Paleoecological determination of the western Biscayne Bay coastal zone salinity regime prior to anthropogenic alterations to the system and estimates of freshwater discharge required to reproduce an estuarine condition
J. Meeder*, P. Harlem and A. Renshaw
Southeast environmental research Center,
Florida International University, Miami, FL.
Historically, freshwater was delivered to the Bay primarily by tidal creek discharge via transverse glades and secondarily by sheet flow generated by local rainfall and from overflow from transverse glade-tidal creek systems. A third freshwater source was springs found along the contact between upland limestone and coastal marl soils. Three types of tidal creeks are identified which are related to origin, transverse glades-sloughs (which produce the largest tidal creek and are usually associated with shoreline projections or points), spring fed and tidal action alone (the smallest). The presence of dead Crassostrea virginica shell accumulations, the common oyster, at larger tidal creeks along the western BB shoreline are strong indicators of an estuarine type environment with typical changing salinities but normal brackish water conditions.
Based upon the salinity requirements of the common oyster throughout its range a pre-development salinity range of between 5 and 19ppt is determined. The lower salinity is the target during the rainy season and the maximum salinity target for the dry season is 19ppt. Salinities over 20ppt result in serious predation and limits oyster production and survivability.
The volume of freshwater discharge delivered through transverse glades is estimated by the crude method of measuring the width of select transverse glades, determining average water depth (based upon the distribution of marl soils of known Hydroperiod) and estimating water velocity. At the lowest water velocities imaginable (1cm/sec) flows through one transverse glade is calculated at 1200 acre ft/day. This suggests that a great deal of freshwater was available to coastal Biscayne Bay prior to drainage and changes in water delivery patterns.
Calculation of freshwater needs in two different coastal basins suggests that no easy single model will work for calculating required freshwater flow to maintain the target salinity regime. This is due to differences in the size of the basin and basin water renewal time.
Jack Meeder, Southeast Environmental Research Center, Florida International University, Miami FL, 305-348-1615 (work); 305-242-9376 (home);