J. Matthew Hoch

A Molecular and Stable Isotopic Approach to Investigate Algal and Detrital Energy Pathways in a Freshwater Marsh

The relative importance of algal and detrital energy pathways remains a central question in wetlands ecology. We used bulk stable isotope analysis and fatty acid composition to investigate the relative contributions of periphyton (algae) and floc (detritus) in a freshwater wetland with the goal of determining the inputs of these resource pools to lower trophic-level consumers. All animal samples revealed fatty acid markers indicative of both microbial (detrital) and algal origins, though the relative contributions varied among species. Vascular plant markers were in low abundance in most consumers. Detritivory is important for chironomids and amphipods, as demonstrated by the enhanced bacterial fatty acids present in both consumers, while algal resources, in the form of periphyton, likely support ephemeropteran larvae. Invertebrates such as amphipods and grass shrimp appear to be important resources for small omnivorous fish, while Poecilia latipinna appear to strongly use periphyton and Ephemeroptera larvae as food sources. Both P. latipinna and Lepomis spp. assimilated small amounts of vascular plant debris, possibly due to unintentional ingestion of floc while foraging for invertebrates and insect larvae. Physid snails, Haitia spp., were characterized by considerably different fatty acid compositions than other taxa examined, and likely play a unique role in Everglades’ food webs.

Migration Strategies Vary in Space, Time, and Among Species in the Small-fish Metacommunity of the Everglades

Spatial ecology and movement strategies of aquatic organisms may limit their response to human-caused drying of wetland habitats. We characterized the movement strategies of the most abundant species of fish in the wetlands of the Everglades (USA) to better understand how they cope with annual fluctuations in aquatic habitat size. Over a six-year period, we used a sampling method designed to measure the density, activity levels, and movement direction of small fishes. We estimated changes in displacement speed and directional bias to identify patterns of movement that different fishes use to disperse over the gradient of disturbance in this environment. Movement of fishes ranged from highly active and directed to passive and random, and varied with hydrological condition (water rising, stable, or dropping). Six of the eight species studied displayed “directed” movement (possibly displaying taxis along environmental gradients) that varied in both speed and directional bias in response to hydrological cues. The remaining two species did not adjust the direction that they moved in response to hydrological cues, but their activity levels increased. Moving with directional bias may improve a fishs chance of early arrival in a newly available habitat or of escaping the risk of desiccation in drying wetlands. Fishes that change activity levels may improve their likelihood of reaching favorable environments by increasing diffusion rates and greater sampling of the environment. Interspecific variation in movement strategies is predicted to play a large role in community structure and may be a primary driver of the dynamics of the Everglades fish metacommunity.