[Rhodes22-list] Expanding Dead Zone in the Gulf of Mexico

[Jim White]

Dave:
   
  Right, and in normal circumstances a homogenous redution in salinity of seawater can be a good thing regarding dissolved oxygen (DO) and Oxygen potential (the ability of water to hold a specific volume of dissolved oxygen). In the case of the dead zone phenomon, freshwater acts as a "blanket", since the two layers don't mix (non-homogenity; effectively a sort of "oil and water mix" combo), therefore available oxygen is correspondingly inhibited to the salt water below. Open seawater gets oxygenated from the air/water interface and is dispersed according to submarine current and processes such as upwellings which move the oxygenated water to varying depths....in fact if there is no significant movement at depth there is a significant reduction in available 02 below the surface....
   
  Also, sea surface temperatures affect the waters ability to absorb and hold oxygen (DO potential), further compounding problems.....Wind and waves help things along....
   
  In the case of the red tide organism, it's been the subject of a lot of study trying to understand the triggering mechanism for "blooms" to occur, and it seems likely that there is not a single cause involved,  red tides happen in areas without any known anthropogenic pollutants. That does not mean of course that we can continue to supply the coastal waters with unlimited nitrogenous based runoff products, other things result from that. It just doesn't make sense to treat the worlds seas as a handy dumping ground.
   
  Hope that explanation helps somewhat. We face the same environmental problems down here (wetland loss, habitat alteration, seawater chemistry changes), and I have worked in this field for a number of years, and continue to consult with the USACE, NOAA, USFWS, TPWD and a lot of other alphabet soup agencies. It is usually an antagonistic effort, and definately counter-productive to any substantive change or policy. 
   
  The coastal oceans are in serious jepordy for a variety of reasons, not the least of which is human overcrowding of shorelines (hey, we all want to live by the sea right?) and over-exploitation of resource, terrestrial and aquatic. Finding solutions is a long term effort, with no quick fixes. 
   
  Unfortunately there is no shortage of junk science and reactionary "solutions" out there, and sometimes it's hard to know what's real....and what ain't. 
  End of soapbox.
   
  Cheers
   
  jw
   
  

DCLewis1 at aol.com wrote:
  
JW,

Thanks for your reply. I didn’t know any of that, it’s really good info.

The problem in the Chesapeake Bay (well, one principal problem) is runoff 
from agriculture and sewage. And there are other problems (invasive species, 
loss of wet lands, you name it). 

Re the red tide: Seems strange to me that the population of alge that causes 
the red tide could simply take off without a lot of some sort of nutrient. 
Clearly, it takes more of something for the little buggers to “bloom”, 
whatever the “it” is, let’s call it a nutrient. But it may well not be “man made
”.

Re freshwater inhibiting oxygen exchange to salt water, that’s news to me. 
I had no idea there was any substantial difference between the oxygen 
carrying capabilities of fresh and salt water - I’d never thought about it. Your 
post prompted me to do a little checking, I see there’s about a 20% between 
the solubility of oxygen in seawater and fresh water. I’m still bothered that 
the freshwater on top has more oxygen, I don’t see how that maps to less 
oxygen in the salt water at depth - its as if the fresh water were scavenging 
oxygen from the salt water, as opposed to being a richer source of oxygen at 
some depth than would otherwise be the case. If it were the inverted situation, 
(fresh water = less oxygen) I could understand how the freshwater could act 
as a barrier for transfer of oxygen from the atmosphere to depth. 

Clearly, you know this stuff - I’m impressed. I hope someone is trying to 
figure it all out and mitigate these large “dead zones” and fish kills. It’
s bad for tourism and bad for fish and other critters that are in, use, or 
eat products from, those waters.

Dave



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