Response of a humic lake ecosystem to an extreme precipitation event: physical, chemical, and biological implications

Abstract

Climate projections indicate that the frequency and severity of extreme precipitation events will increase over the next century. Although a large number of lakes across the globe are systematically monitored, it is rare to have a wide range of ecological indices measured at high enough frequency and over a sufficient time scale to allow characterisation of the response of a lake ecosystem to such events. We present data from Lough Feeagh, a relatively large oligotrophic lake in Ireland, which was at the epicentre of a once in 250-year precipitation event in July 2009 when 50 mm of rain fell in less than 2 hours. The effects of the resulting flood on the water column stability, chemistry, biology, and metabolism were examined, and data from multiple years before and after the event were used to ascertain the significance and longevity of any observed changes. The flood caused the water column to destabilise prematurely and depressed primary production. Bacterial biomass was high in the month after the flood, and zooplankton assemblages in late 2009 were significantly different from those of other years. Changes in all these variables combined to produce lower rates of gross primary production and higher rates of respiration than those measured between 2010 and 2014, resulting in more pronounced negative net ecosystem production than in the other years. Despite all these changes, the normal seasonal cycles resumed in 2010, and it appears that this rare but significant event did not have a long-term impact on the ecosystem functioning of the lake.