Meteorological influences on algal bloom potential in a nutrient‐rich blackwater river

Abstract

1. The effect of variability in rainfall on the potential for algal blooms was examined for the St Johns River in northeast Florida. Water chemistry and phytoplankton data were collected at selected sites monthly from 1993 through 2003. Information on rainfall and estimates of water turnover rates were used in the analyses of trends in phytoplankton biomass. 2. Major trends in rainfall and runoff within the lower St Johns River catchment over the 10-year study period were marked by both significant drought and flood periods. Autumn and winter rainfall patterns were strongly correlated with the range of Pacific sea surface temperature anomalies associated with El Niñ o events and La Niñ a periods. The effect of these major shifts in rainfall was evident in the strong relationship to replacement rates for water within the lower St Johns River. 3. The eutrophic status of the river was reflected in the high concentrations of nitrogen and phosphorus observed at all sampling sites, with total nitrogen concentrations up to 3100 lg L )1 and total phosphorus concentrations up to 180 lg L )1 . 4. While it is clear that the high phytoplankton biomass and frequent blooms that characterize the freshwater portions of the lower St Johns River are fundamentally based on nutrient status, the expression of that potential was strongly correlated to water replacement rates, as revealed by the inverse relationship between phytoplankton biovolume increase and water turnover rate, with an R 2 of 0.80 for the major bloom season. The sensitivity of algal blooms to rainfall patterns over the 10-year study period suggest that longer-term temporal and spatial shifts in rainfall, such as multi-decadal cycles and the global-warming phenomenon, will also influence the frequency and intensity of algal blooms.

Extreme Weather Events and Climate Variability Provide a Lens to How Shallow Lakes May Respond to Climate Change

“…Lake George is another large shallow eutrophic lake in Florida that is subject to intense CyanoHABs, particularly during the warmer months of the year (i.e., May-October) [27]. However, in contrast with Lake Okeechobee, the most intense blooms have occurred during years of below average rainfall and lower water levels [52].…”

“…In part, this pattern is attributable to the flow-through character of the lake, which has a major river inflow at the southern end (i.e., upper St. Johns River) and discharge into the lower St. Johns River at the northern end of the lake. The flow-through regime results in overall shorter water turnover rates in Lake George than Lake Okeechobee, and larger differences in water turnover rates between high rainfall/high discharge periods versus low rainfall/low discharge periods [27]. Low discharge and water turnover rates in Lake George during lower than average rainfall periods increase the potential for the buildup of phytoplankton biomass in several ways [52].…”

“…The recurring presence of N-limited conditions in the lake is reflected in the strong presence of N 2 -fixing cyanobacteria in the lake [27,55]. Recent estimates of the N budget and N 2 fixation rates for Lake George suggest that N 2 fixation contributes over 50% of external loads of N to the lake [54].…”

“…The lake is a part of the Lower St. Johns River that flows from south to north and empties into the Atlantic Ocean 173 km downstream near Jacksonville, FL. The lake is located in a shallow-gradient basin, which results in relatively long water residence time and slow water turnover rates (less than 1-6 times per season) [27]. The peak flushing season in the lake is the fall and early winter, during which color levels can be very high (up to 500 platinum-cobalt units, PCU) in high rainfall years [28].…”

“…The shallowness of the lake results in polymictic conditions. The lake is eutrophic and regularly experiences blooms of cyanobacteria [27,29]. We examined data taken from one sampling location near the major outlet of this lake into the St. Johns River.…”