In aquatic systems, productivity tends to decline rapidly once P addition is stopped. One of the major sources of water pollution in this country was once phosphates in detergents, which were added as water softeners. The phosphate created algal blooms in lakes and rivers, which led to massive fish kills due to anoxia. The result was a dramatic improvement in water quality in a relatively short period of time. Other water pollution legislation has also helped, but the phosphate ban had one of the largest effects.
Marine systems are more often N limited or both N and P limited.
N-fixation in marine systems is low, even with adequate P, evidently because the chemical composition of seawater interferes with nitrogenase activity. The open oceans generally have very low productivity because of nutrient limitations. Consumer influences on primary productivity This topic was touched upon in the last lecture, when multi-level trophic interactions were discussed. Generally, a bottom-up control on an ecosystem is one that affects the base of the food web, e.g., physical factors that influence primary production. When they manipulated bass populations, they found that increased bass had the predicted effect.