Temporal and spatial variability in Pseudo-nitzschia spp. in Alabama coastal waters: A “hot spot” linked to submarine groundwater discharge?

The potentially toxic diatom Pseudo-nitzschia is common in the northern Gulf of Mexico. Seven sites along the Alabama Gulf Coast have been monitored weekly to bi-weekly for Pseudo-nitzschia spp., which were detected in 489 of 829 samples (59%) taken between November 2003 and July 2008. Mean population density peaked at 19.6 ± 3.2 °C but bloom densities (>106 cells L−1) occurred at 20–32 °C. Mean population density peaked at a salinity of 30.1 ± 3.2, with blooms occurring between salinities of 26 and 32. Peaks in abundance occurred in April–May, with secondary peaks in fall. A cluster analysis of the relative frequency distributions of abundance by site showed that Little Lagoon Pass had a strong dissimilarity compared to other sites, due to a higher frequency of bloom densities and a lower frequency of absences. Salinities at Little Lagoon Pass were higher and less variable than at other sites. Pseudo-nitzschia spp. were absent more frequently from sites at the mouths of Perdido and Mobile Bays, where salinity was lower and more variable. Freshwater transport from Baldwin County, which lies between these bays, has previously been shown to be primarily through submarine groundwater discharge into the Gulf of Mexico. Groundwater in Baldwin County has high nitrate concentrations and discharge is most likely to occur adjacent to Little Lagoon. Blooms of Pseudo-nitzschia spp. at Little Lagoon Pass in spring were highly correlated with discharge from the Styx River, a proxy for groundwater discharge. Little Lagoon Pass may therefore be a hot-spot for blooms of Pseudo-nitzschia spp., because local maxima in discharge result in nutrient availability without significant reductions in salinity.