Summary 1. The term ‘meiobenthos’ (or ‘meiofauna’) has been used in the literature in a broad sense to designate collectively small individuals, mainly metazoans, which could be separated from the larger animals by fine sieves of about i mm. or 0.5 mm. mesh. It has been variously defined in terms of the sampling or sorting technique employed. Thus used it indicates a purely statistical category including temporary members which are juvenile stages of the macrofauna, as well as permanent members –species of small adult size. 2. The term has also been used in a narrower sense, referring mainly to the permanent members and restricted to particular animal groups such as Nematoda, Harpacticoida, Gastrotricha, Kinorhyncha, Tardigrada, etc. This usage designates a more natural grouping of small organisms with certain biological characteristics as well as sampling requirements in common, which distinguish them from both larger and smaller organisms. 3. Special collecting and sorting methods are required for meiofauna. Small core tubes are most convenient for collecting, while sorting, often with stained samples, is usually done by elutriation, decanting or repeated sieving. Other techniques are discussed. 4. In the intertidal zone total population numbers range from IIXIO3 to more than 16 × 106 per m.2 On the majority of grounds nematodes are the most numerous group, with harpacticoids second. The highest densities are usually on the softer deposits in sheltered areas. 5. Subtidally the numbers on the shelf range from 4 × 103 to 3.2 × 106 per m.2 and again nematodes and copepods are the main animals, with soft deposits richest. Density declines towards deep water but even in the abyssal zone numbers range from 1.6 to 17.0 × 104 per m.2. 6. Intertidal distribution of meiofauna is determined by temperature and salinity and also by the grain size of the deposit which affects the interstitial space, water content, and availability of food and oxygen. 7. On intertidal muddy deposits the fauna is confined mainly to the upper few centimetres. The main controlling factor on some sediments seems to be reduced oxygen due to poor drainage, and on others the close packing of the particles leading to reduced interstitial space. This restriction to surface layers exposes the fauna to extremes of environmental conditions which in turn limit the intertidal distribution. 8. On intertidal sand, with interstitial space extensive and drainage better, life can be found deeper than 1 m. below the surface and the fauna can migrate vertically to aggregate in areas of optimum conditions. Oxygen is again of prime importance, but migrations are also caused by salinity changes and by interaction between species. 9. On subtidal deposits the fauna is much restricted to the superficial layers, but data on sandy grounds are sparse. 10. On beaches, marked seasonal fluctuations have been observed in the meiofauna, usually correlated with temperature and reproduction. In subtidal areas these changes are much less obvious. 11. The main predators on meiofau...

Plates I-II and Text-figs 1-4)The meiofauna was studied over a 10-year period on a flatfish nursery ground between the high-water mark and a depth of 10 m below low-water springs.The sediment was well sorted sand, with median diameter from 210 to 279 ji in the intertidal area and 160 to 208 ji in the subtidal. It was composed of medium rounded quartz, with the calcium carbonate content mainly 0-25 to 2-20 % by weight. Porosity was 33-39% and the coefficient of permeability ranged from 166 to 233 x io~2 cm per sec, indicating good drainage. The sand was usually over 90 % water-saturated, and seldom less than 60 %. The annual average concentration of particulate organic carbon was 205 /ig/g sand in the intertidal, and 684 fig/g at 5 m depth. Corresponding values for chlorophyll a were 0-75 and 4-50 /*g/g.The most numerous taxonomic groups were nematodes, copepods, turbellarians, gastrotrichs and tardigrades. Oligochaetes, polychaetes, coelenterates and archiannelids occurred in smaller numbers. Total meiofauna individuals averaged from 0-5 x 10 s to 6-7 x 10 6 per m 2 and the corresponding biomass from 279 mg to 1092 mg dry weight per m 2 . There was a distinct zonation of fauna, the richest populations occurring from around the level of mean low-water neaps on the beach to about 1 m depth below low water. The populations were most sparse in both numbers and species above mean highwater neaps.Below midtide level the bulk of the meiofauna was found in the upper 4 cm of sand, but there was a significant vertical migration apparently depending on the state of the tide, and a general downward movement during the winter. At higher beach levels the vertical distribution was less regular. During a 6-year period of detailed observation, significant overall annual change in the populations was detected only once, when, due to severe storms, there was a drastic decline in numbers, but recovery took place the following year.The optimum sediment for interstitial meiofauna appears to be well-sorted sand of median diameter about 230 fi, but other density-independent factors affecting the populations are discussed. On an exposed beach the typical meiofauna comprises a community of of high individual numbers with a stable and permanent set of species of great diversity. This is contrasted with the equivalent macrofauna community, and reasons for the differences in diversity discussed in terms of niche utilization.Meiofauna populations on exposed sandy beaches may be fuelled largely by soluble organic matter via bacteria attached to sand grains. No great interaction was shown with other groups on the beach and no significant transfer of material to higher trophic levels was observed. It is suggested that meiofauna in these circumstances may be largely at the top of a food chain. I N T R O D U C T I O NAs part of a comprehensive investigation of the food web in a sandy bay, a study has been made of the meiofauna, denned here as metazoans passing a 0-5 mm sieve. This paper deals with the composition, abundance and distribution of the...

