Strong interactions between stoichiometric constraints and algal defenses: evidence from population dynamics of Daphnia and algae in phosphorus-limited microcosms.

The dynamic interactions among nutrients, algae and grazers were tested in a 2 × 3 factorial microcosm experiment that manipulated grazers (Daphnia present or absent) and algal composition (single species cultures and mixtures of an undefended and a digestion-resistant green alga). The experiment was run for 25 days in 10-L carboys under mesotrophic conditions that quickly led to strong phosphorus limitation of algal growth (TP is approximately equal to 0.5 μM, N:P 40:1).

Ontogeny of digestion in Daphnia: implications for the effectiveness of algal defenses.

Models of feeding and digestion predict that increased body size should result in longer gut passage time and improved assimilation efficiency. We examined the implications of digestion theory for size-structured interactions in a generalist zooplankton herbivore by studying the relationships between body size, ingestion rate, gut passage time (GPT), assimilation efficiency (AE), and growth rate in a clone of Daphnia pulex feeding on seven taxa of green algae that differed in digestibility. We also tested the effect of varying food concentration on GPT and AE while keeping body size constant.

Stoichiometry in an ecological context: testing for links between Daphnia P-content, growth rate and habitat preference.

We used laboratory experiments with ten Daphnia taxa to test for links between Daphnia P-content, growth rate and habitat preference. The taxa represent a wide range of body sizes and most show distinct preferences for one of three habitats: shallow lakes, deep, stratified lakes or fishless ponds. Previous studies show that taxa from shallow lakes and fishless ponds experience high predation risk and rich food resources, whereas taxa from deep lakes experience low predation risk, strong food limitation and potentially P-deficient resources.

Optimal foraging by a suspension-feeding copepod: responses to short-term and seasonal variation in food resources.

Laboratory radioisotope experiments were used to investigate the effects of phytoplankton seasonal succession on the selectivity and clearance rates of a suspension-feeding copepod in two Indiana lakes. Responses to particle size and quality were tested by allowing adult female Diaptomus birgei feeding in natural seston to select between a small (6×7 μm) flagellate (Chlamydomonas reinhardii) and a large, poor quality food (heat-killed Carteria olivieri, 22×25 μm). Short-term responses were tested in one lake by additional treatments in which copepods were acclimated for 5-6 h in filtered lake water ("starved") or natural seston with added Chlamydomonas ("enriched").

The role of taste in food selection by freshwater zooplankton.

Laboratory experiments with flavored and untreated polystyrene spheres revealed major differences in taste discrimination among diverse taxa of freshwater zooplankton. Copepods showed the strongest responses to flavor treatments. Both nauplii and copepodites of calanoid (Diaptomus) and cyclopoid (Cyclops) copepods selected flavored spheres over untreated ones.