Salinity

Nutrients

The salinity of seawater has not varied much over the past 600 million years, so it is not surprising that the majority of the seaweeds will tolerate only a limited range of salinities. The sublittoral seaweeds of truly marine habitats live in an osmotically constant medium of 30-35 psu. Experimental work with a range of species suggests that the tolerance range for salinity fluctuations over periods of up to 24 h may be 16-50 psu (Lüning, 1990). Growth rates of many seaweeds are maximal at an optimum salinity, but this is usually in the range of 30-35 psu. Kelps are stenohaline seaweeds, in that they do not tolerate wide fluctuations in salinity; in fact, the growth rate may be adversely affected if the kelp plant is subjected to periodic salinity stress. Localised, long term reductions in salinity (possibly due to coastal construction work and drainage alterations) may result in the loss of kelp beds in the affected area.

All kelp species are thought to be efficient absorbers of nitrate and phosphate from seawater. Numerous experiments have been conducted on kelp species throughout the world in order to determine the rates at which kelps are able to take up these major nutrients and to determine the effects of changes in nutrient concentration on the growth rates of kelps (Birkett, 1993). The mechanisms of nutrient uptake are less well understood, as are the mechanisms governing the luxury uptake and storage of nutrients. When nutrients are available to kelps in amounts that are greater than needed to meet immediate metabolic requirements, nutrient uptake will continue (luxury uptake) and the excess is stored in the kelp tissues.

In the laboratory, kelp species respond to increased nutrient levels by taking up the nutrient more rapidly, and the addition of fertiliser to some kelp species (Macrocystis, Laminaria japonica) in situ has been reported to result in increased production (Lüning, 1990). However, all kelp species investigated to date show a similar pattern of responses to excess nutrients in that a maximum rate of uptake is reached at any given nutrient concentration. The maximum uptake rates are governed by the nutrient concentration to which the plant was previously accustomed and by the amount of light available to the plant in the hours preceding the availability of higher nutrient concentrations. There is a complex interaction between nutrient concentrations and their uptake rates, rates of photosynthesis, rates of production of DOM and POM (dissolved and particulate organic matter) and the growth rates of kelps. An increase in nutrient levels in the North Sea and the Irish Sea as well as in coastal waters throughout Europe has been observed over the past 40 - 50 years (long term data sets are few, one is continuing at the Marine Laboratories, Port Erin, Isle of Man).

• The long-term effects of increased nutrient loading on coastal kelp bed communities and the productivity of kelp plants are not known and deserve detailed investigation.

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