Let me begin by saying that the best and easiest way to make your point to a room of scientists is to make a graph. Scientists LOVE graphs. They love graphs more than they love New Balance and The North Face, which is a very telling statement if you’ve ever seen a scientist. Scatter plots, bar graphs, pie charts, whathaveyou. If you can put it in a graph, they’ll love you forever. When we plot nutrient concentration vs. depth in the ocean, we end up with a plot like this:It doesn’t matter if this is a graph of phosphate, nitrogen, or silica, most nutrient curves typically have this shape. Why, you ask? Well, let’s start from the top and move our way down.
Oceanic phytoplankton can only live in “The Mixed Layer’. Remember that phytoplankton are plants so they need sunlight to grow and nutrients to be happy. Depending on how murky your water is, sunlight can only penetrate about 60-100 meters down, at the most. The Mixed Layer is phytoplankton Party Time. Such Party Time, in fact, that they tend to use up every last molecule of available nutrients in the surface ocean – that’s why we have a concentration of practically zero in the top 10s of meters on our fancy graph above.
Particles like poo-poo are loaded with avalible nutrients. When that stuff falls through the mixed layer, the nutrients get recycled back into the water column. Below the mixed layer, the nutrients redissolve and are up for grabs again! It’s at this depth that we see a maxima in nutrient concentration.
The rest of our nutrient profile is pretty bland. As you can see, we have a pretty constant concentration except for a wee increase in the deep sea. This increase is no figment of your imagination or analytical error. It was a real head scratcher for ocean chemists for a long, long time because it shows up everywhere in the world and gets bigger as you move from the Atlantic to the Pacific. This, my friends, is evidence of the Thermohaline Circulation. If you were unfortunate enough to have seen “The Day After Tomorrow”, you would have gotten at least the jist of the Thermohaline Circulation – the deepwater “conveyor belt” that moves water from the surface ocean in the North Atlantic all the way around the world until it pops back up in the North Pacific. This current is super slow, but transfers heat all over the world, and apparently deadly storms, and Dennis Quaid, and then we all move to Mexico. The point is that the deep water is really, really old. Because it’s really old, it’s collected tons of particles that have rained down into it as it makes its way around the world. This gives us that slight bump at the very bottom of our graph.
See, now you know how to explain a nutrient profile to your friends! Use this knowledge wisely, as the power of oceanography is unwieldy at best.