I mean, sewage contains lots of nutrients (nitrogen, minerals, etc.). In the oceans , “upwelling” of mineral-rich bottom waters greatly stimulated fish populations, as the nutrients feed the plankton (at the bottom of the food chain). So, does sewage do the same thing?
When I was a kid, Boston Harbor was heavily polluted (raw sewage was discharged into the harbor). Yet, flounder fishing in the harbor was very popular-Quincy Bay was known for good bottom fishing.
Is sewage bad or good for the ocean?
While the nutrients might be good, many of the components of sewage are still in the process of breaking down, which requires oxygen. Even the presence of so much nutrients in cleaner forms can cause an algal bloom that consumes all available oxygen (we see this with agricultural runoff, for example). If too much oxygen is used, then there won’t be any left for the fish.
And then you have to worry about disease vectors, heavy metals, toxins and any other trash mixed into the sewage. None of these are good for the wildlife or the people who might eat the wildlife.
Bad, because it can cause massive algae blooms, which eventually cause oxygen to be depleted.
Bad, because sewage can contain heavy metals and other pollutants other than nutrients.
In the case you mention it’s possible that nobody ever lab-tested the flounder to see what sort of horrible things were in it, or the currents carried the waste out to sea before it reached the bottom, or just that the harbor was too big for the sewage to make much difference. Hard to say without a scientific study, but mass fish kills due to excess nutrients causing algal blooms is a pretty thorougly documented problem.
It’s OK, as long as we’re talking some handfuls of humans living in small villages. But once we have concentrations, then it’s bad.
In addition to what has been said, excess nutrients can significantly alter community structure, shifting species richness from high to low. Or from an abundance of marketable fish to inedible ones.
Excess nutrients also promote harmful algae blooms (HABs), which cause fish kills. No more fishery for you.
Excess sediments and pathogens can fuck up oyster beds (for both their growth and human consumption). When the oysters are gone, there goes your internal filtering system. So turbidity increases (which screws up the phytoplankton), dissolved organic matter increases (bye bye dissolved oxygen). The influx of carbon will drive down pH levels–causing acidification. More death, more oxygen depletion. And low pH is not only harmful itself, but it can also make certain pollutants more toxic (metals bound in the sediments, in particular).
Sewage contains dissolved metals, which can be toxic at high enough concentrations. The ammonia in wastewater is also toxic.
All of this pertains to raw, untreated sewage. There are many wastewater treatment facilities that discharge into coastal waters and other major waterbodies. But with the exception of combined sewer overflows, the discharges have been treated–often to technological limits–to mitigate significant impacts.
What happens in areas where the continental shelf is narrow? Does raw sewage have any bad effects if is sinks into the abyssal plain? I assume it stays there long enough to break down
When I was a kid and we stayed near the Inlet in Atlantic City one summer, we ate once at a fish bar over the bay. The guy cleaning the fish threw all the waste through a hole in the floor and also had a line there that he pulled up a fish on every once in a while. He claimed the fishing was great because of all that he threw down that hole. Of course, there were no heavy metals or other toxic waste. I had never heard of algal blooms.