Eutrophication, phytoplankton dynamics and nutrient removal in two man-made urban lakes (Palácio de Cristal and Serralves), Porto, Portugal

Urban lakes are prone to eutrophication because of a number of factors, including a high water residence time (slow flushing rate), and high nutrient loads from artificial feeding of waterfowl and fish by visitors and from waterfowl faeces, falling leaves and nutrient run-off from adjacent areas. This study focuses on the ecology of two similar-sized urban lakes in Porto, Portugal, that exhibit some differing characteristics conducive to different trophic states and nutrient removal efficiencies. Although similar in many aspects, the two reservoirs developed quite diverse phytoplankton communities. The higher nitrogen load to Palacio de Cristal Lake could be the result of the higher concentration in its underground influent stream, while a less-oxidized nitrogen form (ammonia) is higher in Serralves Lake. Nitrate concentrations are higher in Palacio de Cristal Lake than in Serralves as a result mainly of high nitrate loads (maximum values above 80 mg L ‐1 ) in its influent stream. Interestingly, however, its nitrate removal efficiency is greater than for Serralves Lake. Cyanobacteria are the dominal phytoplankton In both lakes throughout the year. However, only one genus of this group (Pseudoanabaena mucicola) was the dominant phytoplankton in Palacio de Cristal Lake, while five Cyanobacteria species co-dominated in Serralves lake. Regular monitoring of Cyanobacteria and their toxins in urban recreation lakes is advised.