Major - but rare - spring blooms in 2014 in San Francisco Bay Delta, California, a result of the long-term drought, increased residence time, and altered nutrient loads and forms

Rare spring blooms, >20 mu g l chlorophyll a, were observed in the San Francisco Bay Delta during the drought year of 2014 in both the upper Sacramento River and in Suisun Bay. The upper Sacramento River bloom was dominated by chlorophytes, but biomass and photosynthetic efficiency (based on variable fluorescence, Fv/Fm) precipitously declined downstream when cells were exposed to sewage effluent and NH4+ levels > 70 mu M-N. Further downriver, substantial rates of nitrification occurred, based on increasing levels of NO3- and NO2- in proportion to decreasing NH4+ concentrations, reducing NH4+ levels to <10 mu M-N. The other major tributary, the San Joaquin River, had extremely high nutrient levels (NO3- > 400 mu M-N, PO43- > 13 mu M-P, but NH4+ similar to 2 mu M-N), very low chlorophyll a levels (similar to 3 mu g L-1) and low Fv/Fm values, but elevated bacterial production, suggesting presence of an algal inhibitor, possibly an herbicide. Both rivers converge above Suisun Bay, where elevated NO3 center dot- (>50 mu M-N), sufficient PO43- (>3 mu M-P), and reduced NH4+ levels (as low as 6 mu M-N), and reduced flow created conditions conducive to a spatially large and physiologically healthy (elevated Fv/Fm) diatom bloom dominated by the species Entomoneis sp. We conceptualize this bloom as a window of opportunity response by these diatoms to multiple factors promoted by the drought, including longer residence time for cell growth and biomass accumulation, and longer time for in-river nitrification to occur, reducing sewage-derived NH4+ to a level where diatoms could access NO3- for uptake and growth. We suggest that management practices that favor higher rates of flow may narrow the window of opportunity for phytoplankton growth, potentially leading to low productivity and food limitation for fish. Under high flow, a condition of washout may develop where both chlorophyll and unassimilated nutrients are transported out of the bay, and the phytoplankton that do develop are less favorable in terms of community composition for supporting the upper food web. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).