Temperature Dynamics in a Small Pacific Northwest Estuary and its Sloughs

Abstract Water temperature impacts estuarine ecology and biogeochemistry, but few studies have explored its dynamics, especially at tidal to seasonal timescales. Temperature is controlled by the mixing of river and ocean water and by heat fluxes from the atmosphere, both direct and indirect via bed sediments in intertidal areas. This study investigates temperature variability in the Yaquina Bay estuary, a small, partially to well-mixed estuary in the Pacific Northwest, using 2.5 years of observations. The seasonal cycle, the largest component of temperature variability, ranges up to 15°C in the upper estuary where long summer residence times allow for strong surface heating. Subseasonal variability of up to 6°C is also affected by longer residence times during neap tides in spring and summer, when tidal amplitude explains up to 65% of the subseasonal temperature variance. Diurnal and semidiurnal variability (ranging up to 12°C) depends on the local along-channel temperature gradient. Diurnal variance arises from a combination of tidal advection and local surface heating that varies with tidal-solar phasing, complicated by heat exchange with intertidal sediments in shallow areas. In summer, temperature contributes approximately 25% of the along-channel density gradient. These results demonstrate the sensitivity of estuarine temperature to variations in oceanic, riverine, and atmospheric conditions on timescales from hours to months as well as the strong spatial variability within the estuary. A better understanding of the drivers of temperature in estuaries informs predictions of temperature under changing climate conditions.