Spatiotemporal variability of light attenuation and net ecosystem metabolism in a back-barrier estuary

Abstract. The light climate in back-barrier estuaries is a strong control on phytoplankton and submerged aquatic vegetation (SAV) growth, and ultimately net ecosystem metabolism. However, quantifying the spatiotemporal variability of light attenuation and net ecosystem metabolism over seasonal timescales is difficult due to sampling limitations and dynamic physical and biogeochemical processes. Differences in the dominant primary producer at a given location (e.g., phytoplankton versus SAV) can also determine diel variations in dissolved oxygen and associated ecosystem metabolism. Over a one year period we measured hydrodynamic properties, biogeochemical variables (fDOM, turbidity, chlorophyll-a fluorescence, dissolved oxygen), and photosynthetically active radiation (PAR) at multiple locations in Chincoteague Bay, Maryland/Virginia, USA, a shallow back-barrier estuary. We quantified light attenuation, net ecosystem metabolism, and timescales of variability for several water properties at paired channel-shoal sites along the longitudinal axis of the bay. The channelized sites, which were dominated by fine bed sediment, exhibited slightly higher light attenuation due to increased wind-wave sediment resuspension. Light attenuation due to fDOM was slightly higher in the northern portion of the bay, while attenuation due to chlorophyll-a was only relevant at one channelized site, proximal to nutrient and freshwater loading. Gross primary production and respiration were highest at the vegetated shoal sites, though enhanced production and respiration were also observed at one channelized, nutrient-enriched site. Production and respiration were nearly balanced throughout the year at all sites, but there was a tendency for net autotrophy at shoal sites, especially during periods of high SAV biomass. Shoal sites, where SAV was present, demonstrated a reduction in gross primary production (GPP) when light attenuation was highest, but GPP at adjacent shoal sites where phytoplankton were dominant was less sensitive to light attenuation. This study demonstrates how extensive continuous physical and biological measurements can help determine metabolic properties in a shallow estuary, including differences in metabolism and oxygen variability between SAV and phytoplankton-dominated habitats.