Hélène Cyr

Zooplankton community size structure and taxonomic composition affects size-selective grazing in natural communities

Abstract The body size of an individual zooplankton is well related to its grazing rate and to the range of particle sizes it can ingest, and since cladocerans and copepods feed differently, they follow different relationships. Based on these general patterns in individual organisms, we tested whether the size structure and taxonomic composition of more complex natural zooplankton communities are related to their in situ grazing rate and to the range of algal sizes they graze. We compared community grazing rates on individual algal taxa in two communities dominated by small cladocerans, three communities dominated by large cladocerans and three copepod-dominated communities. Small algae were usually grazed most intensively, but grazing rates were poorly related to algal size alone. The range in size of grazed algae increased with increasing mean zooplankton body size, but differed systematically with their taxonomic composition. Communities dominated by Ceriodaphnia or Holopedium grazed a narrower size range of algae [maximum greatest axial length dimension (GALD)=16–36 μm)] than communities with large biomasses of Bosmina or Daphnia (maximum GALD=28–78 μm). Copepod-dominated communities followed the same general relationship as cladocerans. Daphnia-dominated communities grazed the broadest range of algal sizes, and their total grazing rates were up to 2.4 times their grazing rates on small (<35 μm) “highly edible” algae, a difference of similar magnitude to those found in successful trophic cascade biomanipulations.

Sampling Larval Fish Populations: Choice of Sample Number and Size

Abstract The number and size of larval fish samples are usually determined arbitrarily, despite the influence of these decisions on the precision of abundance estimates and the ability to detect differences among population estimates. Review of the literature suggests that most surveys of larval fish are based on few (median, 4), large (median, 300 m3) samples. To evaluate current sampling designs, we developed a model, based on published data, to predict the variance in larval fish abundance among replicate samples. Inter-replicate variance (s 2) is strongly related to mean abundance (x) as: log10 s 2 = 0.19 + 1.74·log10 x; r 2 = 0.93, P < 0.0001. This relationship was tested with an extensive data set collected in the Hudson River, New York (weekly samples over 14 years in 12 regions of the 250-km-long river), and was found to be general across environments, life history stages, and species. The model was not affected by sample volume, Our analysis shows that half of published studies estimated larval a...

Density-body size relationships in local aquatic communities

Studies of density-body size relationships report generalities in the structure of communities and suggest patterns of energy use among species of different sizes. Most density-body size relationships, however, have been developed from data on populations throughout the world and are expected, therefore, to provide only an approximate description of any specific community. In this study, we test 1) whether log-linear density-body size relationships can also be used to describe local communities and 2) whether these relationships vary systematically with environmental conditions. Our analysis is based on population densities of phytoplankton, zooplankton, zoobenthos and fish species measured in 18 lakes worldwide. Mean annual population density (D, individuals m -2 ) decreases log-linearly with increasing species body size (M, μg fresh mass; log D = a + b x log M), with slopes and intercepts which vary among lakes (b = -1.10 to -0.74, a = 4.5 to 6.9). In contrast with studies which focus on only one group of organisms (e.g. birds, insects, mammals), found that local communities are well described by density-body size relationships (r 2 = 0.78-0.98) when more complete communities are considered. The slopes of density-body size relationships measure the relative density of small to large species, and tend to be shallower (lower relative density of small species) in lakes near cities, but more steeply negative (greater relative density of small species) in deep, highly productive lakes which flush rapidly. The intercepts measure average population density and are positively related to primary productivity. Such differences in size structure among aquatic communities are large enough to suggest shifts, for example, in the relative energy use of small and large species.

