Award: OCE-0962008

Intellectual Merti: Previous studies have documented that blooms of the harmful algal bloom (Pseudo-nitzschia spp.) in Alabama coastal waters are most likely to occur in spring, nearly 3 weeks after a period of increased aquifer discharge. Some of the highest densities of this harmful algae have been measured at Little Lagoon, a semi enclosed body of water in coastal Alabama. Some species of Pseudo-nitzschia produce domoic acid (DA), a neurotoxin that is implicated in contamination of shellfish beds, birds, and poisoning in humans. The occurrences of harmful algal blooms in Little Lagoon have been attributed to submarine groundwater discharge (SGD) that delivers nutrients into the lagoon. While SGD inputs are episodic, input of nutrients from the sediments are continuous, and can support the growth of algae in the water column. The focus of this project was to determine the role of sediments in Little Lagoon in supplying nutrients to the water column and to determine the role of SGD in bloom initiation. SGD nitrogen fluxes were up to 42 times greater than fluxes from the sediments. In contrast to SGD inputs, which were episodic, the sediments, however, provided a continuous source of nutrients for the water column. In this study we were able to demonstrate that in contrast to many other shallow estuarine systems, denitrification, a microbially driven process that transforms bioavailable nitrogen to dinitrogen gas that is then lost from the system, is of minimal importance. This is because because sulfides, which inhibit denitrifers, are present in the sediments throughout the year. Sulfides have also been shown to favor the dissimilatory reduction of nitrate to ammonium (DNRA), a process that we investigated extensively in Little Lagoon. DNRA is also a microbially mediated process that by transforming nitrate to ammonium helps to keep nitrogen in the systems. We found that DNRA was an important process within Little Lagoon, especially in the summer months and the rates exceeded that of denitrification by three orders of magnitude. The conversion of nitrate to the more biologically preferred form of nitrogen, ammonium by DNRA, means that the sediments may contribute to, rather than counteract, eutrophication during warm summers. Comparison of phytoplankton community dynamics with nutrient loads and tracers of SGD showed that there was a high correlation of phytoplankton abundance with a proxy of SGD age, excess 224Ra, during a Pseudo-nitzschia bloom in a wet year but not a dry year. Although there is high delivery of nutrients by SGD, nutrient load and other environmental descriptors such as temperature and salinity had little predictive power alone or in combination. This infers that the role of SGD in bloom initiation is not due to nutrient delivery per se but to ecological disturbance. Because the primary driver of the blooms is a system "reset" rather than nutrient loading alone, the implication is that management of nutrient loading in the groundwater would be unlikely to impact bloom development. Toxicological analysis of samples collected during one bloom demonstrated that the cells contained the toxin domoic acid (58 – 540 pg/ml). The cell toxin quota was inversely correlated with phosphorous and silicate availability, consistent with bloom initiation following flushing of the system with nutrient-rich SGD, followed by elevated toxicity in the maturing bloom as non-nitrogenous nutrients became depleted. Examination of 131 juvenile fish collected during the bloom showed that 98% contained domoic acid in their tissues. This was the first time that it has been documented from the part of the Gulf of Mexico. Broader Aspects During the course of this project, two graduate students received training, using the latest techniques for collection, measurement and interpretation of benthic nutrient fluxes, stable isotope measurements, and detection and understanding of harmful algal bloom dynamics. One earned a...