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Abstract:
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Estuarine wetlands and salt marshes are fundamentally driven by variations in freshwater inflow . In semi -arid salt marshes , such as the Nueces River Delta , TX , the stochastic nature of freshwater inflow events exposes resident organisms to a wide range of environmental conditions . In this study , we investigate (1 ) the relative importance of environmental variables on porewater salinity and (2 ) determination of freshwater inflow needs based on the response of emergent plants to salinity variations . Porewater salinity variations were tracked on a continuous basis with deployed conductivity sensors and on a synoptic basis with soil water extracts . We found that spatial patterns of porewater salinity were characterized by a high degree of variability in creekbank areas (23 .8 ± 7 .68 ) relative to interior marsh areas (44 .2 ± 3 .4 ) . Our observations were used to test a simple model capable of predicting porewater salinities based on environmental variables . Both empirical measurements and model simulations indicated that semiannual tides play a critical role in controlling porewater flushing from precipitation and freshwater inflow events .
Estimation of freshwater inflow needs for the Nueces Delta proceeded in two steps . First , we examined the response of three common emergent plants species (Borrichia frutescens , Spartina alterniflora , and Salicornia virginica ) to variations in salinity . The abundance of one species in particular (S . alterniflora ) was tightly coupled to salinity variations whereby salinities exceeding 25 ± 5 resulted in dramatic declines in coverage . Next , the relationship between freshwater inflow and porewater salinity was examined with respect to the salinity “tolerance” of S . alterniflora . Estimated inflow needs based on maintenance of substantial ( > 20 % ) S . alterniflora coverage was comparable to both previous inflow needs estimates and mean annual inflows observed over the course of the study . The results of this study suggest that S . alterniflora abundance provides a reliable indicator of overall estuarine hydrological condition in the Nueces Delta . |