Chlorophyll a concentration across a trophic gradient of lakes: An estimator of phytoplankton biomass?

Water-column mixing is known to have a decisive impact on plankton communities. The underlying mechanisms depend on the size and depth of the water body, nutrient status and the plankton community structure, and they are well understood for shallow polymictic and deep stratified lakes. Two consecutive mixing events of similar intensity under different levels of herbivory were performed in enclosures in a shallow, but periodically stratified, eutrophic lake, in order to investigate the effects of water-column mixing on bacteria abundance, phytoplankton abundance and diversity, and rotifer abundance and fecundity. When herbivory by filter-feeding zooplankton was low, water-column mixing that provoked a substantial nutrient input into the euphotic zone led to a strong net increase of bacteria and phytoplankton biomass. Phytoplankton diversity was lower in the mixed enclosures than in the undisturbed ones because of the greater contribution of a few fast-growing species. After the second mixing event, at a high biomass of filter-feeding crustaceans, the increase of phytoplankton biomass was lower than after the first mixing, and diversity remained unchanged because enhanced growth of small fast-growing phytoplankton was prevented by zooplankton grazing. Bacterial abundance did not increase after the second mixing, when cladoceran biomass was high. Changes in rotifer fecundity indicated a transmission of the phytoplankton response to the next trophic level. Our results suggest that water-column mixing in shallow eutrophic lakes with periodic stratification has a strong effect on the plankton community via enhanced nutrient availability rather than resuspension or reduced light availability. This fuels the basis of the classic and microbial food chain via enhanced phytoplankton and bacterial growth, but the effects on biomass may be damped by high levels of herbivory.

Effects of water-column mixing on bacteria, phytoplankton, and rotifers under different levels of herbivory in a shallow eutrophic lake.

We conducted grazing experiments to test whether larger-bodied Daphnia puticaria have a different effect from smaller-bodied Daphnia galeata mendotae on the composition of summer algal assemblages in eutrophic lakes. Three separate cubitainer experiments were run for 5 days in a replicated factorial design utilizing two algal community types and the two Daphnia species. Inorganic phosphorus and nitrogen were added to prevent nutrient limitation of the algae. Both edible and inedible size fractions of chlorophyll a increased in cubitainers without Daphnia spp. Grazer addition usually resulted in a reduction in edible chlorophyll; reductions were greater in D.pulicana cubitainers. Grazing by Daphnia spp. on presumed inedible chlorophyll was variable. Algal size was not always a good predictor of grazeability. The results of this study indicate that D.pulicaria, because of its greater filtration potential and ability to ingest larger particles, provides a stronger control on inedible-sized algae when compared to equal numerical densities of D.g.mendotae. However, Aphanizomenon increased as a response to heavy grazing pressure by D.pulicaria on other algal species. This suggests that biomanipulation efforts that promote large-bodied Daphnia may not produce desirable results if nutrient inputs remain high.

/pdf/influence-of-two-daphnia-species-on-summer-phytoplankton-1bcizn7ryx.pdf

Influence of two Daphnia species on summer phytoplankton assemblages from eutrophic lakes

In years of low planktivory by fish, midsummer declines and low population abundances of Daphnia pulicaria in Lake Mendota, WI, USA have been attributed to poor food quality and low food abundance. This hypothesis has been proposed because of midsummer blooms of blue-green algae in this eutrophic urban lake. We tested the hypothesis by performing in situ food manipulations during the midsummer decline of the D. pulicaria population, and during the early autumn low population period. In July, animals held in clear plastic vials containing GF/F filtered lake water exhibited poor survivorship, ceased producing offspring after three days, and had low lipid reserves. Daphnids fed whole-lake water or filtered water enriched with Chlamydomonas survived at a significantly higher rate and produced abundant offspring. In September, there were no significant differences in survivorship among treatments, but animals fed water enriched with Chlamydomonas produced significantly more offspring than animals in the other treatments. The hypothesis of poor food quality causing the midsummer decline of the Daphnia population is not supported by our experiments. Because predation by juvenile fish does not appear to be important in regulating the midsummer Daphnia population during these years, alternate hypotheses for the midsummer decline are suggested. These include increased predation by invertebrate predators such as Leptodora kindtii, a life history shift by daphnids during midsummer to production of resting eggs, and a combination of low levels of planktivory and deteriorating feeding conditions acting together to cause the population decline.

