The seawater levels of the major nutrients (phosphate, nitrate, silicate) are, when considered over long timescales, under biological control. Silicon, the fifth-most abundant element dissolved in river water flowing down into the sea, is, by comparison, largely absent from seawater due to intense biological removal. A similar model to that just described suggests that levels of silicate are also biologically controlledP (not consciously of course, but they are also an emergent property of ecological dynamics). Biological removal also explains why carbon and calcium, the two most abundant dissolved constituents of river water, are relatively so scarce in seawater (Figure 5.8). They are actively removed from seawater in biological residues that fall to the seafloor and get buried; both are removed in CaCO3, carbon also in soft organic tissues. The lack of intense biological removal of elements like sodium and chlorine leaves them to completely dominate sea-salt despite not being the most abundant constituents of the dissolved salt in river water. In general, the biologically-utilised elements are anomalously scarce in seawater (Figure 5.8) .The plankton do not sit in a medium that has been set purely by external forces. Rather it is the plankton themselves (or rather, innumerable generations of their ancestors) which have in large part determined the chemical composition of the medium they inhabit.
Comparison of dissolved substances in river water and in seawater. The top panel shows concentrations in river water, the middle panel concentrations in seawater, and the bottom panel the ratio of the two (mol/mol, on a log scale). Inverted green triangles denote elements which are widely used by marine organisms, either to make new soft tissues or else to make hard parts (out of calcium carbonate CaCO3 or opal SiO2). Black squares denote elements which are not heavily utilised by living organisms. All biologically-utilised elements are exceptionally scarce compared to expectations based on river supply and comparison to non-utilised elements. Aluminium is also surprisingly scarce, in this case because it tends to stick to falling particles and thereby gets rapidly removed from seawater.