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 regu lation of phytoplankton standing crop in Florida's largest river, the St. J ohns. 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 a nd 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 phyto plankton standing crops and key environmental parameters related to nutrien t concentration and light extinction, (2) Estimation of spatial and tempora l patterns of mean light availability in the mixed-layer, (3) Examination o f the correlations between phytoplankton standing crops and light availabil ity, 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. Twe lve sampling sites along a 130 km reach of the river were sampled and analy zed for phytoplankton abundance, water chemistry, and light attenuation ove r a three year period. Our empirical results, along with the outcome of our efforts to model light availability for planktonic production were consist ent with our original hypothesis. Temporal variations in color were strongl y correlated to variability in phytoplankton standing crops. Spatial trends in standing crop were most readily explained through the effects of changi ng basin morphology and flushing rates. The results are discussed in the co ntext of the River Continuum Concept and variations on this theme specific to blackwater ecosystems.