Submarine groundwater discharge: Its measurement and implications for nutrient inputs and biogeochemical processes in the nearshore Coastal Zone

Abstract

Submarine groundwater discharge (SGD) assessments conducted both in the laboratory and at a field site in the northeastern Gulf of Mexico, using a continuous-heat type automated seepage meter (seepmeter) have shown that the device has the potential of providing long-term, high-resolution measurements of SGD. The improvements on the device using a simple inexpensive laboratory set up, have shown that: (1) connecting an extension cable to the seepmeter has a negligible effect on its measuring capability and, (2) influence of very low temperature (≤ 3 ºC) on seepmeter measurements can be accounted for by conducting calibrations at such temperatures prior to field deployments and, (3) salinity had no significant effect on the performance of the seepmeter. Calibration results from fresh water and sea water showed close agreement at a 95% confidence level significance between the data sets from the two media (R2 = 0.98). The observed artifacts on seepmeter measurements associated with Bernoulli-induced flow, the vertically directed flow arising due to water movement across topographic features can significantly be reduced by burying (or submerging) the seepmeter to nearly the same level as the sediment topography. While the study revealed that in general wind speeds > 6 m/s were associated with enhanced SGD measurements in seepmeters with buried and unburied benthic chambers, the influence was greater in the unburied meters, and more pronounced for SGD rates < 2 cm/day. Comparatively, the seepmeter SGD measurements provided data that are comparable to manually-operated seepage meters, the radon geochemical tracer approach, and an electromagnetic (EM) seepage meter. Study of the Sarasota Bay (SB) system revealed SGD advection rates ranging from 0.7 to 24.0 cm/day, except for rare isolated hot spot occurrences where higher rates were observed. In general, SGD estimates were relatively higher in the middle and south regions (5.9 – 24.0 cm/day) compared to the north region (0.7 – 5.9 cm/day). Although no obvious seawater nutrient concentration trend was revealed, the average N/P ratio was higher in the north compared to the middle and south regions of the SB system. The importance of SGD was evident in that about 40% of the regional nutrient fluxes were observed in the north while ~ 60% occurred in the middle and south regions combined. The latter two regions also had the highest overall nutrient flux per water volume ratio, compared to the north region, thus making them potentially more vulnerable to eutrophic conditions. On average, we estimate about 27% of total dissolved N in the SB system was derived via SGD