Abstract

Low-inflow estuaries are highly variable and productive habitats. Organisms in these systems must cope with constantly changing conditions. This study investigates the continuous response of oysters to environmental stimuli in a small, low-inflow, estuary, Los Peñasquitos Lagoon (LPL), in San Diego, California, USA. The oysters were placed on a mooring at near-surface and near-bottom locations and instrumented with noninvasive sensors to measure their shell opening (oyster valve gape), concurrently with physical and chemical measurements (water depth, salinity, temperature, dissolved oxygen, pH, solar radiation, wind velocity, and wave radiation stresses) over 136 days. The time series of each oyster with each variable over tidally averaged and shorter time scales were analyzed using traditional statistical methods. The strongest gape relationships were with salinity and dissolved oxygen (Spearman correlation coefficient 0.6), among the oysters near the water surface. Diurnal phase averaging showed how the surface oysters open and close in phase with insolation, while the near-bottom oysters have a lagged response. Tidal phase averaging further illustrates the distinct gape behavior, with surface oysters closing consistently during low tides, and bottom oysters generally remaining open. A subset of variables that had high Spearman correlations (near surface oxygen and salinity, near-bottom pH, solar irradiance, and near-bottom turbidity) and their 18-hour time lags were used to test a set of multivariate models for predicting surface oyster valve gape. The best performing model (r²= 0.75) was consistent with the correlation analysis, showing salinity and dissolved oxygen to be the most important predictors. Key insights from this study are that oysters in LPL have predictable responses to environmental conditions (particularly salinity and dissolved oxygen) with differing responses to conditions near-surface and near-bottom on tidal and tidally-averaged, and faster than tidal timescales.