Seasonal epilimnetic temperature patterns and trends in a suite of lakes from Wisconsin (USA), Germany, and Finland

Epilimnetic temperatures from early 1980s through 2017 were analyzed for 12 Wisconsin (USA), German, and Finnish lakes. Seasonal temperature metrics exhibited large interannual variability with trends differing among regions. In the Wisconsin lakes, only late summer and fall temperatures increased significantly. In the 2 northeastern German lakes, temperatures increased in all seasons (not all significantly). The 3 Finnish lakes exhibited large spring temperature increases influenced by earlier ice-out; summer and fall temperatures also increased except for Konnevesi (central Finland) in early summer and polar Kevojärvi in midsummer. Kevojärvi also exhibited no increase in maximum recorded temperatures. Earlier records from 4 lakes were analyzed to elucidate longer-term epilimnetic temperature patterns. For Lake Mendota (southern Wisconsin), spring and late fall temperatures have increased modestly but significantly since 1894; summer temperatures have also increased modestly because of higher frequency of warm temperatures in recent summers and not from new record high temperatures. Trout Lake (northern Wisconsin) exhibited warm temperatures in some summers during the 1930s–1940s, similar to warm temperatures in some recent summers. Air–water temperature relationships coupled with long-term regional air temperature data also indicated summer epilimnetic temperatures in the study lakes were likely as warm in the 1930s–1940s as in recent years. Lake data confirmed cooler epilimnetic temperatures occurred in many summers during the 1950s–1980s coincident with cooler air temperatures. Because epilimnetic temperatures have not increased monotonically since 1900, our study supports continued temperature monitoring in lakes with extensive historical data to better understand and predict future effects of climate change on lake ecosystems.

本研究针对美国威斯康星州、德国及芬兰的共12处湖泊，分析了1980年代初至2017年的湖上层水温（epilimnetic temperatures）变化。各季节水温指标呈现出显著的年际波动，且不同区域的水温变化趋势存在明显分化：威斯康星州的湖泊中仅夏末与秋季水温呈现显著上升趋势；德国东北部的2处湖泊各季节水温均出现上升，但并非所有季节的上升均达到显著水平；芬兰的3处湖泊春季水温上升幅度显著，该现象受冰层消融时间提前的影响，除芬兰中部的康涅韦西湖（Konnevesi）初夏时段以及极地凯沃耶尔维湖（Kevojärvi）仲夏时段外，其余夏、秋季水温均呈上升趋势，且凯沃耶尔维湖的实测最高水温亦未出现上升。本研究同时整合了4处湖泊的早期观测记录，以阐明湖上层水温的长期变化格局：以威斯康星州南部的门多塔湖（Lake Mendota）为例，自1894年以来其春季与秋末水温呈现小幅但显著的上升趋势，夏季水温的上升则源于近年暖夏事件发生频率升高，而非创下新的最高水温纪录；威斯康星州北部的特劳特湖（Trout Lake）在1930至1940年代的部分夏季曾出现水温偏高的情况，这与近年部分夏季的水温偏高现象相似。结合区域长期气温数据开展的气-水温关系分析同样表明，本研究涉及的湖泊在1930至1940年代的夏季湖上层水温大概率与近年水平相当；湖泊观测数据证实，1950至1980年代的多数夏季湖上层水温偏低，这与同期区域气温偏低的情况相吻合。鉴于自1900年以来湖上层水温并非呈单调上升趋势，本研究呼吁对拥有丰富历史观测数据的湖泊持续开展水温监测，以更深入地理解并预测气候变化对湖泊生态系统的未来影响。