Initial impacts of Microcystis aeruginosa blooms on the aquatic food web in the San Francisco Estuary

Abstract

The impact of the toxic cyanobacterium Microcystis aeruginosa on estuarine food web production in San Francisco Estuary is unknown. It is hypothesized that Microcystis contributed to a recent decline in pelagic organisms directly through its toxicity or indirectly through its impact on the food web after 1999. In order to evaluate this hypothesis, phytoplankton, cyanobacteria, zooplankton, and fish were collected biweekly at stations throughout the estuary in 2005. Concentrations of the tumor-promoting Microcystis toxin, microcystin, were measured in water, plankton, zooplankton, and fish by a protein phosphatase inhibition assay, and fish health was assessed by histopathology. Microcystis abundance was elevated in the surface layer of the western and central delta and reached a maximum of 32 9 10 9 cells l -1 at Old River in August. Its distribution across the estuary was correlated with a suite of phytoplankton and cyanobacteria species in the surface layer and 1 m depth including Aphanizomenon spp., Aulacoseira granulata, Bacillaria paradoxa, Rhodomonas spp., and Cryptomonas spp. Shifts in the phytoplankton community composition coincided with a decrease in the percentage of diatom and green algal carbon and increase in the percentage of cryptophyte carbon at 1 m depth. Maximum calanoid and cyclopoid copepod carbon coincided with elevated Microcystis abundance, but it was accompanied by a low cladocera to calanoid copepod ratio. Total microcystins were present at all levels of the food web and the greater total microcystins concentration in striped bass than their prey suggested toxins accumulated at higher trophic levels. Histopathology of fish liver tissue suggested the health of two common fish in the estuary, striped bass (Morone saxatilis), and Mississippi silversides (Menidia audens), was impacted by tumor-promoting substances, particularly at stations where total microcystins concentration was elevated. This study suggests that even at low abundance, Microcystis may impact estuarine fishery production through toxic and food web impacts at multiple trophic levels.

Screening Assessment of Cyanobacteria and Cyanotoxins in Southern California Lentic Habitats

“…However, chlorophyll-a was a significant predictor of MCY presence. These results are consistent with other recent studies (Rinta-Kanto et al 2009;Lehman et al 2010;Kudela 2011;Otten et al 2012;McLaughlin et al 2013) that have shown a strong positive correlation between MCY and chlorophyll-a for lakes and other freshwater habitats. Our results also demonstrated a cut point of approximately 15 µg L -1 chlorophyll-a, above which cyanobacteria consistently dominated the phytoplankton assemblage.…”

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

“…(Lehman et al 2008). 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.…”

Emerging health issues of cyanobacterial blooms

“…The distribution of Microcystis which is a well known toxic freshwater genus, has now spread into several estuaries, with possible consequences on fishery production [26]. Indeed, Microcystis shows some salinity tolerance, with no significant variation in growth rate and toxin per cell quota up to 10 g salt L -1 [27].…”