Dynamics of nutrient cycling and related benthic nutrient and oxygen fluxes during a spring phytoplankton bloom in South San Francisco Bay (USA)

Benthic oxygen uptake and nutrient releases of N, P and Si were measured weekly at 2 sites in South San Francisco Bay around the 1996 spring bloom.Exchanges across the sediment-water interface were estimated from whole core incubations performed in the laboratory at in situ temperature and in dark.Fluxes changed significantly on a weekly time scale.Over a period of 15 wk the fluxes of dissolved inorganic N, P and Si ranged from -40 to +200, 0 to 13 and from 30 to 400 pm01 m-' h-' respectively.Sediment oxygen demand increased from 10 before to 64 mg O2 m-2 h-' just after the bloom period.During the bloom, nutrient fluxes represented about 20, 16 and 9 % of the Si.P and N requirements for primary production.Before and after the bloom period, Si fluxes contributed up to 30 and >loo% of this requirement and P and N fluxes up to 15 and 50% respectively.Simple empirical models explain most of the spatial-temporal variability of benthic f l u e s of Si, P and NH, (but not NO3) from 3 predictor variables: sediment porosity, nutrient concentration in bottom waters and chlorophyll content of surficial sediments.These models show that algal blooms influence benthic-pelagc nutrient exchange through 2 processes: (1) depletion of nutrients from the water column (which enhances gradient-driven transports across the sediment-water interface) and (2) sedimentation of labile phytodetritus (which promotes remineralization in or on the surficial sediments].Rates and patterns of nutrient cycling were very different at the shallow and deep study sites, illustrating the challenge of extrapolating measurements of coupled algae-nutrient dynamics to whole ecosystems.