Numerous studies over the past few decades have shown that submarine groundwater discharge (SGD) transports significant quantities of nutrients to estuaries and nearshore oceans worldwide. So far, none of the geochemical and hydrological studies of SGD have demonstrated a clear ecological role. At the same time, studies of microalgal community dynamics have suggested, but not verified, that SGD is an important determinant of community structure. We thus have neither thorough SGD investigations with inferences about ecological responses nor detailed observations of microalgae with only an inferred linkage to SGD. Attempting to assess the role of SGD on microalgal dynamics is complicated by two factors. First, discharge occurs through the benthos, which in near-shore waters is the niche inhabited by benthic microalgae. The microphytobenthos (MPB) can be present at densities orders of magnitude higher than the phytoplankton and have repeatedly been shown to alter nutrient efflux. The role of the MPB as a sink for nutrients will depend on their growth rates, which are in turn largely driven by temperature and light availability. The second complicating factor is that SGD can be highly episodic and its nutrient content very variable. The effect of SGD on the phytoplankton assemblage may be due to nutrient delivery and/or to dilution (reduction in competition and grazing pressure) and altered residence times. The time-scales of SGD and community response are difficult to assess by standard sampling methods.
This project will to investigate the link between SGD and microalgal dynamics in Little Lagoon, Alabama, a model system for such a study. In contrast to most near-shore environments, it is fully accessible; has no riverine inputs; and is large enough to display ecological diversity (c. 14x 0.75 km) yet small enough to be comprehensively sampled on appropriate temporal and spatial scales. The PIs have previously demonstrated that the lagoon is a hot-spot for toxic blooms of the diatom Pseudo-nitzchia spp that are correlated with discharge from the surficial aquifer. This project will use state-of-the-art techniques to assess variability in SGD, the dependence of benthic nutrient fluxes on MPB abundance and productivity, and the response of the phytoplankton to nutrient enrichment and dilution. The work will integrate multiple temporal and spatial scales and will demonstrate both the relative importance of SGD vs. benthic recycling as a source of nutrients, and the role of SGD in structuring the microalgal community.
Broader Impacts: Although this project is geographically restricted, its findings should be far reaching. Groundwater-born nutrient enrichment is now normal where agriculture occurs over porous soils, including in New England, Maryland/Delaware, Florida, Alabama, likely Texas, Yucatan (Mexico), California, Korea, Japan, and the Netherlands etc. The likely dependence of coupling between SGD and phytoplankton composition is likely to be driven by temperature and the frequency/intensity of precipitation, both of which will change in the Northern Hemisphere, according to the IPCC. The phenomenon therefore has wide application. This project will provide training opportunities for three Ph.D. students and the findings will be incorporated into several courses: Physiological Ecology of Microalgae (MacIntyre), Global Biogeochemical Cycles (Mortazavi) and Environmental Radiochemistry (Burnett). Last, this project will build on the PI's active partnership with local citizens, members of the Little Lagoon Preservation Society (LLPS), in bi-weekly monitoring of water quality and microalgal community composition. Members of the PI's lab have presented talks at each of LLPS' quarterly meetings for the past 2 years. These are attended by local stakeholders, local and state political representatives, and members of the press, and have proved to be an effective means for outreach and education on eutrophication, HABs and hypoxia. The relationship has been reported on extensively in the local press and praised as exemplary in an editorial in the region's largest newspaper.
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 Ph.D. and is employed at a Federal government laboratory as a post doctoral researcher. Another female student has defended her dissertation (degree expected in 12/14) and she will be a Science Coordinator at a National Estuarine Research Reserve. A female technician participated in the fieldwork and gained experience in maintaining a research lab. She is pursuing a Masters degree at the University of Alabama. The project built on an existing partnership between the researchers and a local citizens’ group, Little Lagoon Preservation Society (LLPS). Members of LLPS assisted with sample collection in the routing monitoring, and acted as liaisons between the researchers and visiting school groups. During the project, the university researchers presented the results at LLPS’ quarterly meetings. These were attended by local stakeholders: the town council and municipal employees, representatives of local and state government, and members of state and federal agencies tasked with environmental monitoring and protection. LLPS has sustained an independently-funded monitoring program since the end of this project, and has worked actively to broaden the scope of research in the lagoon.
