Coastal Plain Rivers of the Southeastern United States I
Presiding: A Benke, University of Alabama; S W Golladay, Joseph W. Jones Ecological Research Center at Ichauway
NB41F-01 INVITED 08:30h
An Overview of Hydrology, Habitats, Ecology, and Human Impacts in Coastal Plain Rivers of the Southeastern United States
Coastal Plain rivers of the southeastern United States have a different physical template than most conceptual models of rivers, and such differences have a major influence on their ecological characteristics. Although some rivers originate in adjacent mountains or foothills, others arise entirely within the Coastal Plain. Regardless of origin, rivers from Texas to Virginia have many common features. Their Coastal Plain sections have a low gradient, and a tidewater influence extends many kilometers upstream. River beds are primarily shifting sandy beds, but submerged wood (snags) provides sites of high invertebrate diversity and production and a source of cover and food for fish assemblages of close to 100 species. Highest mean discharge occurs in winter and spring, the result of seasonal differences in evapotranspiration, resulting in extended (weeks or months) flooding of wide (several kilometers) floodplain wetlands that provide essential habitat for invertebrate and fish assemblages. Although some Coastal Plain rivers have been spared the impact of damming, those originating in mountains and foothills are typically dammed at and above the Fall Line, resulting in flow regimes that reduce essential floodplain interactions. Snagging and flow regulation have undoubtedly combined to greatly reduce diversity and productivity of most of these rivers.
NB41F-02 INVITED 08:45h
Ecological Importance of Tidal Fresh Waters: Longitudinal Patterns in Biotic Diversity in Mid-Atlantic Coastal Rivers
Tidal freshwaters represent the interface between non-tidal freshwater habitats and estuaries and are a substantial proportion of the coastal habitats on the Atlantic Slope. They typically are considered depauperate compared to upstream riverine and downstream estuarine habitats. The objective of this study was to quantify patterns in fish and macroinvertebrate community composition and biodiversity along the James River, Virginia continuum, testing the prediction that the tidal freshwater section supported a unique species assemblage lower in biodiversity than the other sections of the river. A second objective was to quantify the impact of non-native species of fishes on patterns of community composition along the river continuum. Data were collected from nearly 50 locations along the river. Multivariate analyses identified fish and macroinvertebrate assemblages unique to specific river reaches. The results show that species assemblages shift along a longitudinal gradient. Contrary to predictions, species richness of fishes increased in a downstream direction but peaked in the tidal freshwater reach. Occurrence of non-native species, particularly piscivores, influenced fish community structure along the entire river. The results indicate that tidal freshwater reaches combine characteristics of both riverine and estuarine ecosystems in a manner typical of an ecotone or habitat edge.
NB41F-03 INVITED 09:00h
The St. Johns River, Florida: a Unique Lentic/Lotic Waterbody on the Southeastern Coastal Plain.
The St. Johns River is a 500-km, low-gradient, black water river located in northeast Florida. Tidal effects, reverse flow events, and numerous saline springs result in estuarine conditions sometimes extending far upstream. Invertebrate sampling has occurred aperiodically in the lower mainstem of the river since 1974. In general, fresher areas of the river exhibited higher taxa richness but relatively little difference in diversity. The river shows differences in invertebrate populations throughout the lower estuarine section related to salinity. The watershed is facing tremendous development pressure and is subsequently challenged by typical problems, although little historical change was detected using this 30-year dataset. However, data suggested that there was evidence of localized impairment to the benthic community. Finally, areas with submerged aquatic vegetation (SAV) demonstrated higher richness and diversity than comparable bare areas, underscoring the importance of SAV to the health and productivity of the river.
NB41F-04 INVITED 09:15h
Fish and Invertebrate Assemblages along Hydraulic and Land Use Gradients in the San Jacinto River - Coastal Perspectives on Instream Flow and Habitat Protection.