An in situ test of the effects of food quality on Daphnia population growth

In Lake Hume, a reservoir located in an active agricultural zone of the Murray River catchment, Australia, time series for the abundances of phytoplankton and zooplankton taxa, monitored from 1991 through to 1996, were stationary (without trends), and plankton taxonomic composition did not change. This indicated ecosystem resilience to strong fluctuations in reservoir water level, and to other potential agricultural impacts, for example eutrophication and pollution. Although biological stressors such as introduced fish and invertebrate predators are known to affect planktonic communities and reduce biodiversity in lakes, high densities of planktivorous stages of alien European perch (Perca fluviatilis) and the presence of carp (Cyprinus carpio) did not translate into non-stationary time series or declining trends for plankton in Lake Hume. However, the seasonal successions observed in the reservoir in different years did not conform well to the Plankton Ecology Group (PEG) model. Significant deviations of the Lake Hume successional pattern from the PEG model included maxima for phytoplankton abundance being in winter and the presence of a clear water phase without large zooplankton grazers. The instability of the water level in Lake Hume probably causes the dynamics of most planktonic populations to be less predictable, but did not initiate the declining trends that have been observed in some other Australian reservoirs. Both the PEG model and the present study suggest that hydrology is one of the major drivers of seasonal succession.

Seasonal succession and long-term stability of pelagic community in a productive reservoir

It is becoming increasingly clear that lake plankton communities are regulated by both predation and resources. Top predators, through effects on herbivores, can regulate phytoplankton community structure, biomass, and primary productivity (Carpenter et al. 1985; Carpenter and Kitchell 1988; Vanni and Findlay 1990). Increase in potential limiting nutrients, such as nitrogen and phosphorus, can stimulate phytoplankton production and biomass, and the ratio of limiting nutrients can influence community structure (Schindler 1977; Smith 1983). Recently, lake ecologists have realized that when attempting to explain phytoplankton dynamics, both nutrients and herbivory must be considered. Often the two interactions simultaneously influence phytoplankton, although their relative strengths may vary seasonally (e.g. Sommer et al. 1986; Vanni and Temte 1990). Furthermore, resource effects and herbivore effects may interact in complex ways to influence phytoplankton (Leibold 1980; Sterner 1990; Carpenter et al. 1991)—a case of the interactions themselves interacting (Roughgarden and Diamond 1986).

Herbivory, Nutrients, and Phytoplankton Dynamics in Lake Mendota, 1987–89

The models presented in the three preceding chapters were planned as elements of an integrated ecosystem approach from phosphorus to fishes. The modeling problem was broken into three parts in order to maximize our rate of progress and make best use of the people involved. The modules—piscivory, planktivory, and herbivory-algae-nutrients—have fundamentally different time scales yet strong vertical interactions (Figure 22.1). Within a given nutrient and weather regime, differences in return time cause the upper modules to act as constraints on lower ones (O’Neill et al. 1986). Piscivore dynamics have return times of years (Post and Rudstam, Ch. 19). Stock and harvest policies as well as resource levels must be considered in modeling piscivory. Planktivory by fishes has return times of years, while that by the zooplankter Leptodora has return times of weeks (Luecke el at., Ch. 20). Herbivory, algal growth, and nutrient fluxes have rapid dynamics and short return times of a few days (Vanni et al., Ch. 21).

Modeling the Lake Mendota Ecosystem: Synthesis and Evaluation of Progress

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Herbivory, Nutrients, and Phytoplankton Dynamics in Lake Mendota, 1987–89

Modeling the Lake Mendota Ecosystem: Synthesis and Evaluation of Progress