Nutrient Distributions and Residence Times in St. Andrew Bay, Florida

Water residence time is an important parameter in defining how an estuary functions. The focus of this research was on the residence time and its implications on environmental health. Radium isotopes (223Ra, 224Ra, 226Ra, 228Ra) have proven useful for determining the residence time of waters in river dominated estuaries. This approach was applied to St. Andrew Bay near Panama City, FL to estimate the residence time of this non-river dominated estuary. St. Andrew Bay spans 230 km2 and is comprised of four individual estuaries (North Bay, West Bay, St. Andrew Bay and East Bay). The largest freshwater source to this environment is Deer Point Lake which is a man-made lake sourced by natural springs. It is located at the northern tip of North Bay. This study had three main objectives: 1) determine the relationship between two radium isotope sampling methods, 2) estimate the residence time of the northern portion of St. Andrew Bay, and 3) analyze the nutrient concentrations to assess the ecosystem health. The two radium isotope sampling methods compared were a grab sampling method and a stationary mooring method. Both methods utilize manganese dioxide fibers in the field to extract dissolve radium isotopes from the water column. These fibers are processed in the laboratory using a delayed coincidence counter. The measured activities of 223Ra, ex224Ra and ex226Ra were ratioed and the residence time was calculated using the approach described by Moore (2000). Water was collected and frozen for nutrient analysis during sampling. It was later analyzed for the dissolved inorganic nitrogen and phosphorus concentrations using a Lachat QuikChem 8000. The results of this project indicate grab sampling and mooring sampling methods can be combined. However, the inherent difference between the methods needs to be considered when analyzing the data. The grab samples capture the site's radium activities at the particular moment of sampling while the moorings integrate radium isotopes over a tidal cycle. The residence time calculation was compromised because of the multiple local sources which flow into both North and West Bay. While Deer Point Lake is the largest freshwater source into St. Andrew Bay, the flux of radium isotopes is not large enough to be traced throughout the system. The residence time calculated from the radium activity ratios is likely a lower limit estimation of the actual flushing time of the study area because of local sources. This method indicated there is an overall average residence time of approximately 10 days. Comparatively, a water balance was calculated which indicated a residence time from 7 to 20 days assuming the dominant flushing process is the diurnal tidal pulse. The nutrient concentrations measured throughout the study area were low. The DIN concentrations were between 0.49 µM and 4.00 µM and the SRP concentrations range was 0.47-2.41 µM. These concentrations indicate the system is healthy and able to properly incorporate the nutrients entering the system without affecting the water quality. This balance will be maintained as long as the nutrient concentrations and inputs are carefully monitored.