The effects of changes to estuarine hydrology on system phosphorous retention capacity: the Mondego estuary, Portugal

Abstract

The Mondego estuary is a mainly polyhaline estuary in central Portugal in which eutrophication increased during the last decades of the 20th century. In 1998 the system hydrology was changed, aiming to reverse the eutrophication process. A long environmental monitoring database showed that the mean concentrations of dissolved inorganic phosphorus (DIP) increased by 50%, due to the increase in winter concentrations, i.e., in periods characterized by lower temperature and lower salinity. Given existing background knowledge, especially on the system hydrodynamics, phosphorus dynamics and phosphorus speciation in the estuary, we framed the hypothesis that the significant increase of DIP could be related to different sorption capacities of the sediments. The results highlighted two scenarios: i) Before 1998 the nutrient-rich freshwater input from the upstream cultivated lands entered the system through the area with the highest PO4–P adsorption capacity (Q∗ = 657 μg P g−1 wwt), thus the PO4–P availability in the water column was mostly dependent on the mineralization processes, which is in agreement with previous findings; ii) After 1998, the water residence time diminished from moderate (weeks) to short (days). This change coincided with a diversion of the water to an area with a much lower PO4–P adsorption capacity (Q∗ = 410 μg P g−1 wwt), which represented a decrease in 7.3% of the system adsorption capacity. This means that sediments were not able to adsorb much of the PO4–P and a higher equilibrium concentration occurred in the water column. The sorption study proved to be a valuable tool in testing our hypothesis and provided essential information on the mobility of PO4–P from soils/sediments to the water column.