Netherlands Journal of Sea Research最新文献

Benthic macrofauna was investigated, mainly in clayey silt sediments, on the west and east slopes (65–90 m depth) and at the bottom (≈100 m depth) of a trench in the Skagerrak, western Sweden. The western slope is an underwater delta front area with, at least intermittently, strong bottom currents transporting suspended organic and inorganic particles. In the deeper parts, currents slow down and accumulation is extremely high (mean ≈90 mm·y⁻¹). The benthic community on the western slope was dominated by the passive suspension-feeding brittle star Amphiura filiformis. Its numbers and biomass were much larger than recorded in the Skagerrak and the North Sea. Total community abundance and number of species were significantly larger on the western slope than on the eastern. In the deep part of the trench significantly higher abundance and number of species were recorded than on the slopes. Dominant in the deep were the small polychaetes, Heteromastus filiformis and Paramphinome jeffreysi, both assumed to be sub-surface deposit feeders.

Faunal distribution and richness are discussed in relation to food availability through advective near-bottom processes on the western slope, and in relation to accumulation at the bottom of the trench. The extreme densities and biomasses recorded on the western slope suggest that these communities were limited by space rather than by food. In contrast, similar communities characterized by A. filiformis in the Skagerrak and the North Sea have significantly lower abundance and biomass, and are therefore thought to be food limited.

The disappearance of the whelk from the western Dutch Wadden Sea started in the mid 1920s with a gradual decline due to overfishing and lethal shell damage by fishing gear. When fishery stopped in the early 1970s, tributylin-based (TBT) antifouling paints had become into use. Such paints caused imposex and possibly reproduction failure in the whelk leading to its local extinction. The whelk will disappear from larger parts of its present distribution area if fishery-free areas do not become effective, and if the use of TBT-based paints continues.

In situ grazing of benthic heterotrophic nanoflagellates on bacteria was studied in a wide range of sediment types in the North Sea during summer and winter. Grazing rates were measured using the fluorescently labelled bacteria (FLB) technique. Several factors may potentially influence flagellate grazing, viz. temperature, sediment grain size, bacterial abundance and production, flagellate abundance and biovolume. Flagellate grazing rates were related to these variables. Total grazing of the benthic flagellate population was estimated by multiplying flagellate abundance by maximum/minimum grazing rates. A comparison with the total amount of bacterial cells produced provided an estimate of the impact of flagellate bacterivory on benthic bacterial production.

Individual grazing rates ranged from 0 to 112 bacteria per flagellate per hour. Grazing rates showed no significant differences between summer and winter and did not correlate significantly with any of the above-mentioned variables. Maximum average grazing rates ranged from 15 to 39 bacteria per flagellate per hour. Minimum average values varied between 1 and 5 bacteria per flagellate per hour.

The percentages of benthic bacterial production accounted for by flagellate consumption differ greatly depending on the grazing rate used. Using maximum grazing rates, 2 to 23% of the bacterial production during summer was consumed, while in winter this ranged from 23 to 528%. Minimum estimates were 10 to 20 times lower: 0.2 to 3% in summer and 1 to 50% in winter, higher percentages of the bacterial production were consumed, which was probably due to the relatively lower bacterial production in winter compared to summer.

A previously unrecorded surface trace is described from a towed underwater TV survey of the southern Clyde Sea area. The trace comprises a mound with a single central opening, surrounded by a shallow trench with between 4 and 7 openings. The feature is approximately 10 cm in diameter, and occurred in pairs in the majority (76%) of observations.

Spatial variability in the density of the feature was noted with densities estimated between 0.00134 and 0.0193 per m². The spatial distributions were analysed using quadrat and nearest-neighbour techniques, and were not found to be significantly different from random.