Short-Term and Interannual Variability in Primary Production in the Low-Salinity Zone of the San Francisco Estuary

Abstract

We measured primary production during springsummer 2006-2007 to determine the carbon supply to the low-salinity pelagic food web of the San Francisco Estuary (SFE). Weekly or biweekly samples were taken at three stations of fixed salinity for size-fractionated primary production and biomass, both as chlorophyll and from biovolume based on counts. Error variance in productivity estimates arose mainly from the depth integration of 14 C uptake, showing the importance of productivity measurements at high light levels for estimates of depth-integrated production. Temporal and spatial variability in production were surprisingly small. Combining data from this study with long-term monitoring data, productivity and biomass were variable in time and salinity but without persistent patterns and with infrequent blooms. Production within the lowsalinity zone was unresponsive to variation in freshwater flow, in contrast to findings in other estuaries where nutrient loading drives variability in production and other regions of the SFE where production responds to residence time or to stratification. Estimated annual primary production was only 25 and 31 gC m −2 year −1 during 2006 and 2007, only half of it in cells >5 μm. These results imply that phytoplankton provided poor food web support for higher trophic levels, probably contributing to the long-term decline in fish abundance in the brackish to freshwater region of the estuary.

A biogeochemical model of phytoplankton productivity in an urban estuary: The importance of ammonium and freshwater flow

“…50% of surface irradiance) uptake rate was representative for the euphotic zone. The euphotic zone depth (1.05 m) was calculated from a mean Suisun Secchi depth of 0.3 m and the empirical relationship between Secchi depth and light attenuation coefficient as determined for Suisun Bay by Kimmerer et al (2012). The euphotic zone N uptake was scaled to the ratio of the euphotic zone depth to the mean depth of Suisun Bay (5.61 m) and provided in Peterson et al (1985).…”

“…Studies have concluded that most levels of the food web above the primary producers are food limited (Müller-Solger et al, 2002;Kimmerer et al, 2005;Sobczak et al, 2005;Greene et al, 2011). The estuary has chronically low primary production (Kimmerer et al, 2012) near the bottom of estuaries listed in order of annual primary production (Boynton et al, 1982;Nixon, 1988). Suisun Bay in the northern SFE (Fig.…”

“…Suisun Bay in the northern SFE (Fig. 1) is a center of attention small zooplankton (Glibert et al, 2011) and low primary production (Kimmerer et al, 2012). High summer chlorophyll concentrations in Suisun Bay declined to low levels in 1987.…”

Primary productivity and phytoplankton dynamics in a subtropical estuary: a multiple timescale approach

“…Furthermore, the studies are usually restricted to monthly, bimonthly or even seasonal sampling campaigns, and few have employed approaches focusing on the variability at smaller time scales. More refined investigations have revealed a greater complexity in the temporal variation in primary productivity, composition and biomass of phytoplankton in estuaries (Fujita and Odebrecht 2007, Cloern and Jassby 2010, Kimmerer et al 2012) than was evident in early studies. Small-scale variability may be related to weather events, such as fronts and episodes of heavy rainfall, as well as variations in the tidal level, local currents, concentration of dissolved nutrients and the rates of grazing and competition.…”

“…Located at the interface between the continental and oceanic environments, estuaries are influenced by both and are dynamic ecosystems with high complexity and marked variability in their physicochemical characteristics, which directly affect the rates of primary productivity and the composition and abundance of phytoplankton. This variability is determined by seasonal cycles of solar irradiance, rainfall, winds and tides, which act together in different temporal and spatial scales (Jouenne et al 2005, Abreu et al 2010, Kimmerer et al 2012). …”