Effect of freshwater inflow on macrobenthos productivity and nitrogen losses in Texas estuaries : final report

Abstract

The primary goal of the current research program is to define quantitative relationships between marine resource populations and freshwater inflows to the State's bays and estuaries. However, we know there is year-to-year variability in the population densities and successional events of estuarine communities. This year-to-year variability is apparently driven by long-term, and global-scale climatic events, e.g., El Nino, which affects rates of freshwater inflow. Therefore, this report documents long-term changes in populations and communities that are influenced by freshwater inflow. The best indicator of productivity is the change in biomass of the community over time. A secondary goal of the current research is to quantify loss of nitrogen in Texas estuaries. Nitrogen is the key element limiting productivity. A simple budget would account for nitrogen entering the bay via freshwater inflow, how it is captured and transformed into biomass, and finally how it is lost from the ecosystem. One aspect of nitrogen loss is very poorly understood: How much nitrogen is buried in sediments and lost from the system? We report here nitrogen content changes with respect to sediment depth. Presumably nitrogen is labile in the upper, biologically active, layers of sediment and refractory at depth. Therefore, it is important to determine the sediment depth at which nitrogen content is at a low and constant value. This study is a continuation of freshwater inflow studies that began in 1984. The goals have evolved over the years to reflect the synthesis of new information and the management needs of the Texas Water Development Board (TWDB). The original studies (1984-1986) were designed to determine the effect of inflow on Lavaca Bay. One station used during that study is still being sampled. San Antonio Bay was studied in 1987, and the Nueces Estuary (Nueces and Corpus Christi Bays) were studied in 1988. Long-term studies of the Lavaca-Colorado and Guadalupe Estuaries began in 1990. Our initial conclusions based on one to four years of data were that inflow does increase benthic productivity (Kalke and Montagna, 1991; Montagna and Kalke, 1992; 1995). However, later analysis of the data set over a 5-year period demonstrated that the largest effect may not be on productivity, but may be on community structure (Montagna and Li, 1996). This implies that reduced inflows may not only reduce productivity but may also change the composition of species in an estuary. The complete long-term record now extends over nine years. The completion of this research will take 12 to 20 years, because the trends are driven by long-term climatic events controlled by global climate patterns, e.g, El Nino.