Experimental Perturbations of Whole Lakes as Tests of Hypotheses concerning Ecosystem Structure and Function

Production, respiration, nutrient concentrations and changes in biological communities were monitored for several years in two experimentally eutrophied and two experimentally acidified lakes in the Experimental Lakes Area, Canada. Results were examined to detect changes in ecosystem structure and function that might be sensitive indicators of ecosystem stress. Phytoplankton production in eutrophied lakes was higher than in reference lakes. It decreased quickly to background values when fertilization was terminated. Winter respiration was unchanged, P/R ratios were constant under constant N/P fertilization, but decreased for three years after the N/P ratio in fertilizer was decreased from 14:1 to 5:1 by weight. Phytoplankton production was not affected by acidification to a pH of 5.0. Winter respiration declined at pH values of 5.1 and below, causing P/R to be highest at the lowest pH values. Respiration returned to normal when the pH of the lake was increased to 5.4. Gross periphyton production and respiration both increased with acidification, but P/R ratios declined at pH values below 6.2. Nitrification in both acidified lakes ceased at pH values of 5.4 to 5.7. It recovered when pH was allowed to increase, but with a time lag of one year. No other nutrient cycles appeared to be disrupted. Both eutrophication and acidification caused declines in species diversity among several taxonomic groups. Among phytoplankton, both stresses caused an increase in size of organisms in summer. Among higher taxa, there was not a consistent tendency for either small or large organisms to be favored. Periphyton metabolism was the most sensitive indicator of acidification, followed by taxonomic changes in several groups. Ecosystem-level production and respiration were the most resistant properties to acid stress. The earliest serious changes in ecosystems and food webs occurred when acidification eliminated acid-sensitive organisms that were also the sole occupants of key ecological niches. Such situations occur frequently in northern lakes.