© 1997 Walt Dineen Society

Walt Dineen Society Annual Conference '97

Session VII: Marine Ecology Abstract #: 97705

M. S. Ross, J. F. Meeder, G. Telesnicki, P. L. Ruiz, and J. P. Sah
Southeast Environmental Research Program, Florida International University, Miami, FL 33199


Holocene sea level rise has caused salt water encroachment into coastal areas for at least 6000 years, but in the last century the rate of encroachment has been altered by anthropogenic activities affecting the availability of upstream water resources. Marsh vegetation and soils, and adjacent tree island vegetation were examined in 55 sites SE of the Atlantic Coastal Ridge between Turkey Point and Taylor Slough in 1994-95. Both marsh and tree island vegetation exhibited well-defined compositional and structural gradients with distance to the coast. Since an earlier study by Egler (1952), the boundary of the mixed graminoid-mangrove and sawgrass communities shifted inland by as much as 3.3 km, and a low-productivity band appearing white on B&W and CIR photos moved interiorward by an average of 1.5 km. The shift in this "white zone" was less pronounced in areas receiving fresh water overflow through gaps in the C-111 Canal than in adjacent areas cut off from upstream water sources by roads or levees. Sub-basinal differences in response to sea level rise were also documented from changes in the vertical and horizontal distribution of mollusk assemblages, which include many good indicators of salinity. Subbasinal time lines for the transition from fresh to brackish-water assemblages were developed based on soil accretion rates from Pb210 dating methods. The rates of salt water encroachment in Joe Bay, Highway Creek, and Turkey Point subasins were several times the rate indicated by sea level rise alone, while Taylor Slough exhibited less encroachment than predicted by sea level rise. The timing of the acceleration of saltwater encroachment, when present, also appeared to differ among subbasins.


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