The Suwannee River estuary and the surrounding Big Bend region is one of the productive areas in the Gulf of Mexico and is the site of an emergent clam aquaculture industry. The growth and stability of the clam industry hinges on maintaining a balance between the nutrient inputs which support coastal primary production that feeds clam growth, and avoiding excessive eutrophication that can lead to destructive phenomena, such as harmful algal blooms and periods of anoxia or hypoxia. The Suwannee River drains 28,500 km2 of southern Georgia and north central Florida. Human development in the Suwannee River watershed over the past century has accelerated the rate of eutrophication. Nitrate levels within the Suwannee River are increasing and concentrations near the mouth and in the upper reaches of the estuary now regularly exceed 1 mg/L. The results of recent tests of groundwater indicate that nitrate levels in regions of the Suwannee River drainage basin near recent agricultural development contain higher levels of nitrogen than anticipated and could lead to even higher nitrogen concentrations in future spring outflows. These observations have precipitated widespread concern about the consequences of these changes on the ecological health of the river and the coastal waters of the Big Bend.
The central goal of this study was to define the impact of coastal eutrophication on the sustainability and productivity of native oyster populations and cultured clams. This goal was pursued within the context of three general hypotheses; a) Changes in the abundance and structure of plankton in coastal environments are related to the nutrient composition and volume of Suwannee River outflow, as well as grazing and predation, b) Up to a certain threshold of nutrient loading, increased abundance of planktonic biomass enhances the resources available for the growth of clams and oysters, and c) Beyond this threshold, excessive levels of phytoplankton biomass and shifts to harmful algal species diminish the productivity and stability of clam and oyster populations.
These hypotheses were tested within the context of four research objectives: 1) To determine the correlation between nutrient loads from the Suwannee River outflow and the abundance and composition of near shore plankton, 2) To determine the impact of planktonic predators and grazers on the abundance and structure of the phytoplankton, 3) To define the link between nitrogen and carbon in the Suwannee River and production of clam and oyster biomass, and 4) To determine the effect of plankton abundance and composition on the growth and survival of oysters and clams.
The results of our study revealed a positive correlation between the loading rate of nitrogen and phosphorus from the Suwannee River and the mean standing crop of phytoplankton in the adjacent nearshore environment. Differences in nutrient load were largely attributable to inter-annual changes in outflows from the river, linked to rainfall amounts. The high rainfall year of 1997/1998 resulted in three-fold higher TN and TP load to the estuary than observed in subsequent low rainfall years. Mean phytoplankton biomasses, in the form of chlorophyll concentrations, were three-fold higher in the high rainfall year than the subsequent low rainfall years, matching the change in nutrient load. Nutrient limitation bioassays for phytoplankton production, performed over the study period, showed a predominance of nitrogen limitation in the nearshore environment. The frequency of nitrogen limitation was greatly enhanced during low rainfall periods, suggesting that the inter-annual differences in nitrogen load were primarily responsible for the observed inter-annual changes in phytoplankton biomass.
Imbedded within the broad relationships between nutrient load and mean nearshore phytoplankton biomass, there were smaller-scale spatial and temporal differences in phytoplankton biomass and composition. From a spatial perspective, phytoplankton biomass tended to be higher south of the Suwannee River outflow north of the outflow, reflecting the predominant direction of long-shore water flow, but individual peaks in phytoplankton abundance were observed in various regions within the estuary and not limited to the southern region. Temporally, overall mean phytoplankton standing crops were higher in the warm season than the cold season. However, blooms of certain diatom and dinoflagellate species were also observed in the winter.
Production rate measurements of clam and oyster populations in the nearshore environment revealed important spatial and temporal similarities to those observed for phytoplankton biomass. For example, some clam lease sites north of the Suwannee outflow showed less production over the growth period than leases south of the outflow, supporting the hypothesis that differences in phytoplankton abundance influence the production potential of bivalves. Experiments on the selectivity and rates of grazing of cultured clams showed that certain species of phytoplankton were selectively consumed or grazed at higher rates. These observations indicate that shifts in species composition of the plankton community could alter the efficiency of food utilization and growth of clams.
In terms of the potential harmful effects of algae, significant blooms were observed over the three-year study period, but none were associated with observations of severe depression of oxygen levels in the water column. In addition, none of the observed blooms involved known toxin producing algae. It may be suggested that the relatively unrestricted nature of the Suwannee estuary, combined with river discharge rates, provide a hydrodynamic environment that limits the prolonged buildup and retention of algal blooms, despite the high nutrient concentrations found in the river outflow. However, these general conditions do not preclude the potential for the occurrence of stochastic meteorological events that result in harmful algal blooms. For example, in 2005, strong long shore currents emanating from a region of a strong toxic red tide event resulted in the appearance of significant concentrations of the red tide species Karenia breve in the Suwannee estuary. Such events, though rare, indicate the importance of maintaining a water quality monitoring program in regions of the estuary used for clam and oyster production and harvest.
