Lake Drying

Observation: I wrote decades ago about our dying ponds. This one was a once vibrant lake. In the spring carrying the sounds of swans, ducks, frogs and snipes. When I first came up to this lake about 1996, it still had frogs, and the water was up to the trees. Now it's 3/4 grass. There were no sounds up here this fall and last fall. This spot is a 40 minute walk from the highway up the Mankommen Lake Trail. Wilson Justin

LEO says: Thank you Wilson for your observation. Lake change and lake drying is a phenomena across Alaska, a result of warming temperatures, changes in precipitation, and thawing permafrost. In every climate change assessment performed over the past decade by ANTHC, lake drying has been described. The impacts for fish, birds and wildlife are only just beginning to be understood. In some areas there are also implications for community water supply, and water security. The following consult by Andrew Bach explores the science behind lake change and provides analysis based on the consult and image provided by Wilson.

Huxley College of the Environment Consult: This observation is showing a pond that has suffered a change in hydrology. There are many possible explanations for this change, among them are:

• less water (rain and runoff) entering the pond due to drought conditions, either temporary (just a few dry years) or more permanent due to climate change.
• increased temperatures causing increased evaporation, more likely attributed to global warming.
• surface water diversion, like a tributary stream was diverted by construction or permafrost melting.
• sub-surface water diversion by groundwater pumping, or other groundwater changes related to permafrost melting.

Any of these, or a combination would account for the lower water level. Once the water level goes down exposing the lake bed, it dries out and becomes a not-quite fertile substrate for seed establishment. Because the pond was likely accumulating organic matter from the surrounding forest and the ecosystem in the water, the dry lake bed would have some fertility, but not all the nutrients the native forest needs. Initially grasses are established because grass can withstand nutrient deficiencies that might exist. Also, there are microorganism in soil, such as bacteria and fungus that need to migrate from the surrounding forest soil to build up some needed nutrients that the grass does not require. As we can see in the middle of the photo foreground, a few conifer and other tree seedlings are getting established in the grass near the forest edge (former lake edge), where these microorganisms have moved into. There the soil has built up enough nutrients to allow those tree species to survive. Over the coming years, you should see more and more seedling creeping into the grassland. Eventually those trees will grow large enough to shade out the grass and the grass will disappear, being replaced by a forest with its own under-story species. The seed source of the existing forest is likely not a limiting factor in this process, the seeds are likely being blown out into the middle of the grassy area, but do not have those missing nutrients yet. With time, the forest edge will creep across the grass area. At some point a critical threshold will be past where the soil organisms have migrated all the way across the grass area and the grass will start to be reforested in the center as well.

This process (or versions of it) are the natural end of all lakes and ponds. All lakes and ponds will eventually fill up with sediment (or be drained like this one) and the natural vegetation will take over the former lake area. Many flat bottom valleys around the world are former lake beds. Whether or not this pond will entirely disappear is dependent on if the hydrologic system changes back to providing enough water to sustain the pond, or refill it to its former level. Dr. Andy Bach (Western Washington University)

Resources:

National Geographic Article. October 17, 2006. "Warming May Be Drying Up Alaska's Lakes, Photo Study Says", by Richard A. Lovett

Canadian Journal of Forest Research 41(2):425-428. January 2011. "Wetland drying and succession across the Kenai Peninsula Lowlands, south-central Alaska" (.pdf). by Klein et al. (Source: NRC Research Press)