Download Establishing minimum and maximum freshwater inflows to the

Establishing minimum and maximum freshwater inflows to the Caloosahatchee Estuary,
FL.
Peter H. Doering and Robert H. Chamberlain
South Florida Water Management District, West Palm Beach, FL.
Anthropogenic modification of the Caloosahatchee River and its watershed has altered
the delivery of freshwater to the downstream Caloosahatchee Estuary. The resulting
fluctuations in salinity frequently exceed salinity tolerances of estuarine biota. As a first
step in reducing this variability, the South Florida Water Management District has been
using a resource-based approach to establish minimum and maximum estimates of
freshwater inflows. The approach is a combination of the Valued Ecosystem Component
(VEC) approach, developed by the US Environmental Protection Agency (USEPA,
1987), and the habitat overlap concept, developed by Browder and Moore 1981.
The VEC approach has been modified to focus on critical estuarine habitat. In some
cases that habitat might be physical, such as an open water oligohaline zone. In other
cases the habitat is biological and typified by one or more prominent species (e.g. an
oyster bar, mangrove prop roots, grass beds). It is important to note that these habitats are
distributed along the estuarine salinity gradient with different habitat forming species
occupying different portions of the salinity gradient. The success of these habitat forming
species is critical to the success of the many other species that utilize the habitats and the
salinity zones in which they occur. A major assumption of this approach is that conditions
that are suitable for habitat forming VEC will also be suitable for the species that utilize
them. For the Caloosahatchee, beds of submerged aquatic angiosperm grasses have been
identified as a VEC.
The overlap concept of Browder and Moore (1981) forms the basis for relating
freshwater discharge to VEC or other estuarine resources. In the present application,
freshwater inflow produces a temporal and spatial overlap between grass beds and
physiologically tolerable salinities.
Salinity tolerances of submerged aquatic vegetation were used to make estimates of
inflow limits. Downstream, the marine grass, Halodule wrightii, was used to estimate
maximum inflows, while upstream, a salt-tolerant freshwater species, Vallisneria
americana, was used to estimate minimum flows. Salinity tolerances, determined from
laboratory experiments and field observations combined with results of relationships
between discharge from the Caloosahatchee River and salinity in the estuary indicated:
(1) mean monthly inflows of 300 cfs provided tolerable salinities for upstream SAV, and
(2) maximum mean monthly flows of 2800 cfs or less would avoid mortality of
downstream marine SAV. Analysis of field survey data indicated that this flow range
was not detrimental to other biotic components of the system including zooplankton,
ichthyoplankton and benthic macrofauna.
Literature Cited
Browder, J.A. and D. Moore 1981. A new approach to determining the quantitative
relationship between fishery production and the flow of fresh water to estuaries. p.
403-430 In R.D. Cross and D.L. Williams (eds.), Proceedings of the national
symposium on freshwater inflow to estuaries Volume 1.U.S. Fish and Wildlife
Service, U.S. Dept. of Interior. FWS/OBS-81/04.
U.S. Environmental Protection Agency. 1987. Estuary Program Primer. EPA Office of
Marine and Estuarine Protection. Washington, D.C.
Doering, Peter, South Florida Water Management District, Mail Stop 4820, 3301 Gun
Club Road, West Palm Beach, FL, 33406, Phone:561-682-2772, Fax: 561-640-6815,
[email protected] Restoration Needs and Priorities