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,