Estuarine Influence on Biogeochemical Properties of the Alabama Shelf during the Fall Season

Dzwonkowski, B., Greer, A. T., Briseño-Avena, C., Krause, J. W., Soto, I. M., Hernandez, F. J., Deary, A. L., Wiggert, J. D., Joung, D., Fitzpatrick, P. J., O'Brien, S. J., Dykstra, S. L., Lau, Y. H., Cambazoglu, M. K., Lockridge, G., Howden, S. D., Shiller, A. M., & Graham, W. M. (2017). Estuarine Influence on Biogeochemical Properties of the Alabama Shelf during the Fall Season. Continental Shelf Research. 140, 96-109. DOI:10.1016/j.csr.2017.05.001.

Estuarine-shelf exchange can drive strong gradients in physical and biogeochemical properties in the coastal zone and exert a significant influence on biological processes and patterns. Physical, biogeochemical, and plankton data from an across-shelf transect extending south of Mobile Bay, Alabama, in conjunction with regional time series data, were used to determine the relative importance of estuarine-shelf interactions on the physical-biological structuring of the shelf environment during fall conditions (i.e., well-mixed, low discharge).This period was also characterized by a relatively unique weather event associated with the remnants of Hurricane Patricia, which drove a meteorological flushing of estuarine water onto the shelf. Survey data indicated generally low N:P ratios across the shelf, with slightly elevated dissolved inorganic nitrogen in the Region of Freshwater Influence (ROFI) that extended approximately 30 km offshore. The ROFI had higher values of chlorophyll-a, diatom-specific production, marine snow, and primary productivity, with notable contributions from the larger size cells ( > 5 μm). Furthermore, stratification provided a niche opportunity for Trichodesmium sp. aggregates, a typically oligotrophic cyanobacteria, at the offshore edge of the ROFI. The lens of estuarine water may have limited the vertical extent to which this population was mixed, providing enhanced light availability relative to the well-mixed offshore conditions. Following the biogeochemical trend, the highest zooplankton abundances were also located within the estuarine outflow. While limited in spatial extent, the distinct geochemical and biological characteristics within the ROFI demonstrate the ecological impacts that estuarine-sourced waters can have during periods of generally low productivity in the Mississippi Bight.