Like all plants, phytoplankton require light and dissolved nutrients-primarily nitrates and phosphates-to grow. Obtaining both light and nutrients is problematic. Light is available where nutrients are scarce, and vice versa.
Plants on land solve this problem with roots in the ground to take up nutrients and leaves above ground to absorb light. Phytoplankton in the sea must perform the same functions in a single cell.
Light from the sun penetrates only a short distance into the ocean surface. Consequently, most phytoplankton species live in the the upper 100 meters of clear water.
Nutrients, on the other hand, come from the sea bottom. If dissolved nutrients were constantly supplied at all depths, phytoplankton would flourish in the water column to some distance below the surface, depending on the clarity of the water and the intensity of light striking the surface. Below that, populations would decrease with decreasing light.
In fact, dissolved nutrients are not available at all depths in the open water of the Gulf of Mexico. Rather, dissolved nutrients are usually undetectable in the surface because phytoplankton have incorporated the dissolved nitrate and phosphate into their cells. Nutrient concentrations are low where phyto-plankton abundance is high.
In the surface layer of water, temperature and salinity are uniform because the water is mixed, primarily by wind. The thickness of this layer varies with wind strength and solar heating. Below this mixed layer, temperature decreases rapidly over a short distance. The interval where temperature changes is called a thermocline.
As water temperature decreases, density increases. The sharp change in density at the thermocline creates a barrier, inhibiting mixing of water from the upper layer with that below the barrier, where nutrients are found. Nutrients that support phytoplankton in the mixed layer are mostly recycled from the decay of organic matter within that layer.
As light decreases with depth, phytoplankton growth slows enough to allow dissolved nutrients to build up in the water. The depth where nutrient concentration increases is called the nutricline.
If the mixed layer is deep, the nutricline and thermocline coincide. This occurs in the Gulf of Mexico after deep winter mixing.
If the mixed layer is shallow, the nutricline and thermocline are separated. Sufficient light reaches below the thermocline to support phytoplankton growth, which depletes dissolved nutrients. In the gulf, this situation develops in late summer.
[~12K] Phytoplankton in the water column.
