The effect of different zooplankton grazing patterns resulting from diel vertical migration on phytoplankton growth and composition: a laboratory experiment

Diel vertical migration (DVM) of herbivorous zooplankton is a widespread behavioural phenomenon in freshwater ecosystems. So far only little attention has been paid to the impact of DVM on the phytoplankton community in the epilimnion. Some theoretical models predict that algal population growth in the epilimnion should depend on the herbivores migration and grazing patterns: even if migrating zooplankton consume the same total amount of algae per day in the epilimnion as non-migrating zooplankton, nocturnal grazing should result in enhanced algal growth and favour algal species with high intrinsic growth rates over species with lower intrinsic growth rates. To test these hypotheses we performed experiments in which several algal species were confronted with different feeding regimes of Daphnia. In the experiments algal growth did not only depend on the absolute time of grazing but was comparatively higher when grazing took place only during the night, even when the grazing pressure was the same. Furthermore, algal species with higher intrinsic growth rates had higher advantages when being grazed upon only discontinuously during the night than algal species with a smaller intrinsic growth rate. The grazing pattern itself was an important factor for relative algal performance.     

Intraspecific variation in a predator drives cascading variation in primary producer community composition

Predation has important cascading impacts on primary producer biomass and community composition in many ecosystems. While most studies have focused on the consequences of interspecific or density differences in predators, it is recognized that phenotypic variation within species can have strong and cascading community and ecosystem consequences at lower trophic levels. In coastal New England lakes, both the presence and life history form of the zooplanktivorous fish alewife, Alosa pseudoharengus, have strong influence on the biomass, size structure and community composition of crustacean zooplankton communities. Here we test the hypothesis that alewife presence and life history will have cascading impacts on phytoplankton biomass and community composition in a mesocosm experiment that previously reported strong biomass and compositional differences of crustacean zooplankton communities among alewife treatments. We show that alewife life history led to small but statistically significant differences in phytoplankton community composition among treatments. This compositional difference was driven primarily by an increase in the density of two edible phytoplankton genera associated with lower zooplankton biomass in the anadromous alewife treatment. Our results show that intraspecific variation in a predator can have cascading effects on primary producer communities. However we did not observe significant differences in total algal biomass.     

Differential responses of size‐based functional groups to bottom–up and top–down perturbations in pelagic food webs: a meta‐analysis

We performed a meta‐analysis of 31 lake mesocosm experiments to investigate differences in the responses of pelagic food chains and food webs to nutrient enrichment and fish presence. Trophic levels were divided into size‐based functional groups (phytoplankton into highly edible and poorly edible algae, and zooplankton into small herbivores, large herbivores and omnivorous zooplankton) in the food webs. Our meta‐analysis shows that 1) nutrient enrichment has a positive effect on phytoplankton and zooplankton, while fish presence has a positive effect on phytoplankton and a negative effect on zooplankton in the food chains; 2) nutrient enrichment has a positive effect on highly edible algae and small herbivores, but no effect on poorly edible algae, large herbivores and omnivorous zooplankton in the food webs. Planktivorous fish have a positive effect on highly edible algae and small herbivores, a negative effect on large herbivores and omnivorous zooplankton, and no effect on poorly edible algae. Our meta‐analysis confirms that nutrient enrichment and planktivorous fish affect functional groups differentially within trophic levels, revealing important changes in the functioning of food webs. The analysis of fish effects shows the well‐described trophic cascade in the food chain and reveals two trophic cascades in the food web: one transmitted by large herbivores that benefit highly edible phytoplankton, and one transmitted by omnivorous zooplankton that benefit small herbivores. Comparison between the responses of food webs and simple food chains also shows consistent biomass compensation between functional groups within trophic levels.     

