Why Calcium is More Variable
Some phytoplankton (the foraminifera, coccolithophorids, pteropods,
and heteropods, and also corals and coralline algae) form CaCO3.
The formation of CaCO3 organically or inorganically (evaporation) can locally affect Ca concentrations, particularly in shallow waters. The dissolution of CaCO3 in some regions can also affect Ca and CO32- concentrations.
The main other type of carbon in seawater are the forms of organic carbon, both dissolved and particulate. (It's primarily a matter of what can get through a filter of a certain size, and what can't.) Dissolved organic matter/carbon (DOM, DOC) can sometimes be colored (CDOM), and a variety of semi-polymeric DOC is termed Gelbstoffe, German for "yellow substance". Particulate organic matter/carbon (POM, POC) is larger organic particles from a variety of sources. SeaWiFS data is being used to calculate the amounts of CDOM (which may be correlated with total DOM) and POM in the water column.
Jim Acker(acker@daac.gsfc.nasa.gov)