Drought Effects on the Community Structure of Two Tidal Streams within the San Antonio-Nueces Coastal Basin, Texas: A Multivariate Ecological Assessment of the Mission and Aransas Rivers

Tidal streams are highly productive transitional areas between the freshwater of rivers and the increasing salinities in the estuary, and these areas can serve as important nursery grounds for many fish and shellfish species.  Routine monitoring of tidally influenced segments throughout Texas has revealed that water quality standards are not being met, as dissolved oxygen (DO) measurements in these segments are routinely lower than the established criteria.  Our goal was to further the development of a standardized methodology for assessing ecosystem health within tidal segments.  This new methodology, which relies heavily on multivariate ordination techniques, has successfully been used to recommend aquatic life uses on five different tidal segments throughout the state.  The Mission and Aransas Rivers were studied seasonally from 2007 to 2009 for chemical (physiochemical profiles; 24 hour datasonde deployments; and water / sediment samples); physical (instream flow; landcover / land use; and instream / riparian habitat classification); and biological (nekton sampled with bag seines and trawls; sediment and benthic macroinvertebrate / infaunal) components of ecosystem health.  Climatologically, this study was conducted during an exceptional drought period, and the effects of wind-driven salinity intrusions had a dramatic effect on water quality readings.  Low DO conditions were noted, with hypoxic and at times anoxic conditions encountered in the bottom waters on each study stream. Water chemistry data revealed that proxies for inflow (e.g., chloride, total dissolved solids, total phosphorus, and orthophosphorus) were most important in defining differences between the two study streams.  The nekton communities were comprised of a mixture of highly euryhaline/marine taxa (Clupeidae, Engraulidae, Sciaenidae, Penaeidae, Paleomonidae, Portunidae), and for a number of these taxa, differences in nekton abundance were far more affected by salinity than any other physical or chemical parameter measured, irrespective of season.  The benthics were dominated by polychaetes, oligochaetes, and chironomids, and these groups were common across the salinity gradient-based station design.  The dramatic calendar year variability seen, resulting from the drought, cut across the many different levels of ecological integrity that were measured for this study.  The DO regimes within both streams were heavily influenced by the interaction of temperature, (a general lack of) precipitation, nutrient-loading, and salinity stratification.  Despite the large spatial extents of hypoxic bottom waters, there was little to no relationship between DO measurements and overall community structure.