Diel vertical migration by males,females, copepodids and nauplii in a limnetic population of diaptomus (Copepoda)

Diel vertical migration (DVM) by the sexes and life history stages of the copepod Diaptomus novamexicanus (Herrick) was investigated during two summers in a subalpine lake (Castle Lake, California, USA). Migratory behavior was quantified repeatedly by 1300 and 21–2200 hr sampling in the epilimnion. Population density and vertical distribution were estimated routinely at 1300 hr and at 2200 hr on selected dates. Males had greater night to day biomass differentials and ratios than females. The sexes showed comparable DVM participation (% of total biomass migrating) and amplitudes of migration (distance of ascent). Copepodids displayed higher participation, night/day ratios and amplitudes than younger copepods, but adult participation differed only from that of nauplii. Active participation in high amplitude ascents thus increased with age and was greatest in adult males. Analysis suggests that such relative behaviors probably occur in most limnetic Diaptomus, and that the magnitude of ascent into the epilimnion is associated with daytime vertical distribution.     

The impact of year-to-year changes in the weather on the dynamics of Daphnia in a thermally stratified lake

The factors influencing the seasonal dynamics of Daphnia in a thermally stratified lake (Esthwaite Water) are described and related to long-term changes in the weather. The Daphnia produced three cohorts in the year and the strength of the cohorts was determined by year-to-year variations in the physical characteristics of the lake and the abundance of edible algae. Food was most abundant in early summer when small, fast-growing flagellates were particularly common. In late summer, the phytoplankton community was dominated by large, inedible species but edible forms re-appeared when nutrients were entrained by wind mixing. Examples are presented to demonstrate the effect that year-to-year variations in the weather have on the growth of the phytoplankton and the dynamics of the Daphnia. In ‘good’ years, when the lake stratifies early and there are periods of episodic mixing in summer, there are two ‘pulses’ of edible algae and two strong cohorts of Daphnia. In ‘bad’ years when stratification is delayed and there is little episodic mixing, the growth of the edible algae is suppressed and the Daphnia produce two weak cohorts. The results are discussed in relation to the impact of intermediate disturbances on growth of phytoplankton and current theories of population regulation in Daphnia. The evidence suggests that the dynamics of the Daphnia in the lake are strongly influenced by seasonal variations in the mixing regime, the recycling of nutrients and the episodic growth of edible algae.     

Advantages from diel vertical migration can explain the dominance of Gonyostomum semen (Raphidophyceae) in a small, steeply-stratified humic lake

In late summer, a large flagellated alga, Gonyostomum semen(Raphidophyceae), constituted most of the phytoplankton biomassin a small steeply-stratified humic lake. Its diel verticalmigration (DVM) was very distinct and extended at night intothe anoxic hypolimnion. After midsummer, the depletion of hypolimneticoxygen led to a gradual release of soluble reactive phosphorus(SRP) from the sediment, but one month later, irrespective ofcontinuing stratification, the concentrations again returnedto undetectable levels down to the bottom. As this coincidedwith the rapid increase in G.semen population, the latter wasprobably responsible for the depletion of SRP. The flux of SRPfrom the sediment to the epilimnion was virtually interrupted,making non-migrating phytoplankton dependent only on regeneratedand inflowing inorganic phosphorus. Besides nutrient availability,DVM also benefited G.semen in the reduction of metabolic andgrazing losses. In this lake, the remarkable multiple advantagesof DVM probably explain the dominance of the large G.semen inthe late summer phytoplankton biomass over much smaller algae.     

Responses to fish predation and nutrients by plankton at different levels of taxonomic resolution

1. To improve mechanistic understanding of plankton responses to eutrophication, a mesocosm experiment was performed in the shallow littoral zone of a south Swedish lake, in which nutrient and fish gradients were crossed in a fully factorial design. 2. Food chain theory accurately predicted total biomass development of both phyto‐ and zooplankton. However, separating zooplankton and algae into finer taxonomic groups revealed a variety of responses to both nutrient and fish gradients. 3. That both nutrients and fish are important for phytoplankton dynamics was seen more clearly when viewing each algal group separately, than drawing conclusions only from broad system variables such as chlorophyll a concentration or total phytoplankton biovolume. 4. In some taxa, physiological constraints (e.g. sensitivity to high pH and low concentrations of free CO₂) and differences in competitive ability may be more important for the biomass development than fish predation, grazing by herbivorous zooplankton, and nutrient availability. 5. We conclude that food chain theory accurately predicted responses in system variables, such as total zooplankton or algal biomass, which are shaped by the dynamics of certain strong interactors (‘keystone species’), such as large cladocerans, cyanobacteria and edible algae (<50 μm), whereas responses at finer taxonomic levels cannot be predicted from current theory.     

