| I researched effects of environmental factors on seasonal variation in food web and community structure at Mad Island Marsh, Matagorda Bay during March 1998–August 1999. The research had two major components. First, fish and macroinvertebrate distribution and abundance were analyzed along a longitudinal transect. Species richness and abundance of fishes and macroinvertebrates were highest during late spring and summer, and were lowest during winter and early spring. Sites near the bay supported the most individuals and species. Canonical correspondence analysis indicated two distinct seasonal community compositions. Low temperature and salinity and high dissolved oxygen during winter-spring were associated with high abundance of spot, Gulf menhaden, and Atlantic croaker. When salinity, temperature, and vegetation cover increased during late spring-summer, bay anchovy, several killifishes, pinfish, spotted sea trout, and silver perch were more abundant. Prevalent harsh environmental conditions (e.g., low water depth, high temperature, and low DO) at upper reaches during summer months created spatial variation in community composition. In general, estuarine biota responded to seasonal variation more than spatial variation at the scales examined.; The second component examined seasonal dynamics of the food web based on gut contents analysis of fishes and invertebrates. Data were partitioned for analysis into two seasons (summer, winter). Red drum was the only species classified as a secondary carnivore. The low maximum trophic level obtained in this study emphasizes the importance of detrivores (e. g., striped mullet, Gulf menhaden, and macroinvertebrates) in linking detritus to the top predators, which suggests that energy flows efficiently from primary producers (detritus) to piscivores. Detritus, probably derived from Ruppia maritima, appeared to be the principal production source in this system, and the food web may be regulated by resources rather than consumers.; Species richness and number of trophic links tended to be higher during summer. Connectance and ratios of top, intermediate, and basal species varied little seasonally. The addition of links based on literature data increased web complexity, which suggests that food web properties strongly depend on methodology. Nonetheless, the Mad Island Marsh food web shared many properties with recently published food webs. |
