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: pfrezza@audubon.org) Question 4