The surface of the ocean represents the interface where the atmosphere, hydrosphere, and biosphere are in contact. Autotrophic phytoplankton (organisms that photosynthesize their own food) require light from the sun to pass into the water to power chlorophyll to fix carbon, that is, to synthesize primary production, sugars and carbohydrates, through photosynthesis. Primary production is measured in grams of carbon per m2 per day.
Most primary production occurs in the upper half of the photic zone (the thin surface layer of lighted water that is rarely deeper than 200 m), in the zone called the euphotic zone (meaning good zone). The euphotic zone is where there is enough light so that plants capture more light energy than they require to just make a living (i.e., grow, reproduce) and so can store excess carbohydrates and sugars (inorganic carbon turned into organic or fixed carbon). However, all plants respire in burning food to live, grow, and reproduce. The depth at which primary production equals respiration is called the compensation depth. Above this depth, phytoplankton can make a living; below this depth, they cannot and either die or go into a resting stage to await better light conditions. They can live but cannot grow or reproduce. Some phytoplankton survive the low light or darkness of winter by going into a resting stage and return to the surface to seed the next spring's bloom.
The compensation depth coincides with the depth in the ocean where the light level is 0.1% to 1% of the amount of the sunlight entering the surface of the ocean. This also coincides with the bottom of the euphotic zone. Diatoms have a deeper compensation depth, as low as the 0.1% light depth, than do dinoflagellates, probably closer to the 1% light depth. This is because diatoms are more efficient than dinoflagellates and can photosynthesize in lower light levels. The compensation depth varies greatly from 100 m in the clearest ocean waters to a few meters in turbid or muddy coastal ocean waters. In very productive water with an abundance of nutrients and an explosive algae population, sunlight can penetrate to a depth well under a meter.
The amount of light at ocean depth depends upon the intensity of sunlight passing through the atmosphere and striking the surface of the water, which varies with the time of day, season, angle of the sun above the horizon (solar altitude), cloud cover and thickness, sea waves, and any factor that would affect the transparency of the atmosphere (e.g., haze, smog). Once sunlight penetrates the water, the compensation depth varies with ocean conditions. For example, with an increase in production there is an increase in phytoplankton populations, as well as the numbers of zooplankton that eat the phytoplankton. This creates shade for phytoplankton deeper in the water column, decreasing the available light and raising the level of the compensation depth.
The Secchi disc is the simplest way to estimate the depth to which visible light reaches in the water column. The Secchi disk is simply a plate-like device about 20 cm in diameter hung from a line that is marked off in one-meter increments. The disk is oriented so that it is parallel to the ocean surface. The disk is marked off into alternate black and white quadrants to make it easier to see at depth. It is lowered until the observer can no longer see the disk (cannot distinguish between the black and white quadrants), and that depth is recorded. It is then raised until the disk just reappears, and this depth is recorded. The Secchi depth is the average of these depths. This then, is an estimate of the compensation depth of 0.1% to 1% light depth. It is semi-quantitative, but the method is simple and is still used today. Of course with modern electronics, very precise measurements of the transparency of ocean water to sunlight are made from profiling instruments.
Note also the following:
Increased sunlight also heats water creating a pycnocline along with stratification, which can limit the depth of surface mixing. If the pycnocline were shallower than the compensation depth, then higher productivity would occur because the phytoplankton are held in the euphotic zone, at least until the nutrients run out or the stratification breaks down. Therefore, the sun driven stratification can be helpful until the nutrient supply runs out but then the stratification may seal new nutrients off below the compensation depth and thus hinder productivity.
Tropical waters are clear because of lack of nutrients to fuel primary production. With lower concentrations of phytoplankton in the water, the compensation depth can be quite deep, perhaps 100 m. The Mediterranean Sea and the central subtropical gyres (Chapter 6) are examples. In both cases, downwelling of warm, nutrient poor water occurs so that low productivity results, along with clear water.
Production usually decreases with depth. However, often a zone of slightly lower production is found in the top one to two meters overlying the maximum production depth because solar ultraviolet radiation can penetrate this upper layer. UV interferes with chlorophyll. With the decrease in stratospheric ozone, the level of UV in the upper few meters of the ocean has increased, causing an even more pronounced decrease in surface productivity.
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Prepared by H.J. Niebauer and Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2018, The American Meteorological Society.