No abstract

SUMMARYMeiobenthos, here defined as metazoans passing through a ½ mm screen, is described from core samples collected on muddy grounds at 100-140 m depth in the North Sea and off the west coast of Scotland between February 1962 and August 1963.Sixteen cores of 2·2 cm diameter were collected from Loch Nevis, and 14 from the Fladen ground. The top 7 cm of each core was passed through a 500μ then a 76μ screen. The residues were examined entire, and the nitrate of the finest mesh was subsampled.The fauna consisted mainly of nematodes, kinorhynchs, ostracods, copepods, polychaetes, and lamellibranchs. Between 61 and 97 % of the population consisted of nematodes, but copepods were second in importance and were more abundant than recorded on other subtidal grounds.The fine mesh (76μ) retained almost all specimens of every animal group except Nematoda. Numerically 38% of the nematode population passed through the fine sieve, but this represented only 18% by weight.The mean dry weight of meiobenthos on Fladen was calculated as 0·76 g/m2 and the average number of individuals was 196 × 104/m2.In Loch Nevis the corresponding weight was 1·28 g and the number 101 × 104. The loch had a larger and more varied population of all groups except nematodes, which were particularly abundant on Fladen.The data do not indicate significant population fluctuations throughout the year in the permanent meiobenthos.Compared with other subtidal grounds Nevis and Fladen seem to support much richer populations, and the high values for Fladen are of particular interest since this is a poor macrobenthos area.An attempt is made to indicate the possible level of meiobenthos productivity and it is suggested that there may be competition between meio- and macrobenthos for available organic matter which could result in a less efficient production of fish food.

SUMMARYThe fauna of a muddy area in the northern North Sea (the Fladen ground) is compared with that of a similar area in a Scottish west-coast sea loch (Loch Nevis). These areas were selected because certain similarities in environmental features and in fauna, coupled with their marked difference in biomass, make them particularly suitable for studies of the factors affecting benthic production.For macrobenthos samples (from a grab) a coarse sieve with round holes of 1-3 mm diameter and a fine sieve with meshes of 0-5 mm side were used. Meiobenthos samples (from a corer) were sieved through a 0124 mm mesh and the total residue stained in rose bengal.The Fladen ground, depth 140 m, had annual bottom temperatures between 6-31 and 8-22 C and salinities between 35-20 and 35-2.50. The deposit was mainly of coarse silt with abundant tests of small Foraminifera. The fauna is redescribed. The characteristic species was the foraminifer Saccammina sphaerica. By separating empty tests from living specimens it is shown that this species was not so abundant as had been thought. The main lamellibranch was Thyasira equalis, not T. flexuosa as recorded in previous work. Other important species were the polychaetes Paraonis gracilis, Myriochele heeri, Spiophanes kroyeri, Tharyx sp., Lumbrineris impatiens, and Nephthys incisa. The standing crop of macrobenthos was 6-42 g/m2 wet weight.