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Water's density anomaly — peak density at 4°C, not 0°C — is the single most consequential physical fact in aquatic ecology. It produces lake stratification, ice cover, and the seasonal turnover patterns that drive freshwater productivity.
Most substances are denser as solids than liquids. Water's hydrogen bonding pattern reverses this: ice floats. The 4°C density maximum means that as lakes cool, surface water sinks until 4°C is reached, then stops — surface water below 4°C is less dense and remains on top, eventually freezing. This protects aquatic life from the bottom up and produces spring and fall turnover that mixes nutrients and oxygen through the water column.