The Itaparica reservoir – Aquatic ecosystem functions: Impact, vulnerability and development of an adapted management.

Today multiple use of reservoirs is a public agreement, but conflicts between stakeholders for ecological, economic and sociological reasons are frequent. Sustainability of the aquatic ecosystem functions must be the aim for any reservoir management, and a sufficient water quality, at least mesotrophic conditions, is necessary. However, eutrophication processes of many tropical reservoirs are advanced, and a re-oligotrophication is needed, being successfully applied up to now only to temperate lakes. Studies on Itaparica reservoir, São Francisco river, located in the semi-arid Northeast of Brazil, are carried out to quantify impacts on ecosystem functions and to develop an adapted reservoir management to guarantee sustainability of ecosystem services. Large reservoirs such as the Itaparica possess a high dynamic of water quality due to high inflow. Main disturbance factor of tropical reservoirs are extensive water level changes due to dry and rainy season, as well as operational hydropower conditions. Hydrodynamics simulations were conducted with the software TELEMAC-2D on Icó-Mandantes Bay, a branch of the Itaparica reservoir. Low and high water level scenarios were investigated with the purpose to deeper understand their effect on transport of substances and the exchange of the main stream with the bay. Littoral desiccation areas are of high significance for nutrient release after rewetting by sediment mineralization; after drying sediment composition shifted towards a relatively larger mobile P pool. Increasing water level promotes the water pest (Egeria densa), a pioneer plant. E. densa was found to be the dominant species in the bay, with negative impacts on the aquatic ecosystem services. Growth success of this plant is related to recurrent water level changes. E. densa accumulates large quantities of P and is therefore an important P sink beside the sediment. The trophic level of the reservoir and the Ico-Mandantes bay differ significantly due to reduced water exchange processes. P-chlorophyll a relationship as P use efficiency model was applied for critical P load estimation of 25 μg P L-1. After impoundment, the P load highly exceeded the carrying capacity, because of leaching of the inundated vegetation and sediments; too of high significance is the export of nutrients from the sub-watershed with the dominant Caatinga biome. But lake sediments have a high P absorption capacity, and the trophic level has been changing towards mesotrophic with sediment deposition within the reservoir after the trophic upsurge. Intensive aquaculture fish production with net cages leads to an overcharge of the reservoir. Itaparica reservoir, was found to be a minor source of greenhouse gases in contradiction to higher emissions seen in many other Brazilian reservoirs. Ebullition of methane is the main factor explaining higher emission rates in shallower waters up to 3 m depth. Multiple water uses and sustainability of aquatic ecosystem services are focus of an adapted reservoir management. Hereby key factors are re-oligotrophication, sediment management and blue aquaculture. An advanced and adapted reservoir management lead to a better ecological status with a more stable water level, without seasonal breakdown of Egeria densa, supported by a sediment management with erosion control, sediment abstraction and re-use for soil amendment.