Fish-induced changes in zooplankton grazing on phytoplankton and bacterioplankton: a long-term study in shallow hypertrophic Lake Sobygaard

The impact of fish-mediated changes on the structure and grazingof zooplankton on phytoplankton and bacterioplankton was studiedin Lake Søbygaard during the period 1984–92 bymeans of in vitro grazing experiments (¹⁴C-labelled phytoplankton,³H-labelled bacterioplankton) and model predictions. Measuredzooplankton clearance rates ranged from 0–25 ml l^–1h^–1 on phytoplankton to 0–33 ml l^–1 h^–1on bacterioplankton.The highest rates were found during thesummer when Daphnia spp. were dominant. As the phytoplanktonbiomass was substantially greater than that of bacterioplanktonthroughout the study period, ingestion of phytoplankton was26-fold greater than that of bacterioplankton. Multiple regressionanalysis of the experimental data revealed that Daphnia spp.,Bosmina longirostris and Cyclops vicinus, which were the dominantzooplankton, all contributed significantly to the variationin ingestion of phytoplankton, while only Daphnia spp. contributedsignificantly to that of bacterioplankton. Using estimated meanvalues for clearance and ingestion rates for different zooplankters,we calculated zooplankton grazing on phytoplankton and bacterioplanktonon the basis of monitoring data of lake plankton obtained duringa 9 year study period. Summer mean grazing ranged from 2 to4% of phytoplankton production and 2% of bacterioplankton productionto maxima of 53 and 88%, respectively. The grazing percentagedecreased with increasing density of planktivorous fish caughtin August each year using gill nets and shore-line electrofishing.The changes along a gradient of planktivorous fish abundanceseemed highest for bacterioplankton. Accordingly, the percentagecontribution of bacterioplankton to the total ingestion of thetwo carbon sources decreased from a summer mean value of 8%in Daphnia-dominated communities at lower fish density to 0.7–1.1%at high fish density, when cyclopoid copepods or Bosmina androtifers dominated. Likewise, the percentage of phytoplanktonproduction channelled through the bacteria varied, it beinghighest (5–8%) at high fish densities. It is argued thatthe negative impact of zooplankton grazing on bacterioplanktonin shallow lakes is highest at intermediate phosphorus levels,under which conditions Daphnia dominate the zooplankton community.     

Pigment-specific rates of phytoplankton growth and microzooplankton grazing in a subtropical lagoon

Phytoplankton growth and microzooplankton grazing rates wereevaluated in one station in Bahía Concepción,located in the middle region of the Gulf of California, México.We used high-performance liquid chromatography (HPLC) estimationsof phytoplankton pigment signatures to evaluate the annual variationof taxon-specific grazing and growth rates obtained with thedilution technique. Chlorophyll-a (Chl-a) concentrations variedwidely (0.34–3.32 µg L^–1) and showed two maxima,during late spring and autumn, associated with the transitionbetween mixed and stratified conditions. Phytoplankton growthrates varied seasonally with the lowest rates during summer(range: 0.01–2.55 day^–1 for Chl-a; 0.00–3.84day^–1 for Chl-b; 0.26–3.29 day^–1 for fucoxanthin;0.00–6.27 day^–1 for peridinin; 0.00–4.35 day^–1for zeaxanthin). Microzooplankton grazing was an important lossprocess (range: 0.0–1.89 day^–1 for Chl-a; 0.00–3.12day^–1 for Chl-b; 0.26–3.29 day^–1 for fucoxanthin;0.00–2.03 day^–1 for peridinin; 0.00–3.51 day^–1for zeaxanthin). Average grazing rates accounted 68–89%of estimated average phytoplankton pigment-specific growth rates.The analysis of pigment signatures indicates that diatoms anddinoflagellates were the dominant groups, and contrary to expectationfor typical subtropical lagoons, the specific growth rates inBahía Concepción showed a pronounced seasonalvariability, linked to transitional hydrographic conditions.Our results indicate a close coupling between the communitymicrozooplankton grazing and phytoplankton growth rates, withoutselective feeding behavior. These results suggest that microzooplanktonplay a critical role and may significantly modify the availabilityand efficiency of transfer of energy to higher trophic levels.     

