Influence of Freshwater Inflow in the Brazos River Estuary

The influence of freshwater inflow on estuarine ecology is a topic that receives a lot of attention due to estuaries acting as medians to oceanic and freshwater environments. Freshwater inflow has varying effects depending on the morphology of the estuary, and the Brazos River is one of only three rivers in Texas with a riverine estuary that discharges directly into the Gulf of Mexico. The Environmental Institute of Houston (EIH) has been collecting data to categorize the ecology and hydrology within the estuary since 2012 when the Brazos Basin and Bay Area Stakeholder Committee (BBASC) formally addressed the need for research assessing the impacts of freshwater inflow. Project objectives included: (1) Describing the temporal variation in hydrology in the lower Brazos River, (2) evaluating the relationships between nekton community, freshwater inflow, seasonality and water quality using graphical and statistical methods, (3) characterizing nekton abundance, diversity, and community composition, (4) identifying focal species for different sites, flow tiers and seasons, and (5) identifying any future research needs. Data acquired from the United States Geological Survey (USGS) gage #08116650 was used in order to assess the normal trends of variation in discharge (cfs), as well as determine if data utilized far upstream from the estuary could be used to accurately predict discharge and water quality downstream. Automated monitoring loggers were used to collect long-term data for salinity (psu), temperature (°C), dissolved oxygen (DO) (mg/L), and depth (m) beginning in 2014 in order to create predictive models from daily average discharge. The results indicated that discharge in the Brazos River does exhibit predictable seasonal patterns of increased flow during the spring and reduced flow during the summer, yet still exhibits a huge degree of variation within seasons and between years. The Brazos River is also subject to extreme flow conditions at an increasing rate despite having an average annual discharge of only 7,400 cfs. The regression analysis from the automated loggers and USGS discharge data indicates that salinity and water depth are strongly correlated to flow and react in the form of exponential decay and sigmoidal growth respectively. Temperature proved to not be significantly correlated with flow, but multiple linear regression analysis with DO data demonstrated a complex relationship with flow and temperature. Two-way ANOVAs were used to determine if variations in water quality variables- temperature, salinity, DO, pH, turbidity (NTU), thalweg depth (m), and Secchi disk transparency (m) could be explained by sites, TCEQ flow tiers, or an interaction between both. The results showed that all variables exhibited significant differences (p ≤ 0.05) between flow tiers and sites- the only exception being temperature. The only two variables tested with a significant interaction effect between flow tier and site were salinity and pH. Principal component analysis was also used to determine whether season, flow tier, or spatial differences could explain the variability in water quality between samples. The results indicated that season is the primary driver of variability due to temperature having the highest eigenvector coefficients for each principal component calculated for surface and bottom profile. Pearson correlation analysis was also used to determine how water quality could be used to predict nekton community diversity. Salinity proved to be the most significantly correlated to nekton communities sampled using both an otter trawl and beam trawl. Nekton community metrics in the form of total catch, species richness, Shannon-Wiener diversity, Shannon evenness, Margalef Richness and catch per unit effort (CPUE) were subjected to nonparametric tests in order to test for significant differences between season, site, and flow tier. The results indicated the larval fish community sampled with the beam trawl near the shore was significantly affected by season, while spatial differences and flow tier explained the variation in nekton communities sampled mid-channel with the otter trawl. The results of this study corroborate many of the conclusions drawn from earlier studies on the ecology of the lower Brazos, as well as provide additional evidence of the highly dynamic nature of the Brazos estuary, while also providing further justification for continued and expanded research.