Limnological measures related to climate change in the Hubbard Brook Valley, USA

Long-term monitoring within the Hubbard Brook Valley revealed changes in many climate and limnological variables known to affect the structure and function of aquatic ecosystems. The Hubbard Brook Valley is located in the White Mountains of New Hampshire, USA, and is the site of integrated, long-term ecological, biogeochemical, and hydrological studies. Mean annual temperatures in 2 headwater streams on south-facing slopes of the Hubbard Brook Valley declined similar to 2 degrees C from 1966 to 1983 and then showed a small (similar to 1 degrees C) and variable increase thereafter. No significant change in temperature was observed in 3 north-facing, headwater streams. Reliable records of ice-in and ice-out have been maintained since 1968 for Mirror Lake at the base of the Hubbard Brook Valley. The duration of ice cover on Mirror Lake has declined since 1968 at a significant rate of about -0.475 d/yr as a result of earlier ice-out (0.203 d/yr) rather than later ice-in dates. Since about 1996 the duration of ice cover has become much more variable with a nonsignificant change in ice cover duration. Evaluating how various temperature-sensitive components of aquatic ecosystems in the Hubbard Brook Valley change (amount and timing) in the future will be important to developing a better understanding of limnological responses to climate change.