Complex interactions among biological, physical, and chemical parameters within coastal ecosystems make it difficult to define the impacts of nutrient loading on phytoplankton biomass. Natural and anthropogenic surface runoff and groundwater discharge deliver elevated concentrations of nitrate and phosphorus to the Suwannee River, Florida, USA. These observations have precipitated interest in the potential impact of increased nutrient loads on algal biomass and composition in the Suwannee River and its estuary. To address the consequences of nutrient loading, the objectives of my study were:
• to define the relationships between changes in nutrient loading and phytoplankton biomass.
• to characterize the composition of the phytoplankton community within different hydrogeographical regions of the river and estuary.
• to determine the impact of micro-zooplankton grazers on phytoplankton standing crop in the estuary.

The results of my study indicated that phytoplankton biomass in the Suwannee River estuary was sensitive to changes in nutrient loads from the Suwannee River. For example, phytoplankton in the region seaward of the oyster reefs was most impacted by changes in nutrient loads due to the increased potential for nutrient limitation, specifically nitrogen. When the nutrient-rich freshwater plume was spatially restricted (i.e., low riverine discharge), phytoplankton standing crop remained low. During higher freshwater discharge, the nutrient-rich plume extended seaward and phytoplankton biomass increased. High phytoplankton biomass (>30 micrograms chl a per liter) was also observed in the reef region after periods of southwesterly prevailing winds, which may have increased residence time.
The impact of nutrient loading on phytoplankton biomass was also viewed in the context of loss processes, such as mortality. Within the Suwannee River estuary (Site C2), the micro-zooplankton community was dominated by nauplii, hetero/mixotrophic dinoflagellates, and ciliates. The density of ciliates was comparable to densities found within highly eutrophic freshwater lakes and blackwater systems. Maximum phytoplankton growth rates ranged from 0.45 to 2.7 per day and grazing rate coefficients ranged from 0.12 to 1.4 per day, both of which were within the range reported for other estuarine/nearshore regions around the world. Micro-zooplankton grazing rates impacted phytoplankton standing crop as grazing loss rates of up to 75% per day were observed under laboratory conditions.