Inhibitory effect of Anabaena sp. on in situ filtering rate of Daphnia

We found that in situ filtering rates of Daphnia spp. measured in a lake containing Anabaena were significantly lower than those measured in a filament-free lake. Even after accounting for the depressing effects of high nannoplankton biomass concentration, filtering rates in the lake with Anabaena were 64% lower than those from the filament-free lake. We also found that filtering rates for Daphnia pulex in laboratory experiments were lower when Anabaena was present in experimental beakers than when Chlorella was present. When Anabaena was removed from Three Mile Lake water, filtering rates compared closely with predicted rates based on nannoplankton concentration and carapace length alone. Our analysis indicates that the presence of Anabaena filaments depresses Daphnia grazing rates in general, and that the filaments themselves are ingested at a lower rate than algae such as Chlorella. </jats:p

Type-3 functional response in limnetic suspension-feeders, as demonstrated by in situ grazing rates

Field-measured grazing rates (ml/animal/d) of cladocerans (mostly daphniids) and diaptomids were assembled from various published studies and plotted as a function of corresponding phytoplankton concentration (μg l-1 f.w.). Filtering rates of both zooplankton groups initially increased with seston concentration until maximal grazing rates were observed at approximately 4 × 102 and 1 × 102 μg l-1 for cladocerans and copepods, respectively; at higher algal concentrations, filtering rates of both declined as a function of food concentration. The shape of these curves are most consistent with Holling's (1966) Type 3 functional response. We found little support for the Type 3 functional response in published laboratory studies of Daphnia; most investigators report either a Type 1 or Type 2 response. The one study in which the Type 3 response was observed involved experiments where animals were acclimated at low food concentrations for 24 h, whereas those studies associated with response Types 1 or 2 had acclimation periods of only 1 to 3 h. We therefore assembled relevant data from the literature to examine the effect of acclimation period on the feeding rates of Daphnia at low food concentrations. In the absence of any acclimation, animals filtered at extremely low rates. After 2 h of acclimation, however, filtering rates increased 4 to 5-fold but declined again with longer durations; after > 70 h of pre-conditioning, filtering rates were almost as low as they had been with no acclimation. We also found little support for the Type 3 functional response in published studies of copepods. The only study associated with a Type 3 response involved a marine copepod that had been subjected to a starvation period of 48 h; however, an analysis of the effects of acclimation period did not yield conclusive evidence that filtering rates of freshwater copepods (Diaptomus and Eudiaptomus) decrease significantly with acclimation duration. The low filtering rates associated with long acclimation periods in laboratory experiments appears to be a direct result of animals becoming emaciated from prolonged exposure to low food concentrations, a situation which renders them incapable of high filtering rates. This may explain the Type 3 functional response for field cladocerans, since zooplankton in food-limiting situations are constantly exposed to low food concentrations, and would therefore have low body carbon and consequently less energy to filter-feed. We cannot, however, use this to explain the Type 3 response for field diaptomids, since copepods in the laboratory did not appear to lose body carbon even after 72 h of feeding at very low food levels, and there was inconclusive evidence that either Diaptomus or Eudiaptomus decrease their filtering rates with acclimation period. Although Incipient Limiting Concentrations (ILC) for Daphnia ranged from 1 to 8.5 × 103 μg 1-1, more than half of these fell between 1 and 3 × 103 μg l-1, bracketing the value of 2.7 × 102 μg l-1 for field cladocerans. There was, however, a great deal of variation in reported maximum ingestion rates (MIR), maximum filtering rates (MFR) and ILC values for Daphnia magna. ILC values from the few laboratory studies of freshwater copepods ranged between 0.5 to 2.8 × 103 μg 1-1, and was higher than the ILC value of approximately 0.2 × 103 μg l-1 calculated for field populations of D. minutus. Generally, there was considerable agreement among laboratory studies regarding the shape of grazing-rate and ingestion-rate curves when data were converted to similar units and presented on standardized scales. © 1992 Kluwer Academic Publishers

Sheldon Spectrum and the Plankton Paradox: Two Sides of the Same Coin : A trait-based plankton size-spectrum model

The Sheldon spectrum describes a remarkable regularity in aquatic ecosystems: the biomass density as a function of logarithmic body mass is approximately constant over many orders of magnitude. While size-spectrum models have explained this phenomenon for assemblages of multicellular organisms, this paper introduces a species-resolved size-spectrum model to explain the phenomenon in unicellular plankton. A Sheldon spectrum spanning the cell-size range of unicellular plankton necessarily consists of a large number of coexisting species covering a wide range of characteristic sizes. The coexistence of many phytoplankton species feeding on a small number of resources is known as the Paradox of the Plankton. Our model resolves the paradox by showing that coexistence is facilitated by the allometric scaling of four physiological rates. Two of the allometries have empirical support, the remaining two emerge from predator-prey interactions exactly when the abundances follow a Sheldon spectrum. Our plankton model is a scale-invariant trait-based size-spectrum model: it describes the abundance of phyto- and zooplankton cells as a function of both size and species trait (the maximal size before cell division). It incorporates growth due to resource consumption and predation on smaller cells, death due to predation, and a flexible cell division process. We give analytic solutions at steady state for both the within-species size distributions and the relative abundances across species

Dome patterns in pelagic size spectra reveal strong trophic cascades

In ecological communities, especially the pelagic zones of aquatic ecosystems, certain bodysize ranges are often over-represented compared to others. Community size spectra, the distributions of community biomass over the logarithmic body-mass axis, tend to exhibit regularly spaced local maxima, called "domes", separated by steep troughs. Contrasting established theory, we explain these dome patterns as manifestations of top-down trophic cascades along aquatic food chains. Compiling high quality size-spectrum data and comparing these with a size-spectrum model introduced in this study, we test this theory and develop a detailed picture of the mechanisms by which bottom-up and top-down effects interact to generate dome patterns. Results imply that strong top-down trophic cascades are common in freshwater communities, much more than hitherto demonstrated, and may arise in nutrient rich marine systems as well. Transferring insights from the general theory of nonlinear pattern formation to domes patterns, we provide new interpretations of past lake-manipulation experiments

Managing ecosystem services at Loch Leven, Scotland, UK: actions, impacts and unintended consequences

Rivers, lakes and wetlands are good examples of ecosystems that provide multiple, concurrent, services to mankind. Human society has often exploited these systems by enhancing one ecosystem service at the expense of another. Loch Leven, Scotland, UK, is a good example of this. Over the past 150 years, the lake has been subjected to hydrological modification, fish stocking and pollution control to improve the delivery of key goods and services. This study uses historical records to explore the results of these interventions on the ecosystem services that were targeted for improvement and the knock-on effects on other services provided by the lake. The results suggest that, when management changes are being considered to enhance particular ecosystem services, the potentially damaging effects on other ecosystem services should be taken into account. This requires a better understanding of the role of ecosystem function in delivering ecosystem services, and of the links between multiple ecosystem services, than is currently available. While further research is clearly needed, the value of long term datasets in providing knowledge and understanding through ‘hindsight’ should not be underestimated. The study concludes that successful management actions are likely to be those that incorporate lessons learned from previous decisions