Environmental processes, water quality degradation, and decline of waterbird populations in the Rio Cruces wetland, Chile

Abstract

Changes in wetland ecosystems may result from the interactions of endogenous processes with exogenous factors such as environmental fluctuations and anthropogenic influences. Since mid-2004, the Rio Cruces wetland, a Ramsar site located in southern Chile (40°S), exhibited a sudden increase in mortality and emigration of the largest breeding population of Black-necked swans in the Neotropics, a massive demise of the dominant macrophyte Egeria densa (the main food of swans and several aquatic birds), and a seasonal appearance of turbid waters. We compared annual variation in rainfall, river flow, and radiation over the period 2000–2005 to assess the role of environmental factors on these wetland changes. Those factors, with the exception of a decrease in river flow during 2004, did not show significant inter-annual differences. However, when comparing Landsat images, we found in the visible and near-infrared spectrum, a corresponding increase and decrease in water reflectance for 2005 with respect to 2003 and 2001, respectively. These results may reflect the appearance of turbid waters and the decrease in cover of E. densa. All temporal changes were restricted to the northern and central zones of the wetland. In addition, spatial analysis showed a gradient in turbidity across the wetland waters, which was enhanced by estuarine influence during spring-tides. Censuses of aquatic birds (1999–2005) showed that only herbivorous birds exhibited a pronounced decrease in population abundance after mid-2004, while piscivorous birds continued normal cycling, with even some positive trends in abundance during 2004–2005. Population declines in herbivorous birds may be related to the demise of E. densa and suspension of sediments during periods of reduced river flow (2004) that gave rise to the turbidity in the wetland waters. Environmental changes could be related to changes in water quality after a new pulp mill, built upstream of the wetland, initiated operations during February 2004.