Nitrogen (N) and phosphorus (P) limit primary production in many aquatic ecosystems, with major implications for ecological interactions in plankton communities. Yet it remains unclear how evolution may affect the N∶P stoichiometry of phytoplankton-zooplankton interactions. Here, we address this issue by analyzing an eco-evolutionary model of phytoplankton-zooplankton interactions with explicit nitrogen and phosphorus dynamics. In our model, investment of phytoplankton in nitrogen versus phosphorus uptake is an evolving trait, and zooplankton display selectivity for phytoplankton with N∶P ratios matching their nutritional requirements. We use this model to explore implications of the contrasting N∶P requirements of copepods versus cladocerans. The model predicts that selective zooplankton strongly affect the N∶P ratio of phytoplankton, resulting in deviations from their optimum N∶P ratio. Specifically, selective grazing by nitrogen-demanding copepods favors dominance of phytoplankton with low N∶P ratios, whereas phosphorus-demanding cladocerans favor dominance of phytoplankton with high N∶P ratios. Interestingly, selective grazing by nutritionally balanced zooplankton leads to the occurrence of alternative stable states, where phytoplankton may evolve either low, optimum, or high N∶P ratios, depending on the initial conditions. These results offer a new perspective on commonly observed differences in N∶P stoichiometry between plankton of freshwater and those of marine ecosystems and indicate that selective grazing by zooplankton can have a major impact on the stoichiometric composition of phytoplankton.

AbstractThe traditional and widely used ecomorphological spring classification—pool springs (limnocrenes), seepages (helocrenes), and flowing springs (rheocrenes)—is based mainly on the flow regime at the spring mouth. This clear distinction is based purely on environmental conditions, but how and to what extent these spring typologies are reflected by biological assemblages consisting of different taxonomic groups remains largely untested. Classification of habitats typically is based on one or few taxonomic groups. However, groups are likely to differ in their response to the environment, so different, equally valid classifications might result with different groups. We evaluated the responses and their congruence of a wide range of taxonomic groups to different spring types. Eighty-six springs in the Italian Alps were first classified based on environmental factors only. The consistency of this classification was tested using diatoms, bryophytes, vascular plants, nematodes, mollusks, oligochaetes, water mites, copepods, ostracods, chironomids, stoneflies, and caddisflies. When only environmental variables were used, 7 spring types were distinguished: limnocrenes and helocrenes, low- and high-altitude rheocrenes on carbonate rocks, rheocrenes on siliceous rocks, rheocrenes with high discharge, and hygropetric rheocrenes. This classification was reflected by most taxonomic groups, and many species were characteristic for ≥1 spring type. However, the predictive power of the environment for determining species distribution was generally low, a result suggesting that other factors may play an important role in structuring spring assemblages. Concordance among taxonomic groups was found for 2 macrogroups of organisms: autotrophs (diatoms, bryophytes, and vascular plants) and heterotrophs. This result shows that achieving a general classification of springs relevant across all taxonomic groups would be difficult.

The effects of flow variation on surface invertebrates have been well documented; however, studies involving the subsurface (hyporheic) environment are rare. We assessed the response of hyporheic invertebrates to drying by sampling 5 sets of wells (30, 50, and 100 cm deep) from 8 May 1995 to 4 August 1995 in an intermittent desert stream undergoing drying. Drying began with discontinuity of surface water, and continued through loss of surface flow to recession of the water table (at a rate of 11 cm/wk) to >1 m depth by August. Twenty taxa were collected during the study. Shallow (30- and 50-cm) assemblages consisted mainly of insect larvae and cyclopoid copepods whereas bathynellaceans, isopods, and harpacticoid copepods were the major taxa in deeper (100-cm) sediments. Total abundance of all taxa combined and abundance of many individual taxa differed between depths, but also changed significantly over time. The interaction between date and depth was also significant, indicating that the direction of temporal change was not consistent among well depths. Once surface-water dried, total invertebrate abundance began to increase in wells that remained inundated. Thereafter, as the water table continued to recede, abundance in the 100-cm wells increased while numbers in shallow wells decreased. This pattern supports the hypothesis that the hyporheos migrates into deeper sediments to avoid conditions associated with drying.

Variability in the spatial and temporal patterns of stream biota has been well documented, yet the causes and consequences of this variability are poorly understood. Most early work focused on how changes in mean environmental parameters over space or time influence the abundance or distribution patterns of biota. We argue for an increased focus on the study of variability in these parameters. To date, only a few lotic studies have used variance as a dependent variable i.e., showing that changes in the variance in some parameter can be related to changes in the mean of that or another parameter. These studies have suggested interesting links between ecological patterns and processes at a variety of scales. The use of variance as an independent variable and its experimental manipulation (i.e., hold the mean of some parameter constant while altering its variance) may also lead to new understandings of how pattern and process are linked. We begin by outlining when and why the study of variance may be important to lotic ecology. Next, we suggest a 3-step process in the use of variance in stream studies: 1) documentation of a pattern with respect to variance; 2) examination of descriptive data to explore potential causes of the pattern; and 3) direct manipulation of variance in experiments. We provide an example of this 3-step process using data from preliminary experiments in a warmwater stream that suggested spatial variability in the abundance of chironomids and copepods could be related to flow variability. We close by suggesting that future directions for the use of variance in stream ecology should attempt to couple scale-dependent physical patterns with biological patterns or processes, develop experimental frameworks to determine if species interactions or other biotic processes vary with scale, embrace statistical methods for determining the relative importance of variance-generating processes, develop methods for the quantification of variance in biologically meaningful ways, and focus on the identification and interpretation of domains of homogeneous variance in streams.