Human Impact Assessment on the Subtidal Macrobenthic Community Structure in the Mondego Estuary (Western Portugal)

The Mondego Estuary consists of two arms, north and south. The north arm is deeper, while the south arm is largely silted up, which causes the freshwater to flow primarily by the north arm. Human impact is significant, due to harbour activities, salt-works, aquaculture farms, and nutrient and chemical discharge from agricultural areas. From December 1989 to September 1990 a reference study on the subtidal macrofauna was carried out, in order to characterize the community structure in relation to physicochemical environmental factors. The subtidal macrofauna in the Mondego Estuary appears to be clearly impoverished compared to a reference Portuguese estuary, the Mira, which has been much less exposed to human impacts. The south arm macrobenthic community consists mainly of infaunal species and appears to be more stable and structured, presenting a higher macrofauna abundance. The north arm community is mainly characterized by sparse mobile epibenthic species populations, exhibiting a lower biodiversity and an impoverished macrofuna, compared to the south arm. The subtidal communities appear to be physically controlled, with emphasis on the type of sediment, salinity and currents. Biologically, due to their distinct physicochemical characteristics, the two arms of the estuary constitute different subsystems. Dredging takes place twice a year along the north arm, and time intervals between dredging operations appear to be inadequate to allow macrofauna recovery. Although structurally more stable, the south arm community, due to the feeble water circulation, may be more exposed to environmental changes. Benthic monitoring of the Mondego Estuary biological communities is clearly necessary in order to assess temporal trends and to establish if the apparent environmental changes are reversible