Effects of extreme flow events on community composition and habitat complexity of groundwater dominated systems
Extreme flow events in the form of major floods and droughts are primary drivers in structuring aquatic habitats and communities. While floods and droughts can directly alter aquatic biota by displacement or by increased mortalities, extreme flow events can have an indirect and prolonged effect on biota by altering habitat complexity, which in turn delays the recovery of the aquatic biota. Interrelationships among extreme flow events, habitat complexity, and fish communities are established in hydrologically variable rivers (run-off dominated). Less known are the interrelationships among stream flow extremes, habitat complexity, and fish communities in hydrologically stable rivers (groundwater dominated). The purpose of this study was to assess how extreme flow events effect habitat complexity and the fish community within two hydrologically stable rivers in Texas (San Marcos and Comal rivers) using a 9-year dataset. Habitats (N = 4,863) within 12 reaches of the rivers were categorized as high, moderate, or low complexity along a depth, vegetation, substrate, and current velocity gradient. Following or during extreme flow events, shifts in habitat complexity were not evident except in low complexity habitats that shifted towards greater complexity during drought. Among a total of 43 fish species and 135,199 individuals observed, densities of surface water fishes (Gambusia), pelagic generalist fishes (Lepomis, Micropterus, Herichthys), pelagic fluvial fishes (Dionda, Notropis, and Astyanax), and benthic fishes (Etheostoma) generally were greater in high and moderate habitat complexity reaches than in lower habitat complexity reaches and generally unaffected by extreme flow events with few exceptions. These results indicated that habitat complexity in hydrologically stable rivers support greater densities and diversity of fishes similar to hydrologically variable rivers, but habitat complexity and fish community were more resistant and resilient to extreme flow events in hydrologically stable rivers than in hydrologically variable rivers.
habitat complexity, extreme flow event, flood, drought, spring systems, groundwater dominated systems, fish
Tivin, J. (2023). Effects of extreme flow events on community composition and habitat complexity of groundwater dominated systems (Unpublished thesis). Texas State University, San Marcos, Texas.