Effects of Catchment Development on the Trophic Status of a Deep and a Shallow Reservoir in Portugal

Abstract

Although catchment nutrient flux regulation is considered vital for controlling eutrophication, direct measurement of runoff loads is difficult. Shallow eutrophic Divor and smaller deep mesotrophic Apartadura, with different catchment uses but almost identical lake-to-watershed area ratios, were studied. Ratios suggest both reservoirs should be oligo- to meso-trophic. Divor catchment intensive livestock farming opposes to subsistence farming in Apartadura catchment. Runoff coefficients indicated that cattle accounted for 75% of TP mass export in Divor but only about a third in Apartadura, where goats and sheep dominated. In both watersheds agriculture, including pasture, dominated TP export over woodland/brush cover. Because of the large agricultural component, people contributed <1% TP loading to Divor and 2–4% to Apartadura, reflecting population densities. Despite differences in watershed use and lake morphometry, reservoirs showed similar behaviour (summer hypolimnion anoxia, photic zone algal biomass). Transparent Apartadura was dominated by large algal particles and probably P-limited, while turbid Divor contained surplus phosphorus because of large amounts of fine suspended clay particles. The differences in mean depth, not watershed loading, apparently accounted for most of the differences in trophic state. Based on results, management actions to reduce eutrophication in the two reservoirs will differ. Divor drainage will primarily require better manure disposal and fertilizer use reduction but will probably not show trophic state improvement. Apartadura drainage will require diffuse-source P-control (constructed wetlands, riparian buffers) and it should respond by regaining an oxygenated hypolimnion. Stakeholder voluntary cooperation as well as modifying people's perceptions regarding water resources will be needed to comply with the legally binding EU Water Directives.

Key Words:

• reservoir
• catchment
• in-lake processes
• phosphorus
• trophic state
• land use