Detecting water quality improvement along the Catalan coast (Spain)using stress-specific biochemical seagrass indicators

Abstract

Evaluating the efficacy of management actions to improve environmental quality is often difficult becausethere may be considerable lags before ecosystem management actions translate into measurable indi-cator responses. These delays make it difficult to justify often-expensive remedial actions to preventeutrophication. Therefore, it is critical to identify reliable, rapid and sensitive indicators to detect degra-dation and environmental quality improvement. We evaluate the efficacy of a set of indicators basedon the seagrass Posidonia oceanica to reliably and quickly detect ecosystem improvements using a 7-year (2003–2010) dataset of 10 stations along the Catalan coast (north-western Mediterranean Sea). Inthe Catalan region, environmental agencies have invested heavily on wastewater treatment, resultingin significant reductions (ca. 75%) in the BOD5discharged to coastal waters from 2003 to 2010. Theseimprovements were clearly reflected at the regional level (i.e. for all the stations averaged) in six bio-chemical seagrass indicators from our dataset. These indicators were directly related to eutrophication(nitrogen, 15N, phosphorus and total non-structural carbohydrates content in rhizomes, 34S and 13C inseagrass rhizomes and N content in epiphytes). In contrast, seagrass structural indicators, related to sea-grass abundance or meadow structure (density, cover) did not show any sign of overall recovery duringthe monitored period. These results confirm that biochemical seagrass indicators are the most sensitiveto water quality improvements within management time-scales (7–10 years) for slow-growing specieslike P. oceanica. Given the budgetary restrictions under which most management actions operate, theavailability of decision-support tools that function at appropriate time-scales is crucial to help managersvalidate the relative success of their remedial efforts. Our results indicate that low inertia, biochemicalseagrass indicators fit this task, and can be a robust set of tools to include in monitoring programmes.