Ecosystem response to freshwater inflow: determining a link between freshwater pumping regimes, salinity, and benthic macrofauna

Abstract

The Nueces River Basin is one of the 15 major river basins in Texas, and is an important water supply for the Nueces-Rio Grande Coastal Basin area. The construction of two large reservoir dams in the Nueces River Basin has reduced the amount of freshwater reaching the Nueces Estuary by 99% from that of its historical flows. The reduction of freshwater to the marsh has created a reverse estuary condition, where lowest salinity values are near Nueces Bay and the highest are in the upper delta. The City of Corpus Christi has been required to provide not less than 185 million cubic meters (151,000 ac-ft} of water per year to the Nueces Estuary by a combination of releases, spills, and return flows to maintain ecological health and productivity of living marine resources. The City constructed a pump station and pipeline (RBP} to convey up to 3.7 x 10^6 m^3 (3,000 ac-ft) of freshwater directly into the Nueces Delta at Rincon Bayou. Inflow into Rincon Bayou is dependent upon pumped inflow with salinity and depth regimes in the Nueces Delta being controlled through management release actions. Haphazard pumping release, along with drought conditions, cause the salinity in Rincon Bayou to fluctuate from fresh to hypersaline, and hypersaline to fresh in very short time periods. The presence of benthos was represented by indicator species that were determined by the most numerically dominant species: Streblospio benedicti, Chironomidae larvae, and Laeonereis culveri. The biological responses of the indicator species to three physical variables (salinity, temperature, and depth) were examined. The optimal ranges in Rincon Bayou during the current study were determined by combining the ranges for the indicator species. The optimal salinity was between 1 and 15 psu for biomass and 1 and 14 psu for abundance, and the optimal depth range between 0.05 m and 0.2 m (2 - 7.9 inches}. There are ·several management recommendations that can be made for Station C in Rincon Bayou: 1) to improve ecological stability: inflows should be a trickle, not a flood, releases should be continuous and not haphazard, only one pump should be used at a time, and releases should no' t be dependent on pass-through requirements; 2} to maximize ecological function: salinity should be maintained under 20 psu, and water depth should be maintained between 0.05 m and 0.2 m; 3) to maintain ranges: inflows rates on the order of ≥ 0.00102 m^3/s (0.084 ac-ft/day) are required to maintain salinities ≤ 20 psu, inflows on the order of ≤ 0.689 m^3/s (48.261 ac-ft/day) are required to maintain a depth ≤ 0.5 m, and inflow on the order of 0.41 m^3/s (28.72 ac-ft/day) will obtain an optimal value for both salinity a·t 2.2 psu and depth at 0.2 m (7.9 inches).