Microcystin in the southern San Francisco Bay-Delta Estuary and possible negative impact on the food web

Background/Question/Methods

California’s Sacramento-San Joaquin Delta is an expansive fresh-water tidal estuary that is part of the San Francisco Estuary, the largest estuary on the US Pacific coast.  Recently, there has been a collapse in the pelagic fish community in the San Francisco Estuary, known as the pelagic organism decline (POD).  Blooms of the cyanobacteria Microcystis, which often produce the cyanotoxin microcystin, were first documented in the Sacramento-San Joaquin Delta in 1999.  Cyanotoxins have been suspected as one contributing factor to POD.  It has been proposed that microcystin could be contributing to POD directly though poisoning of fish or indirectly by affecting zooplankton and other food sources.  Copepods, especially E. affinis and P. forbesi are important food sources for larval POD species.   Delta and longfin smelt, threatened species, feed on rotifers and other small organisms the first weeks after hatching.

In this study, we documented the spatial and temporal extent of cyanobacteria blooms in the Southern Bay-Delta Estuary (SBDE), an area further east and upstream in the San Francisco Estuary than previously studied.  Copepod, rotifer and total zooplankton abundance was examined in comparison to the spatial distribution of microcystin.  The environmental factors associated with cyanobacteria blooms in the SBDE were investigated.

Results/Conclusions

Microcystis and other cyanobacteria were common in the SBDE and a large, persistent Microcystis bloom was observed in the summer of 2012, but not in 2009 or 2011.  There was a strong relationship between microcystin and cyanobacteria biomass (r²=0.74).    Total zooplankton density and rotifer and cladoceran mass and density were significantly lower when microcystin concentrations were >0.02 µg/L (alpha=0.05, n=142).  The effect of microcystin on copepods, if any, is not definitive at this point.  In July and August of 2012, microcystin concentrations were measured above California EPA recreational advisory limits (0.8 µg/L) and World Health Organization drinking water limits (1.0 µg/L), with a maximum observed concentration of 2.1 µg/L.  In previous years, maximum observed microcystin concentrations were below 100 ng/L in this study and in studies by others (2004,2005, 2007, and 2008).  We compared water quality, flow and climate conditions of 2012, when the persistent bloom occurred, to 2009 and 2011.  Significantly higher phosphorus concentrations and temperatures occurred in 2012 corresponding to higher cyanotoxins concentrations, but cause and effect cannot yet be determined.