Seasonal growth and photoinhibition in Plocamium cartilagineum (Rhodophyta) off the Isle of Man

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Abstract
Length frequency determinations in subtidal populations of Plocamium cartilagineum provided information on growth of upright thalli in relation to season and depth. At 2 m below lowest astronomical tide (LAT), under the laminarian canopy, growth started in March and was exponential until June, with a relative growth rate in weight of 0.026 per day. Under wave exposed conditions few mature uprights survived into the following winter. In deeper water, under the laminarian canopy, growth was slower and deeper still (13–17 m) mature uprights survived throughout the year. Clearance of the phaeophycean canopy allowed faster growth over an 8 m depth range but in early summer appeared to reduce the growth at 1 m below LAT. Uprights held on blocks at 0.5 m below LAT and in tanks exposed to reduced mid-summer daylight grew more slowly than uprights receiving less irradiance. It seemed that 0.5 mmol m−2 S−1 was inhibitory. These data agree with the concept of the species as ‘shade loving’, with a maximum relative growth rate which is slow compared with currently cultivated red algae but probably fairly typical of subtidal undergrowth species. At most depths, however, it is light limited for much of the year.