The importance of inter-annual variability for management

Abstract

In regions with high inter-annual climatic variability, lake management strategies should be based on scientific data covering the maximum range of lake dynamics. Lake Sanabria is a monomictic lake located in a gneiss-dominated glacial valley at 1000 ma. m. s. l. in NW Spain. A three-year study was carried out during 1987-89 to obtain the basic ecological knowledge required for a sustainable lake management. Water and phytoplankton samples were collected from several depths of the water column at the deepest zone of the lake. Temperature and Secchi disk visibility, and total and dissolved inorganic nitrogen and phosphorus, chlorophyll a, and phytoplankton species composition and abundance (in laboratory) were determined monthly following standard limnological methods. Stratification began in March-April, with the thermocline stabilized at between 6-10 m, and lasted until November-December. The euphotic depth was higher than the mixing depth throughout the stratification period, except in the month previous to the water column mixing. The water residence time, estimated from the volume of the lake and the inflows and outflows, was between 0.48-0.67 years. However, during the months of highest rainfall (September-February) it was up to three times greater than during the same months of low rainy years. Nutrient concentrations in the water were very low throughout the year (52 μg l-1 dissolved inorganic nitrogen, 6.1 μg l-1 soluble reactive phosphorus, and 1.45 mg l-1 soluble reactive silica) and dominated by the dissolved organic fractions, which was related to the inputs of organic materials from the densely forested catchment. The colour of the lake, 21-41 mg Pt l-1, is also indicative of the relatively high amount of organic substances dissolved in the waters of the lake. A bloom of cyanobacteria (Microcystis flos-aquae, Aphanothece clathrata, Merismopedia tenuisssima, Synechococcus sp.) persisted in most of the water column during August 1988-March 1989, associated with the high residence time of the water. In contrast, the cyanobacteria bloom was limited to the mesolimnion for 3-5 months during 1987 and 1989. A model is proposed for the seasonal variation of the phytoplankton, which differentiates the seasonal trend during years of high water turnover from that of years with long residence time. Lake Sanabria is oligotrophic, based on a range of criteria (Secchi disk visibility, chlorophyll a concentration, total phosphorus concentration). However, during years of long water residence time (e.g. 1988), the annual input of total phosphorus is critical to maintain the characteristics of the lake. Short-term strategies for the management of lakes should take into account the inter-annual variability rather than averages as indicators to prevent the impact of increased nutrient inputs derived from changes in land and water use in the catchment area. Long-term strategies should also consider the potential changes in the lake's dynamics, derived from the changes in the rainfall pattern caused by climatic changes.