Abstract
Increasing residential and commercial development along the coasts of southwest Florida has resulted in lower water quality in local estuaries. Estero Bay's primary tributaries have experienced hypoxic conditions as well as elevated turbidity and nutrient concentrations. This study aimed to characterize the sources of dissolved organic matter (DOM) and inorganic nutrients to southern Estero Bay. Water samples were collected monthly along two tributary transects within Estero Bay on the Estero and Imperial rivers. Elemental ratios of DOM (as dissolved organic carbon, nitrogen, and phosphorus; DOC, DON, DOP, respectively) had C:P (700-5000) and C:N (14-31) ratios greater than Redfield, indicating highly-refractory influences from vascular plants, while organic N:P ratios were between 23-42. The presence of the humic-derived terrestrial organic matter fluorescence peak from chromophoric dissolved organic matter analyses in all areas of Estero Bay suggests that mangrove detritus may be the primary source of DOM in the estuary. Ratios of C:P and N:P showed seasonality with greatest values occurring in the dry season, suggesting increased inputs of P during periods of heavy rainfall while C:N ratios did not vary significantly throughout the year. Overall concentrations of dissolved inorganic nutrients in the Imperial River were greater than in the Estero River. Mixing curve models suggested that there was a source of DOC within the Imperial River throughout the year and within the Estero River during the dry season only. Dissolved organic nitrogen was conservatively mixed along both transects indicating that much of it is refractory, while DOP was conservatively mixed in the Imperial River and a source within the Estero River. Benthic flux chamber experiments in two fringing mangroves showed that the low-impact backwater site (Fish Trap Bay), was an annual source of DOC and a sink for DOP to the water column, while Big Hickory Pass, an outlet to the Gulf, showed the opposite trend. These results suggest the possibility of no net benthic flux of DOC and DOP in the estuary. Both sites were sinks for DON indicating a highly N-limited system, and were also sinks for DIN and DIP annually. Together with higher DOM concentrations at lower salinities, these results indicate that much of the DOMin the estuary may be derived from anthropogenic terrestrial sources or perhaps more predominately, from benthic mangrove remineralization and decomposition of detrital material.