William R. DeMott

A strain of the green alga Oocystis isolated from a lake showed a decline in digestion defenses and an increase in growth rate over 3 years of culture. Changes over time were corroborated by comparisons between a sub-strain grown continuously in light and two sub-strains kept refrigerated in the dark most of the time. Continuous culture in light leads to a sharper decline in grazer defenses and an increase in growth rate. The decline in digestive defenses was evidenced by increases in carbon assimilation efficiency and juvenile growth rate of Daphnia feeding on the alga as well as decreases in the incidence and thickness of the protective gelatinous sheath of the alga. Moreover, two strains of Oocystis from culture collections isolated decades earlier showed no evidence of grazer defenses in comparison with a high quality control alga. At high algal concentrations, Daphnia juvenile growth rate increased from 0.10 to 0.38 day⁻¹ for field-isolated Oocystis over time in culture and ranged from 0.57 to 0.61 day⁻¹ for the three undefended algae. Our experiments suggest that grazing favors the evolution of digestion defenses in Oocystis at the cost of slower growth and show that caution is needed when using cultured algae in food chain experiments.

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 ≅ 0.5 μM, N:P 40:1). Four-day Daphnia juvenile growth assays tested for Daphnia P-limitation and nutrient-dependent or grazer-induced algal defenses. The maximal algal growth rate of undefended Ankistrodesmus (mean ± SE for three replicate microcosms; 0.92 ± 0.02 day⁻¹) was higher than for defended Oocystis (0.62 ± 0.03 day⁻¹), but by day 6, algal growth was strongly P-limited in all six treatments (molar C:P ratio >900). The P-deficient algae were poor quality resources in all three algal treatments. However, Daphnia population growth, reproduction, and survival were much lower in the digestion-resistant treatment even though growth assays provided evidence for Daphnia P-limitation in only the undefended and mixed treatments. Growth assays provided little or no support for simple threshold element ratio (TER) models that fail to consider algae defenses that result in viable gut passage. Our results show that strong P-limitation of algal growth enhances the defenses of a digestion-resistant alga, favoring high abundance of well-defended algae and energy limitation of zooplankton growth.

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. Food quality varied markedly among algal taxa, with mean juvenile growth rates at high food concentrations (1–2 mg/L) ranging from 0.10 to 0.61 d⁻¹. Juvenile growth rate for high food concentrations was highly correlated with juvenile AE (r² = 0.96), verifying the importance of digestibility for food quality. AE, measured with ¹⁴C‐labeled algae, increased with increasing age and body size for each of four digestion‐resistant taxa but did not vary with age and body size for three readily digested algae. GPT decreased with decreasing body size, supporting the hypothesis that shorter GPT in juveniles leads to lower AE for digestion‐resistant resources. Lower food concentrations led to increased GPT and improved AE for juveniles feeding on two digestion‐resistant algae, providing further support for a role of longer gut retention in overcoming digestion defenses. The results suggest that increased abundance of digestion‐resistant food will lead to growth and recruitment bottlenecks for juvenile herbivores, but that the effectiveness of digestion defenses will be decreased when large‐bodied grazers predominate and when low food concentrations result in longer gut passage times. Gut processing constraints may favor either high concentrations of slow‐growing, digestion‐resistant resources or low concentrations of fast‐growing, undefended resources.

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. Thus, we predicted higher P-content and higher maximal growth rates in taxa from ponds and shallow lakes and lower P-content, lower maximal growth but reduced sensitivity to P-limitation in taxa preferring stratified lakes. In each of 25 experiments, a clonal Daphnia cohort was cultured for 4 days on a P-sufficient (molar C:P ratio 70) or a P-deficient (C:P 1,000) diet of a green alga at a high concentration (1 mg C l⁻¹). The P-content of adult Daphnia fed the P-sufficient diet ranged from 1.52 to 1.22% mass. Small-bodied taxa from shallow lakes had higher P-content than larger-bodied taxa from deep lakes or fishless ponds. However, we found a nonsignificant negative correlation between P-content and growth on the P-sufficient diet, rather than the positive relationship predicted by the growth rate hypothesis. The P-deficient diet resulted in declines in both growth rate and P-content compared with the P-sufficient controls and the extent of the declines differed between taxa. Taxa from ponds showed a marginally greater decline in growth with the P-deficient diet compared with taxa from shallow or deep lakes. However, contrary to stoichiometric theory, no relationship was found between a species’ P-content and growth depression on the P-deficient diet. Although we found evidence for habitat adaptations, our results show that factors other than Daphnia P-content are important in determining differences between Daphnia species in both maximal growth rate and sensitivity to P-limited growth.