Ingestion of phytoplankton by the micro- and mesozooplankton communities in a productive subtropical estuary

Grazing on phytoplankton by the micro- and mesozooplankton communitieswas measured during four cruises in a shallow (1.5 m) productive(up to 6 g C m^–2 day^–1 estuary in the northern Gulfof Mexico. Grazing-induced mortality on phytoplankton was alwayshigh and >95% of the grazing was by the microzooplanktoncommunity The grazing contribution from the mesozooplanktoncommunity, comprised primarily of Acartia tonsa, is believedto be small because populations were kept low by predation andadvective losses. A simple model is developed to describe phytoplankton-zooplanktoninteractions in this estuary. Attempts to understand the distributionand abundance of phytoplankton in estuaries must include estimatesof grazer-induced mortality on the phytoplankton.     

Estimation of phytoplankton loss rates from daily photosynthetic rates and observed blomass changes in Lake Constance

Potential growth rates of phytoplankton biomass were estimatedyear-round from production rates and biomass and were comparedwith observed changes in euphotic phytoplankton biomass. Potentialgrowth was always greater than observed growth. The discrepancybetween both is attributed to losses. Relative loss rates showedwide seasonal fluctuations with highest values during the springbloom and autumnal phytoplankton maximum, respectively. Lossrates of photoassimilated carbon showed one peak in late Maywhich lead to a clear-water phase. Relative loss rates werehighly correlated with potential growth rates whereas observedgrowth rates were not. This suggests that most losses occurimmediately after the production process and do not lead toincreases in biomass. During the spring bloom grazing by zooplanktonis the single most important factor leading to losses from thephytoplankton community. During that time, 80–98% of overalllosses can be accounted for by grazing, sedimentation and wash-outcombined. During brief periods in summer and autumn, sedimentationrates comprised >50% of overall losses. In autumn only 30–40%of overall losses were due to the above-mentioned processes.Residual losses can be attributed to respiration, lysis andbacterial remineralization. Grazing, respiration and lysis leadto recycling of carbon and nutrients. Sedimentation rate measurementssuggest an average euphotic carbon regeneration rate of 85%.For the transfer efficiency of carbon along the food chain therelative significance of respiratory losses in overall lossesis of fundamental importance. ¹Dedicated to Professor Elster on his 75th birthday. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982.     

High abundance of picoplankton-ingesting ciliates during late fall in Lake Kinneret, Israel

Small, aloricate ciliates dominated the biomass of heterotrophicprotists throughout the water column at the end of the periodof stratification in Lake Kinneret, Israel The integrated biomassof cilates was 5–20 times that of heterotrophic flagellatesDuring incubation experiments, ciliate growth rates in cpilimneticwater corresponded to population doubling times of 9.6–19.4h, while flagellate populations showed no growth. Most of thealiates were small forms (10–30 µm long), includingscuticocihates, choreotnchs, Coleps spp. and Colpoda spp., andappeared to be consuming bacteria, coccoid cyanobacteria, and<5 µm eukaryotic algae. Grazing rates of cihate assemblageson picoplankton in the epilimnion, as determined by the uptakeof fluorescently labeled bacteria and cyanobactena, ranged from62 to 86 nl cell^–1 h^–1 Colpoda steini, isolatedfrom lakewater, grew on a cultured freshwater Synechococcussp with a doubling time of 4.5 h, and a gross growth efficiencyof 48% The estimated daily requirements of ciliates for growthapproximately equalled total phytoplankton production. We calculatedthat ciliates in the epilimnion were clearing 4–10% ofthe bacterioplankton and cyanobactenal standing stocks per daySince this would not be sufficient food consumption to meetdaily carbon requirements of the aliates, it is likely thatthese organisms were also grazing a significant amount of autotrophicand heterotrophic eukaryotic cells in Lake Kinneret.     

Laboratory-measured grazing and ingestion rates of the salp, Thalia democratica Forskal, and the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

