Hydrodynamic influences on interannual chlorophyll variability in an estuary: Upper San Francisco Bay-Delta (California, U.S.A.)

Diversion of estuarine water for human use, in conjunction with river inflow, is the main underlying cause of interannual chlorophyll variability in the landward portion of the San Francisco Bay-Delta estuary, i.e. Suisun Bay and the Sacramento-San Joaquin Delta. The principal feature of year-to-year change is high chlorophyll variability within the specific conductance range of 2–10 mS cm−1 (salinity 1–6), the approximate boundaries of an estuarine turbidity maximum (ETM). This variability stems from two separate hydrodynamic forces: (1) the strength of flow into the ETM, which affects primary productivity, losses to benthic macroinvertebrates, washout of phytoplankton cells, and chlorophyll loading; and (2) export of water from the estuary for human use, which independently affects the loading of chlorophyll from upstream sources into the ETM. In addition to the evidence for (1) presented in previous, largely descriptive studies, the simultaneous existence of both pathways is supported here by statistical models and material budgets. The source of phytoplankton organic matter from upstream was investigated further. Estimates were made for primary productivity levels in the Delta. These estimates are consistent with a mass balance for chlorophyll that suggests the Delta usually augments the flow of phytoplankton biomass from the rivers. If water were diverted from upstream of the Delta, rather than from within the Delta, the mass balance implies that both chlorophyll outflow from the Delta into the Bay, as well as primary productivity within the Delta, would increase. The primary productivity estimates also indicate that, on average, most of the phytoplankton emerging from the Delta is actually produced within the Delta, not upstream in the rivers; furthermore, most of the Delta primary productivity is respired or consumed within the Delta. A second, smaller mode of variability is as a result of phytoplankton blooms in the San Joaquin River, which were quite large and variable prior to 1978 but have been relatively small since that time. The underlying processes have not been identified in detail, but are suspected to be hydrodynamic in nature.