Long-term changes in apparent uptake of silica in the San Francisco estuary

Silica distributions in estuaries can be used to infer uptake, particularly that due to production by diatoms. I used data from two long-term monitoring programs to infer long-term patterns of diatom production during summer in the river-dominated northern San Francisco estuary. This region was most heavily affected by grazing by the clam Potamocorbula amurensis, introduced in 1986. Production based on dissolved silica (Dsi) uptake was high during the 1970s and early 1980s except during a drought in 1976-1977. From 1987 to 2002 production averaged 1% and 17% of the preclam values from the two monitoring programs. Low values in 1976-1977 and after 1986 are consistent with a loss of diatom production due to benthic grazing. There is no evidence of a relation between DSi-based production and either freshwater flow or temperature, and therefore climate change is an unlikely influence on the long-term trend in diatom production in this part of the estuary. Primary production calculated from chlorophyll and extinction coefficient was of a similar magnitude to that calculated from DSi depletion, indicating that most production was due to diatoms during Summers of the preclam period.