Grazing and ingestion rates of laboratory-born Thalia democraticaaggregates and Dolioletta gegenbauri gonozooids, phorozooidsand oozooids were determined while fed Isochrysis galbana (4–5µm diameter) alone or in combination with Peridinium trochoideum(16–18 µm diameter) at concentrations of 0.15–0.70mm³ x 1^–1. Grazing rates (ml x zooid^–1 x 24 h ^–1)ranged from 10 to 355, and at zooid weights greater than 5 µgcarbon were in order oozooid > gonozooid > aggregate.Grazing rates increased exponentially with increasing zooidweight. Weight-specific grazing rates (ml x µgC^–1x 24 h^–1) were independent of the four-fold initial foodconcentration. Mean weight-specific grazing rates increasedlinearly with increasing zooid weight for the aggregates andoozooids, but gonozooid mean rates were independent of zooidweight. Aggregate and gonozooid ingestion rates (10⁶ µm³x zooid^–1 x 24 h^–1) ranged from 4 to 134 while oozooidrates ranged from 3 to 67. All ingestion rates were independentof the initial food concentration but increased linearly withincreasing zooid weight at similar rates. All mean weight-specificingestion rates (ml x µgC^–1 x 24 h^–1) wereindependent of zooid weight. The mean aggregate daily ration(µgC ingested x µg body C^–1) was 59% and themean doliolid ration was 132%. Field studies indicate that normalconcentrations of D. gegenbauri in the Georgia Bight clear theirresident water volume (1 m³) in about 4 months, but that highlyconcentrated, swarm populations which occur along thermohalinefronts clear their resident water volume in less than 1 day. ¹Current address: MacLaren Plansearch Ltd., P.O.Box 13250, sta.A.,St.John's, Nfld. A1B 4A5     

Microzooplankton grazing in a coastal embayment off Concepcion, Chile, (~36{degrees} S) during non-upwelling conditions

The impact of grazing by natural assemblages of microzooplanktonwas estimated in an upwelling area (Concepción, Chile)during the non-upwelling season in 2003 and 2004. Seawater dilutionexperiments using chlorophyll a (Chl a) as a tracer were usedto estimate daily rates of phytoplankton growth and microzooplanktongrazing. Initial Chl a concentrations ranged from 0.4 to 1.4mg Chl a m^–3 and phytoplankton prey biomass and abundancewere numerically dominated by components <20 µm. Phytoplanktongrowth and microzooplankton grazing rates were 0.19–0.25day^–1 and 0.26–0.52 day ^–1, respectively.These results suggest that microzooplankton exert a significantremoval of primary production (>100%) during the non-upwellingperiod.     

Bacterial growth and losses due to bacterivory in a mesotrophic lake

Bacterial secondary production and rates of bacterivory weredetermined from samples collected from mesotrophic Lake Arlington.Bacterial production and losses were determined by comparingthe growth of natural bacterial assemblages in the presenceof predators (unfiltered samples) to growth in the absence ofpredators (water filtered through 1.0 (im porosity filters).Growth rates of heterotrophic nanoflagellates (HNF) were estimatedfrom growth in the absence of predators (water filtered through5.0 µm porojity filters). Bacterial growth rates rangedbetween 0.002 and 0.069 h^–1 and averaged 0.026 h^–1.HNF grew at rates ranging between 0.003 and 0.107 h^–1and averaged 0.028 h^– Grazing rates ranged between 0.002and 0.043 h^–1, and averaged 0.018 h. The annual averagerate of bacterial biomass synthesis was 3.2 –g Cl^–h^–1 and {small tilde}69% of this production was grazed.Temporal changes in growth and grazing rates suggest a tightlycoupled predator-prey linkage in this lake. ¹Present address: Hydrobiological Institute, Czech Academy ofSciences, Na sddkach 7, 370 05 teski Budjovice, Czech Republic     

Size-fractionated mesozooplankton biomass, metabolism and grazing along a 50{degrees}N-30{degrees}S transect of the Atlantic Ocean

Size-fractionated mesozooplankton grazing and metabolism wereinvestigated along the wide latitudinal range (50°N–30°S)covered during the Atlantic Meridional Transect (AMT) 11 cruise.Five different oceanic provinces were traversed in this cruise:North Atlantic Drift (NADR), North Atlantic Subtropical Gyral(NAST), Canary Coastal (CNRY), Eastern Tropical Atlantic (ETRA),and South Atlantic Gyral (SATL). CNRY and ETRA were affectedby the upwelling Mauritanian and equatorial respectively andprimary production in these provinces was higher than in theoligotrophic subtropical gyres (NAST and SATL). Both mesozooplanktonand phytoplankton biomass were highest around the equator. Theamount of chlorophyll a ingested daily by copepods was noticeablyhigher in mesotrophic than in oligotrophic provinces as shownby the spatial distribution of gut content values and the highabundances of copepods recorded at the equator. Grazing impactalong the transect ranged from 0.2 to 5.6% of the phytoplanktonstanding stock and from 1.6 to 14.5% of primary production.If only phytoplankton >2 µm are considered, the rangesare 1.0–19.4% (stock) and 3.4–44.7% (primary production).Grazing impact upon both phytoplankton biomass and primary productionfollowed a spatial distribution similar to that of chlorophylla ingestion, with higher values in upwelling zones than in thegyres. Weight-specific rates of respiration and NH₄⁺ and PO₄^3–excretion showed large variability both along the transect andwithin provinces, but did not differ between provinces. Therefore,zooplankton assemblages inhabiting the different provinces visitedin the AMT 11 seem to be adapted to the prevailing thermal conditions.Given the substantial proportion of nitrogen and phosphorusthat are supplied to primary producers through the excretoryactivity of mesozooplankton (the percentage of nitrogen andphosphorus requirements of phytoplankton accounted for by mesozooplanktonexcretion was >30% in all the provinces) it follows thatthey may play a crucial role as nutrient regenerators, especiallyin the oligotrophic gyres where regenerated production dominates.     

