Effects of Microhabitats on Stable Isotopic Composition of Biota in the Florida
Everglades
Scott D. Wankel, and Carol Kendall
U. S. Geological Survey, Menlo Park, CA
Paul McCormick, and Robert Shuford
S. F. Water Management District, West Palm Beach, FL
Attached algae (periphyton) serve an extremely important role in the Everglades
ecosystem as a source of organic carbon to foodwebs. As important primary
producers within the wetland ecosystem, they establish the isotopic composition
of the base of the foodweb. However, this initial algal composition is established
by the composition of the inorganic nutrients (DIC and DIN) in the water column,
the compositions of which can vary significantly. Subsequently, the detrital
foodweb begins with the decomposition of this plant material (composed of both
emergent macrophytes and periphyton). The bacteria decomposing the detritus
may have a lower 13C value than the original plant material – imparting a lower
13C signature to organisms consuming the detritus and assimilating the lighter
bacterial carbon. Thus, foodwebs capitalizing on detrital components will exhibit
isotopic variability due both to the variations in the original plant material and the
processes decomposing the detrital material itself. An understanding of how these
processes and linkages operate is critical to any interpretation of stable isotope
data from the Everglades and to evaluating the success of the restoration process.
Aquatic vegetation and detritus samples were collected from various sites along
two parallel transects in WCA 2A (Sites E1, E4, E5, F1, F4, F5, and U3).
Sampling was carried out during the wet season (September 1998) and the dry
season (March 1999) in order to discern any seasonal variations in isotopic
composition. While the data indicate very little difference in 13C and 15N
between seasons, there was a strong spatial trend moving from the canal into the
marsh center. 15N values of vegetation (living and dead) tend to decrease (from
around +5‰ to around 0‰) with distance from the canal, while 13C values show
an increase with distance from the canal (from –30‰ to –26‰). Shrimp and
other invertebrate samples collected at the same times also show an ~2‰ decrease
in 15N and ~5‰ increase in 13C along the same gradient.
In addition to the sampling of the primary vegetation and detrital matter at each
site, benthic macroinvertebrates were collected within several microhabitats at
each site. Microhabitats sampled at each site varied but included open-water
sloughs, cattail marsh, sawgrass marsh and spikerush stands. The microhabitats
were not further than 100m apart within each site. At each site, there were
differences between the 13C and 15N of invertebrates from different habitats. At
U3, for example, there was a 0.5 to 1‰ variation in 13C and a 1 to 2‰ variation
in 15N between marsh types (fig. 1). However, the only generalization that can
be made for the microhabitat variations between sites was that invertebrates had
lower 13C values in sloughs than in cattail or sawgrass marshes. The differences
in isotopic composition of benthic macroinvertebrates within microhabitats at
each site suggest localized influences, perhaps due to the relative rates of
photosynthesis and respiration.
Figure 1. Differences in the 13C and 15N of selected invertebrates collected
during the dry season (March 1999) at 3 different types of microhabitats
(spikerush marsh, sawgrass marsh, open slough) near site U3 in WCA2A.
Additionally, in October 1997, algal growth experiments were conducted at site
U3, a pristine marsh site within WCA 2A. Plexiglass plates were submerged
within three slough-wet prairie habitats and allowed to colonize with algae for 8
weeks. Site U3-1 was a slough with abundant water lilies whereas the other two
sites were spikerush marshes. Weekly samples collected from each site were
analyzed for 13C and 15N, as well as diatom species composition (fig. 2). 13C
of the algae ranged from –32‰ to –27‰, while the 15N ranged from +2‰ to
+6‰. Isotopic compositions showed discrete temporal trends over the 8-week
experiment that correlated well with changes in the dominant diatom species.
These data are consistent with spatial variation in local microhabitat
biogeochemistry and species composition controlling bulk algal isotopic
composition.
Figure 2. Changes in 13C and 15N of algae growing on plates at 3 locations near
site U3 in WCA2A.
Scott, Wankel, U.S. Geological Survey, 345 Middlefield Road, MS 434, Menlo
Park, CA, 94025, Phone: 650-329-4303, Fax: 650-329-5590, sdwankel@usgs.gov