Physical, biological and chemical characteristics of Canadian and Danish lakes during summer

1. Winter temperatures differ markedly on the Canadian prairies compared with Denmark. Between 1 January 1998 and 31 December 2002, average weekly and monthly temperatures did not drop below 0 °C in the vicinity of Silkeborg, Denmark. Over this same time, weekly average temperatures near Calgary, Alberta, Canada, often dropped below -10 °C for 3-5 weeks and the average monthly temperature was below 0 °C for 2-4 months. Accordingly, winter ice conditions in shallow lakes in Canada and Denmark differed considerably.
2. To assess the implications of winter climate for lake biotic structure and function we compared a number of variables that describe the chemistry and biology of shallow Canadian and Danish lakes that had been chosen to have similar morphometries.
3. The Danish lakes had a fourfold higher ratio of chlorophyll-a: total phosphorus (TP). Zooplankton : phytoplankton carbon was related to TP and fish abundance in Danish lakes but not in Canadian lakes. There was no significant difference in the ratio log total zooplankton biomass : log TP and the Canadian lakes had a significantly higher proportion of cladocerans that were Daphnia. These differences correspond well with the fact that the Danish lakes have more abundant and diverse fish communities than the Canadian lakes.
4. Our results suggest that severe Canadian winters lead to anoxia under ice and more depauperate fish communities, and stronger zooplankton control on phytoplankton in shallow prairie lakes compared with shallow Danish lakes. If climate change leads to warmer winters and a shorter duration of ice cover, we predict that shallow Canadian prairie lakes will experience increased survivorship of planktivores and stronger control of zooplankton. This, in turn, might decrease zooplankton control on phytoplankton, leading to 'greener' lakes on the Canadian prairies.

1. 加拿大草原地区的冬季气温与丹麦存在显著差异。1998年1月1日至2002年12月31日期间，丹麦锡尔克堡（Silkeborg）周边区域的周均、月均气温均未低于0℃。同期，加拿大阿尔伯塔省卡尔加里（Calgary）附近的周均气温常跌破-10℃，持续时长达3至5周，且月均气温低于0℃的时段为2至4个月。因此，加拿大与丹麦浅水湖泊的冬季冰况差异悬殊。
2. 为评估冬季气候对湖泊生物群落结构与功能的影响，我们对比了多项描述加拿大与丹麦浅水湖泊化学及生物学特征的变量；本次研究选取的两类湖泊，均具有相似的形态计量特征（morphometries）。
3. 丹麦湖泊的叶绿素a（chlorophyll-a）与总磷（TP）比值较加拿大湖泊高出4倍。在丹麦湖泊中，浮游动物（Zooplankton）与浮游植物（phytoplankton）的碳含量比值与总磷（TP）及鱼类丰度存在相关性，但加拿大湖泊中未观察到此关联。log(总浮游动物生物量)与log(总磷)的比值无显著组间差异，而加拿大湖泊的枝角类（cladocerans）中水蚤属（Daphnia）的占比显著更高。上述差异与“丹麦湖泊的鱼类群落丰度更高、多样性更丰富”的事实高度契合。
4. 研究结果显示，与丹麦浅水湖泊相比，加拿大的严寒冬季会引发冰下缺氧，鱼类群落更为贫瘠，且浮游动物对浮游植物的管控作用更强。若气候变化导致冬季气温升高、冰盖持续时长缩短，我们预测加拿大草原地区的浅水湖泊中，浮游生物食性鱼类（planktivores）的存活率将上升，对浮游动物的管控作用也会增强。这反过来可能削弱浮游动物对浮游植物的管控，最终使加拿大草原地区的湖泊出现“更绿”的现象（即水体富营养化程度加剧）。