Robert W. Sterner

The Role of Grazers in Phytoplankton Succession

At times, freshwater zooplankton consume phytoplankton populations at rates similar to or faster than that at which they are growing (Hargrave and Geen, 1970; Gulati, 1975; Horn, 1981; Persson, 1985; Borsheim and Anderson, 1987). Such high losses certainly must help direct seasonal succession as they force a subset of algal species to suffer high mortality rates. Some studies have concluded that losses in general (Kalff and Knoechel, 1978; Reynolds, et al., 1982) and grazing losses in particular (Porter, 1973, 1976, 1977; Lynch and Shapiro, 1981; Crumpton and Wetzel, 1982; Kerfoot, 1987) are important in seasonal succession. In addition, the influence of zooplankton on algal succession is not limited to their selective effect on algal numbers. Zooplankton also interact indirectly with phytoplankton by making some nutrients more available to them (Gliwicz, 1975; Lehman, 1980a, b; Redfield, 1980; Lehman and Scavia, 1982; Sterner, 1986a). Zooplankton thus act not only as predators in the classic sense, but they also have an effect on the competition among algae (Elser et al., 1988).

Stoichiometric relationships among producers, consumers and nutrient cycling in pelagic ecosystems

Most ecosystem models consolidate members of food-webs, e.g. species, into a small number of functional components. Each of these is then described by a single state variable such as biomass. When a multivariate approach incorporating multiple substances within components is substituted for this univariate one, a ‘stoichiometric’ model is formed. Here we show that the Nitrogen:Phosphorus ratio within zooplankton herbivores varies substantially intraspecifically but not intraspecifically. By using stoichiometric theory and recent measurements of the N:P ratio within different zooplankton taxa, we calculate large differences in ratios of nutrients recycled by different zooplankton species. Finally, we demonstrate that N:P stoichiometry can successfully account for shifts in N- and P-limitation previously observed in whole-lake experiments. Species stoichiometry merges food-web dynamics with biogeochemical cycles to yield new insights.

Resource competition during seasonal succession toward dominance by cyanobacteria

An annual replacement of eucaryotic algae by nostocalean cyanobacteria was studied by monitoring the algal and zooplankton communities as well as various nutrient components during three growing seasons in Pleasant Pond, Minnesota. Short-term factorial experiments consisting of nitrogen and phosphorus enrichments and macrograzer removal were performed, and algal population growth was observed to determine whether nitrogen, phosphorus, or both were potentially limiting to different phytoplankton taxa. The algal taxa that were consumed by the dominant grazer, Daphnia pulex, were also determined. Competition for nitrogen together with grazing on the inferior competitors occurred. Several other potential mechanisms -a changing nitrogen-to-phosphorus supply ratio, in- creasing grazing pressure, and competition for silicon-did not appear to be important. Nitrogen was more limiting than phosphorus. At least some nonheterocystous species were nitrogen limited in every experiment. During the transition to dominance by Nos- tocales, Nostocales did not usually respond to nitrogen additions, whereas many other species did. This taxonomic difference in nutrient limitation suggests that some of the species unable to produce heterocysts were being outcompeted for nitrogen. Daphnia grazers consumed Anabaenaflos-aquae, Fragilaria sp., gelatinous greens, pen- nate diatoms, and Scenedesmus spp. Algal responses to added nitrogen and to grazers were often interdependent. A behavioral indirect effect via the grazers functional response could explain this dependence.

Elemental ratios and the uptake and release of nutrients by phytoplankton and bacteria in three lakes of the Canadian shield

The dynamics of carbon (C), nitrogen (N), and phosphorus (P), elemental ratios, and dark uptake/release of N and P in bacterial and phytoplankton size fractions were studied during summer 1992 in three lakes of contrasting food web structure and trophic status (L240, L110, L227). We wished to determine if phytoplankton and bacteria differed in their elemental characteristics and to evaluate whether the functional role of bacteria in nutrient cycling (i.e., as sink or source) depended on bacterial elemental characteristics. Bacterial contributions to total suspended particulate material and to fluxes of nutrients in the dark were substantial and varied for different elements. This indicated that some techniques for assaying phytoplankton physiological condition are compromised by bacterial contributions. C/N ratios were generally less variable than C/P and N/P ratios. Both elemental ratios and biomass-normalized N and P flux indicated that phytoplankton growth in each lake was predominantly P-limited, although in L227 these data reflect the dominance of N-fixing cyanobacteria, and N was likely limiting early in the sampling season. In L227, phytoplankton N/P ratio and biomass-normalized N flux were negatively correlated, indicating that flux data were likely a reasonable measure of the N status of the phytoplankton. However, for L227 phytoplankton, P-flux per unit biomass was a hyperbolic function of N/P, suggesting that the dominant L227 cyanobacteria have a limited uptake and storage capacity and that P-flux per unit biomass may not be a good gauge of the P-limitation status of phytoplankton in this situation. Examination of N-flux data in the bacterial size fraction relative to the N/P ratio of the bacteria revealed a threshold N/P ratio (∼22:1 N/P, by atoms), below which, bacteria took up and sequestered added N, and above which, N was released. Thus, the functional role of bacteria in N cycling in these ecosystems depended on their N/P stoichiometry.

Nutrient enrichment and nutrient regeneration stimulate bacterioplankton growth.

Bacterial abundance results from predatory losses of individuals and replacement of losses through growth. Growth depends on sustained input of organic substrates and mineral nutrients. In this work we tested the hypothesis that bacterial growth in two oligotrophic Canadian shield lakes was limited by nitrogen (N) or phosphorus (P). We also determined whether consumer-regenerated resources contributed substantially to net bacterial growth. Two types of dilution assays were conducted to determine the response of bacteria to nutrient enrichment: diluted whole water (DWW, 1:9 whole/filtered with 0.2 μm of filtered lake water) and diluted fractionated water (DFW, 1.0 μm prefiltered then diluted as above). Replicate bottles in each dilution assay received either N (50 μm), P (10 μm), or both N and P enrichments. Controls received no nutrients. Resource-saturated growth rates and grazing rates were estimated from a standard dilution-growth approach. Bacterial growth was stimulated by addition of P alone and in combination with N. Consumers regenerated sufficient resources to support up to half the bacterial growth rate, but the benefit derived from consumers was minor when compared to mortality.

Joint variation of zooplankton and seston stoichiometry in lakes and reservoirs

Summary We tested several stoichiometric hypotheses for thejoint dependence in element ratios in zooplanktonand their resources using direct chemical measure-ments of zooplankton (>80 µm) and seston. Our fivefield sites included three small boreal lakes, a warmwater reservoir and Lake Superior. Zooplankton ele-ment pools were generally not large compared to thetotal seston. We found a tight homeostasis in bulkzooplankton element ratios compared to theirresources. Weak correlations consistent with stoichi-ometric hypotheses were seen. Zooplankton C:P andC:N were very slightly positively correlated withseston ratios, but the zooplankton N:P ratio wasnegatively correlated with the same ratio in theseston. Acknowledgements Financial support came from the National ScienceFoundation, NOAA, and the University of Minne-sota. Fig. 3. Expansion of the vertical scale of Fig. 2A.Horizontal dotted lines indicate reported values ofC:P for a high and a low C:P zooplankton taxon(from S TERNER et al. 1992). The wavy line is a dis-tance-weighted least squares regression. The solidinclined line represents an hypothetical region ofexclusion (high seston C:P precludes dominance bylow C:P zooplankton).