Lakes are full of color, their surfaces often appearing in shades of blue, gray, or brown. Some, though, turn a sickly green because of large amounts of nutrients draining off agricultural and urban landscapes.
Excess nutrients such as phosphorus can lead to nuisance algal blooms that produce toxins and deplete oxygen. As phosphorus flows into a lake, it eventually settles into the top layers of sediment. From there, it either cycles back up into the water column or is stored permanently in deeper sediment layers. Current models do not well represent the complexity of nutrient cycling processes involving sediment.
In a new study, Markelov et al. merged two existing models of nutrient movement in lakes. They updated the new model to account for additional chemical reactions that occur in sediments and in the water column to track nutrient concentrations over time in Lake Vansjø, Norway. They then ran simulations to determine how the lake’s biogeochemistry might change under several scenarios.
Their results suggest that as the climate warms, algal blooms will worsen. The duration of annual ice cover on the lake will decrease, and prolonged wind exposure will further mix the water column. This mixing tends to shorten the length of low-oxygen periods, but algal blooms last longer, and more phosphorus cycles up from the sediment.
The researchers also tested potential management options. They found that reducing phosphorus flow into the lake by half would decrease the contribution of sediment to the lake’s phosphorus levels by approximately 20% in a decade. But even if phosphorus inputs were halted entirely, decreasing this contribution by 50% would take up to 200 years.
Another management tactic is to add iron to the water to bind and store phosphorus in the sediment so algae cannot use it. The researchers considered a 20-year treatment plan with this approach and found that it could be effective at Lake Vansjø, where more than half of the phosphorus present is stored in the sediment. Within a decade, dissolved phosphorus levels in this simulation dropped by nearly half, and algae biomass decreased by a third. However, once the treatment was stopped, algal biomass returned to pretreatment levels within 15 years.
The new model is an important step in evaluating treatment techniques for individual lakes, the researchers noted. Future models will have to capture algal dynamics in greater detail by including, for example, the pH of water in sediments and other factors that influence nutrient limitations. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2019JG005254, 2019)
—Elizabeth Thompson, Science Writer