I wouldn't be so quick to blaim run off without evidence that this is the main cause. Diving in the ocean in northern Europe you experience the same thing: you have a choice between 4 degrees C and clear water or 15 degrees C and murky green. Runoff isn't a significant cause of plankton blooming in the North Atlantic. Temperature is.

Still, it requires nitrogen to get an algae bloom, right? Has to come from somewhere.

Phosphorus is the limiting nutrient, not nitrogen. But yes, it's nearly all from ag and wastewater.

In the ocean, iron plays the role of phosphorus.

And it looks like the evidence for run off causing algal blooms in the Great Lakes is pretty convincing.

I swam in Lake Ontario near Kingston from the mid-80's up to a few years ago and never had an issue, although after heavy rains in summer there could be pretty high E. coli load near shore due mostly to ducks and geese (solution, swim off a dock).

Fecal matter from birds--like the scads of corrmorants around Kingston--is rich in nutrients, and while agricultural runnoff can be a problem Lake Ontario doesn't suffer from it particularly, and even without it there are plenty of altnernative nutrient sources.

I have a photobook of Chicago that has a picture of north avenue beach in the 1960s with thousands of people on it.

I think a city's relationship with its waterfront has to do with what was built on land next to the water. In Toronto the lakefront was industrial until recently. In Chicago the industry was clustered along the river and the lakefront has consisted of parks and beaches for a century.

And now some of our beaches have Blue Flag certification. Incredible.