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A 3-week diversion of the Orange County Sanitation District effluent discharge into nearshore waters off Newport Beach, CA constituted a considerable injection of secondarily-treated effluent into the coastal ecosystem. Thelocation 1.6 km from shore, shallow water de m),volume and nutrient content 8 liters day-1 of effluent with inorganic nitrogenconcentration>2 mM) during the diversionraised concerns regarding the potential for stimulating phytoplankton blooms and, in particular, blooms of toxic species. Remarkably, phytoplankton standing stocksduring the event and shortly thereafter did not reachvalues associated even withminor blooms historically observedin the region (generally <5 g l-1), although shifts in community compositionwere observed. Diatom abundances increased early during the diversion, dinoflagellates,phototrophic picoplanktonic eukaryotes and other algae increased mid-diversion, and cyanobacteria (Synechococcus, Prochlorococcus) increased near the end of the diversion. Concentrations of domoic acid(a phycotoxin commonly present in the area) remained near or below detectionthroughout the diversion, and abundances of potentially-harmful algal species were unresponsive. Bacterial biomass increased during the diversion, and equaled or exceeded total phytoplankton biomass in most samples.Abundances of microbial grazerswere also elevatedduring the diversion. We speculate that nutrient uptake by the bacterial biomass, acting in concert with or a response to a negative effect of disinfection byproducts associated with chlorination on phytoplankton physiology, played a significant role in muting the response of the phytoplankton to nutrients released in the effluent.

Distinguishing between local, anthropogenic nutrient inputs and large‐scale climatic forcing as drivers of coastal phytoplankton biomass is critical to developing effective nutrient management strategies. Here we assess the relative importance of these two drivers by comparing trends in chlorophyll‐a between shallow coastal (0.1–16.5 km) and deep offshore (17–700 km) areas, hypothesizing that coastal regions influenced by anthropogenic nutrient inputs may have different spatial and temporal patterns in chlorophyll‐a concentration from offshore regions where coastal inputs are less influential. Quarterly conductivity‐temperature‐depth (CTD) fluorescence measurements collected from three southern California continental shelf regions since 1998 were compared to chlorophyll‐a data from the more offshore California Cooperative Fisheries Investigations (CalCOFI) program. The trends in the coastal zone were similar to those offshore, with a gradual increase of chlorophyll‐a biomass and shallowing of its maximum layer since the beginning of observations, followed by chlorophyll‐a declining and deepening from 2010 to present. An exception was the northern coastal part of SCB, where chlorophyll‐a continued increasing after 2010. The long‐term increase in chlorophyll‐a prior to 2010 was correlated with increased nitrate concentrations in deep waters, while the recent decline was associated with deepening of the upper mixed layer, both linked to the low‐frequency climatic cycles of the Pacific Decadal Oscillation and North Pacific Gyre Oscillation. These large‐scale factors affecting the physical structure of the water column may also influence the delivery of nutrients from deep ocean outfalls to the euphotic zone, making it difficult to distinguish the effects of anthropogenic inputs on chlorophyll along the coast.