Trophic links in the lowland River Meuse (Belgium): assessing the role of bacteria and protozoans in planktonic food webs

Trophic interactions within the plankton of the lowland RiverMeuse (Belgium) were measured in spring and summer 2001. Consumptionof bacteria by protozoa was measured by monitoring the disappearanceof ³H-thymidine-labelled bacteria. Metazooplankton bacterivorywas assessed using 0.5-µm fluorescent microparticles (FMPs),and predation of metazooplankton on ciliates was measured usingnatural ciliate assemblages labelled with FMPs as tracer food.Grazing of metazooplankton on flagellates was determined throughin situ incubations with manipulated metazooplankton densities.Protozooplankton bacterivory varied between 6.08 and 53.90 mgC m^–3 day^–1 (i.e. from 0.12 to 0.86 g C^–1bacteria g C^–1 protozoa day^–1). Metazooplankton,essentially rotifers, grazing on bacteria was negligible comparedwith grazing by protozoa (1000 times lower). Predation of rotiferson heterotrophic flagellates (HFs) was generally low (on average1.77 mg C m^–3 day^–1, i.e. 0.084 g C^–1 flagellatesg C^–1 rotifers day^–1), the higher contribution ofHF in the diet of rotifers being observed when Keratella cochleariswas the dominant metazooplankter. Predation of rotifers on ciliateswas low in spring samples (0.56 mg C m^–3 day^–1,i.e. 0.014 g C^–1 ciliates g C^–1 rotifers day^–1)in contrast to measurements performed in July (8.72 mg C m^–3day^–1, i.e. 0.242 g C^–1 ciliates g C^–1 rotifersday^–1). The proportion of protozoa in the diet of rotiferswas low compared with that of phytoplankton (<30% of totalcarbon ingestion) except when phytoplankton biomass decreasedbelow the incipient limiting level (ILL) of the main metazooplantonicspecies. In such conditions, protozoa (mainly ciliates) constituted50% of total rotifer diet. These results give evidence thatmicrobial organisms play a significant role within the planktonicfood web of a eutrophic lowland river, ciliates providing analternative food for metazooplankton when phytoplankton becomesscarce.     

Carbon Fixation, Ammonium Uptake and Regeneration in an Equatorial Lake: Biological Versus Physical Control

Ammonium uptake and regeneration were measured in the euphoticzone of Petit Saut Lake, French Guyana, to examine nitrogencycling in this recently flooded equatorial forest environment.Am-monium regeneration rates were extremely high (mostly inthe range 1–6 µmol N l^–1 h^–1), and aredue to the very high grazing rates of the microzooplankton,which consumed between 56 and 95% of the phytoplankton productionin any given incubation. These regeneration rates were aboutan order of magnitude higher than the net ammonium uptake rates.This imbalance is probably due to dissolved organic nitrogenrelease during grazing. At the bottom of the euphotic zone (4–5m), photosynthetic bacteria are responsible for ammonium uptake.Diffusion-driven ammonium fluxes are an order of magnitude lowerthan biologically driven fluxes. Therefore, ammonium fluxesare dominated by biology rather than by physics in this lake.     

