Effects of simulated climate warming on macrophytes in freshwater microcosm communities

We monitored macrophyte communities (Lagarosiphon major, Elodea nuttallii, Potamogeton natans) growing in freshwater microcosms that mimicked north temperate shallow lake and pond environments. The microcosms were subjected to a 2 years simulation of two climate warming regimes (3 degreesC above ambient year-round and 3 degreesC above ambient during the summer-only), in combination with a nutrient addition (eutrophication) treatment and the presence or absence of fish. Throughout the experiment, total macrophyte abundance remained relatively high and was unaffected by warming. However, the proportion of each community made up by L. major and the growth rate of L. major increased under continuous warming. Warming did not significantly influence the abundance or growth rate of E. nuttallii. For P. natans, flowering occurred earlier in the season under continuous warming and floating leaf surface area increased under both warming treatments. The effects of nutrient addition and the presence of fish were almost exclusively warming-independent and, where of statistical significance, tended to increase and decrease macrophyte abundance, respectively. Only the growth rate of E. nuttalld was significantly increased by nutrient addition. Our observations suggest that, as a functional component of north temperate shallow lake and pond ecosystems, elodeid macrophyte communities may be broadly resilient to the small increases in temperature associated with climate warming, even when these temperature increases occur in combination with increased nutrient loading and the presence of fish. Nevertheless, it may be of some concern that the species apparently most favoured by warming in our microcosms, L. major, is an exotic which has the known potential to become problematic as a weed in temperate waters. (C) 2002 Elsevier Science B.V. All rights reserved.