Tuesday, July 17, 2012

Warming temperatures leading to additional lake complications?

This past winter, Third Sister Lake didn't get a complete freeze, and it was possible, as the spring approached, to see algae growing in some of the ice. In March, while we were experiencing temperatures normally part of June weather, I wrote about some of the possible impacts that such weather might bring if they become the norm. I also wrote about some of the possible impacts to Third Sister Lake that such warming might bring.

Well, add to all that one more impact brought on by warming: decreased lake turnover. In a new paper out in the journal Climate Change, Swiss scientists have shown how increased warming in Swiss lakes have decreased lake turnover, and this has brought about increased levels of Burgundy blood algae.

The paper's abstract states:
Anthropogenic-induced changes in nutrient ratios have increased the susceptibility of large temperate lakes to several effects of rising air temperatures and the resulting heating of water bodies1. First, warming leads to stronger thermal stratification, thus impeding natural complete water turnover (holomixis), which compensates for oxygen deficits in the deep zones2, 3. Second, increased water temperatures and nutrient concentrations can directly favour the growth of harmful algae4, 5, 6. Thus, lake-restoration programmes have focused on reducing nutrients to limit toxic algal blooms7. Here we present evidence that the ubiquitous8, 9, 10 harmful cyanobacterium Planktothrix rubescens has become the dominant species in a large lake during the past four decades, although the phosphorus content of the ecosystem decreased fivefold. However, the nitrogen input was not diminished concomitantly, favouring this non-N2-fixing cyanobacterium owing to increased N:P ratios10. P. rubescens contains gas vesicles that allow for buoyancy to accumulate within the depth of optimal irradiance11. As the toxic cyanobacterium has low consumption by predators12, water turnover represents the main mechanism of seasonal population control. Thus, unidirectional lake-restoration measures13 in parallel with recurrent absence of holomixis owing to lake warming may lead to similar undesired effects that have formerly emerged from fertilization.
This type of impact is problematic for Third Sister Lake, not because of Planktothrix (or because of other cyanobacteria, at least to my knowledge), but because Third Sister Lake is already a lake that doesn't completely turn over. From a 2001 paper by Bridgeman et al., the finding of lake stability (i.e., the resistance to turnover) in Third Sister Lake was that:
Salt-laden water entering Third Sister Lake [during the winter] from [the business lots to the southeast] could eventually lead to the formation of a chemocline. Large vertical gradients of salinity may potentially prolong stratification or altogether prevent mixing. ... If Third Sister Lake follows a similar pattern [to First and Second Sister Lakes], the contribution of a chloride gradient to stability would reach a maximum in March or April ... and the lake ... will remain [thermally stratified] until mid-November.
The paper also reported that - due to forest growth from 1904 - Third Sister Lake didn't fully turn over in the spring (i.e., following ice-out), even back in 1986. The authors of the 2001 paper concluded that:
Given that the lake is now more sheltered [than in earlier decades] and may require about 60% more wind energy to overcome a chlorine gradient, complete spring mixing, which was a common event at least into the 1940s and occurred occasionally until the early 1980s, is now probably rare.
And it's likely to remain rare if increased temperatures become the norm. (That is, unless a significant number of trees fall down, thus increasing the lake's wind exposure.)

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