Dynamics of microplankton communities at the ice-edge zone of the Lazarev Sea during a summer drogue study

Microzooplankton grazing and community structure were investigatedin the austral summer of 1995 during a Southern Ocean Drogueand Ocean Flux Study (SODOFS) at the ice-edge zone of the LazarevSea. Grazing was estimated at the surface chlorophyll maximum(5–10 m) by employing the sequential dilution technique.Chlorophyll a concentrations were dominated by chainformingmicrophytoplankton (>20 µm) of the genera Chaetocerosand Nitzschia. Microzooplankton were numerically dominated byaloricate ciliates and dinoflagellates (Protoperidinium sp.,Amphisoleta sp. and Gymnodinium sp.). Instantaneous growth ratesof nanophytoplankton (<20 µm) varied between 0.019and 0.080 day^–1, equivalent to between 0.03 and 0.12 chlorophylldoublings day^–1. Instantaneous grazing rates of microzooplanktonon nanophytoplankton varied from 0.012 to 0.052 day^–1.This corresponds to a nanophytoplankton daily loss of between1.3 and 7.0% (mean = 3.76%) of the initial standing stock, andbetween 45 and 97% (mean = 70.37%) of the daily potential production.Growth rates of microphytoplankton (>20 µm) were lower,varying between 0.011 and 0.070 day^–1, equivalent to 0.015–0.097chlorophyll doublings day^–1. At only three of the 10 stationsdid grazing by microzooplankton result in a decrease in microphytoplanktonconcentration. At these stations instantaneous grazing ratesof microzooplankton on microphytoplankton ranged between 0.009and 0.015 day^–1, equivalent to a daily loss of <1.56%(mean = 1.11%) of initial standing stock and <40% (mean =28.55%) of the potential production. Time series grazing experimentsconducted at 6 h intervals did not show any diel patterns ofgrazing by microzooplankton. Our data show that microzooplanktongrazing at the ice edge were not sufficient to prevent chlorophylla accumulation in regions dominated by rnicrophytoplankton.Here, the major biological routes for the uptake of carbon thereforeappear to be grazing by metazoans or the sedimentation of phytoplanktoncells. Under these conditions, the biological pump will be relativelyefficient in the drawdown of atmospheric CO₂.     

Biomass, feeding and production of Noctiluca scintillans in the Seto Inland Sea, Japan

The abundance and biomass of the large heterotrophic dinoflagellateNoctiluca scintillans, together with the changes in its potentialprey items, were monitored in the Seto Inland Sea, Japan, duringsummer 1997 (17 July-11 August). Growth and grazing rates ofNscintillans fed natural plankton populations were also measuredeight and seven times, respectively, during the survey period.The abundance and biomass of N scintillans averaged over thewater column (19 m) were in the range 1–345 cells 1^–1(temporalaverage = 93 cell1^–1) and 0.1–49.6 µg C l^–1(temporalaverage = 13.8 µg C l^–1; three times higher thanthat of calanoid copepods during the same period). Noctilucascintillans populations followed the changes in phytoplankton:N.scintillans biomass was increasing during the period of diatomblooms and was at a plateau or decreasing during periods oflow chlorophyll a. The growth rates of N.scintillans (µ)were also consistent with the wax and wane of the N.scintillanspopulation: N.scintillans showed highest growth rates duringdiatom blooms. A simple relationship between µ and chlorophylla concentration was established, and the production of N.scintillanswas estimated using this relationship and the measured biomass.The estimated production averaged over the water column wasin the range >0.1–5.2 µg C l^–1 day^–1(temporalaverage = 1.4 µg C l^–1 day^–1; 64% of the productionof calanoid copepods during the same period). Diatom clearancerates by N.scintillans were in the range 0.10–0.35 mlcell^–1 day^–1, and the phytoplankton population clearanceby N.scintillans was >12% day^–1. Thus, although thefeeding pressure of N.scintillans on phytoplankton standingstock was low, N.scintillans was an important member of themesozooplank-ton in terms of biomass and production in the SetoInland Sea during summer.     

