Effects of land use changes on streams in terrestrial-aquatic palm ecotones (Morichals) of the Orinoco lowlands

Abstract

Streams running through terrestrial-aquatic palm ecotones (Morichals) are widely distributed in the Orinoco basin and the Brazilian Cerrado; however, little is known about how these ecosystems respond to human actions. We investigated the effects of land use changes on water chemistry and benthic chlorophyll in five loworder streams in the Orinoco lowlands, Venezuela. Current land use and land use changes from 1977 to 2007 were analyzed for the ecotone and catchment areas above each sampling site. At monthly intervals over a 16-month period, discharge was measured and samples were collected at each stream for water chemistry and benthic chlorophyll. Multivariate techniques were used to analyze spatial and temporal variation in streams and their relationship with land use. Changes in land use over 30 years were characterized by loss of savanna vegetation and increase in agricultural and urban areas. Multivariate analyses indicated that spatial and temporal variations observed in the streams were related to current ecotonal forest and to rates of change in catchment land use between 1977 and 2007. Impacts of land use change on stream variables depended primarily on the rate of savanna loss, increase in fallow land and the rate of increase of urban area. Streams draining catchments with a lower proportion of ecotonal forest, greater rates of savanna loss and increase in fallow land showed higher chlorophyll and nitrate concentrations. Higher phosphorus and suspended solids concentrations, runoff and conductivity were observed in streams with higher rates of increase in urban lands in their catchments. Our results suggest that ecotonal vegetation and the magnitude of land use change have implications for stream condition.