decline of the algal biomass (Hansen et al., 1986; Riemann andS0ndergaard, 1986a). System pertubations leading to dynamic changes inphytoplankton production and biomass resulted in large short-term changes ingrowth yield.

In the calculations of growth yield we have assumed that the TTI method is ameasure of bacterial net production, and we have used a constant respiratoryquotient of 1 (Bolter, 1982). Other authors have used 0.85 (Bell and Kuparinen,1984) or 0.77 (Sepers, 1981; Laanbroek and Verplanke, 1986a). The statedgrowth yields are accordingly minimum values, which would increase if a lowerquotient was used (see Table I).

The precision of the bacterial production measurements is another factoraffecting the calculated growth yields. The results by Riemann and S0ndergaard(1984) showed that three diel measurements with [3H]thymidine provide a 95%confidence limit of ±42% of the mean. Accordingly, the growth yield valueshave an approximate coefficient of variation of 20% increasing to about ~25%when the precision of the oxygen measurements are included.

Including minimum and maximum respiratory quotients and the overallcoefficient of variation, the growth yields ranged from 22 to 42% in Lake Hylkeand from 28 to 56% in Frederiksborg Slotss0. Assuming similar precision inother studies our values are probably larger than the 25% found during a springdiatom bloom in Lake Erken (Bell and Kuparinen, 1984) and the 26 and 6%measured in spring and winter in a tidal and a stagnant coastal basin byLaanbroek and Verplanke (1986b).

Conclusions

An evaluation of the substrate demand of bacterioplankton depends on theaccuracy of the respiration and the net production measurements. Previous

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assumptions of a very high growth yield of 70% (Williams, 1981b), theconversion factors applied to the TTI method and estimates of bacterial biomasshave changed substantially over the previous five years. Most changes havetended to increase the relative importance of the bacterioplankton.

The present results from two eutrophic Danish lakes may contribute to thistrend with respect to the contribution of the bacteria to overall planktonmetabolism, which amounted to ~50%. The results also support the suggestionsof a bacterial growth yield lower than 50-70%. In fact, the results make it morereasonable to suggest a value close to or less than 35%.

Bacterial growth yield is variable and is likely to change as a function of thequality of the organic substrates and nutrient availability.

Acknowledgements

We thank D.J.W.Moriarty for useful comments and W.Martinsen, B.Pihlkjaerand J.Bargholtz for technical assistance. The study was partly supported by theDanish Natural Science Research Council (11-1816, 11-4758) and the Fresh-water Laboratory, National Agency of Environmental Protection.

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Documents

Respiration in eutrophic lakes: the contribution of bacterioplankton and bacterial growth yield