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

QUIRÓS, ROLANDO; BOVERI, MARIA; PETRACCHI, CRISTIAN; RENNELLA, ARMANDO M.; ROSSO, JUAN JOSE; SOSNOVSKY, ALEJANDRO; VON BERNARD, HUGO

Congreso; Eutrofización en Sudamérica: causas, consecuencias y tecnologías para manejo y control; 2006

Agriculturehas being the most important driver for grassland and wetland conversion intoagricultural 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 andsedimentation, stream regulation, channelization and wetland desiccation,urbanization, and surface water eutrophication. The intensity of land use foragriculture has been increasing gradually since the last decades of the 19thcentury, but it has been intensifying heavily during thelast fifteenyears. Fertilizers and pesticides use has increased more thanfour times between 1991-1993 and 2000-2002. As expected, pampean wetland changeand degradation has been intensifying even more during the last few years, and highnutrient loads usually enter to the already naturally eutrophic shallow lakes.Thelakes of the Pampas wetlands (?lagunas?) are very shallow, polimyctic, andhighly fluctuating in salinity and water renewal time. They are situated innutrient rich soil drainages, and its trophic state vary from meso-eutrophic tohighly hypertrophic depending on land use intensity. Within the permanent largelakes, two extreme types can be identified: a) ?clear? lakes, macrophytedominated, with waters relatively transparent, and a high relative abundance ofpiscivorous fish, and b) green water ?turbid? lakes, phytoplankton dominated,with high abundance of planktivorous fish. However, most of the lakes areneither completely ?clear?, nor entirely ?turbid?. Rather, a continuousdistribution between a definitely ?clear? state and highly ?turbid? lake stateis found. The relative abundance of ?turbid? lakes is greater in zones with thehigher intensity of land use. On the other hand, the ?clear? lakes are situatedin the remnant of the less disturbed Pampa grasslands and wetlands. Very shallow lakes are the major component ofthe Pampas wetland system where pampean agroecosystems are included. Thepristine state of the majority of these lakes was probably one of clear waterand a rich aquatic vegetation. However, nutrient loading from agriculture andurbanization has changed this situation in most cases. In a large portion ofthe pampean wetlands, the shallow lakes have shifted gradually from clear toturbid, and with the increase in algal turbidity, submerged plants have largelydisappeared.Theenvironmental management for pampean agroecosystems and urbanizations iscurrently highly deficient. The poor management is highly responsible for bothlake nutrient enrichment with its associated high biomass of phytoplankton, andthe current reduction in weighted biodiversity for the wetland lakes. Ourresults support that the primordial state for pampean lakes was one sufferinglow environmental pressure from natural wetlands and grasslands, with clearwaters and macrophyte dominated.Restoration of non-vegetated turbid shallow lakes to a more clearvegetated state is not easy. Reduction of the nutrient loading may have littleeffect in the short term, as during the period of nutrient enrichment a largeamount of phosphorus has been adsorbed to the sediments. After the reduction ofnutrient loads to shallow lakes, phosphorus concentration in the water columnusually did not drop immediately, and phosphorus release from the anoxicsediment becomes an important nutrient source for phytoplankton and anunfavorable habitat for macrophyte development. At the phytoplankton level, aseveral decades delayed lake response to nutrient load reduction can beexpected. However, in order to accelerate the lake recovery process, severalkind of very expensive lake internal measures can be applied.To continue with the presentnon-action measure for the pampean wetland management is to condemn pampeanlakes to a certain future. A gloomy future of very high nutrientlevels 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 inmore reductive environments, and extended fish and avian mortalities and humanhealth hazards.