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.
Florida State University Project OCE0961970 Intellectual Merit This study demonstrated the utility of radon and radium isotopes in identifying areas where submarine groundwater discharge (SGD) is qualitatively important as well as quantifying SGD rates. Both resistivity and radioisotope measurements suggested that the strongest groundwater source was at the east end of the study site, Little Lagoon, Alabama, an area where toxic diatom blooms occur commonly. We surveyed the lagoon several times making continuous measurements of temperature, salinity and radon (222Rn), which is much higher in groundwater than surface waters so serves as a groundwater tracer. Figure 1 shows an example of the results from one of these surveys. The aerial distribution of both radon and salinity can be used to interpret where groundwater inputs are important. The areas with relatively high radon and low salinity are most likely influenced by groundwater inputs. After these surveys were completed, follow up sampling was able to focus on the areas of interest identified by the tracers. We used natural radium isotopes (223Ra, 224Ra, 226Ra, and 228Ra) to estimate the residence time of the lagoon waters (about 11 days on average) and to identify two different groundwater sources (deep and shallow aquifers). We constructed a three-end-member mixing model to calculate the fractions of lagoon water samples contributed by the two different groundwater sources as well as by the Gulf of Mexico. Figure 2 shows how the 228Ra/226Ra activity ratios are very different between the shallow and deep groundwater sources. Combining the estimated fractions with a 226Ra mass balance, we were able to estimate an overall average shallow and deep groundwater discharges of 1.22±0.53 and 1.59±0.20 m3 s-1, respectively (total discharge = 2.81±0.57 m3 s-1). SGD rates based on the Rn mass balance model ranged from 0.60 to 2.87 m3 s-1 and displayed a temporal pattern in general agreement with the water table elevation in the area. We observed well-defined relationships between nutrients and chlorophyll-a (Figure 3) during periods when there was a diatom bloom (April 2010) and when no bloom was present (March 2011). A well-defined relationship between radium (groundwater-derived) and nutrients was clear during the April 2010 sampling, while no relationship existed between the same parameters during the drier March 2011 period when no bloom occurred. A multivariate analysis was applied to examine possible relationships between SGD, nutrient concentrations, and other environmental factors. Results indicated that groundwater discharge is a likely driver of algal blooms in Little Lagoon. Broader Aspects During the course of this project, two graduate students received training, in both the laboratory and the field, of the latest techniques for collection, measurement and interpretation of natural radioisotopes. One of these students (Ni Su) earned a Ph.D. based in part on the results she collected during this project. That student is now a postdoctoral researcher. Another student (Kristen Eller) finished a M.S. based partly on results from the project and she is now employed as an Environmental Specialist by the Florida Department of Environmental Protection. During the fieldwork on the lagoon, we worked closely with a citizen’s group that lives in the area. The Little Lagoon Preservation Society (LLPS) has an interest in maintaining good water quality within the lagoon and they volunteered to assist us in several ways including use of private boats, docks, sample collection and even bacterial counts. Interactions between the project scientists and the LLPS volunteers were responsible for much of the project’s success.