Diel vertical and horizontal distribution of crustacean zooplankton and young of the year fish in a sub-alpine lake: an approach based on high frequency sampling

Understanding the spatial dynamics of predators and their preyis one of the most important goals in aquatic ecology. We studiedspatial and temporal onshore–offshore distribution patternsin young of the year (YOY) Eurasian perch (Perca fluviatilis)and crustacean zooplankton (Daphnia hyalina, Cyclops prealpinus)along a transect in Lake Annecy (France). Our study representsa first attempt at coupling hydroacoustic fish survey and highfrequency zooplankton recording to assess simultaneously thelarge-scale distribution patterns of YOY fish and their zooplanktonprey over a diel cycle (day, dusk and night sampling). We hypothesizedthat the spatial distribution of zooplankton could be shapedby both anti-predator behaviour (horizontal and vertical migrations)and predation losses. Fish biomass, size structure and dietwere assessed from split-beam echosounding and net trawlingsamples, whereas crustacean abundances were estimated with asmall modified Longhurst–Hardy continuous plankton recorder.We evaluated the diel changes in the spatial distribution patternsof fish and zooplankton and determined the overlap between theirdistributions. Fish biomass was dominated by YOY perch in upperwarmer layers and salmonids (Coregonus lavaretus and Salvelinusalpinus) in the colder and oxygenated deep layers. YOY perchwere aggregated in dense schools in the epilimnion during theday and dispersed at night. Fish biomass was distributed alonga strong increasing onshore–offshore gradient at night,whereas crustacean prey showed a decreasing gradient. This onshore–offshorenegative gradient in crustacean distribution, expressed on ashorter scale during the day, shifted toward the surface watersat night. A distinct kinetic of diel vertical migration (DVM)patterns was exhibited by daphnid and cyclopoid populationsand resulted in distinct vulnerability to perch predation. Spatio-temporaldistribution of crustaceans in Lake Annecy during the diel cyclestudy was probably shaped both by predation loss to YOY perchand by anti-predator behaviour (DVM, DHM) by zooplankton. Theimplications for fine-scale studies of fish-zooplankton interactionsare discussed.     

Interaction effects of zooplankton and CO2 on phytoplankton communities and the deep chlorophyll maximum

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Movement of plankton through lake-stream systems

1. River plankton are often assumed to come from upstream lakes, but the factors controlling the movement of plankton between lakes and rivers into outflow streams are unclear. We tested the possibility that the physical structure of the littoral zone near the lake outlet (depth, presence of macrophytes) and diurnal differences in plankton composition at the lake surface influence the movement of plankton from the lake into the stream and determine their persistence downstream. 2. Zooplankton and phytoplankton biomass, community composition and mean body size were compared between two deep lakes without macrophytes at the lake edge and two shallow lakes with macrophytes at the lake edge. Samples were collected day and night on three dates, in the lake centre, in the littoral zone adjacent to the lake outlet, at the outlet and at two sites downstream in Algonquin Park, Ontario, Canada. 3. The morphology of lake edges clearly affects the movement of lake zooplankton into outlet streams. Outlets draining deeper littoral zones had higher zooplankton biomass than shallow littoral outlets (P < 0.0001), but these differences disappeared within 50 m downstream of the lake. There was no difference in mean zooplankton body size among lake outlets or between littoral and outlet samples. However, shallow littoral zones were dominated by cyclopoid copepods and deeper littoral zones were dominated by Bosmina longirostris. In contrast, phytoplankton biomass entering the outlet was similar to that found within the lake and did not vary with lake outlet morphology. These effects were consistent across several sampling weeks and were not affected by surface zooplankton biomass changes associated with diurnal vertical migration in the lake centre. 4. A comparison with published river zooplankton data suggests that zooplankton are rapidly eliminated from shallow outlet streams (≤1 m deep) but persist in most deeper outlet rivers (≥2 m deep). Because the depth of an outlet river determines downstream zooplankton community development, the contribution of lakes to river plankton communities may be influenced by the location of each lake within the drainage basin. These findings suggest that lake and outflow physical structure influences connection strength between spatially successive habitats.     

