A cold phase of the East Pacific triggers new phytoplankton blooms in San Francisco Bay

Abstract

Ecological observations sustained over decades often reveal abrupt changes in biological communities that signal altered ecosystem states. We report a large shift in the biological communities of San Francisco Bay, first detected as increasing phytoplankton biomass and occurrences of new seasonal blooms that began in 1999. This phytoplankton increase is paradoxical because it occurred in an era of decreasing wastewater nutrient inputs and reduced nitrogen and phosphorus concentrations, contrary to the guiding paradigm that algal biomass in estuaries increases in proportion to nutrient inputs from their watersheds. Coincidental changes included sharp declines in the abundance of bivalve mollusks, the key phytoplankton consumers in this estuary, and record high abundances of several bivalve predators: Bay shrimp, English sole, and Dungeness crab. The phytoplankton increase is consistent with a trophic cascade resulting from heightened predation on bivalves and suppression of their filtration control on phytoplankton growth. These community changes in San Francisco Bay across three trophic levels followed a state change in the California Current System characterized by increased upwelling intensity, amplified primary production, and strengthened southerly flows. These diagnostic features of the East Pacific "cold phase" lead to strong recruitment and immigration of juvenile flatfish and crustaceans into estuaries where they feed and develop. This study, built from three decades of observation, reveals a previously unrecognized mechanism of ocean-estuary connectivity. Interdecadal oceanic regime changes can propagate into estuaries, altering their community structure and efficiency of transforming land-derived nutrients into algal biomass.

Fig. 1.

Indicators of phytoplankton change in South SFB. (A) Occurrences of autumn blooms (August through December Chl-a >10) since 1999. (B) Increases in August through December Chl-a (interquartile ranges are shown as bars) and gross primary production (GPP) (open circles). (C) Ten-year windowed trends showing statistically significant (P < 0.05; filled squares) Chl-a increases after 1999. (D) Annual inputs of dissolved inorganic nitrogen and phosphorus from the San Jose–Santa Clara Wastewater Treatment Plant [N. Van Keuren (City of San Jose), personal communication], the largest municipal discharger to South SFB.

Fig. 2.

Monthly trends of water quality in South SFB based on USGS measurements of DIN and DRP for the years 1990–2005, and surface salinity, water temperature, Chl-a, and SPM for the years 1978–2005. Units are percent change y⁻¹. Darker bars indicate statistically significant trends (P < 0.05).

Fig. 3.

Indices of biological community change within SFB and physical changes in the adjacent California Current. (A) Annual median biomass of filter-feeding bivalves across shallow habitats in South SFB; numbers above squares indicate sample number per year. (B) Mean annual catch ha⁻¹, normalized to 1980–2005 averages, of English sole, Bay shrimp, and Dungeness crab, from monthly sampling across the marine domains of SFB. (C) Anomalies in upwelling intensity computed by the National Oceanic and Atmospheric Administration from atmospheric pressure fields. (D) Sea surface temperature measured at the Farallon Islands. The bottom series (C and D) are 12-mo running averages of deviations from 1977–2005 monthly means.