Final report : studies of freshwater inflow effects on the Lavaca River Delta and Lavaca Bay, Texas

Abstract

The Texas Water Development Board (TWDB) is concerned with management of surface freshwater resources. They must plan on water use by urban populations, industry, and agriculture. In addition they must also consider the needs of Texas bays and estuaries that have evolved to receive freshwater input. In order to better understand these needs, the TWDB has been conducting and sponsoring research on the freshwater requirements of bays and estuaries in both impounded and non-impounded drainage basins. The TWDB contracted with the University of Texas at Austin's Marine Science Institute (UTMSI) for one such project. Officials from TWDB and UTMSI met and worked out the components of a two year, multidisciplinary study on selected sites in the upper Lavaca-Tres Palacios Estuary and parts of Matagorda Bay. Data were collected on 14 sampling trips between November 1984 and August 1986. The primary goals were to obtain an environmental assessment of the upper Lavaca Bay after completion of the Palmetto Bend reservoir project on the Navidad River (dam closed in 1980, forming Lake Texana); and to document the use of the lower river delta as a nursery area for finfish and shellfish. The study had several components that are reported as separate chapters within this report. The broad objective of this and similar studies is addressed by three questions. What happens when freshwater is introduced into the estuary? What happens when freshwater is withheld from the estuary? How much freshwater must be introduced to forestall the negative effects of withholding it? These questions have little meaning unless there is a clear understanding of what processes are being studied and what temporal and spatial scales are being considered. There is a crucial relationship between the scales of physical forcing and biological response that is dependent on the generation times and mobilities of the organisms in question (Haury et al. 1979; Lewis and Platt 1982). Because the diverse biological components of an estuarine ecosystem have vastly different lifespans and capacity for movement, the answers to the three questions above would depend in large part on ecological perspective. A reasonable approach would be to look at the temporal scale of a year and the spatial scale encompassing the drainage basin. Appropriate biological components for study would include larger organisms that integrate their environment and may have some economic importance: finfish, shrimp, and benthic macrofauna. Unfortunately, many of the effects of physical forcing (i.e. freshwater input) on higher trophic levels are likely to be indirectly expressed through influences on the productivity and taxonomic composition of food resources. So, to answer our three questions in the appropriate context for management (relatively long term, large scale, higher trophic levels), it is necessary to answer the same questions for lower trophic levels on appropriate scales for each biological component. In addition, the nature of biological coupling between producers and consumers must be determined. For example, what is the relationship between primary production and fish production? The problem assumes immense proportions. Ideally, a study of the freshwater requirements of an estuary would look at statistical relationships between state variables (e.g. salinity, chlorophyll, zooplankton abundance, fisheries yield) and also the dynamic processes linking the variables (e.g. light-limitation of primary production, feeding habits of juvenile fish, etc.). This two-year study with approximately bimonthly sampling was by necessity constrained. Systematic sampling provided good records of a large number of variables over limited temporal and spatial scales but process-oriented studies were beyond the scope of the contract. Many process-oriented questions are now being addressed in a project recently initiated in San Antonio Bay.From the University of Texas at Austin Marine Science Institute, Port Aransas, Texas ... to Texas Water Development Board ... Austin, Texas ...December 1986Marine Scienc