Factors Affecting the Abundance of Phytoplankton in a Restricted Subtropical Lagoon, the Indian River Lagoon, Florida, USA

Phytoplankton dynamics in the Indian River Lagoon was examined from the point of view of several key controlling factors, including: nutrient limitation, light availability, temperature, wind, hydrodynamic restriction and grazing. Water was collected at eight sampling sites in the lagoon on a monthly basis for a two and a half year period. Samples were analysed for nutrient content and phytoplankton abundance. Site measurements of salinity and light extinction were made at all sites. Regional data on rainfall, wind and water temperature was obtained for correlation analyses. Bioassays were performed to determine nutrient limiting status and a series of preliminary grazing experiments were carried out on selected samples. The results indicate that the Indian River Lagoon is a nutrient-rich environment where phytoplankton standing crops are often held below their potential by several key loss functions, including hydraulic flushing and grazing. Spatial patterns of phytoplankton abundance generally reflected the degree of restriction to water turnover in different regions of the lagoon, with higher mean abundances in restricted regions. Spatial and temporal patterns of nutrient content and limitation suggest that patterns of external nutrient loading also play a significant role in phytoplankton dynamics. High phosphorus levels in the southern portion of the lagoon contribute to the predominance of nitrogen limitation in the region. In contrast, relatively high N/P ratios in the northern portions of the lagoon contribute to greater potential for phosphorus limitation of phytoplankton growth. As might be expected from the subtropical location of the lagoon, temporal patterns of phytoplankton abundance appear to be less strictly dependent on season than in temperate habitats, and more closely linked to variations in weather conditions, like rainfall (including storm events). The latter considerations bring into play issues like temporal variation in salinity and wind-induced mixing. The high light flux and shallow depth in the lagoon also presents the potential for photoinhibition.