Connections to Tidal Marsh and Restored Salt Ponds Drive Seasonal and Spatial Variability in Ecosystem Metabolic Rates in Lower South San Francisco Bay

Abstract

We characterized five years of seasonal metabolic patterns in two tidal sloughs in Lower South San Francisco Bay, one surrounded by a broad tidal marsh and the other connected to restored salt ponds (ponds). Despite comparable seasonal dissolved oxygen (DO) patterns, the two sloughs exhibited markedly different seasonal metabolic rates. The in-depth analysis of these small intertidal systems reveals the complexity of processes at work in the fringing habitats of larger estuaries. In the marsh-connected slough, respiration rates peaked in summer (~10 g {mathrm{O}}_{2}/{mathrm{m}}^{2}/mathrm{d}), coincident with highly dynamic net ecosystem production rates (NEP). The peaks in DO consumption were correlated with temperature and tidal elevation. The most negative NEP rates were associated with the highest nighttime tides, when strong over-marsh respiration rates cannot be balanced by production. Thus, the combination of warmer water and the coincidence of higher-high tides with dark hours during summer may largely explain seasonal DO patterns in the marsh-connected slough. In the pond-connected slough, strong net heterotrophy (NEP < -20 g {mathrm{O}}_{2}/{mathrm{m}}^{2}/mathrm{d}) correlated with peak phytoplankton production and export from the adjacent ponds in spring, consistent with the hypothesis that organic matter exported from the pond exerts oxygen demand in the turbid slough. The impact of the elevated spring respiration rates appears to be offset by the super-oxygen-saturated water entering the slough from the highly productive pond. Oxygen minima in the slough (< 5 mathrm{g}/{mathrm{m}}^{3}) occurred in summer when outflow from the pond was lower in DO, coinciding with weaker, but still very high, in-slough respiration (~ 10 g {mathrm{O}}_{2}/{mathrm{m}}^{2}/mathrm{d}).