RIASSUNTO
Preamble: The nature of weed areas around Britain.
Extensive tracts of the inter-tidal and sub-littoral zones of coasts display an array of seaweeds, which may be profuse enough to form weed 'forests' within the marine ecosystem, (19,20,21) providing some protection against bed-scour, tending to obscure bottom morphology, and acting partly as sediment-traps. The weed areas often coincide with areas where resource development is taking place, or is anticipated.
Much of the available literature relating to seaweed concerns plant characteristics and spatial distribution. The dominant inter-tidal weeds (or wracks) either belong to the genus ""Fucus"" or are closely related to it. Much sub-littoral rock around Britain is covered by large brown seaweeds, the larger ones mostly belonging to the genus ""Laminaria"" because of their flat lamina or frond. The frond develops from a stalk or stripe which is partially flexible, strong enough to resist normal wave and tidal action, and attached to rock, boulders, shingle or gravel by a holdfast. The nature of the substrate is important since an increase in the percentage of the rocky substratum generally means an increase of seaweed.
The spatial distribution and productivity of seaweeds (which reproduce from spores provided by the fronds) below low tide level depends largely on the extent to which sunlight penetrates the water, this being influenced by water depth and prevailing turbidity levels. For example, high sunshine figures for 1955 led to outbursts of weed growth around South-East England. At high latitudes, seaweeds may survive down to about 60mi and in tropical and sub-tropical regions, where sunlight penetrates far deeper, seaweeds may survive even to some 183m. Some species may even occur in excess of 305m. In relatively clear British waters, seaweeds can develop down to c.50m in summer, 10-15m in winter. (13) Brown and red algae, which have other pigments besides chlorophyll, can survive with less sunlight and in deeper water.
Of 800 kinds of attached weed around Britain, laminarians (which are found in all oceans) are the largest, and generaly occur in depths of 2-15 m. The most important species of the genus Laminaria, Laminaria hyperborean, may form 'forests' in the L.W.S. to 6-21 m depth range, normally inhabiting fairly exposed coasts, on stable bottoms and frond occurs between January and June, growth during the remainder of the year normally being very slow. In general, there is a decrease in stripe production, (for all laminarians) from North-East to South-West England, with a subsidiary decrease up the English Channel. About April-May, the old frond gets torn off during gales, leaving new ones to develop. The older plants of these species in the forest normally survive for about 10 years. ""L. Digitata"", a similar species, grows in a zone just above L.W.S., often covering the lower foreshore, and has a more flexible stripe.