The effects of long-term climate variability on the trophodynamics of an estuarine ecosystem in southern South America

The trophodynamics of the Río de la Plata ecosystem over a long time scale (from 1948 to 2008) were simulated using a food-web model forced by two environmental factors. The Atlantic Meridional Mode (meridional sea surface temperature anomalies) was used as regional forcing, and the Río de la Plata (RdlP) runoff was applied as local forcing. The entire food web was impacted by the regional forcing on a decadal scale; at the inter-annual scale, this remote factor had partial effects on the base of the food web. The RdlP runoff impacted primary producers and secondary consumers at the inter-annual scale. The higher effects of the local forcing were temporally coupled with seven of the strongest El Niño events from 1950 to 2008 (1957-1958, 1965-1966, 1972-1973, 1982-1983, 1986-1987, 1991-1992, 1997-1998). In contrast, the lower effects of RdlP runoff on the food web were coupled with six of the strongest La Niña events since 1950 (1950-1951, 1954-1956, 1964, 1970-1971, 1974-1975, 1988-1989). Total system biomass (trophic web attribute) and a measure of system entropy (holistic indicator) were used to identify ecosystem degradation. The entropy and total biomass of the RdlP ecosystem showed two opposite phases: before and after the early 1970s. During the period 1948-1971, the system showed high entropy and low total biomass, indicating high degradation. This cycle was reversed after 1972, and prevailed until the beginning of the 2000s. During this new cycle, the system entropy decreased and it was compensated by an increase in total system biomass. A sustainable entropy gain occurred after 2003, suggesting a new period of ecosystem degradation. The findings are discussed in light of temporal changes in the structural properties of this coastal ecosystem.