Across northern Gulf of Mexico estuaries, freshwater inflow drives productivity, and remains a dominant factor affecting the success of oyster populations. Freshwater inflow influences salinity, a key factor determining oyster population success, and may also alter food quality and quantity. Suspended particulate organic matter (SPOM), which is generally composed of detritus, microphytobenthos, and phytoplankton, provides an important food source for oysters. In estuarine systems, freshwater inflow provides limiting nutrients of terrestrial origin, spurring autochthonous phytoplankton production and producing higher quality organic matter, as measured by carbon/chlorophyll a ratios.

We combined published and unpublished isotopic (δ13C and δ15N) data on oysters, SPOM, and surface sediment organic matter (SSOM) from studies across the Texas Coastal Bend region between 2011-2017. Salinity is used as a proxy for freshwater inflow, assuming lower estuarine salinities reflect periods of higher inflow. Results indicate that over a salinity range of 13 to 40, oyster δ13C values were enhanced as salinity increased. Specifically, mean oyster δ13C values ranged from -23.5 ± 0.6 at mean salinity 21, to -16.6 ± 0.7 at mean salinity 35. At all reefs, mean SSOM δ13C values were significantly more enriched than SPOM δ13C values, and SSOM contribution to oyster assimilation was higher. SPOM contribution was increased following lower salinities associated with freshwater inflow events. The carbon/chlorophyll a ratio, a proxy for the quality of organic matter, was significantly related to salinity (p<0.05). Changes in freshwater inflow can cascade to changes in the composition of SPOM, as indicated by the changing δ13C value and carbon/chlorophyll a ratio, respectively. Ultimately, this may affect the general functioning of the reef via energy transfer to higher trophic levels. However, this impact of freshwater inflow on oyster populations will likely reach a threshold as salinities become significantly reduced.
 

The Northern Gulf of Mexico, including Mobile Bay, AL, is home to one of few remaining harvestable oyster populations in the U.S., and knowledge of oyster population connectivity is needed to inform restoration and management activities of remaining populations. Along with data on settlement patterns, trace elemental (TE) signatures within shells of oysters have potential to serve as natural geochemical tags to define larval origins and infer population connectivity. To determine oyster settlement patterns, settlement plates were deployed in the Mobile Bay/Mississippi Sound (MB/MS) System bi-monthly from May-September 2014 and 2016. To determine oyster larval origins and population connectivity, we quantified TE concentrations (Mg, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Sr, Cd, Ba, Pb) in native adult oysters and newly settled spat (on 2016 settlement plates) along a ~40 km freshwater gradient and compared TE values in larval shell (origin) to settled and adult shell (grow-out sites). Overall, oyster settlement was higher in 2016 (70 ± 19 oysters plate-1) compared to 2014 (18 ± 6 oysters plate-1), with maximum settlement occurring at higher salinity sites in MS in both years (2014: 174 oysters plate-1; 2016: 1,137 oysters plate-1). Settlement increased exponentially from July until September (end of field sampling) each year, suggesting larvae were available for settlement post sampling. Linear discriminant function analysis (LDA) identified spatially distinct TE signatures, with Fe, Mn, and Cr accounting for 88% and Sr accounting for 69% of the variation among sites in adult and settled shell, respectively. Preliminary classification results from LDA determined that larvae originated from the Portersville Bay area in northeastern MS, rather than MB. Future restoration efforts and oyster farming/harvesting operations in the MB/MS system may benefit from grow-out in these areas that naturally promote oyster recruitment and by targeting projects to enhance/conserve natural reefs and associated broodstock.

The eastern oyster (C. virginica) is an important foundation species within coastal ecosystems. Oysters improve water quality, provide refuge for smaller fish and invertebrates, and serve as an important fisheries resource. The successful early recruitment of eastern oyster is vital for resupplying adult populations in the face of harvesting. Healthy coastal oyster populations are also challenged by various sources of stress, including pollution, freshwater input, hypoxia, disease, and predation pressure. Thus, considerable efforts have been dedicated toward oyster restoration within the northern Gulf of Mexico. The objective of our study is to evaluate the role and importance of early recruitment to oyster restoration success in western Mississippi Sound. During the summer 2018 recruitment period, we compared larval supply and spat settlement among eight sites, including four recently restored inshore reef sites (2 limestone, 2 relic shell), two historic unrestored inshore reef sites, an unrestored offshore reef site, and a previously restored offshore reef site (1 limestone). Variation in recruitment success will be quantified relative to region, restoration materials, and the degree of background predation on post-settlement oysters. Local oyster spawning stock biomass will also be considered as a potential source of early recruits. Preliminary results from zooplankton samples as well as from spat settlement plates both with and without predator exclusion cages will be discussed.