My nitrates are at zero, why is my pond green? I love this question, but people aren't usually looking for the why. They want to know how to clear it up. Too bad, cause here is the why:
The short answer is that bacteria built the living world, they are everywhere, and they are really, really good at what they do. To give you the long answer I'm going to have to throw a little science at you, just because I'm a scientist.
We'll start with the Redfield Ratio. The Redfield Ratio is the historical average atomic ratio of Carbon, Nitrogen and Phosphorus in oceanic phytoplankton, not to belittle the importance of silicon, iron and other trace elements. It goes a little something like this:
This number can vary quite a bit between species, seasons, and nutrient availability. So your average oceanic phytoplankter will, at its favorite temperature and salinity, with no limiting nutrients and plenty of light grow in accordance with these atomic proportions. Compared to inland systems the ocean is fairly consistent in many of its parameters. As such recreating the study on North American freshwater lakes yields quite different and highly variable ratios:
So what do these numbers mean? The buggers making your water green, growing in their prime conditions will strive towards that perfect body, made up of some hundreds carbon to tens nitrogen to one phosphorus. Generally there will be no lack of carbon sources in your water, which is why photosynthesis works. Bacteria rework dissolved carbon dioxide and water via light to make sugars, energy for growth. Now, atmospheric nitrogen will dissolve into water too, though luckily this is inorganic nitrogen, not usually a viable source for bacteria. Cyanobacteria can fix atmospheric nitrogen and make it available, but there are already plenty of organic nitrogen and phosphorus for your pond. Uneaten food, fish waste, and organic debris are the main sources of these elements in your pond, and it takes relatively little nitrogen and very little phosphorus to achieve what we'll call balanced growth.
The growth rate of bacteria is often referred to as doubling time, 1 cell becomes 2, 4, 16, 32, 64, etc. Again depending on available light, temperature and nutrients this can vary between 0.3-3 times per day. Being that a single drop of water may contain one million bacteria, it is easy to see how things can get out of hand quickly.
I used the term balanced growth before, so now we'll get into unbalanced growth. When low light and temperature begin to inhibit photosynthesis the algae continue to take up whatever nutrients are available, and they will store them regardless of that perfect body ratio. Some will even recycle their photosystems and focus on primarily metabolic pathways to energy. When spring time hits, the sun is shining and the temperature is rising you've got a fat and happy colony ready to turn their photosystems up to eleven and make it one hell of a doubling season.
So why aren't you reading nitrates in your green water? Your natural biofilter is healthy and those little buggers are eating it up as fast as your fish can put it down. They aren’t just floating around waiting on some phosphorus or trace elements. They’ve been around a long time, and they’re really, really good at what they do.