The effects of dietary Microcystis aeruginosa and microcystin on the copepods of the upper San Francisco Estuary

Abstract

1. Increasing blooms of Microcystis aeruginosa have unknown impacts on the copepods Eurytemora affinis and Pseudodiaptomus forbesi, which are the dominant zooplankters and key prey species for endangered larval fish in the upper San Francisco Estuary. 2. Laboratory feeding experiments were designed to measure the effect of Microcystis on copepod survival and to distinguish the effects of toxicity and nutrition. In a series of survival tests, copepods were fed a mixed diet of algae plus one of two strains of Microcystis, either producing (MC+) or lacking microcystin (MC)). 3. Microcystis significantly reduced survival even when it was a small proportion of the diet, indicating that toxicity was the major cause of mortality. Contrary to expectation, however, the MC+ strain did not result in higher mortality, suggesting that non-MC metabolites of Microcystis can be toxic to copepods. 4. Across treatments, survival of P. forbesi was greater than that of E. affinis, although the two copepods responded differently to both the ratio and the strain of Microcystis in their food. Survival of P. forbesi was greater on the MC+ strain and was inversely proportional to the ratio of dietary Microcystis (MC+ or MC)). In contrast, survival of E. affinis declined similarly across treatments and was not related to the proportion or strain of dietary Microcystis. Results indicate that the copepod P. forbesi can coexist with Microcystis while the other copepod E. affinis cannot. 5. Regardless of species, dietary Microcystis caused significant mortality to copepods, and it may cause adverse impacts to the potentially food-limited zooplankton community of the San Francisco Estuary. These impacts may not be related to the cellular MC concentration because Microcystis contains other metabolites that negatively affect copepods.

Microcystins derived from lysing Microcystis cells do not cause negative effects on crustacean zooplankton in Lake Taihu, China

“…However, we cannot conclude that the sensitivity of different species to microcystins does not significantly differ because of rather low concentrations of MCs in the present study. Our observations, therefore, support previous studies that these zooplankton species can coexist with one another during the periods when cyanobacterial blooms are toxic (Ger et al 2010). Moreover, we also know from previous studies that different species may evolve physiological and behavioral adaptations to coexist with toxic blooms (DeMott et al 1991).…”

Shifts in Zooplankton Community Structure: Implications for Food Web Processes in the Upper San Francisco Estuary

“…Reduced food availability for mysids was further suggested by the decreasing length of the mysid population as immature mysids depend on phytoplankton (Siegfried et al 1979) and mysid growth is a (Hansson et al 1990). Moreover, the replacement of nutritious diatoms with small-sized flagellates and cyanobacteria (Lehman 2000;Kimmerer 2005) and increasing frequencies of Microcystis aeruginosa blooms (Lehman et al 2008a, b) may have intensified the effect of the overall food limitation for primary consumers in the upper Estuary (Ger et al 2010). In addition to exploitative competition for shared food resources, direct clam predation on small-sized taxa and early life stages contributed to the declines in zooplankton.…”

A Conceptual Model of the Aquatic Food Web of the Upper San Francisco Estuary

“…Harmful algal blooms may be related to high concentrations of NH 4 (Lehman et al 2010). Microcystis is known to be toxic to both invertebrates and vertebrates; although toxicity has been demonstrated on copepods in the laboratory, it is unknown whether it can cause population-level effects on zooplankton abundance (Ger et al 2009(Ger et al , 2010.…”