Meiofaunal nematodes and the decomposition of kelp

Several aspects of the involvement of free-living nematodes in the decomposition of the kelp Laminaria saccharina have been investigated. The seasonal pattern of growth and decay of the L. saccharina population at a site on the Isle of Cumbrae was followed. Brightfield, fluorescence and scanning electron microscopy were used to study the distribution of all epiphytes from bacterial size upwards on fronds of different year-groups over a period of one year. The areas of decomposing tissue at the frond tip had a distinctive microbial community: bacteria of several morphological types, yeasts, diatoms, flagellates and ciliates. Most epiphytes on the intact frond surface (diatoms, rod-shaped bacteria) were most abundant on the older, apical part of the frond. In the spring and early summer a network of filaments of an ectocarpoid alga developed at the frond tip, and around these filaments accumulated a mucilage matrix containing bacteria, diatoms, flagellates, ciliates, sediment grains, faecal pellets etc. The distribution of hydroids, bryozoans etc. is described. The distribution and abundance of nematodes were studied over the same period. Four species dominated the fauna: Monhystera disjuncta, M. refringens, Chromadora nudicapitata and Theristus acer. Nematodes were virtually absent from the meristem and mid-frond regions. The first three of the above species are of similar body size, were present throughout the year, both on the tip surface and in the decomposing tissue, and bred continuously. An attempt is made to relate their seasonal changes in abundance to those of particular epiphytes, considering also data from gut contents analyses. T. acer is a larger nematode. As the ectocarpoid filaments grew in spring, progressively larger T. acer colonized the frond. One cycle of T.acer breeding took place, and then the filaments died, the material trapped around them dispersed and T. acer disappeared from the frond. Gut contents analyses suggested that T. acer fed on mucilage and bacteria. M. disjuncta appeared to be the nematode most closely associated with decomposing tissue. This species was cultured and its feeding 32 on bacteria isolated from the frond was investigated using P as a label. There was significant uptake of label from only one of the four bacterial strains tested. Comparison of the gut retention time determined by microscopic observation, with the time-course of uptake of label showed that assimilation was occurring. Uptake of label from solution was insignificant. L. saccharina plants were allowed to decompose below the photic zone, and changes in the nematodes and epiphytes were followed. The rate and process of decomposition were essentially the same as those of distal tissue loss in living plants. A preliminary attempt at respirometric measurement of decomposition rates in the laboratory is described