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).
Description
A Thesis Paper Submitted in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE in the Environmental Science Program at Texas A&M University- Corpus Christi in Corpus Christi, Texas.