Processes governing phytoplankton blooms in estuaries. II: The role of horizontal transport

The development and distribution of phytoplankton blooms in estuaries are f unctions of both local conditions (i.e, the production-loss balance for a w ater column at a particular spatial location) and large-scale horizontal tr ansport. In this study, the second of a 2-paper series, we use a depth-aver aged hydrodynamic-biological model to identify transport-related mechanisms impacting phytoplankton biomass accumulation and distribution on a system level. We chose South San Francisco Bay as a model domain. since its combin ation of a deep channel surrounded by broad shoals is typical of drowned-ri ver estuaries. Five general mechanisms involving interaction of horizontal transport with variability in local conditions are discussed. Residual (on the order of days to weeks) transport mechanisms affecting bloom developmen t and location include residence time/export, import, and the role of deep channel regions as conduits for mass transport. Interactions occurring on t idal time scales, i.e. (on the order of hours) include the phasing of later al oscillatory tidal flow relative to temporal changes in local net phytopl ankton growth rates, as well as lateral sloshing of shoal-derived biomass i nto deep channel regions during ebb and back into shallow regions during fl ood tide. Based on these results, we conclude that: (1) while local conditi ons control whether a bloom is possible, the combination of transport and s patial-temporal variability in local conditions determines if and where a b loom will actually occur; (2) tidal-time-scale physical-biological interact ions provide important mechanisms for bloom development and evolution. As a result of both subtidal and tidal-time-scale transport processes. peak bio mass may not be observed where local conditions are most favorable to phyto plankton production, and inherently unproductive areas may be regions of hi gh biomass accumulation.