Anthropologic Modification of Water Flows in South Florida Is a Leading Cause of Environmental

The Caloosahatchee Estuary: Applied Research to Determine Freshwater Inflow Limits

Robert H. Chamberlain and Peter H. Doering

South Florida Water Management District, West Palm Beach, Florida

Anthropologic modification of water flows is a leading cause of environmental problems that extends throughout South Florida. The physical evidence of this is perhaps best exemplified by the multitude of canals that have proliferated from the primary system established as part of the Central and South Florida Project. This web of artificial waterways has enabled the alteration of predevelopment watershed hydrology and increased diversion away from historical ecosystems in order to serve and protect human related activities. Modification of river discharge to coastal areas by diversion, water withdraws, channelization, and damming has dramatically changed the timing and magnitude of the freshwater supply to South Florida estuaries. The impacts from these changes include decreased bio-diversity, alteration and loss of livable habitat, excessive stratification, hypoxia, eutrophication, and possibly fish disease. As the human need for water increases, the amount of freshwater required to ecologically sustain estuaries has become an increasingly important issue for resource managers. The current and future challenge to managers and restoration efforts is how to allow human manipulation of freshwater, while at the same time satisfying the needs of the estuarine environment. A first step in meeting this challenge is to estimate estuarine requirements for freshwater. In this report, we present the scientific approach employed by the South Florida Water Management District to address the issue of proper water allocation from the Caloosahatchee River to the downstream estuary.

Like most of south Florida’s estuarine watershed, the Caloosahatchee system has been greatly altered. The river has been channelized (C-43) and 20% more water flows to the estuary on the average because of a connection made to Lake Okeechobee for flood control protection. The river also has been deepened and three water control structures were added to better convey water movement from the watershed and the Lake. The last structure, Franklin Lock and Dam (S-79), was completed in 1966 and prevents saltwater encroachment upriver, effectively truncating the upstream extent of the estuary and its associated oligohaline zone. The public operation and private use of this water control infrastructure can result in both diversion of water away form the estuary during dry conditions and to the estuary from the watershed and Lake during wet periods. Long term records for S-79 indicate huge fluctuations in water flows can occur. These discharges can range from 0 to well above 12,000 ft3 per second. Enough water annually passes through S-79 to fill the volume of the estuary (3.6 billionft3) over 8 to 9 times.

In 1985, the District began an estuary research program to determine the optimum quantity of water needed by the Caloosahatchee Estuary to protect key biota. These species, or Valued Ecosystem Components (VECs), helps sustain the ecological structure and function of the estuary by providing food, living space, and foraging sites for other naturally occurring estuarine species. Oysters and submerged aquatic vegetation (SAV) are VEC examples. Therefore, limits of water quantity and related water quality that protect these species should lead to a healthy and diverse estuarine ecosystem. Adoption of this resource-based strategy requires specific information, which includes identifying key biota, their location, and their response to changes in water quantity and quality. The more comprehensive and scientifically based this information, then the more reliable and defendable are the resulting recommended inflows.

For the Caloosahatchee, as the research program evolved, a semi-tiered approach for data acquisition developed. A more detailed description of the research results and program linkage will be provided in the presentation. However, in summary:

1) A literature and information review lead to selecting SAV as the original VEC and also confirmed the importance of flow from S-79 as a major influence on estuarine biota throughout the estuary, from S-79 to the Gulf of Mexico.

2)Based on historical information, a sampling program was designed that

initially included seagrass, as well as other biota. Seven sampling areas were selected that extended along the longitudinal and salinity axis of the estuary (approximately 75 km), from near the structure to southern Pine Island Sound. All sampling was conducted monthly, beginning in 1986, with plans to continue until a predetermined range of flows was adequately represented. Therefore, having daily measured inflows at S-79, the major source of freshwater, is paramount for both understanding the influence of flow on biota and water quality, as well as ensuring a range of flows is adequately sampled that fully represents the conditions encountered by the estuary. This is the link that made the science applied and applicable to our research goals. This critical link is often lacking in other estuaries that now are trying to develop flow recommendations as part of the Everglades Restoration effort and Southwest Florida Feasibility Study (SWFFS).

As expected, analysis of water quality information confirmed that salinity is highly variable due to the large fluctuation in freshwater delivery. This variability at the head and mouth of the estuary often exceeds the salinity tolerance of oligohaline and marine species, implicating salinity as a major stressor that is clearly linked to freshwater inflow volume. Sampling of SAV along the longitudinal axis of the estuary, which overlaps with the fluctuating salinity patterns caused by changing flows, helped determine a preferred salinity envelope for the entire estuary. The results from sampling the other biotic groups indicate that diversity, distribution, and abundance increases at all stations, from the inner estuary out to San Carlos Bay, when salinity zones are equitable in size and salinity is supportive of all the SAV species along the longitudinal axis.

3)Field and lab experiments were conducted on Caloosahatchee SAV to assess

salinity tolerances under controlled conditions. The majority of these experiments isolated salinity as the single treatment factor. The experiments analyzed to date support the results from the field survey, confirming the importance and strong influence of salinity on SAV. Field and laboratory research was begun under contract in 1999 to assess oysters as an additional VEC.

4)Recognizing the importance of salinity in the Caloosahatchee and its

correlation to flow lead to the installation of continuous salinity sensors in 1992 along the longitudinal axis of the estuary. A steady state salinity model was developed in 1996 that predicted salinity based on flows and allowed the development of the current recommendations. Work is underway to develop a fully hydrodynamic model that eventually includes non-conservative water quality constituents. This will allow the evaluation of potential water quality changes within the recommended flow limits and help address concerns related to eutrophication and the possible need to reduce nutrient loading to the estuary.

5)Simply promoting the presence and good health of SAV may not fully provide

the desirable results. Therefore, it is important to understand how salinity and flow affect the morphological characteristics of SAV beds and their use by associated biota. Contractual efforts are underway to help address these issues. Similar contractual work is being conducted for oysters.

6)Finally, institution of a management strategy is weakened without subsequent

evaluation or monitoring. In the Caloosahatchee, long term monitoring includes hydroacoustic sampling of SAV. In addition, aerial surveys have been completed and routine updates planned. The salinity sensors will continue to provide instantaneous information that can be compared with continuous flow data from S-79 and data from an ongoing water quality monitoring program. Additional monitoring efforts are being considered as part of the SWFFS, based on conceptual model development that link stressors and species of special concern.

Robert Chamberlain, South Florida Water Management District, 3301 Gun Club Rd, West Palm Beach, FL, 33406, Phone: 561-338-1668,