Will small diversions of freshwater inflow affect water quality?

Freshwater inflow is a vital component of an estuary, as several ecological relationships exist between the level of inflow and aspects of estuary function. For future management, it is necessary to know how diversions of freshwater inflow may affect both water quality and ecosystem function within estuaries. The purpose of this study was to examine the effects of variation in freshwater inflow to make inference about water quality variables and estuary function during low inflow periods. This study focused on three bays: Carancahua Bay, San Antonio Bay (including Guadalupe Bay), and Tres Palacios Bay. Data was collected monthly via water quality sampling, and with continuous and discrete multiparameter sondes. Acoustic doppler current profilers (ADCPs) collected current speed and direction daily. Hourly precipitation and wind data was collected from the National Climatic Data Center (NCDC). Daily discharge was collected from two USGS flow gages. Freshwater inflow is responsible for driving nutrient concentrations and salinity ranges, as demonstrated by a principal component analysis. Based on results of this study, San Antonio Bay requires a large amount of freshwater inflow change (above 10,000 ac-ft/mo) to yield changes in water quality response, because it typically receives large volumes of inflow. Conversely, Carancahua Bay and Tres-Palacios Bay both require smaller volumes of freshwater inflow (less than 10,000 ac-ft/mo) to have large (i.e., 30% change) effects on water quality response because these bays receive lesser amounts of inflow. Freshwater inflow also alters net ecosystem metabolism (NEM) of an estuary. Freshwater inflow and salinity both had a significant but weak correlation to NEM when lagged, due to a time lag experienced between drivers and estuary response. The flow-to-water quality concept created in this study provides a generic framework that can be applied by managers and policy-makers to analyze how specific amounts of flow diverted from, or added to, specific bays may alter water quality conditions.