Los efectos de la agriculturización del humedal pampeano sobre la eutrofización de sus lagunas

QUIRÓS, R.; BOVERI, M.B.; PETRACCHI, C.A.; RENNELLA, A.M.; ROSSO, J.J.; SOSNOVSKY A.; VON BERNARD, H.T.

Eutrofização na América do Sul: Causas, conseqüências e tecnologías de gestão

Tundisi, J.G.; Matsumura-Tundisi, T.; Sidagis Galli, C.

Lugar: Sao Carlo, Brasil; Año: 2006; p. 1 - 16

Abstract Agriculture has being the most important driver for grassland and wetland conversion into agricultural land at the Pampas. During more than a hundred years, the Pampas, one of the last world’s largest less developed grassland and wetland ecosystem, has been facing various threats, such as agriculturization, erosion and sedimentation, stream regulation, channelization and wetland desiccation, urbanization, and surface water eutrophication. The intensity of land use for agriculture has been increasing gradually since the last decades of the 19th century, but it has been intensifying heavily during the last fifteen years. Fertilizers and pesticides use has increased more than four times between 1991-1993 and 2000-2002. As expected, pampean wetland change and degradation has been intensifying even more during the last few years, and high nutrient loads usually enter to the already naturally eutrophic shallow lakes. The lakes of the Pampas wetlands (“lagunas”) are very shallow, polimyctic, and highly fluctuating in salinity and water renewal time. They are situated in nutrient rich soil drainages, and its trophic state vary from meso-eutrophic to highly hypertrophic depending on land use intensity. Within the permanent large lakes, two extreme types can be identified: a) “clear” lakes, macrophyte dominated, with waters relatively transparent, and a high relative abundance of piscivorous fish, and b) green water “turbid” lakes, phytoplankton dominated, with high abundance of planktivorous fish. However, most of the lakes are neither completely “clear”, nor entirely “turbid”. Rather, a continuous distribution between a definitely “clear” state and highly “turbid” lake state is found. The relative abundance of “turbid” lakes is greater in zones with the higher intensity of land use. On the other hand, the “clear” lakes are situated in the remnant of the less disturbed Pampa grasslands and wetlands. Very shallow lakes are the major component of the Pampas wetland system where pampean agroecosystems are included. The pristine state of the majority of these lakes was probably one of clear water and a rich aquatic vegetation. However, nutrient loading from agriculture and urbanization has changed this situation in most cases. In a large portion of the pampean wetlands, the shallow lakes have shifted gradually from clear to turbid, and with the increase in algal turbidity, submerged plants have largely disappeared. The environmental management for pampean agroecosystems and urbanizations is currently highly deficient. The poor management is highly responsible for both lake nutrient enrichment with its associated high biomass of phytoplankton, and the current reduction in weighted biodiversity for the wetland lakes. Our results support that the primordial state for pampean lakes was one suffering low environmental pressure from natural wetlands and grasslands, with clear waters and macrophyte dominated. Restoration of non-vegetated turbid shallow lakes to a more clear vegetated state is not easy. Reduction of the nutrient loading may have little effect in the short term, as during the period of nutrient enrichment a large amount of phosphorus has been adsorbed to the sediments. After the reduction of nutrient loads to shallow lakes, phosphorus concentration in the water column usually did not drop immediately, and phosphorus release from the anoxic sediment becomes an important nutrient source for phytoplankton and an unfavorable habitat for macrophyte development. At the phytoplankton level, a several decades delayed lake response to nutrient load reduction can be expected. However, in order to accelerate the lake recovery process, several kind of very expensive lake internal measures can be applied. To continue with the present non-action measure for the pampean wetland management is to condemn pampean lakes to a certain future. A gloomy future of very high nutrient levels due to agriculture and urbanization, phytoplankton dominated highly “turbid” lakes, with hypoxic and anoxic phosphorus saturated sediments, simplified vegetal and animal communities, high levels of organic matter in more reductive environments, and extended fish and avian mortalities and human health hazards.