Nanoheterotroph grazing on bacteria and cyanobacteria in oxic and suboxic waters in coastal upwelling areas off northern Chile

The vertical distribution and abundance of microbial assemblagesand the grazing of nanoheterotrophs upon prokaryotes in oxicand suboxic waters were examined in two coastal upwelling areasoff northern Chile where a shallow Oxygen Minimum Zone (OMZ)is characteristic. Prokaryotic prey included bacterioplanktonand cyanobacteria (Synechococcus); both displayed a bimodaldistribution, with abundance maxima above and within the upperOMZ. Flagellates numerically dominated the nanoplankton andwere mostly concentrated in the oxic layer. Mean ingestion ratesof cyanobacteria by nanoflagellates (vacuole content method)ranged from 0.2 to 1.1 cells flagellate^–1 h^–1 andmean consumption rates (34–160 cells mL^–1 h^–1)were four times higher in the oxic layer. With the selectiveinhibitors technique, specific grazing rates on bacteria werelow (<0.1 h^–1) and consumption did not control bacterialproduction in the surface layer but did so in the suboxic layer(accounting for >100% of bacterial production). With thesame method, the specific grazing rate on cyanobacteria rangedbetween zero and 0.23 h^–1 with no clear differences betweenoxygen conditions; prey growth and production were always higherthan the grazing pressure (accounting for <17% of cyanobacterialproduction). The impact of grazing by nanoheterotrophs in regulatingthe production of prokaryotes in oxic and suboxic waters inthis region is discussed.     

Measurement of in situ growth rates of phytoplankton under conditions of simulated turbulence

In situ measurement of the growth rates of planktonic populationscan be improved by using dialysis chambers (‘cage cultures’)to avoid shifts in the chemical environment during incubation.Vertical mixing and small-scale turbulence affect the growthof planktonic populations, there fore natural mixing conditionsshould be simulated as closely as possible during the incubation.A new device is described here which combines the advantagesof a dialysis chamber with a programmable vertical mixing regime.Realistic phytoplankton growth rates can thus be measured insitu under con ditions of vertical mixing and small-scale turbulence.The chamber made of transparent, UV-transmitting acrylic glasswas fitted at both ends with permeable polycarbonate membranes.It was moved vertically through the water column by a pocket-sizedlift and rotated simultaneously on its central axis. The methodwas applied to two typos of experiments on growth and lossesof phytoplankton in the River Severn, UK. The first one comparedchanges in biovolume of phytoplankton in a water parcel flowingdownstream (6% h^–1 decline) with those in a simultaneouslyincubated dialysis chamber moved between water surface and riverbottom (7% h^–1 increase). The difference equates to algallosses prevented in the chamber but suffered along the river(mainly sedimentation and grazing of benthic filter feeders).Loss rate of diatoms was three times higher than those of chlorophytes.In another experiment growth of phytoplankton from the mainstream and lateral dead zone was compared under different mixingconditions. Algae from the main stream grew faster than fromthe dead zone. Only cryptophytes preferred calm conditions,all the other algal groups grew faster in chambers moved throughthe water column than in stationary ones. Further possible applicationsin both standing and flowing waters are discussed.     

Uptake and regeneration of inorganic nitrogen in coastal waters influenced by the Mississippi River spatial and seasonal variations

The Mississippi and Atchafalaya Rivers introduce large amountsof nutrients to surface waters of the northern Gulf of Mexico.This paper reports the most complete data to date on inorganicnitrogen uptake and regeneration in a broad range of coastalenvironments influenced by the river water, along with informationon nutrient concentrations and including pico-, nano-, and microplanktonspecies composition. Nitrate in surface waters is greatly reducednear the river plume, at salinities between 5 and 25 PSU, wherethe largest variance in uptake rates was observed, and was coincidentwith peaks in surface chlorophyll. Despite the depletion ofnitrate, nitrogen limitation was a rare event during the study,because of relatively high ammonium concentrations (>1 µmolNH₄⁺ I^–1 and regeneration rates. Two contrasting situationscharacterize the seasonal nitrogen dynamics in surface shelfwaters. High nitrate input during the spring caused a largebloom in which the cells were well adapted to use nitrate.Thedominant phytoplankton species were chain forming diatoms, alsoreported in sediment-trap studies in the area. Ammonium regenerationonly accounted for a small fraction of the nitrogen requirementsduring the bloom. In contrast, the low flow of river water duringsummer resulted in low nitrate concentrations in surface water.In this case phytoplankton productivity was highly reduced andmay depend greatly on ‘in sita’ ammonium regeneration.     

