Correlation of Karenia brevis presence in the eastern Gulf of Mexico with rainfall and riverine flow.

Counts of Karenia brevis from the eastern Gulf of Mexico between 1953 and 1998, binned by months and converted to presence/absence and duration, were analyzed for cross-correlation with rainfall and riverine flow. The region was divided into three sub-areas for analysis to accommodate geographic variation in potential forcing factors. The duration of the presence of K. brevis in northern Florida waters was not significantly related to either rainfall or flows. The central Florida region recorded significant correlations of K. brevis durations with a number of central Florida rainfall stations and with flows of most central and southern rivers examined. Lags were short (less than two months), if any, indicating relatively direct linkages. Relationships of K. brevis in southern Florida waters to rainfall, while similar to the central region, were significantly correlated for only a few south Florida rainfall stations and primarily with flows from the Peace River in south central Florida. Cross-correlations of K. brevis duration with departures of flow or rainfall from seasonal means were generally not significant. Introduction In the Gulf of Mexico, the dominant organism responsible for red tides is Karenia brevis and its presence has been inferred as early as 1844 (Feinstein, 1956). A number of mechanisms for the perceived recent increases in blooms have been proposed, including an increase in nutrient loadings to coastal waters by both anthropogenic activities and long-term climatic cycles. This work examined presence of K. brevis in the eastern Gulf of Mexico for relationships to rainfall and riverine flows as surrogates for nutrient or trace requirement loadings. The implicit null hypothesis was that K. brevis presence and duration was uncorrelated with these dependent variables. Materials and Methods A 46-year record of counts of K. brevis (1953-1998), assembled by the Florida Marine Research Institute from multiple sources (FWC-FMRI, 2001), were segregated by region (Fig. lA) and reduced to presence/absence by month to avoid biases due to varying sampling effort and uncertainties as to reporting units of cell concentrations. Data were available from 75% of the months for Region II, but for only 10% and 52% of the months for Region I and Region III, respectively. Missing data were conservatively replaced with "absence. " A three-month duration parameter was computed by region as the number of months of the following 3 months in which K. brevis was present. Duration parameters were similarly computed for Figure 1 A Regions of the eastern Gulf of Mexico. B Locations of riverine flow stations. C Locations of rainfall stations. 6,9,12,18, and 24 month accumulation periods. Flows were obtained from 22 US Geological Survey stations (Fig. IB, Table 1). Monthly rainfalls were obtained from 41 National Weather Service cooperating stations from Pensacola to Key West (Fig. 1 C), and within the contributing watershed. Cumulative flows and rainfalls over a variety of periods (3, 6, 12, and 24 months), as well as anomalies (departure from monthly means), were also investigated. K. brevis duration periods and hydrologic accumulation periods were selected to represent a range between relatively immediate to improbably long linkages between K. brevis Table 1 Riverine flow stations