Extent of acute, chronic, and nutritional impacts of Microcystis aeruginosa blooms on the calanoid copepods of the upper San Francisco Estuary.

摘要

Microcystis is a globally occurring toxic cyanobacteria that has become a regular feature of the upper San Francisco Estuary (SFE) in the past decade. Its toxicity comes from secondary metabolites including hepatotoxic microcystins as well as several other less defined toxins. The copepods Eurytemora affinis and Pseudodiaptomus forbesi are the dominant zooplankton and the principle food source for larval fish in the upper SFE, where abundances of pelagic organisms including phytoplankton, zooplankton as well as endangered pelagic fish have reached record low levels. The potential negative impact of Microcystis blooms on copepods has raised concern for further depletion of high quality fish food, prompting this investigation of how Microcystis impacts copepod populations under controlled laboratory conditions, with implications for the SFE. Chapter 1 evaluates the toxicity and post exposure effects of dissolved microcystin on copepod populations. Chapter 2 explores the nutritional and dietary impacts of Microcystis and its microcystin toxins on copepod survival when given as the sole food and in a mixed diet with other nutritious algae. Chapter 3 quantifies the ingestion of Microcystis cells and the role of dietary microcystin ingestion on copepod survival. Two different strains of Microcystis (microcystin producing and non-producing) were used as a food source to distinguish effects of microcystin and non-microcystin toxicity on copepod feeding and survival. Results indicate that extended periods (>24 hr) of dissolved microcystin levels above 0.2 mg/L will cause toxic impacts (48 hr LC-50 = 0.5 -- 1.5 mg/L), though such high concentrations are probably very rare. In contrast, even the smallest proportion of dietary Microcystis (10% of diet by mass) caused rapid and significant mortality in both copepods, suggesting strong impacts in the SPE. Moreover, microcystin lacking strains of Microcystis resulted in the same or higher mortality, highlighting the importance of the non-microcystin toxins for copepods. Microcystis cells were ingested by both copepod species, but E. affinis had an order of magnitude higher rate compared to P. forbesi. Although copepods can partially adapt to dietary Microcystis through selective feeding, results strongly suggest that Microcystis can impact copepod populations in the SFE by reducing their overall food quality.