Developing a bioassessment framework to inform tidal stream management along a hydrologically variable coast

Tidal streams are spatiotemporally varying areas that encompass tidally influenced limnetic and oligohaline zones within estuaries. These areas are important for many biogeochemical processes and for the life cycles of many fishery species. However, tidal streams are also susceptible to impairment from coastal development and watershed-derived runoff, which potentially affects faunal assemblages within the ecosystem. This study developed indices of biotic integrity (IBIs) for nekton and benthic macroinfauna in tidal streams along the southern Texas coast. Fifteen tidal stream sites with mean salinities ranging from 0.4 to 11.9 were classified as degraded if their surrounding land use was > 20 % urban or agricultural, watershed population density was > 50 km−2, and nutrient and chlorophyll concentrations exceeded specific screening limits. Otherwise, sites were classified as reference. Nekton and benthic macroinfauna communities were then sampled at these fifteen stream sites in 2020 and 2021. Historical metrics and metrics derived from multivariate analyses were considered for inclusion in the IBIs, and were assessed for collinearity, redundancy, suitability for score assignment, and agreement with historical literature. Nine univariate nektonic metrics (including total abundance, number of invertebrate taxa, and the percent abundance of five species, one family, and one functional group) and six benthic macroinfauna metrics (including Shannon’s diversity, total abundance and biomass, and the percent abundance of two taxa and one functional group) were incorporated into separate nektonic and benthic IBIs. Mean IBI scores of reference sites were greater than degraded sites by 42 % for nekton and 30 % for benthic macroinfauna. Seven out of eight reference sites had greater mean nekton IBI scores than the mean scores of all seven degraded sites, while four of eight reference sites had greater benthic IBI scores than all degraded sites. However, overlap in the ranges of scores calculated for degraded and reference sites occurred, which is likely caused by spatiotemporal variability, including stream size variation and the changing climatic and biogeographical gradient along the southern Texas coast. The IBIs developed in this study represent an important preliminary step in bioassessment development for Texas tidal streams, and will help to provide a useful tool for coastal environmental management.