How Long Has Hypoxia Been A Problem In Long Island Sound -- A Further Explanation From Varekamp and Stacey
Joop responded first. He conducts his studies by taking core samples of sediments, identifying what the core samples contain, figuring out which time period each layer of sediment represents, and then making inferences. Here’s what he said:
… in general, we find in our records that evidence for eutrophication and possible hypoxia start in the early 1800's, much earlier than thought by most people. However, our cores from near Execution Rock [off New Rochelle] show evidence for low level eutrophication going back much further, suggesting that the narrowest section of the Sound may have suffered from summer hypoxia for most of its life time.
Another interesting side issue for the extreme western Sound is that the east river was blasted out in the mid 1800's to improve ships’ passage through Hell's Gate, which would have increased nutrient fluxes from the Hudson into the Sound but also may have ventilated the extreme western Sound a bit better.
We have been somewhat puzzled by the early onset of eutrophication, given that population densities were much lower at the time and so sewage input must have been much more modest as well. We are currently pursuing the idea that the large scale harvesting of oysters may have played a role as well. Before the early 1800's there were many more oysters than during the 1800's (large scale harvesting) and since these are filter feeders, removal of oysters may show up in the records as eutrophication of the water column (more little organisms that settle to the bottom to be oxidized instead of ending up in 'oyster flesh'). We still have a way to go there, but this was inspired by the book 'Oysters' that we recently laid our hands on.
[Here's what I wrote about Mark Kurlansky's Book "Oysters." And here's what I wrote about oysters and water quality.]
So Professor Varekamp thinks that the far western end of the Sound might have been troubled by hypoxia (roughly 3 or 3.5 milligrams per liter of oxygen or less) of a greater intensity and for far longer than is indicated by Paul Stacey’s calculation from 1990 that dissolved oxygen levels never fell below about 5.5 milligrams per liter.
Later in the afternoon Paul responded as well, prefacing his remarks with a note that his work is old and that he happily changes his mind when newer and better data come in. Here’s what Paul said:
I've always been a little concerned that the natural condition of the Sound may have been more hypoxic than the LIS 3.0 model [the one he relied on 15 or so years ago -- TA] determined. The environmental "historians" have sometimes referred to legends from Native Americans suggesting that crab jubilees and hypoxia fish kills may have occurred before the European invasion in some estuaries. I don't have anything substantive on that, though. Kent Mountford who writes for the Chesapeake Bay Journal (Past is Prologue column) might have some more info on that if you'd like to follow up.
The SWEM model [one that’s in the works, I guess -- TA] does (preliminarily) suggest that natural conditions for hypoxia may have been in the 4-5 mg/L range. And there are likely to have been some harbors and embayments that would have been more susceptible to low DO. Shallow areas that got pretty warm in the summer and had high carbon inputs from a tributary that were not well flushed would, I think, always have experienced some hypoxia. Offshore, though, maybe not.
That reminded me that in 1679, a Dutch fellow (not Joop Varekamp) was exploring the shore of Staten Island. He found what sounds like a pretty good bunker kill:
“Lying rotting upon the shore were thousands of fish called marsbancken, which are about the size of a common carp. These fish swim close together in large schools, and are pursued so by other fish that they are forced upon the shore in order to avoid the mouths of their enemies, and when the water falls they are left there to die, food for the eagles and other birds of prey."
About 150 years later Henry David Thoreau was on Staten Island and found the same thing.
All this of course is interesting but also dangerous. I’m not suggesting that that the possibility that hypoxia is an old phenomenon should in any way diminish our efforts to control contemporary hypoxia. I don’t think either Paul Stacey or Joop Varekamp are suggesting that dissolved oxygen levels routinely fell close to zero in the western end of the Sound, or that hypoxia extended as far east as the Housatonic River and beyond. Nor do historical anecdotes of crab jubilees or bunker kills in any way equate with the devastating fish kills of 1987, when pretty much any fish in the western half of the Sound died. The really bad hypoxia that we have now is still our problem to fix.