Re: Why is deep Atlantic Ocean water phosphorus-poor?

Dear Erwin
I am sorry for late response.  Here is the explanation.
I have copied this text from a Review titled," The Oceanic Phosphorus Cycle" written by Adina 
Paytan, and Karen McLaughlin and published in the Journal - Chemical Reviews 2007, 107, 563-
576.
If you are interested you can get this article and read more about it.
Best
Shashank

Pytan and McLaughlin (2007) The ocean phosphorus cycle. Chem. Rev. 107: 563-576.
Marine phytoplankton and autotrophic bacteria take up P from solution for their metabolic 
needs, mostly as orthophosphate (HPO42-, PO43-). Heterotrophic bacteria are responsible for 
much of the DOP hydrolysis and conversion back to DIP; however, phytoplankton and autotrophic
bacteria can also hydrolyze organic P compounds when their P demand is not satisfied by 
inorganic orthophosphate. Much of the Dissolved Inorganic Phosphate (DIP) uptake takes place in 
the sunlit upper zone of the water column (euphotic zone), where marine photosynthesis takes 
place; hydrolysis of organic P (both particulate and dissolved) to DIP occurs throughout the water 
column. Accordingly, DIP (measured as SRP) depth profiles in the oceans exhibit a “nutrient 
trend” such
that surface waters are depleted due to intense biological uptake in the euphotic zone and 
concentrations increase with depth as a result of conversion of organic P forms to DIP
(also called regeneration). Within deep waters, an increase in DIP concentrations is also observed 
with increasing deep water age due to continuous accumulation of sinking particulate matter and 
its regeneration (e.g., the cumulative accumulation during deep water mass aging and
transit). At present, deep water that forms in the North Atlantic flows through the deep ocean 
basins to the North Pacific: a process that takes about 1500 years. Thus, waters in the deep 
Pacific are older than waters in the deep Atlantic. As a result, at present, the DIP concentration in
the deep Pacific water is higher than that in the deep Atlantic (e.g., increased concentration 
along the deep ocean circulation path), or in other words, the dissolved phosphorus in deep 
atlantic is lower than in other oceans.