Low water + low inorganic sediment inflow (drought) = increased sunlight penetration to bottom substrate.
Increased sunlight penetration to substrate means ideal conditions for submergent vegetation.
Significant growth of agressive submergent vegetation means decreased available nutrients available for single celled algae.
But, nutrients don't have to be limited to what's in sediment. Nutrients can be dissolved in water as well - look at hydrophonics.
