I used to wonder why plants reduced pearling the day I dosed iron, manganese, or CSM. It never made any sense to me but I’ve been able to repeat it many times, though not consistently. Only after a large water change was performed do the plants pearl at a higher rate.
Others have made this same observation as well. Tom Barr stated that after he performed a “large water change in the morning right after the lights come on, then late in the day, there is mad pearling and obviously better growth than any other day of the week. I’ve measured plant growth to confirm this also.” Why water change day seems to grow aquatic plants better – BarrReport.com
One of the assumptions was that the new water contained high concentrations of CO2 and that the influx resulted in improved photosynthesis. However, this wouldn’t explain why the same thing happens with water that has very low CO2, e.g. my tap water has less than 0.02mg/L so the increased pearling couldn’t possibly be from an increase of CO2.
So what explains why whe water change improved photosynthesis and plant growth?
According to many experiments described in the plant science literature, the first sign of toxicity is reduced photosynthesis. Excesses of certain nutrients impair the physiological processes that prevent carbon dioxide assimilation which results in less oxygen produced. We would observe that plants don’t pearl as much as would be expected even as both light and CO2 are high. This is why dosing the micronutrients at excessively high concentrations reduced pearling. Only when the concentrations of the excess nutrients are reduced to more optimal levels, such as after a large water change, will photosynthesis increase. It’s not the water change that causes the increased photosynthesis and growth. It’s the reduction of toxic concentrations of nutrients.
Under the conditions of high light, 100+PAR, and high CO2, 40+mg/L, plants should pearl a lot more and growth should also be very fast. However, in my tanks with high light and low CO2 (<10mg/L) plants still pearl and grow very fast, faster than should be expected at such low CO2 levels. The difference is that the nutrients are closer to within the tolerance levels that plants require and can function normally even at low CO2 levels.