Seasonal growth of two subtidal species of rhodophyta off the isle of man

Abstract

Whole plants of Delesseria sanguinea (Huds.) Lamour. and Odonthalia dentata (L.) Lyngb. were collected from 2 m below lowest astronomical tide on Port Erin breakwater and 1 to 17 m off Spanish Head, Isle of Man, Irish Sea. In shallow water there was a four- and three-fold seasonal variation respectively in mean plant weight in the two species. Individual blades were measurable by mid-February and doubled in weight about every 2 wk between then and late March. Growth was slow by June but continued until August. In Odonthalia there was a loss of whole blades in late summer but little reduction in blade size while in Delesseria both losses occurred in autumn. In deeper water blades of each species grew more slowly, particularly early in the year. A minimum age could be ascertained for each part of Odonthalia plants and from this it was apparent that few blades survived the first year but those that did lost little weight. This species was shown to accumulate tissue over the years, older plants being capable of bearing more tissue of each year's growth. In shallow water Odonthalia plants probably often lived > 5 yr and Delesseria plants probably longer. In deeper water plants were shorter lived and smaller. There was slender evidence for some dependence on stored material for early spring growth.