Ecological history affects zooplankton community responses to acidification

The effects of ecological history are frequently ignored in attempts to pre dict community responses to environmental change. In this study, we explore d the, possibility that ecological history can cause differences in communi ty responses to perturbation using parallel acidification experiments in th ree sites with different PH histories in the Northern Highland Lake Distric t of Wisconsin, USA. In Trout Lake, high acid neutralizing capacity had his torically buffered changes in pH. In contrast, the two basins of Little Roc k Lake (Little Rock-Reference and Little Rock-Treatment) had experienced se asonal fluctuations in pH. Furthermore, the two lake basins were separated with a curtain and Little Rock-Treatment was experimentally acidified in th e late 1980s. In each site, we conducted mesocosm experiments to compare zo oplankton community dynamics in control (ambient pH) and acidified (pH 4.7) treatments. Zooplankton community responses were strongest in Trout Lake a nd weakest in Little Rock-Treatment suggesting that ecological history affe cted responses to acidification. In part, variation in community sensitivit y to acidification was driven by differences in species composition. Howeve r, the results of a reciprocal transplant experiment indicated that changes in the acid tolerance of populations during past acidification events may make zooplankton communities less sensitive to subsequent pH stress. Our st udy highlights the role that ecological history may play in community-level responses to environmental change.