@hurlumerlu asked so: why are lakes (NOT ALWAYS) murkier in the spring? The answer is...... long. I tried to keep it concise.
so lakes (not all but many) go through cycles of stratification and turnover. Lakes are stratified in summer and winter (if they freeze) and experience turnover during fall and spring where all the water mixes together (if the lake doesn't freeze it usually stays mixed through winter, too). Stratification is when a body of water splits into separate layers, namely the epilimnion (upper layer, warm) and the hypolimnion (lower layer, cold). This occurs because cold water is denser than warm water (unless it's ice, which is less dense than warm water). There's more to explain here but we're rapidly getting into the weeds of this already weedy question.
Nutrients have a habit of concentrating at the bottom of lakes because 1) dead things float down there and 2) most stuff lives in the epilimnion because that's where light is available. Many lakes that stratify are deep enough for loss of light to be a serious factor deeper down. No light means no algae or plants, and you need primary producers to support an ecosystem. These living things deplete nutrients, meaning the epilimnion is generally pretty lacking in the nutrient department
BUT! during spring the entire lake mixes, bringing all those nutrients up to the photosynthetically active part of the lake. With the fresh input of nutrients and the extra light that comes in spring (which is why this doesn't happen in fall), we get the "spring bloom" of algae, which makes the lake murkier! it doesn't stay like this through summer, though, because 1) as mentioned, algae eats through the available nutrients and 2) algal bloom means a boom in herbivores, who eat much of the algae and we enter what is called the "clear water phase" during summer! just in time for swimming <3
(almost none of this applies in places that are warm year-round btw.)
