Oligotrophic patterns in southern Chilean lakes: the relevance of nutrients and mixing depth

Southern Chilean monomictic, temperate lakes are generally oligotrophic with high water transparency, low productivity and some apparent resistance to disturbances such as phosphorus additions. This paper attempts to explain low chlorophyll-a values using descriptive and experimental approaches. Three different scales are used, (a) a micro scale both in space and time, with experimental manipulations of N, P and total light in 24 enclosures in lake Llanquilme, (b) a longer time scale analysing 18 months of data in several sites within lake Llanquihue grouped as salmon farming sites, town bays and control sites, and (c) a broader time scale (9 years) by monitoring lakes Puyehue, Rupanco and Llanquilme, all in the Araucanian lake region and Yelcho lake in the north Patagonian region. In the Llanquihue in-lake sampling, total phosphorus values varied between I and 12 mug L-1 between sites, showing marginal site effects, P = 0.09 (salmon sites had greater values), however, chlorophyll a (Chl-a) concentrations were generally low (< 2 mu g L-1) and did not show site effects. Dissolved inorganic nitrogen (DIN) concentrations were lower than 30 mu g L-1 and showed a better predictive power than Chl-a. The analysis between lakes showed the lowest mean Chl-a values in Lake Yelcho (0.7 mu g L-1) and the highest in lake Puyehue (2.1 mu g L-1). Similar patterns were found for mean total P values, however there were no clear temporal trends through the 9 year sampling. The N/P molar ratio for the dissolved forms was generally below 20. In the enclosure experiment N had a positive significant effect on Chl-a concentration but neither P nor light had significant effects. Another outstanding result was that Chl-a values in the control bags were significantly higher than values in the lake. In all the lakes TN values did not exceed 20 mu mol L-1 and TP did not exceed 0.8 mu mol L-1 and thus N limitation could be proposed as playing an important role in regulation Chl-a and productivity. Another key-regulating factor of productivity and biomass could relate to the deep mixing which is a particular characteristic of Southern Hemisphere lakes, indeed, mixing depth has a negative effect and yields the best predictive power on Chl-a. This effect could also explain Chl-a increase in control bags with respect to lake since the epilimnion is artificially reduced in the former. If mixing depth influences productivity, then lake food webs should be linked to the benthos rather than to the pelagic zone particularly in the deepest mixing lakes.