FRESHWATER INFLOW AND ESTUARINE PRODUCTION IN THE CALOOSAHATCHEE RIVER AND ESTUARY

Increasing human population and the accompanying changes in land and water use alter the timing and amount of freshwater delivered to estuaries. Urbanization and development of coastal watersheds increases the area of impervious surfaces, causing runoff to leave the landscape more quickly and enters estuaries downstream in a pulse: peak flows become higher and the lag between rainfall and runoff is greatly reduced, a condition referred to as flashiness. In estuaries where seasonal rainfall often determines the degree of freshwater inflow (i.e., relatively low groundwater contributions), estuarine tributaries become more flashy with increasing watershed development. In contrast, water control structures can interrupt the delivery of freshwater to estuaries downstream, altering not only the amount and quality of freshwater, but also the timing of its delivery. Furthermore, climate change is expected to increase global precipitation, though precipitation may actually decrease on a regional scale.

In a typical, healthy estuary there is a predictable progression in ecohydrology moving downstream along the salinity gradient. Upstream, freshwater inflow creates low salinity zones and brings with it nutrients, detritus, colored dissolved organic matter (CDOM), and suspended particles (turbidity). Net upstream moving currents converging with net downstream moving currents often trap large amounts of particulate matter near the landward extent of the salinity intrusion. This region of greatest concentration of suspended particles is referred to as the estuarine turbidity maximum (ETM). Nutrient-fueled phytoplankton production often occurs immediately downstream of the ETM, which suppresses photosynthesis by reducing light penetration. Zooplankton, including larval fishes, shrimps and crabs, can be found in greatest concentrations downstream of the phytoplankton upon which many of them graze, and zooplankton predators, such a jellyfishes, occur downstream of the zooplankton maximum. As phytoplankton are grazed or senesce and settle from the water column, light is able to penetrate to the bottom, especially in shallow estuaries, where it can drive photosynthesis by benthic microalgae, macroalgae, and seagrasses. A number benthic invertebrates and bottom-dependent zooplankton are attracted to these organic-rich phytoplankton deposits, and, in turn, serve as prey for juveniles of a number of estuarine dependent fishes. In fact, fishes that depend on estuaries during some stage of their lives comprise 70% of the recreational and 90-98% of the commercial catch in Gulf of Mexico waters.

Altered freshwater inflow upsets this predictable pattern of estuarine production. Too much freshwater can result in high turbidity and color, suppressing phytoplankton production or moving it downstream. Because high inflow also brings with it high nutrient concentrations, phytoplankton are able to bloom in areas where light is not limiting, but such blooms can limit light penetration to the bottom, shutting down benthic primary production. Both of these effects can reduce the amount of habitat available for juvenile fishes. Too little freshwater can starve an estuary of vital nutrients required for primary production and the sediments it needs to create and sustain wetlands and to stabilize the coast. In estuaries impacted by water control structures upstream, members of the estuarine plankton community can stack up on top of one another, with phytoplankton, zooplankton, and gelatinous predators such as jellyfishes and comb jellies all blooming immediately downstream of a lock, dam, or weir. The resulting feeding frenzy by the voracious jellies can devastate populations of larval fishes.

This project seeks to establish linkages between variability in freshwater inflow and ecosystem condition in the Caloosahatchee river and Estuary by characterizing and quantifying the responses of nanoplankton, phytoplankton, fish-prey (zooplankton), gelatinous predators, benthic microalgae, colored dissolved organic matter (CDOM), water quality, and the Estuarine Turbidity maximum (ETM) to variation in freshwater inflow and by identifying relationships among these responses. Monthly sampling is being conducted at night on an incoming tide at 14 stations along the length of the Caloosahatchee River Estuary from Point Ybel off Sanibel Island to the WP Franklin Lock and Dam just east of Olga. The resulting relationships can be used to assess ecosystem change, to predict future environmental impact based on projected inflow alterations and climate change, and to develop targets for ecosystem restoration or enhancement.

News Press Article

Pinnacle Magazine: Fall 2008

This project is a collaboration between FGCU and the University of South Florida and is funded by the South Florida Water Management District and by a Congressional Award through the U.S. Department of Education.