Plundered nutrients

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Phytoplankton Plunder Nutrients

Analysis using models: An understanding the overwhelming importance of biological removal in setting nutrient concentrations in the ocean can be obtained by experimenting with the three different nutrient models. This experiment can be carried out with either the phosphate-only (P), the nitrate-phosphate (NP), or the silicate-phosphate (SiP) models. From the control panel of a model, select 'Model Parameters' and then change the growth rate of all phytoplankton in the model to a low value (less than the constant mortality rate, so that there is no hope of the phytoplankton populations doing anything but remorselessly decline over time). Click 'Apply and Close' to save the parameter changes and then click 'Run Model' on the main control panel. In the resulting plots you should see a striking effect, that the concentrations of all nutrients in all boxes increase steadily over time. This is because they have been released from the otherwise incessant removal pressure by phytoplankton (you should also notice a plummeting of phytoplankton numbers at the beginning of the run). With phytoplankton populations no longer able to hold nutrient concentrations down, the nutrients can escape to higher values.

What happens in the NP and SiP models when only one of the two phytoplankton groups are prevented from growing?

A second way of examining this controlling exhaustion pressure by phytoplankton is to artificially raise up the nutrient concentrations and then examine the response of the system.

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 (e.g. shells 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.

Comparing different elements: 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. We can get an idea of the importance of the plankton influence over nutrient levels from the figure to the right. 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. Biological removal also explains why carbon and calcium, the two most abundant dissolved constituents of river water, are relatively scarce in seawater (see figure to right). About 50% of the dry weight of plankton is made up of carbon. Both carbon and calcium are removed when calcium carbonate shells are formed in the surface ocean and sink to become permanently buried (and thereby lost from the ocean) in seafloor sediments.

the plankton themselves (or rather, innumerable generations of their ancestors) which have in large part determined the chemical composition of the medium they inhabit.