Benthic communities in urban waterways may be influenced by changing nutrient loads, carbon sources, salinity, temperature, and dissolved oxygen that result from large-scale ecological or climatic changes. Traditional assessment of benthic health focuses on a triad analysis to determine impairment. The sediment quality triad assessment that is typically conducted to judge benthic impairment is the integration of benthic community metrics, sediment toxicity test results, and sediment chemistry. However, the potentially confounding impacts of larger scale ecological and climate driven changes are not included in the assessment. Consideration of these factors is of particular importance in coastal and estuarine urban areas where negative ecological changes such as eutrophication, oxygen deficiency, ocean acidification, and changes in salinity due to shift in river/precipitation input have already been documented. The challenge is to determine the ecological health of a benthic community given the multitude of impinging factors. In addition, commonly used categorical scoring systems do not allow the quantitative assessment of benthic conditions in the context of changing environmental parameters. We present a case study from Puget Sound (WA) using PSEMP data, on the use of statistical likelihood analysis, which allows an objective, quantitative, and easily interpretable determination of the likelihood of community impairment from several independent lines of evidence. In addition, we show the formulation of an environmentally responsive reference area that takes into account specific environmental factors influencing the benthic community such as depth, sediment grain size and total organic carbon. The likelihood analysis then evaluates the relative probabilities that the observed community condition is representative of either an unimpaired or an impaired condition. Further, the results from each line of evidence or biological response are quantitatively combined into an integrated weight of evidence for impairment.
Results/Conclusions
Likelihood-based statistical methods provided a direct assessment of the relative probability for each line of evidence and a means for their quantitative integration into a combined weight of evidence for benthic impairment for toxicity test results and benthic community metrics (abundance, richness and dominance). Results showed that while some areas had undergone benthic habitat degradation (e.g. Liberty Bay) others showed improvement (e.g. Commencement Bay) when an environmentally responsive reference value was taken into account.