2016 Drought Highlights: Role of watersheds in lake condition

Looking down view tube at Secchi disk

Precipitation and flows were well below normal in spring and summer of 2016. For example, at Morses Pond in Wellesley, MA, there was no winter snow pack to speak of and precipitation in May through August was about half of the average for the previous decade (7.2 vs 14.2 inches). The situation was similar all over New England, and while evaporation exceeded precipitation during summer and caused low water levels, the reduction of nutrient inputs also resulted in high (sometimes record) water clarity. Blooms of algae were less common in lakes that are tightly linked to watershed inputs on a seasonal basis, which includes most impoundments on streams and river and other lakes with watersheds more than about 20 times the area of the lake. Phosphorus and chlorophyll-a concentrations were lower than average for a respective 72 and 80% of lakes surveyed by LEA in Maine, leading to Secchi transparency values higher than average for 72% of surveyed lakes. Unless internal recycling is the dominant source of phosphorus to a lake, reduced precipitation translates into less runoff, lower nutrient inputs, and higher water clarity.

The importance of a watershed to lake condition is clearly demonstrated, but that importance is mediated through two key processes: weather pattern and land management. In 2016 the weather did a lot of what we strive to do with land management, minimizing the transport of nutrients and other contaminants to lakes. We can’t control the weather, and having less water entering our lakes has its downsides (e.g., lower water levels, more impact from rooted plants), but the importance of watershed management to minimize nutrient inputs when the weather is not cooperating is underscored. If we can’t put a dome over our watershed and only open the roof when we want the water, we have to manage the watershed to limit inputs to the lake.

But what is the potential for watershed management to provide the benefits observed in 2016 as a result of low precipitation? The better than average conditions were associated with precipitation about 50% below normal. Nutrient loading is not necessarily proportional to water inputs, and we would expect disproportionately more loading with larger storms, but it seems reasonable to assume that we would need at least a 50% reduction of loading through watershed management to reap the same benefits provided by the 2016 weather pattern. Based on years of evaluation by the USEPA, phosphorus removal by best management practices rarely averages more than 50%, although well designed infiltration facilities can achieve 90% reductions. However, not all watershed soils are suitable for infiltration systems, so what all this means functionally is that we will be hard pressed to provide the level of watershed management necessary to maintain the conditions we observed in 2016.

We can view 2016 as setting the bar for potential lake condition with regard to nutrients, algae and water clarity. Low precipitation limited inputs, and while there were some negative effects of having less water, the water quality was about as high as could be expected in New England lakes. If your lake was not appreciably better than in other recent years, internal loading sources were most likely dominant or there is another source (e.g., direct discharge or extensive storm water piping that limits load reduction on the way to the lake) that requires attention. Yet for those lakes that did exhibit better than average conditions in 2016, maintaining those conditions by watershed management will require superior effort, as the practical limits to best management practices will necessitate application all over the watershed to achieve the level of loading reduction experienced in 2016.