Silicon is present in seawater chiefly as silicate ions and possibly sometimes minute traces of colloidal silica. It is a constituent of the diatom cell wall, some radiolarian skeletons and some sponge spicules.
The concentration of silicate at the surface is usually low, but increases with depth to between 1 and 5 mg Si/€ in deep ocean water, the highest values being in the deep Pacific. In the English Channel, surface concentrations of 200-400 [ig Si/€ have been recorded during the winter, falling rapidly to as low as 10 ¡xg/( during the spring diatom peak. During the summer months, surface concentrations often show rapid and considerable fluctuations.
Although much of the silicate incorporated in the diatom cell wall is probably returned to the water fairly quickly after death, siliceous deposits of planktonic origin cover large areas of the sea-bed (see page 216).
Ferric hydroxide is almost insoluble within the pH range of seawater. The amount of iron in true solution is probably not more than 2 ¡g Fe/€, but there are appreciable quantities of iron in particulate form as colloidal micelles, mainly ferric hydroxide and traces of ferric phosphate, ferricitrate or haematin. This particulate iron can be removed from seawater by ultrafiltration.
Estimates of total iron vary considerably with time, place, and different techniques of measurement, generally within the limits of 3-70 ¡g Fe/€. Inshore water usually contains much more iron than oceanic water, especially in the vicinity of estuaries where river water often transports relatively large quantities of both dissolved and particulate iron. In the English Channel, fluctuations from about 3 to 150 ¡g Fe/€ have been recorded, high values being obtained at the surface and near the bottom. Surface values usually show marked seasonal reductions following peak periods of diatom growth.
There is probably a continual loss of iron from seawater, and accumulation at the bottom, due to adsorption on sinking detritus and sedimentation. Iron is an essential plant nutrient, and also has various roles in animal physiology. It is a component of the cytochrome enzyme system and the blood pigments haemoglobin (vertebrates, some annelids and some molluscs), haemerythrin (some molluscs and crustacea) and chlorocruorin (some annelids). The amount of iron in solution seems inadequate to support rapid plant growth, and it is possible that marine plants can utilize particulate iron in some way, perhaps by gradual solution of particles adsorbed on the cell wall, or even by actual ingestion by certain plants which have exposed protoplasm.
Manganese is a plant nutrient which, like iron, is probably present mainly in particulate form as oxide micelles, in amounts between 0.3 and 10 ¡g Mn/€. In deep water the surface layers may become depleted of particulate and adsorbed iron and manganese by losses through sinking, and this may limit the amount of plant growth that can be supported. There is often more iron and manganese in neritic water due to replenishment by land drainage. In experimental cultures, enrichment of samples of ocean water by addition of iron and manganese sometimes results in a considerably increased growth. Recent experiments in which iron sulphate was added directly to areas in the Pacific where phytoplankton biomass was low, demonstrated greatly increased phytoplankton growth (see Section 9.6.4).
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