Urban runoff is one of the most threatening hazards to coastal ecosystems worldwide.  In Southern California, stormwater is of particular concern because pollutants that accumulate in the watershed during the dry season are likely to be released en masse to coastal waters during the rainy season. Runoff contains high concentrations of human pathogens, organic matter, and inorganic nutrients, creating public health hazards and the potential to fuel harmful algal blooms. The aim of this study was to characterize stormwater plume development through time, linking changes in the physical and chemical parameters to changes in phytoplankton dynamics. We focused on plumes within Santa Monica Bay, CA, a coastal system that is subject to rapid pulses of untreated runoff from the urbanized watershed of. We expected that runoff would stimulate phytoplankton production, and that phytoplankton dynamics would change with time during plume evolution. Specifically, initial inputs of runoff would stimulate the growth of small diatoms, resulting in increases of biogenic silica. After nutrient depletion, we expected that dinoflagellates and other large cells would be more prevalent within stormwater plumes. We tested these biological predictions by sampling stormwater plumes sequentially through time and measuring bulk phytoplankton parameters including chlorophyll a, biogenic silica, and primary productivity.

Results/Conclusions

Low salinity surface plumes were apparent after rain events of varying magnitudes, ranging from 1.5 to 9 centimeters of precipitation. These stormwater plumes persisted in Santa Monica Bay for 2-5 days over spatial scales of 15 km. Major rain events led to increases in primary productivity, phytoplankton biomass, and biogenic silica. Diatoms, due to their high initial abundances, numerically outcompeted dinoflagellates for new nutrients associated with runoff, and there was little change in the major phytoplankton groups with plume development. We suggest that as regulations of stormwater runoff are developed in the region they incorporate dissolved nutrients in addition to conventional parameters such as toxins and metals.