Population density and community size structure : comparison of aquatic and terrestrial systems

The size structure of aquatic communities is generally measured using size spectra, an approach which is tedious or inapplicable in benthic and terrestrial communities. This has inhibited comparison of size structure of aquatic and terrestrial communities. This study uses an approach more common among terrestrial ecologists to develop a general density-body size relationship for lacustrine communities, based on mean annual population densities for dominant species of phytoplankton, zooplankton, zoobenthos and fish measured in 18 lakes worldwide. Overall, mean annual population density (D, individuals m -2 ) decreases log-linearly with increasing species body size (M, μg fresh mass) as D = 4 x 10 5 . M -0.89 (n = 280, r 2 = 0.92), although the exponent appeared smaller (-0.55 ± 0.04) within broad taxonomic groups (algae, invertebrates). We found that density-body size relationships for dominant species are quantitatively similar to size spectra, a pattern which suggests that density-body size relationships may provide an interesting alternative to size spectra for the prediction of ecosystem processes. These relationships also suggest that aquatic species reach, on average, 6-60 times higher densities than terrestrial species, depending on their body size and on their thermoregulatory system (ectotherms vs endotherms). The implications of these differences in size structure for size-related patterns of energy use and other processes depend on which physiological groups (unicells, ectotherms, endotherms) are being compared.

Allometric Theory: Extrapolations from Individuals to Communities

Physiological rates of individual organisms are well related to their body size. These allometric relationships suggest that ecological rates should also be related to the size structure of organisms in populations, communities, and ecosystems. We describe size distributions of zooplankton communities and explore the implications of such dis? tributions on community grazing rates. Ninety zooplankton communities, varying in bio? mass and in size distributions, were sampled in 28 lakes in northeastern North America and their grazing rates were predicted with an allometric equation. Zooplankton size dis? tributions vary in shape but, on average, can be described as bimodal. Predicted community mass-specific grazing rate decreases with increasing mean body size (r2 = 0.92) and is only slightly affected by the shape of community size distributions. Community biomass on the other hand increases with mean body size (r2 = 0.39). Total zooplankton grazing rate is expected to be higher in communities dominated by large zooplankton, but this relationship is obscured (r2 = 0.15) by temporal and spatial variability in zooplankton biomass. Although body size is a powerful predictor of individual physiological rates, its importance is expected to be largely masked at the level of communities.

Phosphorus sorption experiments and the potential for internal phosphorus loading in littoral areas of a stratified lake.

The exchange of phosphorus (P) during the resuspension of sediments into shallow (oxic) waters of deep stratified lakes is regulated by equilibrium dynamics. In this study, we compared the P-sorption characteristics of sediments from 17 shallow and deep littoral sites in an oligo-mesotrophic lake. Zero Equilibrium P Concentration (EPC(0)) ranged from 0.2 to 5 microgPL(-1). EPC(0) did not vary with sediment characteristics, but increased with increasing sediment-to-water ratios (SWR). Buffering capacity also increased with increasing SWR up to 1 gL(-1), at which point P concentrations were buffered almost perfectly. Therefore, internal P loading in littoral areas may depend primarily on the intensity and duration of sediment resuspension instead of sediment composition, and is expected to be spatially and temporally patchy. Maximum P-sorption capacity (S(max)) varied with chemical composition of the sediments, but was generally low, indicating a limited capacity of littoral sediments to retain external inputs of P.

Distribution of Exopolymeric Substances in the Littoral Sediments of an Oligotrophic Lake

Bacteria and algae release exopolymeric substances (EPS) that perform a wide range of important functions in aquatic and terrestrial systems. In this study we measured EPS in sediments at nine littoral sites around a shallow oligotrophic basin, and tested whether the concentration and composition of EPS was related to sediment characteristics. The concentrations of both loosely bound (colloidal) and tightly bound (capsular) EPS carbohydrates ranged up to ~800 µg glucose equiv. cm−2 and were well within the range of concentrations reported from marine intertidal flats, where EPS play an important role in stabilizing sediments, affecting nutrient exchanges between sediments and the water column, feeding benthic invertebrates, and sequestering and increasing the transfer of contaminants to food webs. Proteins were an important component of the EPS in these littoral sediments, with protein:carbohydrate ratios of ~0.4. In summer, the concentrations of most EPS fractions were positively related (P < 0.05) to the porewater and organic matter content of the sediments. Capsular EPS concentrations were lower in the fall, with a simultaneous increase in colloidal proteins but not in colloidal carbohydrates. This suggests that the carbohydrates in this colloidal EPS may be more labile than the proteins. Our results suggest that exopolymeric substances could be an important, but neglected, component of littoral sediments in lakes.

