Applying quality status criteria to a temperate estuary before and after the mitigation measures to reduce eutrophication symptoms

Abstract

The Mondego estuary is a well-described polyhaline type of transitional water located at the North Atlantic Ocean Ecoregion, where cultural eutrophication progressed over the last decades of the 20th century. Consequently, and due to huge productivity of Ulva spp. Zostera noltii meadows were severely reduced causing the whole ecosystem to become impoverished in terms of macrofaunal abundance, biomass and species richness with a concomitant lowering of secondary production. In 1998, experimental mitigation measures were implemented, via changes in hydrology to increase circulation and diversion of nutrient rich freshwater inflow, to reverse the process in the most affected area of the estuary - its south arm. Thus, the system quality status was assessed before and after 1998, over a ten year period. The OSPAR comprehensive procedure, the first phase of the US-NEEA procedure and the proposed EU-WFD physicochemical status criteria were applied to the data before and after the modifications and all show that the system health has improved. Nonetheless, the annual means of the oxidised forms of nitrogen and of phosphate were not reduced. In fact, applying criteria used in classifying the nutrient levels in transitional waters and the Baltic sea trophic condition, the system has not improved. Meaning that, to look forward to a "higher" quality status, future measures should also consider longer term solutions such as improved agriculture practices in the Mondego River valley through environmental friendly technological solutions that will reduce the nutrient loads to this system.