Fish and macroinvertebrates in wetlands are known to consume micro-crustaceans and rotifers, potentially impacting zooplankton assemblages within macrophyte beds and in adjacent open water. However, determining whether invertebrate taxa within vegetated areas are planktonic is difficult because most sampling devices are likely to capture both truly planktonic organisms as well as those closely associated with macrophyte surfaces. This study examines seasonal changes in type of prey consumed by the Eastern Mosquitofish Gambusia holbrooki in a small beaver-created wetland (Cobb Co., Georgia), and assesses the degree to which prey taxa vary in planktonic behavior by using traps that passively sample plankton moving through the water column. Aquatic insects were dominant in mosquitofish guts in the autumn 2012. Diet shifted to the small cladoceran Chydorus sp. in spring 2013, then to a mix of larger cladoceran taxa and terrestrial insects in the summer, and to another small cladoceran Bosmina longirostris in the second fall. During the winter of both years, fewer prey were taken and no single taxon dominated. Both species of small cladocerans showed planktonic tendencies and shifts to these small cladocerans tended to reflect seasonal changes in abundance. Mosquitofish electivity estimates were positive for both taxa, though were quite negative for other planktonic taxa (copepods and ostracods). Benthic prey availability did not change dramatically over seasons, and mosquitofish diet breadth did not decline during periods of high cladoceran consumption. Results indicate that Gambusia holbrooki actively and differentially take taxa that are planktonic, demonstrating the potential of G. holbrooki to alter plankton assemblages.
The Effects of Nutrient Cycling by the Exotic Catfish, Pterygoplichthys Disjunctivus, on Algal Growth in a Central Florida Spring
Nutrient recycling by fish plays a large role in the availability of nutrients, such as nitrogen and phosphorus, in freshwater aquatic systems. The invasive exotic catfish, Pterygoplichthys disjunctivus, has infiltrated central Florida springs and may have contributed to algal overgrowth in these springs due to its digestive by-products. In this research, we focused on whether algae would grow on microscope slides as a direct result of the presence of P. disjunctivus feces in situ by implanting an array in Volusia Blue Spring to measure algal growth. The algal accumulation in control and experimental treatments was compared using dry mass as well as spectrometry for chlorophyll a. Algae also were grown over the course of four weeks in a growth medium to measure growth rates of viable algal cells in the feces. There was significantly more growth of algae in the presence of P. disjunctivus feces than in controls. In addition to promoting algal growth, P. disjunctivus feces were found to contain viable algal cells that were thriving after being excreted. The addition of nutrients and viable cells due to the presence of feces could be a contributing factor to eutrophication and ecological shifts in Volusia Blue Spring.
Preliminary Assessment of the Ecological Effects of Natural Channel Design Stream Restoration Projects in West Tennessee
Natural channel design (NCD) principles are increasingly used in stream restoration projects that aim to remediate degraded stream channels. NCD projects have been constructed in West Tennessee to reverse stream channelization, reestablish channels through valley-plugged systems, and reduce localized flooding. The ecological effects of these projects are poorly understood and our goal was to assess how fish assemblage composition differed between restored and channelized streams as related to physical habitats. A control/impact sampling design (three streams, two treatments, and three replicates per treatment) was used to inventory instream physical habitat and collect fish assemblage data at eighteen sites. Fish samples were summarized as the relative abundance of each reproductive guild (number of individuals in guild/total number of individuals in sample). Canonical correspondence analysis was used to determine how reproductive guild composition was affected by physical habitat variables at channelized and restored sites. Ass emblage composition in restored streams was positively associated with mean water depth and negatively associated with number of observed channel units; these streams had higher occurrences of guarding behaviors, substrate generalists, and vegetation spawners. Assemblage composition varied among channelized streams with one having a similar composition to restored streams, another being dominated by species with high parental investment and low channel complexity with few flowing water habitats, and the third having a high abundance of non-guarding lithophils associated with flowing water habitats and number of channel units. Hair restoration projects generally affected fish assemblage composition (with one exception) and the specific effects appeared to be system dependent.
1 Dept of Biological Sciences, University of Tennessee at Martin, Martin, TN; 2 Tennessee Chapter of The Nature Conservancy, Jackson, TN
29 • Nicole M. Sadecky, Luke K. Sadecky, Zachary J. Loughman
