Ecological Succession and Fragmentation in a Reservoir: Effects of Sedimentation on Habitats and Fish Communities

摘要

While processes of depositional filling and ecological succession in natural lakes have been well described, these concepts are relatively new and seldom applied to reservoirs, especially at the landscape scale. However, ecological time has been sufficient to allow us to see successional processes in many reservoir systems. Illustrative of such processes, Lake Texoma is a 36,000-ha reservoir located in southern Oklahoma and northern Texas, and patterns of depositional filling and subsequent processesare apparent in the up-lake ends (there are two large-river tributaries) of this system. Completed in 1944, Lake Texoma has a drainage area of more than 100,000 km~2, most of which is highly erodable agricultural lands. We used historic aerial photographs, geographic information systems technology, and field measurements to examine a variety of surface and habitat features and analyzed experimental gill-net samples using ordination techniques to characterize the fish communities in portions of the reservoir most affected by sedimentation. Extensive sedimentation and accretion of sediments above water level has effectively resulted in surface area reduction, cove isolation, fragmentation of lacustrine habitats, morphometric changes, and establishment of terrestrial vegetation on newly deposited lands. Most notably, sedimentation has led to the development of linear bars of deposition above normal pool elevation that have blocked mouths of coves, bisected large areas of the reservoir, and fragmented several pools. In our study site alone, 332 ha (surface area) of reservoir has experienced accretion of land above the water level. Reservoir fragments had lower shoreline development values (mean = 2.21) than comparable control sites (mean = 3.39). Depositional shorelines associated with sedimentation exhibited lower gradients than nondepositional shorelines (mean = 2.0% versus 4.2%, respectively), and habitat heterogeneity was lower along depositional shorelines than along nondepositional shorelines. Fish communities in isolated reservoir fragments appeared to be distinct from fish communities in nonfragmented habitats. This change in community structure may be driven by an appreciable reduction of pelagic species from fragmented sites, as these siteshave limited or no connectivity to the main body of the reservoir. With respect to the newly deposited lands, ecological succession of vegetation followed a progression from mud flats to dense, nearly monotypic stands of black willow Salix nigra forestswithin a few years. These habitat changes had strong implications to the fish communities as well as to adjacent terrestrial wildlife communities and will likely pose many challenges, and perhaps opportunities, for natural resource managers.