Predicting chlorophyll vertical distribution in response to epilimnetic nutrient enrichment in small stratified lakes

Light-limited metalimnetic phytoplankton communities are thoughtto be negatively impacted by epilimnetic nutrient enrichmentbecause of shading by increased epilimnetic phytoplankton biomass.We tested this expectation with a dynamic simulation model thatwas calibrated to three lakes undergoing whole-lake nutrientand food web manipulations. Total areal chlorophyll increaseddue to nutrient enrichment in each lake, but the magnitude ofthe response varied between lakes. Modeling experiments, whichallowed analysis of separate components of each lake's responseto nutrient enrichment, indicated that the response to enrichmentdepended on lake water color and food web structure. In weaklystained lakes ({small tilde}10 mg Pt 1^–1, k₄ = 0.4 m^–1),metalimnetic chlorophyll was stimulated by nutrient enrichmentup to moderate levels (1 µg Pt1^–1 day^–1).In more strongly colored lakes (25 mg Pt 1^–1, k₄ = 1.0),metalimnetic chlorophyll responded negatively to nutrient enrichmentat all P loading rates. Food web structure, as expressed byrates of zooplanktivory, interacted with water color in twoways. One impact was through direct grazing losses on metalimneticchlorophyll. The other process involved was indirect impactfrom grazing on epilimnetic phytoplankton, which reduced shadingon metalimnetic chlorophyll. Vertical redistribution of chlorophyllbetween the epilimnion and the metalimnion led to little accumulationof areal chlorophyll with increased P loading over limited rangesof water color and nutrient input rates. Model predictions maybe most effectively tested with whole-lake experiments contrastingfood web structure, water color and nutrient loading.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

Pheopigment dynamics,zooplankton grazing rates and the autumnal ammonium peak in a Mediterranean lagoon

The dilution technique was used to estimate chlorophyll and pheopigment, net and gross production as well as zooplankton grazing over a 12-month period in a coastal lagoon in Southern France. Chlorophyll a (Cha) based gross growth rates of phytoplankton ranged from undetectable in February to 2.6 day⁻¹ in June, corresponding to 3.8 divisions per day. Cha-based grazing rates ranged from undetectable in February to 1.1 d⁻¹ in June. The seasonal growth pattern of picoplankton was similar to that of the whole community, with a peak in July, corresponding to four divisions per day. Grazing processes represented from 20 to 150% of the phytoplankton daily growth, and the grazing pressure was stronger on small phytoplankton cells than on larger cells. Gross growth rates of phytoplankton were related to zooplankton grazing rates, and both were related to water temperature. Mesozooplankton which escaped sampling or oysters had to be also invoked as additional sinks for the primary production. In the fall, pheopigment concentrations greater than chlorophyll concentrations coincided with high ammonium levels in the water column. Pheopigment a production rates were highly correlated to chlorophyll -based microzooplankton grazing rates. The pheopigment a to chlorophyll a ratio was correlated with ammonium concentrations and could be used an index of the balance between ammonium supply (degradation) and demand (uptake by phytoplankton). In addition, pheopigment degradation rates in absence of grazing could be related to irradiance, indicating photo-degradation of these compounds.     

Carbon dynamics in the 'grazing food chain' of a subtropical lake

Studies were conducted over a 13 month period at four pelagicsites in eutrophic Lake Okeechobee, Florida (USA), in orderto quantify carbon (C) uptake rates by size-fractionated phytoplankton,and subsequent transfers of C to zooplankton. This was accomplishedusing laboratory ¹⁴C tracer methods and natural plankton assemblages.The annual biomass of picoplankton (<2 µm), nanoplankton(2–20 µm) and microplankton (<20 µm averaged60, 389 and 100 µg C 1^–1 respectively, while correspondingrates of C uptake averaged 7, 51 and 13 µg C1^–1h^–1. The biomass of microzooplankton (40–200 µm)and macrozooplankton (<200 µm averaged 18 and 60 µgC 1^–1, respectively, while C uptake rates by these herbivoregroups averaged 2 and 3 µg C 1^–1 h^–1. Therewere no strong seasonal patterns in any of the plankton metrics.The ratio of zooplankton to phytoplankton C uptake averaged7% over the course of the study. This low value is typical ofthat observed in eutrophic temperate lakes with small zooplanktonand large inedible phytoplankton, and indicates ineffectiveC transfer in the grazing food chain. On a single occasion,there was a high density (<40 1^–1) of Daphnia lumholrzii,a large-bodied exotic cladoceran. At that time, zooplanktoncommunity C uptake was <20 µg C 1^–1 h^–1and the ratio of zooplankton to phytoplankton C uptake was near30%. If D.lumholrzii proliferates in Lake Okeechobee and theother Florida lakes where it has recently been observed, itmay substantially alter planktonic C dynamics.