Natural bodies of water are also subject to a much richer and every changing biological influence than our little tanks.
Bioload on our farm pond in early spring is next to nothing. Few if any water birds, reptiles and amphibians are just coming out of hibernation, most of the fish population dies back during winter and even the livestock don't spend time there--they'd rather get their water from the heated tubs. No rain to wash nutrients into the pond, and the small amount of snow melt typically gets absorbed into the immediate soil.
Fast forward a few months: amphibians are laying massive quantities of eggs, micro-organism populations are flourishing, fish are breeding, ducks are nesting in the cat-tails, raccoons and possums are on the banks every night, spring rains are washing in carrying livestock waste and nutrients from the decaying plant material thatched under the new pasture growth.
A few months further along, the cattails and surface plants are shading 80% of the water surface, the rains have stopped and the water levels are starting to drop, the massive number of tadpoles are now young frogs who've moved out into the mudflats under the cat tails and weeds--and predators are rapidly thinning their numbers.
Exact scientific explanations are great--but exponentially more difficult as the variables increase.