Anthropogenic inputs of CO2 into the atmosphere are lowering estuarine pH globally in a process known as estuarine acidification. Shell-forming organisms are especially susceptible to the consequent changes in water pH and alkalinity. However, coastal upwelling and runoff from terrestrial watersheds can decrease pH and alkalinity in dramatic and spatially complex ways and on much shorter and biologically relevant time-scales. Native and commercial oyster populations may already be impacted by these pulse acidification events with potentially substantial consequences to local economies and estuarine biodiversity. To quantify these impacts, we outplanted lab-reared larval and juvenile native (Ostrea lurida) and commercial (Crassostrea gigas) oysters to sites within Tomales Bay, CA. We measured growth and survival of outplanted oysters after upwelling and freshwater run-off events. We concurrently measured water chemistry (pH, alkalinity, salinity, temperature, DO) and biological activity (chlorophyll a) at relevant spatial and temporal scales to quantify the movement of upwelled water and freshwater run-off within the bay and its impact on the oysters.
We found that the impacts of upwelled water and terrestrial run-off were highly spatially complex. We observed sharp decreases in pH and alkalinity at the entrance of the bay during upwelling and at sites near the river-mouth during run-off events. Water chemistry was also highly variable between sites adjacent to the shore and those in the middle channel. This spatially complex pattern of water chemistry had measurable consequences for the native and commercial oyster populations, and likely other species sensitive to altered pH and alkalinity. We observed higher growth rates when water was warmer (seasonal) and lower growth and larval calcification rates when oysters were outplanted in upwelling or run-off conditions. Mortality was low and non-significantly different across the experiment, suggesting a possible sub-lethal population impact of pulse acidification events with possible consequences for the quality of habitat created by these foundation species. Our study highlights the potential biological importance of changes in water chemistry at small spatial and temporal scales as ocean waters become increasingly acidic and patterns of upwelling and runoff become more variable due to future climate change.