Plankton dynamics in a nearshore coastal environment: Responses to short-term environmental fluctuations and top-down control

Nearshore coastal environments are common sites of episodic and stochastic events that rapidly alter the physics and chemistry of the water column and thereby affect plankton biomass and community structure. The dominant factors controlling the manner in which the pelagic communities responds to these changes are still poorly understood owing in part to their episodic nature and in part because they often rely on observations made at low temporal/spatial resolution and correlations between instantaneously measured conditions to draw relationships between the biological and physico-chemical parameters. King Harbor is a small, semi-enclosed recreational embayment within Santa Monica Bay. Nonlinear time-series and linear multiple regression analyses of a high-resolution, year-long dataset allowed an in-depth investigation of the relationship between various chemical/physical factors and algal biomass. A significant relationship with the tidal cycle was manifest primarily as increased biomass and bloom initiation during or in the days following neap tide, especially in the hydrodynamically-constrained northern basin of the harbor. Local histories of dissolved nitrate and salinity were also significant predictors of increased biomass, as was reduced wind speed.;A major fish mortality in King Harbor 8 March 2011 killed approximately 170 tons of fish (Pacific sardine) and garnered international attention as a marine system out of balance. In situ sensors present in the harbor prior to, during and after the event revealed rapid decreases in dissolved oxygen in surface waters from 7-9 March 2011, coincident with the fish mortality event. Continuous and automated observations provided evidence that fish respiration, exacerbated by an incursion of upwelled low-oxygen water, was the immediate cause of fish mortality. The hydrodynamically-constrained northern basin transitioned to extreme and sustained hypoxic conditions while spatially-variable hypoxia was also observed throughout the harbor and adjacent bay for more than ten days following the event. Initial recovery of dissolved oxygen in the harbor was facilitated by storm-mediated mixing of the water column. A trophic shift was observed throughout King Harbor concomitant with dramatic changes in water column chemistry associated with the fish kill. Relative abundances of bacterivorous ciliates increased up to > 80% in the weeks following the fish kill. Multivariate analyses also revealed significant temporal dissimilarity in microplankton community composition and trophic structure within King Harbor during hypoxia, subsequent storm-mediated mixing events, and following the storm. Finally, dramatically reduced photosynthetic yield by the phytoplankton in the northern basis indicated severe physiological stress of the phototroph population during the extreme hypoxia.;Top-down control on phytoplankton populations was examined through experimental studies of the interactions between the heterotrophic dinoflagellate Noctiluca scintillans and several red-tide forming algal taxa that were common in King Harbor, with special focus on the toxic dinoflagellate, Alexandrium catenella. A range of N. scintillans growth rates were observed on diets of several red tide-forming taxa (-0.08 to 0.79 d-1). Growth was negligible when offered A. catenella as prey, but growth of N. scintillans on the harmful raphidophyte Heterosigma akashiwo (0.79 d -1) were among the highest published. Although growth on A. catenella was negligible, N. scintillans ingested the toxic dinoflagellate at rates comparable to rates for H. akashiwo , but substantially lower than previously published reports for N. scintillans feeding on A. minutum. Positive growth of N. scintillans was obtained in the presence of high concentrations (16.73 ng ml-1) of purified dissolved saxitoxin, indicating that toxin alone was not responsible for poor growth of the predator on A. catenella. However, H. akashiwo grown in the presence of A. catenella culture (and to a lesser extent, filtrate) exhibited deleterious effects. These results indicate that A. catenella was not sufficient for growth and that allelopathy may further indirectly affect N. scintillans in nature through reduction in the availability of high-quality prey.