1. Laboratory experiments were used to study the feeding, growth and reproduction of five daphnids in mixtures of a toxic cyanobacterium, Microcystis aeruginosa, and a green alga, Scenedesmus acutus. The mixtures included 0%, 20%, 50%, 80% and 100% Microcystis with a total food concentration of 0.5 mg C L⁻⁻¹ in each treatment. The feeding rate was measured after 1 and 24 h of acclimatization to the mixtures.2. Toxic Microcystis inhibited feeding in all the species, but they exhibited an unexpected diversity and complexity in the pattern of feeding inhibition. Daphnia magna exhibited the strongest inhibition of feeding after 1 h of exposure to toxic food, but had substantially recovered after 24 h in the same mixtures. This pattern of inhibition and recovery may balance the benefits of reduced ingestion of toxin with the disadvantage of a reduced energy intake.3. All five daphnids grew quickly in the Scenedesmus control, whereas growth and reproduction declined with an increasing proportion of the toxic alga in the diet. Daphnia pulicaria showed the least inhibition of growth and reproduction, D. pulex showed the strongest inhibition and the three remaining species exhibited intermediate sensitivity.4. Estimates of gross growth efficiency (GGE; growth/ingestion) provided a means for discriminating between the effects of feeding inhibition and direct toxicity on zooplankton growth. Daphnia pulex exhibited a sharp decline in GGE, suggesting that growth inhibition was a result of both feeding inhibition and direct toxicity. In contrast, D. magna exhibited a nearly constant GGE, indicating that feeding inhibition accounted for its decline in growth. However, two Daphnia species (i.e. D. pulicaria and D. galeata) exhibited improbable increases in GGE with toxic cyanobacteria, suggesting that their feeding rates were underestimated.Growth assays with sensitive and resistant zooplankton species are proposed for testing the potential impacts toxic cyanobacteria in lakes.

We used laboratory experiments to study the effects of changing food abundance on the selective feeding of two heterotrophic nanoflagellates on particles of similar size but differing nutritional quality (latex beads and fluorescently stained bacteria). Both Bodo saltans and Spumella sp. exhibited similar shifts in selectivity with changes in food concentration. Flagellates that were cultured under food‐limiting conditions showed a modest but significant preference for beads when both particles were offered simultaneously. However, both flagellates exhibited strong discrimination against the inert beads within 30–60 min after the addition of a satiating concentration of live bacteria. As bacterial abundance declined over 24 h, discrimination against the inert beads gradually relaxed. The observed pattern of concentration‐dependent selectivity is in agreement with the predictions of optimal diet models.

SUMMARY: 1. Food selection experiments were conducted by acclimating calanoid copepods (Eudiaptomus spp.) in suspensions of natural seston and then adding pairs of dual‐labelled (¹⁴C/³²P) algae. Each feeding trial measured selectivity between a small, high‐quality reference alga, Chlamydomonas reinhardii, and a test alga that differed in size and/or food quality. The influence of food concentration on food selection was tested by using seston from two lakes with contrasting food abundance and by including treatments with filtered lake water ('starved’) and seston diluted with filtered water or enriched with cultured algae.2. Copepods that had been starved or acclimated to natural seston with low food abundance preferred the larger of two labelled algae, regardless of the nutritional quality of the algae. In agreement with the predictions of an optimal diet model, however, copepods that were acclimated to high food conditions discriminated against low‐quality foods, including digestion‐resistant algae and dead algae.3. Selectivity coefficients showed excellent agreement with a previous study involving the same taxa of copepods and labelled algae but in which the copepods had been acclimated to pairs of cultured algae rather than natural seston. Thus, these comparisons emphasize the importance of food availability in modulating copepod selectivity for foods that differ in nutritional quality and suggest that such behaviour occurs in nature.