We’ve had two papers come out recently that strongly imply phosphorus is entering lakes though atmospheric pathways and that this flux has been increasing in recent decades. If you don’t know why you should care about phosphorus in lakes, here is an internet hole for you.
Though both papers are circumstantial they are provocative and hint at an undocumented pathway for one of the most important ecosystem nutrients. In the first paper, “Is atmospheric phosphorus pollution altering global lake stoichiometry?” we compare measured deposition chemistries from mountain environments around the world to lake chemistries in the same regions. Mountain lakes are really good indicators of atmospheric nutrient deposition because their catchments tend to be small, steep, and lack vegetation. This means the rainwater that ultimately ends up in the lake has very little time to take up nutrients from the catchment. As a result, lake water in these lakes looks like the regional rainwater. If phosphorus emissions are increasing due to human activity (e.g. desertification/biomass burning/oil and gas operations), then we would expect to see greater deposition rates of phosphorus, and, greater lake water phosphorus around regions where this type of human activity occurs. That is what we found, but correlation does not equal causation, which is why the title has a question mark.
In the second paper, “Continental-scale Increase in Lake and Stream Phosphorus: Are Oligotrophic Systems Disappearing in the United States?“ data from 1000’s of rivers and lakes in the continental US were examined for changes in chemistry though three sampling periods over the last decade. The lead authors at the EPA noted that the lakes and rivers that changed the most, were the lakes and rivers that were the least impacted by human activity in their watershed. If the phosphorus is not coming from human activity IN the watershed, perhaps it is coming from OUT of the watershed? i.e. from the atmosphere. However, there is another potential explanation. Even though rainfall has not increased, rain intensity may have increased, which can lead to greater erosion rates in the catchment increasing the flux of phosphorus to these systems. A lack of monitoring data prevent us from determining what is the primary cause of increasing phosphorus concentrations. Perhaps it is both?
Though neither of these papers provides a smoking gun for the atmospheric pathway, they indicate that more research and more data are needed. Many government organizations around the world collect wet deposition, but do not measure phosphorus. Many organizations measure atmospheric aerosols, but only the fraction smaller than 10 micrometers, which may not increase during a dust even, and aerosols that are collected at any size are also rarely analyzed for phosphorus content. There are reasons for these omissions, for example, dust is hard to measure, samples are often contaminated with bird poop, and soluble reactive phosphorus will start to disappear into biological organisms in your sample as soon as it is collected. But there are some obvious solutions to these challenges. Given the importance of phosphorus to aquatic ecosystems, this is clearly an issue that should be investigated further.
Here are links to two articles written about these papers: