Nutrient turnover by freshwater bacterivorous flagellates: differences between a heterotrophic and a mixotrophic chrysophyte

The patterns of nutrient turnover by the freshwater bacterivorous chrysophycean flagellates Ochromonas sp. and Spumella sp. were compared in laboratory experiments. The exclusive phagotroph Spumella sp, released soluble reactive phosphorus (SRP) and ammonium when feeding on bacteria and stimulated the growth of P-limited algae, obviously by making phosphorus available to them. The mixotroph Ochromonas sp, released SRP and ammonium in the dark or at high bacterial densities in the light when phagotrophic nutrition prevailed, but it took up mineral nutrients when it grew primarily phototrophically. In P-limited situations, it retained bacterial P for its own photosynthetic growth and did not stimulate other phytoplankton. The results suggest basic differences in the patterns of nutrient turnover by mixotrophs and by exclusive phagotroph

Algal nutrient limitation affects rotifer growth rate but not ingestion rate

The food algae Scenedesmus acutus and Cyclotella meneghiniana were grown in chemostats in a modified WC medium at identical growth rates (0.4 d(-1)) but under different nutrient regimes. Nitrate or phosphate was reduced to produce N- or P-limited algae. Brachionus rubens did not differentiate between nonnutrient-limited and nutrient-limited Scenedesmus and ingested both at comparable rates. The rotifers reached highest maximum growth rates with nonnutrient-limited Scenedesmus. N-limited Scenedesmus permitted similar growth rates at low food concentrations (below the incipient limiting level, ILL), but maximum growth rates at high food concentrations (above the ILL) were significantly reduced. With regard to the high N content of Brachionus, direct mineral N deficiency of the food appears to be possible. P-limited algae (Scenedesmus and Cyclotella) permitted no positive growth rates of the rotifers at all. P-limited algae that were short-term enriched with phosphate contained similar amounts of P as nonnutrient-limited algae but still were of lower nutritional quality, thus suggesting the importance of essential biochemical constituent

Grazing and nutrient influences of Daphnia and Eudiaptomus on phytoplankton in laboratory microcosms

In two experiments, top-down and bottom-up influences of the herbivorous crustaceans Daphnia pulicaria and Eudiaptomus graciloides on phytoplankton were compared in laboratory microcosms. In a long-term experiment (63 days), both grazers were able to establish populations. The Daphnia population exerted stronger grazing pressure, whereas Eudiaptomus fed more selectively. Daphnia retained relatively more phosphorus (P) and thus caused algal P limitation; with Eudiaptomus as a grazer, both nitrogen (N) and P remained limiting. In a short-term experiment (1 day), N and P release rates and algal-specific grazing rates by both consumers were measured. In this experiment, the increase in concentrations of soluble mineral nutrients (Delta NH4:Delta SRP) also supported the notion that Daphnia mobilized relatively less P than Eudiaptomus. Dry weight-specific grazing patterns did not differ greatly in this experiment. The present experiments and data from the literature suggest that Daphnia populations influence phytoplankton communities not only by the strong grazing pressure they are able to exert, but also by immobilizing large proportions of mineral nutrients (mainly P) within their biomas

Influence of metazoan zooplankton on the microbial community before and after the onset of the spring clear-water phase in Lake Constance (Bodensee)

Shortly before and shortly after the onset of the spring clear-water phase, we took plankton samples from the epilimnion of Lake Constance and incubated them in the laboratory under in situ conditions. In three duplicated treatments, mesozooplankton gt 100 mu-m were removed, left at natural densities or enriched nine-fold. Before the clear-water phase, cyclopoid copepods dominated the mesozooplankton, bacteria probably were controlled by high heterotrophic flagellate densities, and the most important phytoplankton grazers apparently were in the fraction lt 100 mu (ciliates, rotifers). In the copepod enrichments, these small grazers were decimated and chlorophyll a and autotrophic picoplankton reached the highest concentrations. Heterotrophic flagellates did not respond clearly to the different treatments. After the onset of the clear-water phase, Daphnia dominated the mesozooplankton and controlled most components of the microbial food web as well as the phytoplankton. All biomass parameters of the phytoplankton, autotrophic picoplankton, heterotrophic flagellates, and ciliates increased when Daphnia were removed. Bacterial abundances did not respond clearly to the removal of mesozooplankton, because protozoans became important bacterial grazers when they were released from the control by Daphni

Make EU trade with Brazil sustainable

Brazil, home to one of the planet's last great forests, is currently in trade negotiations with its second largest trading partner, the European Union (EU). We urge the EU to seize this critical opportunity to ensure that Brazil protects human rights and the environment