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...

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The long-term effects of diesel and four different low-toxicity oil-based drilling-mud cuttings on the chemistry and benthic fauna of a marine sediment were compared. Eighteen tanks (mesocosms) containing beach sediment were deployed in a control and five treatment, three replicate experimental set-up. The drill cuttings used had total oil concentrations ranging from 6 to 15% and were added to the tanks in quantities calculated to be representative of hydrocarbon levels measured in sediments 400-500 m from platforms in the North Sea. Selected parameters monitored at varied intervals throughout the experiment were: redox profiles, sulphide concentrations, hydrocarbon concentration and meiofaunal abundance. Numbers of macrofaunal organisms evacuating the sediments in the seven days after treatment application were also recorded. Redox measurements showed all sediments other than controls to be significantly reduced, but no differences were observed between treatments. Sediments became most reduced after 3 months and showed recovery thereafter, approaching control values after 15 months. Sulphide levels peaked at about 3 months and declined thereafter, but were still elevated above control levels after 15 months. Highest concentrations were recorded in treatments with the highest oil content. Mesobenthic meiofaunal abundance was significantly reduced in all cuttings treatments, but effects from individual treatments were indistinguishable. Epi/ endobenthic copepod abundance increased markedly in all low-toxicity treatments, and particularly in those with lower oil content, but remained similar to control levels in diesel treatments throughout the experiment. In the seven days after treatments, sediment evacuation rates by Tellina were highest in diesel and tended to reflect oil concentrations in low-toxicity treatments. The results show that in equal oil concentrations diesel and low-toxicity oil-based drilling-mud cuttings have indistinguishable effects on sediment chemistry, but that, even after 15 months of weathering, diesel-based cuttings are demonstrably more toxic to benthic fauna.

The impact of hydrocarbon discharges on the intertidal and subtidal meiobenthos of the North Sea is examined primarily by a consideration of two field investigations. The first study examines the effects of an oil refinery discharge on intertidal meiofauna in the Firth of Forth, while the second describes the impact of oil platform discharges on the surrounding meiobenthos. The impact of the refinery effluent is only clearly distinguishable upstream of the discharge, as downstream the effects are confused with those of a second petrochemical discharge. The meiofaunal community is only strongly affected on the upper shore and this appears to be chiefly the result of an organic enrichment effect causing a raising of the redox potential discontinuity (RPD) layer. All meiofaunal taxa examined are sharply reduced in density and species richness within 320 m of the discharge but at 600-900 m from the discharge meiofaunal densities are enhanced or depressed, relative to clean sediments, dependent upon the seasonal pattern of the RPD layer. Farther down the shore the impact is only felt at most by a slight reduction in species richness and subtle change in species abundance patterns on the middle shore for a distance of about 600 m. The meiofaunal responses to the petrochemical discharges seem similar to those described for the macrofauna in the same area, although a small meiofaunal population persists in the most polluted sediments in the absence of macrofauna. The discharge of drilling cuttings, contaminated with oil-based drilling mud, was found to strongly modify meiofaunal densities within 800 m of the Beryl A Platform. Nematode densities are strongly reduced in the vicinity of the platform and it is thought that the impact on this infaunal taxon may be due to slow degradation within the sediment of toxic fractions of the diesel base of the drilling mud. By contrast copepod densities were greatly enhanced in one survey and the difference in impact is considered to be due to the epibenthic habit of the species involved, enabling them to flourish in conditions of high food or low predation and competition or all three. The species involved seem typical members of meiofaunal communities of organically enriched sediments. Some improvement in meiofaunal densities throughout the period 1984-85 is thought to be possibly the result of a switch from diesel-based to lowtoxicity drilling muds. It is concluded from these and other studies that hydrocarbon discharges into the North Sea are unlikely to be causing extensive damage to meiofaunal communities.

SynopisThe coastal areas in the Firth of Clyde, of which Irvine Bay is a representative example, receive important amounts of industrial and urban wastes which are a cause of economic and environmental concern.Surveys of the benthos of Irvine Bay revealed a varied fauna dominated by large densities of opportunistic polychaetes such as Chaetozone and Spio and nematodes whose distribution correlated very strongly with the high organic carbon levels in the bay. The high densities and large biomass recorded in Irvine Bay were found to be 2–3 times higher than values from Loch Ewe, a clean area with normal carbon values.All evidence from this survey points towards a situation in which the fauna is enhanced, yet does not suffer a species reduction.