A modelling assessment of potential for eutrophication of Lake Riñihue, Chile
Campos, Hugo; Hamilton, David P.; Villalobos, Lorena; Imberger, Jörg; Javam, Abi
Archiv für Hydrobiologie Volume 151 Number 1 (2001), p. 101 - 125
published: Mar 23, 2001
ArtNo. ESP141015101012, Price: 29.00 €
Lake Riñihue is a large (A = 90 km2), deep (zmax = 323 m) lake in the high rainfall region of the Andean pre-cordillera of southern Chile. A combination of daily meteorological readings, monthly measurements of inflows and water column composition, and a 7-month period of thermistor chain readings over a 2.5-year study period were used to assess phyiscal and biogeochemical characteristics of Lake Riñihue and potential for change in trophic status. A coupled hydrodynamic-water quality model, DYRESM-WQ, was used to reproduce the observed changes in water temperature in the lake over this period. The model output yielded insights into the timing of stratification and mixis. Lake Riñihue did not mix completely over the 2.5-year period of the study (1995-1997). From late spring to autumn, a well developed thermocline was observed at depths of 20-40m. Thermistor chain readings indicated that the thermocline position was quite variable on a day-to-day basis. In late winter the thermocline deepened rapidly in response to a sustained net negative heat flux, due largely to reduced solar radiation. Biological and chemical variables in DYRESM-WQ were calibrated over a period of 7 months and a further period of 2 years was used for validation. The simulation output reinforced the oligotrophic nature of Lake Riñihue ([chl-a] <6 mg/m-3), the general timing of changes in chlorophyll-a and the depletion of dissolved inorganic nutrients from the surface layer during summer. The DYRESM-WQ model and a bulk loading model were used to assess the potential for eutrophication of Lake Riñihue in response to increases in nutrient loads from inflows. The response to doubling of phosphorus (PO4-P and TP) concentrations in the inflows was to increase mean levels of depth-integrated chlorophyll-a near the water surface (0-5 8 m), from 1.36 mg/m3 to 1.95 mg/m3. The simulated 1.4-fold increase in chl-a concentrations in response to the increased phosphorus load was considerably less than that predicted from a combination of a bulk loading model for TP and a regression against chl-a (2.8-fold increase). The DYRESM-WQ model indicated that the trophic response to increased nutrient loading would be complicated by growth limitation by light and nitrogen.