As folks have noted, I think that we need to clarify that most of the elements on this list are NOT trace elements.
Major Elements on this list
Calcium, magnesium, potassium, strontium, boron
Minor elements on this list
Iodine (barium is borderline minor/trace)
Trace elements on this list
iron, manganese, (barium is borderline minor/trace)
Major ions are those above about 1 ppm (or by a different definition, above about 0.05 mM), and are grouped together because they can have a measurable impact on salinity. There are 11 major ions in seawater, and their concentration in the ocean generally only changes as salinity changes.
Minor ions are those at about 0.05 to 50 uM (about 5 ppb to 5 ppm).
Trace elements are those below 50 nM (about 5 ppb). Some of these vary considerably with location and depth since they are turned over rapidly by various process, especially biological processes. Only iron and manganese on this list are true trace elements.