The main features of seasonal variability in the external forcing and dynamics of a deep mountain lake (Redó, Pyrenees)

Lake Redo, a dimictic oligotrophic mountain lake, was monitored for two complete years from July 1996 to July 1998. The main seasonal variations in the physical, chemical and biological parameters are described, with special emphasis on the comparison of external forcing (weather and atmospheric deposition) with internal lake dynamics. Annual mean air temperature was estimated to be 3.6 °C. The duration of ice cover on the lake was 4.5 months in 1996/97 and 5.8 months in 1997/98. The lake water was very ionpoor (mean annual conductivity 12 μS cm-1); however, ion concentrations in the lake were higher than in the precipitation, the differences being due mainly to Ca2+ and bicarbonates originating in the catchment. NH4 + was the main ion in the precipitation, with an average concentration of 17 μM, while in the lake it was always below 3 μM. However, the concentration of dissolved inorganic nitrogen always exceeded that of soluble reactive phosphorus by two or three orders of magnitude, so the latter is likely to be the limiting nutrient for phytoplankton growth. Four main production episodes were identified, occurring during spring and autumn overturn, in the upper hypolimnion during summer stratification, and under the ice at the beginning of the ice-covered period. The highest chlorophyll-a concentrations (1.2-2.2 μg l-1) were attained during spring overturn; concentrations of chlorophyll-c were high during both spring and autumn overturn, while chlorophyll-b was comparatively important in the upper hypolimnion during the stratification period. Daphnia pulicaria was the most abundant macrozooplankton species; its abundance was highest during the icecovered period, when its biomass was comparable to the measured sestonic particulate carbon concentration. The Daphnia maximum was associated with higher concentrations of NH4 + and dissolved organic carbon, suggesting that it may play an important role in the pelagic biogeochemical compartment of the lake under ice. Winter respiration rates for the lake were estimated to be 339 mg O2 m-2 d-1 for 1996/97 and 281 mg O2 m-2 d-1 for 1997/98.

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