Trophic parameters and method validation in Lake Riñihue (North Patagonia: Chile) from 1978 through 1997

Lake Riñihue is the last in a chain of glacially derived oligotrophic lakes in southern Chile and was the subject of limnological research by Dr. Hugo Campos and co-workers at the Universidad Austral de Chile, Valdivia, Chile, since the 1970’s. Prof. Campos died in the field in 1998, and this paper now presents a review of these data with comments on methodology relevant to trophic parameters of this lake. Data are presented on inorganic nutrients, chlorophyll a, phytoplankton biomass, primary production and Secchi depth obtained over a period of 21 years between 1978 and 1997 in order to evaluate potential changes in these parameters over time. Non-parametric Wilcoxon rank sum tests were applied to determine temporal trends and revealed: (1) total phosphorus showed a significant increase (TP: 2.7 mg P L-1 to 6.5 mg P L-1), (2) Secchi depth, soluble reactive phosphorus and primary production did not change significantly, (3) chlorophyll a changed significantly among different years, but did not reveal a clear temporal trend, (4) nitrate showed an upward trend and increased concentrations during the early 1990 period, although this data remain in doubt due to methodological anomalies, and (5) the phytoplankton biomass showed significantly higher values during the 1980 period. The significant increase in total phosphorus may reflect increasing usage of grassland within the catchment area. A number of anomalies discovered in historical data sets related to methodology are analysed and discussed within the context of the study.