A multimetric nekton index for monitoring, managing and communicating ecosystem health status in an urbanized Gulf of Mexico estuary

Abstract

Biological assessments have been used for decades to determine ecological conditions in aquatic environments, yet they have not been extensively applied in estuaries that serve as transition zones between freshwater and marine environments. We present the development and validation of a nekton (fish and selected macroinvertebrate) index for annual monitoring of ecosystem health in Tampa Bay, Florida. We relied on long-term fisheries independent monitoring data of the early recruit and juvenile life history stages of nekton in Florida’s inshore waters. A set of metrics that included measures of abundance, species diversity, trophic structure, and taxa of commercial or recreational importance were explored, and a subset was selected via statistical models. Reference conditions specific to each season and management section of the bay were established from the long-term dataset. The final Tampa Bay Nekton Index included five metrics: the total number of taxa, the number of benthic taxa, the number of recreational/commercial fishery taxa, the number of feeding guilds, and the Shannon-Weiner diversity index. Nekton index scores were calculated for each sample and averaged by bay management section and year and then a “stoplight” color-coding system, based on quantiles, was used to group index scores for communication and management. In general, Tampa Bay’s nekton community appears to be resistant to large-scale changes in functional structure. The index was sensitive to a prolonged red tide event but eventually returned to preperturbation levels, indicating nekton community resilience. This index will be incorporated into monitoring and managing strategies of the local estuary program. Because this index was developed specifically for Tampa Bay and relies on bay-specific reference conditions, the index cannot be directly applied to other systems, but the methodology is transferrable so similar indices could be developed for other ecosystems with long-term monitoring data. Furthermore, using regional data, the index could be expanded/developed to assess health status among estuaries to inform decisions on prioritization of limited resources.