Light availability and variations in phytoplankton standing crops in a nutrient-rich blackwater river

In this paper we examine the potential role of light limitation in the regulation of phytoplankton standing crop in Florida's largest river, the St. Johns. We hypothesized that spatial and temporal patterns in standing crops of phytoplankton are strongly affected by variations in light availability in the mixed layer, particularly as they relate to river basin morphology and changes in water color, which reach high levels in the St. Johns River. This hypothesis was examined within the context of four principal research objectives: (1) Determination of the spatial and temporal patterns of phytoplankton standing crops and key environmental parameters related to nutrient concentration and light extinction, (2) Estimation of spatial and temporal patterns of mean light availability in the mixed-layer, (3) Examination of the correlations between phytoplankton standing crops and light availability, and (4) Evaluation of the relative role of different limiting factors on the regulation of phytoplankton standing crop. The results of this study revealed a relationship between standing crops and light availability. Twelve sampling sites along a 130 km reach of the river were sampled and analyzed for phytoplankton abundance, water chemistry, and light attenuation over a three year period. Our empirical results, along with the outcome of our efforts to model light availability for planktonic production were consistent with our original hypothesis. Temporal variations in color were strongly correlated to variability in phytoplankton standing crops. Spatial trends in standing crop were most readily explained through the effects of changing basin morphology and flushing rates. The results are discussed in the context of the River Continuum Concept and variations on this theme specific to blackwater ecosystems.