Monitoring phosphorus (P) concentration in water and sediments in the Castanhão reservoir, under intensive aquaculture, in Northeastern Brazil showed internal process to dominate P cycling following a reduction of reservoir volume due to an extended drought period. A strong negative correlation between soluble reactive phosphorus in surface waters (SRPs) with the reservoir volume results from diminishing dilution capacity; bottom water SRP showed no significant correlation with volume and imply SRP remobilization to surface layers. Total suspended solids (TSS) showed a significant correlation with chlorophyll-a, suggesting change primary productivity following SRP enrichment of surface waters and living cells dominating the TSS. As a result, eutrophication, as established by a trophic state index, was triggered in the reservoir probably enforced by intensive fish farming effluents, whose nutrients accumulated in bottom waters and which became available due to breaking of the thermocline. Since low rainfall periods are typical of the semiarid region and tend to be more frequent and stronger due to climate change, multiple use of reservoirs in NE Brazil should be reevaluated.
Keywords: Aquaculture; Geochemistry; Northeastern Brazil; Reservoir; Sediments.