Climate change impacts quantitatively assessed

Some impacts of climate change have known for years, but others are still surfacing. About a decade ago a representative of the USGS in Maine presented data at the NECNALMS conference for the date that ice cover broke up for multiple lakes in Maine. While there was variability typical of systems influenced by climate change, it was very clear that the date the ice was going out was getting earlier over decades. In about a 60 year period the average ice out date for the 2000s was about 2-3 weeks earlier than it had been in the 1950s. This should have been impressive enough in its own right, but apparently few other than ice fisherman really took notice.

Now we have another interesting measure that might be scarier. Oxygen consumption is an important feature in lakes, causing oxygen to become depleted (called anoxia) in many lakes deep enough to stratify, at which point oxygen can’t rapidly move downward from upper waters and decomposition gradually removes  oxygen from the bottom up to the boundary point, called the thermocline. Fish like trout that need cold water (<21 oC) but high oxygen (>5 mg/L) can get “squeezed”, faced with water too warm above and water with too little oxygen below. There may be no “trout water” during late summer, causing mortality. Further, loss of oxygen in deep water can allow phosphorus bound to iron to be released into the water column where it can support algae blooms, most often cyanobacteria that are favored by this type of release. In shallow water, high oxygen demand is indicative of elevated decomposition, and while complete loss of oxygen in water <15 ft deep is rare, that decomposition releases phosphorus that can fuel algae blooms. So oxygen consumption matters a lot to lake condition.

Data from Long Pond in Brewster and Harwich on Cape Cod, collected by the Natural Resources Department as part of a very useful water quality monitoring program, were plotted in an effort to understand the variation in oxygen consumption observed over time. What was found was that a relatively small difference in temperature, brought on by warmer spring air temperatures, resulted in a major increases in the rate of oxygen consumption, or oxygen demand (see figure). Oxygen demand below about 550 mg/m2/day is considered unlikely to cause severe anoxia, while values higher than 1000 mg/m2/day usually cause most of the bottom water layer to become anoxic in August and values greater than 2000 mg/m2/day will typically cause anoxia in July. Long Pond has an oxygen depletion problem, and it worsens appreciably with increasing temperatures in the deepest water.

Using a statistical technique called regression, the portion of the variability attributable to any tested factor can be evaluated. For Long Pond, and probably many other lakes, change in temperature explains much of the variation in oxygen demand (62% for Long Pond, a high percentage for a single factor). And this doesn’t require a large change either; the range of oxygen demand in Long Pond more than doubles for a temperature increase of only 3 Co (5.4 Fo)! The influence of the current direction of climate change is pushing our lakes toward a higher metabolism, almost like they have a fever, and the implications for all users, human or otherwise, are not good.

Oxygen demand as a function of temperature in Long Pond, MA.