Factors influencing primary production and respiration in Grand Bay National Estuarine Research Reserve

Author(s): Caffrey, J.; Amacker, K.; Murrell, M.; Woodrey, M.

Advances in technology have greatly increased our ability to collect water quality data over a variety of space and time scales. For example, commercially available data sondes deployed to collect time series of temperature, salinity and dissolved oxygen data can capture events from hourly (tidal and diurnal) time scales to seasonal time scales capturing freshwater runoff and algal blooms. Analysis of data from these long-term deployments provides insights into the relative importance of anthropogenic and external drivers on estuarine ecosystem function. Dissolved oxygen time series have been used to estimate daily gross production, respiration and net ecosystem metabolism (NEM). However, long-term estimates of primary production and ecosystem respiration are rare in the estuarine literature, yet they provide fundamental information about the trophic status of these sensitive environments. When collected consistently, this approach makes it possible to resolve long-term trends, but perhaps more importantly, it provides a historical benchmark against which future patterns may be evaluated. Grand Bay National Estuarine Research Reserve is a small and relatively pristine estuary in the northern Gulf of Mexico. Freshwater input into the estuary is primarily local runoff from bayous and tidal creeks, including Bayou Cumbest, Bayou Heron, and Bangs Lake Nutrient loading to Grand Bay is relatively small, with ambient nutrient concentrations often below detection. Primary production was calculated from diurnal dissolved oxygen data sonde data using Odum’s open water method. Primary production and respiration was highest in the summer and rates of these processes were highly correlated. Despite interannual patterns in freshwater flow and salinity, variability in metabolic rates was low, perhaps reflecting shifts in the relative importance of benthic and phytoplankton productivity, during different flow regimes. Primary production and water column chlorophyll a in Grand Bay following a 2005 phosphorus spill was similar that from other years. The lack of stimulation by phosphorus is consistent with nutrient addition bioassay experiments performed at Bangs Lake and Point aux Chenes. Samples collected bimonthly showed that nitrogen rather than phosphorus stimulated phytoplankton growth at both locations.

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