Polychaete assemblages as indicators of habitat recovery in a temperate estuary under eutrophication

Abstract

From 1980 to 2000, the Mondego estuary (Portugal) suffered dramatic changes due to eutrophication, leading to a decline in seagrass beds and changes in community structure, namely a decline in species richness and replacement of herbivores by detritivores. Because the conservation status of the estuary was being compromised, a restoration project was implemented in 1998 in order to restore the original natural seagrass habitat. This project included several restoration/management measures, including the protection of the remaining seagrass patches and improvement of the hydraulic conditions through modifying sluice opening regimes, thereby reducing the nutrient loading in the southern arm of the estuary. In addition, several stakeholder forums were convened to inform local people of the ecological and economic importance of the area. In this paper we evaluate the success of this management plan as judged by changes in the polychaete assemblages. In the year following implementation of the restoration measures, the total biomass of polychaetes increased markedly, a pattern also seen in the overall macrobenthic community. Biomass recovery was faster and more pronounced compared to density recovery, probably because many larger species were able to attain adult body size (e.g. Hediste diversicolor). The polychaetes Alkmaria romijni and Capitella capitata, considered as indicators of nutrient enrichment and pollution of estuarine systems, gradually declined over time. A general increase in species diversity was observed after the application of the restoration/management measures. These changes indicate that the restoration project has been successful and that polychaetes can be considered representative indicators of the wider macrobenthic community, with considerable savings in time and effort for monitoring programmes.