Biomanipulation development in Norway

Since 1974 several studies have been carried out in Norway to investigate the interactions between planktivorous fish, zooplankton, phytoplankton and water chemistry. Since 1978 a long-term national research program has been conducted by the Norwegian Council for Scientific and Industrial Research (NTNF). In this program several whole lake manipulations of the fish stocks have been performed to test hypotheses about trophic interactions. It was predicted that manipulations of planktivorous fish populations, might also improve water quality in lakes undergoing eutrophication. Two examples are given to illustrate the achieved results. I: Whole lake fertilization experiment (1974–1978) carried out by Langeland and Reinertsen. The results revealed the importance of top-down effects in the lake ecosystem. When cladocerans dominated, the zooplankton community was able to maintain a more or less constant phytoplankton biomass and a rather low phytoplankton production even when nutrient levels were increased. During years with rotifer dominance, algal biomass and productivity increased, despite the low amounts of added nutrients. II: Experiment performed by Reinertsen, Jensen, Koksvik, Langeland and Olsen in the eutrophic Lake Haugatjern, total elimination of the fish populations by rotenone in late 1980, resulted in a 4-fold decrease in the algal biomass. The species composition changed from the dominance of large-sizedAnabaena flos-aquae andStaurastrum luetkemuelleri to smaller, fastgrowing species and gelatinous green algae. The results are discussed in relation to management of inland waters by combined techniques of biomanipulation and reduced external nutrient supply which increase food-chain efficiency.     

Browning reduces the availability—but not the transfer—of essential fatty acids in temperate lakes

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Effects of nitrogen enrichment on zooplankton biomass and N:P recycling ratios across a DOC gradient in northern-latitude lakes

We used data from whole-lake studies to assess how changes in food quantity (phytoplankton biomass) and quality (phytoplankton community composition, seston C:P and N:P) with N fertilization affect zooplankton biomass, community composition and C:N:P stoichiometry, and their N:P recycling ratio along a gradient in lake DOC concentrations. We found that despite major differences in phytoplankton biomass with DOC (unimodal distributions, especially with N fertilization), no major differences in zooplankton biomass were detectable. Instead, phytoplankton to zooplankton biomass ratios were high, especially at intermediate DOC and after N fertilization, implying low trophic transfer efficiencies. An explanation for the observed low phytoplankton resource use, and biomass responses in zooplankton, was dominance of colony forming chlorophytes of reduced edibility at intermediate lake DOC, combined with reduced phytoplankton mineral quality (enhanced seston N:P) with N fertilization. N fertilization, however, increased zooplankton N:P recycling ratios, with largest impact at low DOC where phytoplankton benefitted from light sufficiently to cause enhanced seston N:P. Our results suggest that although N enrichment and increased phytoplankton biomass do not necessarily increase zooplankton biomass, bottom-up effects may still impact zooplankton and their N:P recycling ratio through promotion of phytoplankton species of low edibility and altered mineral quality.

     

Planktonic food chains of a highly humic lake

The development and metabolism of the plankton of a highly humic lake were followed over the vernal primary production maximum. The study was made in a mesocosm in which large filter feeders, typical of this lake in summer, were absent. During the rising phase of phytoplankton, the community was predominantly autotrophic. The most important constituents in the algal biomass were a dinoflagellate, Gymnodinium sp. (40–50%), and a prasinophycean, Scourfieldia cordiformis (7%). The biomasses of Chlamydomonas spp. and Chrysococcus spp. reached their maxima a few days later and Cryptomonas sp. became most abundant at the end of the experiment. After the phytoplankton maximum, about one week from the beginning ofthe experiment, grazing of algae by phagotrophic protozoans and phosphate depletion led to a rapid decrease of algal biomass and the community became predominantly heterotrophic. In spite of a large variation in algal biomass and primary production, the biomass of bacteria remained of the same order of magnitude as in algae both before and after the algal maximum. Bacteria were mostly responsible for the plankton respiration, which also showed no dependence on primary production. Since exudation by phytoplankton was also low, the nutrition of bacterioplankton was probably mainly based on allochthonous dissolved organic matter rather than or primary production. Thus the production of bacteria was an additional food source for higher trophic levels along with phytoplankton. Because filter feeding zooplankton was absent in the experiment, protozoans were the only grazers utilizing algae and bacteria. Essentially all growth of bacteria was used by bacterivores.     

