Eutrophication, oxygen status and nutrient fluxes in a macrotidal estuarine reservoir: the case of the Ria Penfeld, Bay of Brest

Abstract

Excessive nutrient inputs are generally perceived to be responsible for the eutrophication of estuarine and coastal systems. Here, we show that the alteration of estuarine water circulation may be a very critical driver in the case of macrotidal systems. For this study, we intensively sampled the upstream part of the Ria Penfeld in June 2021, in a period of maximum irradiance and after a period of ~7 days of closure of the Kervallon dam gate. We observed that the physico-chemical structure of the Ria Penfeld reservoir (RPR), with very high biomass (chl-a values up to 120 µg L−1) and anoxic conditions in the bottom, was radically different from what is usually observed in the other estuaries of the region. The very high net primary production (~1700 mg C m−2 d−1) in the RPR was associated with an important removal of nutrients, especially phosphate, resulting in a strong nutritional imbalance (molar N:Si:P ratio of 2100:1500:1) for the water exported to the downstream estuary.

Keywords

• Estuary
• eutrophication
• dam
• nutrients
• primary production
• oxygen