| Estuaries represent a continuum ranging from freshwater to marine water, influenced by the amount of freshwater inflow from tributary rivers. Freshwater inflow influences salinity, dissolved oxygen, nutrient transport, and sediment loading. Growing human populations have placed greater demands for freshwater for use in agricultural irrigation, industrial processes, and drinking water. When river water discharges are modified and reduced to meet these demands, this has an impact on the estuarine biota downstream as well as instream uses by riverine fauna. The impact of modified freshwater inflow on various aspects of estuarine ecology including salinity, nutrients, sediment, primary productivity and nekton communities have been studied in many estuaries around the world, including those in the Gulf of Mexico. Many of these are classical lagoon or barrier island type estuaries; however, few studies have been conducted in "riverine" type estuaries. In the Northwestern Gulf of Mexico, there are only three riverine estuaries: the Brazos, lower Colorado River and lower Rio Grande.;Research was needed to determine if (1) water quality and nekton communities in the lower Brazos River have changed since last rigorously surveyed nearly 40 years ago; (2) how these subtropical communities have changed both temporally and spatially in terms of areas of the river utilized; and (3) how Brazos River communities were affected by alterations in freshwater inflow and associated water quality variables. This data is critically needed by resource managers to understand the impact that has been made on the riverine, estuarine and near shore marine ecosystem due to changes in freshwater inflow and water quality management.;Freshwater inflow from the current study on the Brazos River was compared to historical data compiled by the United States Geological Survey (USGS) and Texas Water Development Board (TWDB). The temperature, salinity and dissolved oxygen data from our current study period were compared to historical data collected by Texas Parks and Wildlife (TPWD) during the 1970's to determine how similar the water quality data was for each time period. Regression analysis was used to compare differences in biological responses (abundance, richness, and diversity) to gradients in temperature, salinity, turbidity and dissolved oxygen. Multivariate analyses were used to compare species assemblages, hydrology, and water quality between time periods and locations.;Research began on the Brazos River in January of 2012 and concluded in December of 2012. Based on this current data we determined that: 1) recent nekton assemblage at the mouth of the river exhibited 60% similarity with communities sampled at the same site during the 1970's; 2) these changes in the nekton community are mostly regulated by freshwater inflow; 3) diversity and richness was highest at the sites closest to the Gulf of Mexico; and 4) several mechanisms are likely responsible for these observed patterns including freshwater inflow directly affecting organisms via altered salinity regimes, and indirectly through modification in sediment transport and nutrients. The relative influence of freshwater inflow on overall nekton diversity and productivity during short and long time periods and the potential impacts on nearshore marine water productivity and utilization by estuarine and marine organisms are discussed. |
