Marine Biology

4. Nutirents

The concentration of phosphates and nitrates, which are the two major plant nutrients, has been recoginsed as one of the major factors limiting primary production in aquatic systems. It is known that during photosynthesis phytoplankton absorb these nutrients for the formation of particulate organic matter. Due to this absorption, the concentration of the nutrients in the euphotic zone decreases and this naturally limits further organic production. A certain amount of nutrients utilized by phytoplankton are however, regenerated by bacterial activity within the euphotic zone itself. But a good amount is lost from the euphotic zone as a result of the sinking of phytoplankton as well as through consumption by zooplankton inhabiting deeper levels during day time. Thus, much of the nutrients absorb from the euphotic layer are transferred to the deeper zones of the aquatic systems (marine or lakes), where they are regenerated. The nutrients that accumulate in the deeper levels, particularly in the oceans, are mostly returned to the surface waters by vertical mixing process such as upwelling, eddy diffusion, vertical convection and wind mixing. In addition to this, land drainage and river influx also contribute to the replenishment of nutrients of the surface waters, at least of the coastal areas.

Seasonal and regional variations of primary production are often attributed to the influence of these plant nutrients. The cycle of phytoplankton in temperate latitudes, with marked peak in spring and decrease in production during summer, has been correlated with the changes in nutrient levels. During winter, owing to vertical mixing, the euphotic zone is enriched with nutrients which result in high productivity during spring. On the other hand, in summer, owing to the increase in temperature and the consequent formation of the thermocline, the nutrient replenishment in the euphotic zone by vertical mixing is prevented and this results in the decrease of production.

In the tropical waters, a permanent thermocline is present throughout the year and consequently, replenishment of nutrients by normal mixing process does not take place. Thus, short supply of nutrients in the tropical waters seems to impose a restriction on the rate of primary production. The high productivity recorded in the waters of the south-west coast of India, especially during the south-west monsoon period, has been correlated with the upwellings in this region, which bring about high nutrient concentration in the surface waters. However, the individual species requirements for phosphates and nitrates are not fully known.