The process you are talking about is called eutrophication (
Eutrophication - Wikipedia, the free encyclopedia), a subject that I know well because my dad was a eutrophication researcher for many years and I grew up in a research camp where (among other things) we induced eutrophication in various bodies of water through artificial methods.
In a nutshell, if you increase the nutrient load on a given body of water past a certain tipping point, a bunch of organisms such as algae and plankton suddenly think that they are at an all-you-can-eat buffet and they reproduce like mad. This has the unfortunate side effect of blacking all of the sunlight and using up all of the oxygen dissolved in the water, and that in turn means that everything dependent on that oxygen (like fish and crabs) will die. It's pretty cool to see how effective it is.
The risk of eutrophication is a real one, but it is heavily influenced by the dilution of the nutrients in the water. In a basin with relatively low water exchange and a relatively high human nutrient load (e.g. Puget Sound) this is a real problem. In a basin with very high water exchange (e.g. Straight of Juan de Fuca), a major city like Victoria can dump millions of gallons of straight nutrients (AKA people poop) into the water with lower effects. As an aside, note that I did not say "none", and in fact the world's oceans are paying the price for this.
This is the major reason that we now have "phosphate free" soaps and detergents. Turns out that while phosphates are not directly dangerous to the environment, they are a great way to cause eutrophication. Same for agricultural fertilizers and animal wastes (pig poop in particular).
For all of you "whales poop in the ocean so it's OK" folks, you're sort of right but sort of not. It's not that whale poop isn't a problem, it's that when a whale in the middle of the Pacific poops it gets diluted by the whole ocean. If a bunch of whales stayed in the same place and pooped, you'd get eutrophication. And for those of you who live in places like Monterrey, you've got it now from seal poop. But, in the great cycle of life, the eutrophication leads to loss of prey, and eventually the seal population will die off. Unfortunately, when humans cause eutrophication, they don't die off as easily, they just find a new place to pollute. (It's a joke, a JOKE!).
When it comes to food overboard, I have always kept the total nutrient load in mind. If I'm in a sheltered bay without much water exchange, and with lots of boats, then it's not a good idea. On the other hand, north of Nanaimo, in a secluded anchorage, with no other boats around, the load that pleasure boats put on the ecosystem is minimal. It toss every bit of food scrap I generate overboard. Of course, the same goes of overboard sewage discharge -- fine on the middle of Johnstone Straight, not so good in Montague Harbour.
Scott Welch
Island Eagle