Marine Biology

Dissolved Nutrients

Nutrients, like nitrogen (N), phosphorous (P), and potassium (K), are important for plant growth. The level of dissolved nutrients increases from animal faeces and decomposition (bacteria, fungi). Surface water often may be lacking in nutrients because faeces and dead matter tend to settle to the bottom of the ocean. Most decomposition is thus at the bottom of the ocean. In the oceans, most surface water is separated from bottom water by a thermocline (seasonal in temperate and marginal polar regions, constant in tropics) which means that once surface nutrients get used up (by the plants there), they become a limiting factor for the growth of new plants. Plants must be at the surface for the light. Nutrients are returned to surface waters by a special type of current called 'upwelling' and it is in these areas of upwelling that we find the highest productivity of marine life.

Silica and iron may also be considered important marine nutrients as their lack can limit the amount of productivity in an area. Silica is needed by diatoms (one of the main phytoplankton that forms the base of many marine food chains). Iron is just recently being discovered to be a limiting factor for phytoplankton.