A 2-year experimental study on nutrient and predator influences on food web constituents in a shallow lake of north-west Spain

1. A 2‐year study was carried out on the roles of nutrients and fish in determining the plankton communities of a shallow lake in north‐west Spain. Outcomes were different each year depending on the initial conditions, especially of macrophyte biomass. In 1998 estimated initial ‘per cent water volume inhabited’ (PVI) by submerged macrophytes was about 35%. Phytoplankton biomass estimated as chlorophyll a was strongly controlled by fish, whereas effects of nutrient enrichment were not significant. In 1999 estimated PVI was 80%, no fish effect was observed on phytoplankton biomass, but nutrients had significant effects. Water temperatures were higher in 1998 than in 1999. 2. In the 1998 experiment, cladoceran populations were controlled by fish and cyanobacteria were the dominant phytoplankton group. There were no differences between effects of low (4 g fresh mass m^?2) and high (20 g fresh mass m^?2) fish density on total zooplankton biomass, but zooplankton biomass was higher in the absence of fish. With the high plant density in 1999, fish failed to control any group of the zooplankton community. 3. Total biovolume of phytoplankton strongly decreased with increased nutrient concentrations in 1998, although chlorophyll a concentrations did not significantly change. At higher nutrient concentrations, flagellate algae became more abundant with likely growth rates that could have overcompensated cladoceran feeding rates. This change in phytoplankton community composition may have been because of increases in the DIN : SRP ratio. Both chlorophyll a concentration and total phytoplankton biovolume increased significantly with nutrients in the 1999 experiment. 4. A strong decline of submerged macrophytes was observed in both years as nutrients increased, resulting in shading by periphyton. This shading effect could account for the plant decline despite lower water turbidity at the very high nutrient levels in 1998.     

Field estimates of zooplankton community grazing in the limed Lago d'Orta (Italy) by the use of a radioisotope-free technique

Grazing experiments were carried out in a limed lake (Lago d'Orta) during the periods, 23–24 March and 28–29 June 1990. A simple in situ technique based on cell counts before and after incubation was used in order to evaluate the impact of the natural zooplankton community on the lake phytoplankton. An estimation of the daily consumption by the natural zooplankton community showed that about 4.6% and 1.04% of the standing algal biomass was removed during March and June respectively. The differences in grazing activities were likely due to the difference in zooplankton community structure and to the surrounding environmental conditions, mainly influenced by the liming intervention, started in the lake water since 1989.     

Predators, resources, and trophic chains in the regulation of plankton population and biomass in oligothrophic lakes

The relative strength of "top-down" versus "bottom-up" control of plankton community structure and biomass in two small oligotrophic lakes (with and without fish), located near the Polar circle (Russia), has been investigated for two years, 1996 and 1997. The comparative analyses of zooplankton biomass and species abundance showed strong negative effect of fish, stickeback (Pungitius pungitius L.), on the zooplankton community species, size structure and biomass of particular prey species but no effect on the biomass of the whole trophic level. An intensive predation in Verkhneye lake has lead to: 1) sixfold decline in biomass of large cladoceran Holopedium gibberum comparing to the lake lacking predator, 2) shift in the size mode in zooplankton community and the replacement of the typical large grazers by small species--Bosmina longirostris and rotifers. Their abundance and biomass even increased, demonstrating the stimulating effect of fish on the "inefficient" and unprofitable prey organisms. The analysis of contributions of different factors into the cladoceran's birth rate changes was applied to demonstrate the relative impact of predators and resources on zooplankton abundance. An occasional introduction of the stickleback to Vodoprovodnoye lake (the reference lake in 1996) in summer 1997 lead to drastic canges in this ecosystem: devastating decrease of zooplankton biomass and complete elimination of five previously dominant grazer species. The abundance of edible phytoplankton was slightly higher in the lake with fish in 1996 and considerably higher in the lake where fish has appeared in 1997 showing the prevailing "top-down" control of phytoplankton in oligotrophic ecosystem. The reasons of trophic cascade appearance in oligotrophic lakes are also discussed.     