The San Jacinto River system lies just east of Houston, Texas in the Coastal Plains. The watershed is bounded on the north primarily by rural and agricultural land, on the east by the Sam Houston National Forest (SHNF), and on the west by the city of Houston. The SHNF is made up of fragmented forest imbedded in a matrix of rural farm land and oil and gas development that leads to increased nutrient supplies and unexpected brine spills into streams of the East Fork San Jacinto. Two major impoundments, Lake Conroe on the upper West Fork of the San Jacinto, and Lake Houston on the main river below the East Fork, were constructed primarily to supply water to the Houston Metropolitan area. A valuable fishery exists in Lake Conroe, however off-channel gravel dredging upstream of Lake Houston appears to have increased erosion and led to turbid river conditions. This has a major influence on the ecosystem, such that the fishery in Lake Houston is less productive than that of Conroe, and the level of chemical treatment to flocculate suspended silt and provide potable water is unattainable without exceeding permitted limits for flocculants. This has repercussions to other watersheds. Coastal reservoirs are being built to capture presently unallocated water during high-flow events, and water transfer projects from other drainages are being built or planned in order to provide water to Houston. Solutions to such problems will require cooperation among stakeholders from a range of disciplines. We present this small coastal system as a model for the complex direct and indirect effects on coastal river systems that have not only ecological but major economic and human health impacts.
NB41F-05 INVITED 09:30h
Diversity and Life Histories in Freshwater Mussel Communities of the Gulf Coastal Plain, USA
The Gulf Coastal Plain supports a diverse mussel fauna including many endemic species. Richness among drainages was associated strongly and positively with watershed size. Assemblage similarity among drainages identified three major faunal groupings: Pontchartrain-Pearl-Pascagoula-Mobile; Escambia-Choctawhatchee; and Apalachicola-Ochlockonee-Suwannee. The Escambia-Choctawhatchee showed greater affinity to the Apalachicola than to the Mobile Basin. Patterns of mussel assemblages among drainages were associated strongly with fish assemblages suggesting two non-mutually exclusive hypotheses: 1) biogeographic history affected both groups similarly, and 2) the fish host relationship was important in shaping mussel communities. Based on interspecific variation in life history traits including host use, longevity, offspring size, and fecundity, we established seven guilds to represent regional diversity in life history strategies. The number of guilds decreased from west to east indicating reduced ecological complexity. For widely represented guilds, drainages showed either 1) similar guild composition because of replacement by ecologically similar species, or 2) a shift in dominance among guilds along a west-east continuum. This dichotomy cannot be reconciled currently because data are lacking for numerous species of Elliptio, a dominant genus in eastern Gulf Coastal Plain mussel communities. This information gap illustrates the abundant opportunities for ecological research in the region.
NB41F-06 INVITED 09:45h
A Mosaic of Riparian Landforms Determine Wood Debris Recruitment and Redistribution During Floods in a Southeastern Coastal Plain Stream
In southeastern Coastal Plain streams, wood debris can be very abundant and is recruited from streamside forests. Since 1994, measurements of downed trees (species, dbh, orientation, distance from baseflow channel, and condition) were made across replicated riparian landforms in a 5th order stream. Annually, the fate of these trees was determined and newly recruited trees were noted. More constrained reaches had greatest mortality. Generally, recruitment was greatest in years with substantial floods. Tree mortality was correlated with the maximum daily flow during the period preceding surveys. This relationship was strongest for sand ridges (r2=0.942) and terraces r2=0.915). Flood characteristics also influenced debris recruitment. A 1994 flood caused by a tropical storm caused a rapid rise in stream flow. Much of the debris recruited during this flood was from toppled trees and was oriented parallel to the stream channel. A 1998 flood had more gradually rising flows with no relationship between landform and debris characteristics. These results indicate that wood recruitment dynamics in Coastal Plain streams are complex. Wood recruitment rates are controlled by cyclical variations in climate interacting with a mosaic of riparian landforms. Infrequent floods are critical in the maintenance of the instream debris pool.