Watershed Wanderings by Pam Saunders
Lake Windermere Ambassadors
The transition from summer into fall results in interesting and dynamic changes in lake ecology. Including temperature fluctuations, changes in the activities of various organisms, and changes in light availably to the water column. Let’s look at these aspects a little closer.
During the fall months, there is a noticeable drop in water temperatures in lakes. With this brings changes to organisms and fish activity levels and can affect their distribution. For example, they may become more active as water temperatures cool. Migration and reproduction changes in some species (like salmon) can be observed where they may swim upstream to spawn in tributaries or gather in specific areas of the lake to find mates or lay eggs.
In addition to changes in fish activity, Lakes are often visited by a variety of wildlife in the fall. Birds and waterfowl may use lakes (or wetlands) as stopover locations and may gather in groups on the lake before migrating south to their winter grounds.
Aquatic plants also undergo changes in the fall. Some species may wither back, while others may continue to grow until temperatures drop substantially. These changes in aquatic plants can affect the availability of food and habitat for aquatic organisms.
Over the course of the summer, many lakes go through a process called stratification. Where the warmer water nearer to the surface and the cooler, denser, water below will form distinct layers, with little opportunity for mixing between them. However, as we enter fall these lakes will undergo turnover (remixing of these stratified layers) as the surface layers begin to cool down and the weather starts to change.
This mixing can bring nutrients from the bottom layers up to the surface, effecting nutrient availability and can potentially result in algae blooms. Summer stratification and fall mixing is not usually seen in Lake Windermere. This is due to the lake’s shallow depth, higher wind potential, and flow through rate of the water. For these reasons we don’t see a lot of stratification occurring on our lake.
In the fall, leaf litter input is usually higher as deciduous tree surrounding lakes shed their leaves. This results in an influx of organic material into the water which can serve as a food source and habitat for aquatic organisms and insects. As this leaf litter decomposes it will add to the lake’s natural nutrient cycling.
Regarding Lake Windermere, this process is usually insignificant, due to flow through rate, or 47 day residency of the water. Depending on specific lake conditions, fall can be a time when certain types of algae thrive because of the mixing up of stratified nutrients and new supplies of leaf litter decomposition. Some species of algae can undergo a period of rapid growth (blooms) which can impact water quality and have ecological consequences.
Fall brings changes in human activity on or around lakes as well. Generally, we observe a reduction in water sports and swimming and an increase in fishing. How do you enjoy lake recreation in the fall? Let us know on our website www.lakeambassadors.ca
