Identifying an unhealthy lake involves looking for several key ecological "red flags" that indicate a breakdown in water quality. One of the most obvious signs is a harmful algal bloom, which often appears as thick green "pea soup" or blue-green scum on the surface; these can produce toxins dangerous to humans and pets. Another indicator is poor water clarity or excessive murkiness, which may suggest high levels of suspended solids or nutrient runoff. The presence of dead fish or a lack of varied wildlife is a severe warning of low dissolved oxygen levels or chemical contamination. You should also watch for a strong foul odor, similar to rotten eggs, which indicates anaerobic decomposition at the bottom. Excessive invasive weed growth can also be a sign that the lake is "eutrophic," meaning it has too many nutrients (like nitrogen and phosphorus) which eventually choke out native species and deplete oxygen. Finally, high levels of foamy suds that don't disappear quickly can indicate the presence of detergents or high concentrations of organic surfactants, suggesting that the lake's natural filtering system is overwhelmed by human or agricultural impact.