Heterotrophic Utilisation of Mucilage Released During Fragmentation of Kelp (Ecklonia maxima and Laminana pallida) II. Differential Utilisation of Dissolved Organic Components from Kelp Mucilage

Abstract

Mannitol, and to a lesser extent sugars plus alginates, released in mucilage during fragmentation of kelp is utilised by bacteria in the water column. Incubation of 6.4 g 1-' dried mucilage from Ecklonia maxima and 7.2g 1-' from Laminaria pallida at 10 "C results in a utilisation of more than 50 % of the mannitol within 48 h whereas alginates plus sugars reach 50 % of their initial concentration in 6-10 d. There are, however, marked seasonal differences in conversion efficiency. In the winter, incubation media were colonised by small cocci and rods and the increase in biomass of bacteria per unit carbon loss was approximately 12 %. Incubation of the same mucilage from L. pallida in seawater collected during the summer, however, resulted in the development of a population of large rods with a biomass about 3 X that of the smaller rods which predominated in the wlnter incubation experiments. The increase in biomass of bacteria per unit carbon loss amounted to as much as 29.4 % in the summer incubation experiments. The annual dry weight of muc~lage production during fragmentation from a small kelp bed of 700 ha is 1458.38 X 104 kg and estimated to be capable of supporting a dry biomass of approximately 30 X 104 kg bacteria and 3 X 104 kg dry biomass of flagellates and ciliates. The high density of filter-and deposit-feeding organisms which characterise the kelp bed suggests that the community as a whole is largely dependent on the flow of energy through these initial stages of the decomposer food chain based on kelp.