Resuspension of algal cells by benthivorous fish boosts phytoplankton biomass and alters community structure in shallow lakes

1. Positive effects of fish on algal biomass have variously been attributed to cascading top‐down effects and to nutrient enrichment by fish excretion. 2. Here, we used a combination of field and laboratory approaches to test an additional hypothesis, namely that the physical resuspension of settled algal cells by fish enhances algal biomass and alters community composition. 3. A multi‐lake survey showed that phytoplankton biomass and the fraction of motile algae increased with the concentration of inorganic suspended solids. This correlation could not be explained by wind‐induced resuspension because of the small size of the lakes. 4. In an enclosure experiment, chlorophyll‐a concentration, phytoplankton abundance and inorganic suspended solids increased significantly in the presence of Cyprinus carpio (common carp), but only if the fish had access to the sediment. No such effects were seen when a net prevented carp reaching the sediment. 5. The effects of enhanced nutrients and reduced zooplankton grazing as a result of fish feeding could not (fully) explain these observations, suggesting that the resuspension by carp of settled algae from a surface film on the sediment was the major factor in the outcome of the experiment. 6. An increase in diatoms and green algae (organisms with a relatively large sedimentation velocity) only in enclosures where carp could reach the sediment supported this view. 7. Several lines of evidence indicate that fish‐induced resuspension of algal cells from the sediment is an important mechanism that affects phytoplankton biomass and community composition in shallow lakes.     

<Emphasis Type="Italic">Daphnia</Emphasis> versus copepod impact on summer phytoplankton: functional compensation at both trophic levels

Here we report on a mesocom study performed to compare the top-down impact of microphagous and macrophagous zooplankton on phytoplankton. We exposed a species-rich, summer phytoplankton assemblage from the mesotrophic Lake Schöhsee (Germany) to logarithmically scaled abundance gradients of the microphagous cladoceran Daphnia hyalina×galeata and of a macrophagous copepod assemblage. Total phytoplankton biomass, chlorophyll a and primary production showed only a weak or even insignificant response to zooplankton density in both gradients. In contrast to the weak responses of bulk parameters, both zooplankton groups exerted a strong and contrasting influence on the phytoplankton species composition. The copepods suppressed large phytoplankton, while nanoplanktonic algae increased with increasing copepod density. Daphnia suppressed small algae, while larger species compensated in terms of biomass for the losses. Autotrophic picoplankton declined with zooplankton density in both gradients. Gelatinous, colonial algae were fostered by both zooplankton functional groups, while medium-sized (ca. 3,000 µm³), non-gelatinous algae were suppressed by both. The impact of a functionally mixed zooplankton assemblage became evident when Daphnia began to invade and grow in copepod mesocosms after ca. 10 days. Contrary to the impact of a single functional group, the combined impact of both zooplankton groups led to a substantial decline in total phytoplankton biomass.     

Covariation between zooplankton community composition and cyanobacterial community dynamics in Lake Blaarmeersen (Belgium)

The cyanobacterial community composition in the mesotrophic Lake Blaarmeersen was determined by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene fragments during two consecutive years to assess the importance of different classes of explanatory variables (bottom-up and top-down factors, physical variables and phytoplankton) in cyanobacterial community dynamics. The most dominant cyanobacteria in Lake Blaarmeersen were Synechococcus (three genotypes), Limnothrix redekei and Anabaena/Aphanizomenon. Analyses of Similarity revealed that the cyanobacterial community in Lake Blaarmeersen differed significantly between the growing season and the winter season as well as between the epilimnion and hypolimnion during the stratified periods. Mantel tests revealed significant correlations between the DGGE data and bottom-up factors, physical variables, the phytoplankton community composition and, interestingly, the zooplankton community composition. In general, the zooplankton community composition (especially the cladoceran community) was more important in structuring the cyanobacterial community than the total zooplankton biomass. This study shows that grazing zooplankton communities can have a relatively strong impact on the cyanobacterial community dynamics and that this impact can be equally important as bottom-up processes regulated by nutrient concentrations and/or physical variables.