Does inter-annual variability in population density increase with time ?

Temporal variability in population density has been shown to increase with increasing length of a study. This has serious implications for ecological studies and suggests that obtaining an objective measure of temporal variability may not be possible. To test this hypothesis more generally, I calculated inter-annual variability, using the standard deviation of the logarithm of annual mean population density (STD(log D)), for 70 populations of phytoplankton, zooplankton and fish which have been sampled over 10 to 51 consecutive yr in lakes around the world. STD(log D) fluctuates most widely during the first 3-6 yr of a study, but these changes do not necessarily represent long-term trends in temporal variability. Median changes in STD(log D) over time are small («0.1 unit, even over a logarithmic time scale) and decrease as more years are included in the analysis. The inter-annual variability of most freshwater populations appears to reach an asymptotic value. Several populations, however, show an alternation of plateaux and of sudden increases in STD(log D), suggesting that the asymptotic values found in most populations may be temporary and may not extrapolate beyond the temporal scale of this study (10-50 yr). Overall, the increases in inter-annual variability are expected to be small (< 0.1 units, on average, per 25 yr). The pattern and the magnitude of these changes did not differ significantly between algae, zooplankton and fish, and were similar to those reported for birds and mammals. Increases in temporal variability may only be important for questions addressed over long periods of time.

Winds and the distribution of nearshore phytoplankton in a stratified lake

The distribution of phytoplankton in lakes is notoriously patchy and dynamic, but wind-driven currents and algal buoyancy/motility are thought to determine where algae accumulate. In this study, nearshore phytoplankton were sampled from different parts of a lake basin twice a day for 4-5 consecutive days, in the spring and in late summer, to test whether short-term changes in phytoplankton biomass and community composition can be predicted from wind-driven currents. On windy days, phytoplankton biomass was higher at downwind than at upwind nearshore sites, and the magnitude of this difference increased linearly with increasing wind speed. However, contrary to the generally assumed downwind phytoplankton aggregations, these differences were mostly due to upwelling activity and the dilution of phytoplankton at upwind nearshore sites. The distribution of individual taxa was also related to wind speed, but only during late stratification (except for cryptophytes), and these relationships were consistent with the buoyancy and motility of each group. On windy days, large diatoms and cyanobacteria concentrated upwind, neutrally buoyant taxa (green algae, small diatoms) were homogeneously distributed, and motile taxa (cryptophytes, chrysophytes, dinoflagellates) concentrated downwind. Predictable differences in the biomass and composition of phytoplankton communities could affect the efficiency of trophic transfers in nearshore areas.

Wind-driven physical processes and sediment characteristics affect the distribution and nutrient limitation of nearshore phytoplankton in a stratified low-productivity lake

Wind-driven physical processes are expected to affect the distribution and composition of phytoplankton communities and to determine their access to nearshore nutrients. We examined the effect of wind-driven physical processes (surface waves, seiching activity) on the distribution of phytoplankton biomass, their growth rate, and nutrient limitation. We sampled nearshore and offshore phytoplankton on 11 days during the pre-, early-, mid-, and late-stratification periods. Phytoplankton biomass (measured as chlorophyll concentration) accumulated downwind, but growth rate was usually higher at upwind than at downwind sites. This suggests that the quantity and quality of algal food sources for higher trophic levels may vary in predictable but opposite ways. Wind-driven surface waves and upwelling activity were associated with changes in phytoplankton nutrient limitation in nearshore areas, but these differences were site specific. Our results suggest that wind-driven physical processes and sediment characteristics are both important in determining internal nutrient loading and phytoplankton nutrient limitation in nearshore areas. On windy days, nutrient limitation of offshore phytoplankton at the lake surface was always related to the conditions found upwind, suggesting rapid exchanges between nearshore and offshore areas. Wind-driven physical processes affect the distribution and nutrient limitation of phytoplankton in lakes, and are likely to influence the efficiency of trophic transfers in planktonic communities. These wind-driven processes should be included more specifically into food web models.