Interpreting ecological response to changes in freshwater delivery

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Interpreting ecological response to changes in freshwater delivery by periphyton
communities in the Biscayne Coastal Wetlands.
Anna Wachnicka, Evelyn Gaiser, Pablo Ruiz and Mike Ross
Florida International University, Miami, FL
The coastal wetland communities of Biscayne National Park have been cut off
from sheet flow for decades resulting in the current vegetation mosaic, which is
considerably altered from the landscape present in the 1940’s. Within this span of
time (~60 years), the tall fringe forest adjacent to the coast has broadened, and the
mixed graminoid-mangrove zone further inland has been replaced by a dense
monoculture of dwarf mangroves extending to the base of the L-31E levee—a
structure built in the mid 1960’s to minimize the impact of storm tidal waters.
This structure, along with several other canals, have effectively divided the area
into coastal saltwater and interior freshwater ecosystems with no functional
hydrological connection between the two. In an attempt to restore coastal
wetlands and to mitigate the effects of canal point discharge into Biscayne Bay,
the L-31E Freshwater Re-diversion Pilot Project was initiated in 1993 on a section
of coastal Biscayne Bay (~25 ha) between the Mowry and the Military Canal. The
treatment consisted of diverting water from the L-31E Canal into the mangrove
swamp. At the same time, a monitoring program was initiated to track the
treatment’s effects. The results of the Pilot Project will help to guide proposed
large-scale water re-diversions. Evaluation of the environmental consequences of
such hydrologic alteration to coastal wetlands requires at least two elements: (1)
recognition of biotic response to hydrologically-related parameters and (2)
establishment of restoration success criteria in measurable abiotic or biotic units.
The research presented here focuses on periphyton in the Biscayne Coastal
Wetlands, in particular, quantifying the relationship of periphyton community
attributes to environmental gradients (ie., hydroperiod, salinity, nutrients).
Periphyton communities are an important component of the Everglades coastal
ecosystems and respond quickly in composition and function to changes in
salinity regime. They can be useful in monitoring the pace and effects of
saltwater intrusion associated with hydrologic manipulations, and developing
quantitative performance measures as tools to evaluate the proposed water flow
changes. In particular, diatoms are abundant in periphyton mats and, because
they live in ornamented tests that remain taxonomically distinguishable in
sediment deposits for decades to millennia, sequences of their sedimented remains
can provide useful inferences about past fluctuations. These paleoecological data
can aid in establishing a baseline on natural environmental variability that is
essential in formulating realistic management goals. The goals of this ongoing
study are to: (1) determine relationships between periphyton attributes
(composition and function) and environmental gradients in the Biscayne Coastal
Wetlands and (2) apply relationships in calibrating past environments (i.e., sealevel rise, saltwater encroachment) from fossil assemblages in sediment cores.
The work presented here details the results of our efforts toward the primary goal
of defining ecological relationships in this basin.
Periphyton was collected from three coastal wetland basins between the Mowry
and Military canals near Biscayne National Park. Basin 1 is a treatment wetland,
which receives freshwater from the L-31E during periods when canal stages are
high . Basins 2 and 4 are control basins and are effectively blocked from
freshwater inputs by the L-31E canal. Three sampling stations in each basin were
distributed in the dwarf mangrove forest adjacent to the canal levee, the interior
dwarf mangrove forest and between the dwarf and fringing mangrove forest.
Periphyton production was measured bimonthly for 2 years by incubating
replicate artificial substrates for 2 months at each station. Natural periphyton
biomass was measured at each visit by sub-sampling a measured area of the
periphyton mat. Periphyton biomass was measured by chlorophyll a, dry and ashfree dry mass; subsamples were removed for measurement of TC, TN and TP
content; and diatom assemblage composition was assessed microscopically. At
each bi-monthly visit, water depth, salinity, pH, water temperature and
conductivity were measured at each station.
Biomass accumulation rates exhibited similar temporal fluctuations among sites
but no seasonal pattern was detected. Lack of pattern could be due to highly
variable water inputs due to periodic flooding and ponding of tidal water.
Variation in mat biomass was higher among sites than among dates. Biomass was
highest in the western station of Basin 1 and decreased toward the easternmost
sites, bordering on the taller fringing mangrove forests. Besides receiving
freshwater pulses, western sites of basin 1 are more open and less shaded by
mangrove canopy than the eastern sites. The same pattern is true for Basins 2 and
4, where shading from the mangrove canopy likely prohibits light from
penetrating to the sediment layer, thereby stimulating periphyton growth.
Concentrations of nutrients in Everglades periphyton mat tissue often provide a
highly sensitive index of the quantities of nutrients delivered to the marsh system.
We found that distance to the canal had a significant effect on P accumulation in
periphyton in Blocks 1 and 4, where TP was lowest in the western sites and
increased eastward. This pattern has been observed in coastal mangrove wetlands
elsewhere.
Over 200 diatom taxa representing 34 genera were identified from material
collected from the L-31E wetlands. While there was significant compositional
overlap among the 3 wetlands, based on taxon relative abundances, ordination
revealed a unique community for each wetland. For instance, the westernmost
community in Basin 1, which receives the most freshwater, was distinctly
separable from other communities in the basin, as well as in Basins 2 and 4
(Figure 1).
Figure 1. Ordination showing among and within-site differences in diatom
species composition in periphyton mats in the 3 coastal basins.
West
Central
East
Basin 1
Axis 2
Basin
2
NaCl
Basin 4
Axis 1
In summary, we found periphyton production and species composition to be
reliable indicators of ecological response to changes in freshwater delivery in the
saline Everglades. If monitored on a regular basin, periphyton production should
provide a good early indication of the ecological trajectory of implemented
changes in water delivery to these areas. In addition, diatom indicators will be
important not only in monitoring changes but also in paleoecological studies that
retrospectively infer past environmental conditions, providing a natural-system
ecological baseline for restoration targets.
Anna Wachnicka, Southeast Environmental Research Center, Florida
International University, Miami, FL, 33199,
Phone: 305-348-1284, Fax: 305-348-4096, [email protected], Ecology
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