Limnetica 40
That twenty years is nothing for gravel-pit limnology
The past twenty years have witnessed a rise in the analysis of long-term limnological data given the need to know whether global warming is affecting freshwaters, and if so, how. Previous studies indicated that processes affecting ecosystem functioning may develop slowly, thus requiring long-term study, as in the case of the eutrophication-oligotrophication gradient.Here we report annual averages and seasonal variability of meteorological data (air temperature, solar radiation, rainfall) and the main limnological variables (lake level, water temperature, euphotic depth, mixing depth, nitrogen and phosphorus compounds and phytoplankton biomass) for a gravel-pit, seepage lake close to Madrid (Central Spain), collected monthly from 1992 to 2018. Linear trends (both upward and downward), long-term constancy, regime shifts, piecewise patterns and mixed patterns (constant and piecewise) were found for the variables tested, but their variability did not increase over time. Water warming and decreased lake levels were related with atmospheric warming. In turn, these physical changes covaried with nitrate increase and decreases in total phosphorus and phytoplankton biomass. However, the weak relationships between climate- and limnological variables (for example, the lack of rainfall effects on lake level) would indicate that other factors, such as land use, were more influential. Broadly speaking, it would appear that the observed limnological patterns were greatly influenced by the local increase in novel lake basins through mining activities, which led to a decrease in lake level until the year 2000, and the ecological stabilization of a recently created ecosystem, which is an analogous process to that occurring after reservoir construction. The above processes might have partly offset global warming effects in this lake but twenty years is ALMOST nothing within the framework of this fascinating puzzle. Hence longer studies are necessary to gain an accurate picture of gravel-pit limnology, which is urgently needed.
