Not all of these nutrients are in short supply. Calcium and carbon are used to make calcium carbonate skeletons in plants and animals and both are found in such abundance that they are never exhausted and so cannot restrict growth. Phosphate, nitrate and silica are also basic building blocks needed by plants in the sea and once they have been used up the phytoplankton growth stops. In many areas, phosphates and nitrates are spread on the land to fertilise fields. However, heavy rain can do a very good job in washing these down to the sea where they end up fertilising the surrounding waters.
Sewage can have a similar enriching effect.
Both will cause phytoplankton blooms which may enhance the available food in the sea, but can also be unpleasant as was the case in 1993 when much of the North Eastern seaboard of Italy suffered from algal mats washing up on its shore. These then rotted down, which was unpleasant for the human population and life-threatening for any marine animals caught beneath it. As a result, it is in these surface waters that the nutrients are most heavily used. In spring in British coastal waters these nutrient salts are rapidly used up by increasing numbers of, at first, phytoplankton and then by the zooplankton that feed on them. The importance of this process becomes clear when the reproductive patterns of coastal wildlife are looked at in more detail.