I don't see why an excessive algal growth model would fail to predict a large hypoxic zone around the mouth of the Mississippi. In fact, the models apparently do predict it, though I can't say if they get the right answer for the wrong reason. The Mississippi watershed is vast, and an enormous portion of that watershed is dedicated to intensive agriculture with heavy fertilizer use. Also, the algae are presumably dispersed beyond where they grow and die, and the hypoxic water presumably disperses beyond where decomposition occurs, so the hypoxic zone need not be restricted to locations with enough nutrients to support algal blooms.
Given that the hypoxic zone is pretty much on the continental shelf and away from the Gulf's major currents (based on maps you cna find at http://oceancurrents.rsmas.miami.edu/caribbean/caribbean-cs.html and http://www.epa.gov/gmpo/nutrient/hypoxia_pressrelease.html), it seems plausible that the Gulf in the vicinity of the mouth of the Mississippi is stagnant enough to allow the hypoxic zone to grow quite large. Regarding fish farms to solve the problem, I'm skeptical that they would work. Fish farms usually add to nutrient loads because the operators feed the fish rather than counting on the river to bring enough food in. Also, you mentioned keeping the fish in lakes connected to the river, I believe. It seems to me that, every growing season, these lakes would sprout algal blooms, go hypoxic, and kill off the fish. But then, I'm a terrestrial biologist. I don't know quite enough about aquatic ecology to answer the original question satisfactorally, but I don't think I've yet heard of a miracle cure for everything that actually worked. Jim
