Distribution and Abundance Patterns of Submerged Aquatic Vegetation in Response to Changing Salinity in the Mangrove Ecotone of Northeastern Florida Bay
Peter E. Frezza, and Jerome J. Lorenz
National Audubon Society, Tavernier, FL
Within the mangrove ecotone of northeastern Florida Bay, historic flow and salinity patterns have been altered by the construction and operation of a series of canals known as the South Dade Conveyance System (SDCS). Changes in ecotonal salinity caused by operation of the SDCS have most likely altered the seasonal and inter-annual patterns of submerged aquatic vegetation (SAV). A routine surveying program was established in 1996 to determine any correlations between salinity and SAV distribution patterns and also to characterize seasonal patterns of SAV community structure. Surveying was conducted at 3 sites, located in the coastal wetlands of northeastern Florida Bay in the Taylor Slough/C-111 drainage area. At each site, 6 fixed stations along a salinity gradient, ending at Florida Bay were sampled approximately every 6 weeks. SAV were quantified using the point intercept percent cover method. Species abundance and community composition was determined by analyzing 12 randomly selected 0.25m² quadrats at each station. Salinity, temperature, water depth, and water clarity were also measured at each station on day of sample.
SAV communities consisted of a variety of species ranging from freshwater marsh plants and algae to euryhaline seagrasses. Stations of similar mean salinity had similar assemblages of vegetation. Upstream stations (mean salinity ranging 3.5-8.6psu) consisted primarily of a mixed assemblage of Utricularia sp., Najas marina, Chara hornemanii, and Ruppia maritima. Downstream stations (mean salinity ranging 10.4-15.5psu) were dominated by Halodule wrightii, or a mixture of Halodule and Ruppia. Preliminary results indicate that there was a direct seasonal relationship between salinity fluctuation and die-back and re-growth of vegetation. SAV abundance at nearly all stations, excepting those near or in Florida Bay, appeared to indicate strong negative correlations with salinity. Relatively rapid increases in salinity on the wetlands at the onset of the dry season resulted in severe or complete die-off of vegetation. Recolonization and continued growth occurred with lowered salinities throughout the wet season. Since 1996, overall mean salinity at all stations has steadily increased. Over the same period, upstream stations dominated by freshwater plants and algae have proportionally decreased in total SAV abundance while downstream stations near or in Florida Bay have showed an opposite response. This downstream response may be explained by the continued expansion of Halodule and resulting displacement of lower salinity species. These preliminary analyses indicate that salinity may be the principal controlling factor in the submerged plant community and a trend towards a less stable salinity regime can negatively affect the composition of these communities in the ecotone.
Peter E. Frezza – National Audubon Society, 115 Indian Mound Trail, Tavernier, FL (Phone: 305-852-5318; E-mail: email@example.com) Question 4