Oceanography (2+1)

Silicates

Silica is required as a nutrient by all organisms, but it is especially necessary for the formation of skeletons for many diatoms and some other organisms. Regeneration of silica appears to be due to chemical rather than bacterial action. Silica that has passed through the digestive tract of some marine organism seems to be broken down much faster than silica not subjected to an organism’s digestive processes. Turnover time for silica is not known but some estimates indicate that the turnover rate is faster than for phosphate and nitrate.

Silica concentrations vary widely in the oceans. The variation in frustules-forming diatom populations and the difference between the regeneration processes for silica and for phosphate and nitrate cause silica distribution to differ somewhat from distribution for phosphate and nitrate. The vertical distribution of silica is shown for the various oceans in figure. In upper layers where silica is used by biological organisms, concentrations of less than 0.014ppm are often found. Deep waters of the North Atlantic Ocean contains about 0.98ppm silica, while North Pacific contain much more, about 4.8ppm. Deep water in the Antarctic Ocean and the Indian Ocean contains about 3.4ppm. Generally there is no intermediate ocean depth at which a maximum silica concentrations occurs, as there is with phosphate and nitrate. The concentration of silica increases with increasing depth in deep waters.

Variation of silicon

The reduction of silicon in the surface layer is the result of biological activity and the sinking of the organisms of the skeletal remains. Diatoms accumulate silicon from the sea and is returned to the sea after death of the organism. The skeletal remains forms permanent constituent of the sediments. Siliceous sediments in form of diatom are found in higher latitudes and in other regions, radiolarian skeletons form the major portion of the sediments.