The Sarasota Bay Estuary Program recently completed its second regional study on tidal creek nutrient dynamics. Both studies were funded through the Environmental Protection Agency (EPA) Region 4 Wetlands Program Development Grant program. The first study resulted in the development of a nutrient management framework for southwest Florida tidal creeks. This follow-up study focused on:
- validating outcomes of the initial study,
- refining the nutrient management framework and
- identifying additional indicators of tidal creek condition to refine nutrient targets and thresholds that protect the biological integrity of these critical natural resources.
The current study also produced additional indicators to help managers pinpoint potential causes of nutrient imbalance. These indicators include:
- A chlorophyll to nitrogen threshold ratio no greater than 15, above which indicates creeks that may be physically altered or have their hydrologic connection to the estuary cut off.
- A trophic state index score less than 60, which places a creek in the “fair” category for estuarine waters.
- A macrophye index (similar to Florida Department of Environmental Protection’s rapid periphyton index) based a frequency of occurrence of less than 50% of samples with macrophytes present.
- A total nitrogen annual geometric average concentration of 1.1 mg/l, based on a weight-of-evidence that higher concentrations are associated with an increased frequency of creeks in the “caution” category.
- A nitrate ratio less than one between source (fresh) and estuarine water. This indicates that dissolved inorganic nitrogen should be assimilated quickly within the tidal portion of the creek and there should not be nitrate sources within the creek.
In addition to indicator development nutrient management framework refinement, a wealth of scientific information was generated to accelerate the research into causes, extent and potential remediation of nutrient pollution to tidal creek wetlands. Results confirmed that nutrient addition to regional creeks is common. Nutrient dynamics can be characterized by three expected phases.
- Dry Season Phase: Low volume of freshwater is delivered to the tidal portion of the creek under “base flow” conditions. Source water nutrients tend to be lower than average.
- Pulse Wet Phase: Characterized by stormwater runoff (and stormwater infrastructure). Nutrient pulses are delivered to the tidal portion of the creek and attenuated by the differential between source and tidal water concentrations. Salinity is a decent surrogate to estimate the dilution of nutrients within the tidal portion of the creek.
- Saturated Wet Phase: The entire creek is essentially freshwater; there is no gradient in nutrient concentrations.
Tidal creeks remain under-sampled by routine water quality monitoring programs This study developed protocols and sampling strategies suggesting how a minimal amount of effort can be valuable to evaluate tidal creek condition. Recommendations are provided for minimal sampling requirements necessary to include additional tidal creeks currently without data into this assessment, as well as future efforts to enhance the development of biological and landscape metrics to further refine the assessments of tidal creek condition. Until such time, the revised management framework and development of additional indicators of tidal creek condition serve as valuable decision support tools that can be used to prioritize resource management responses to increase the stewardship and protection of these critical coastal habitats.
Nutrient Management Report Card
The nutrient management Report Card was revised to serve as a screening tool to identify creeks with elevated nutrient concentrations that could then be prioritized for further evaluation. This revised framework remains numerically based though it is not designed nor intended to explicitly identify regulatory impairments.
Study collaborators include the Tampa Bay and Coastal and Heartland Estuary Programs, Mote Marine Laboratory, and six regional county governments.