Rob Witbaard

The consequences of changes in abundance of Callianassa subterranea and Amphiura filiformis on sediment erosion at the Frisian Front (south-eastern North Sea)

In the early 1990’s a conspicuous shift took place in the density of two key macrofauna species at the Frisian Front (SE North Sea). The density of the formerly dominant brittlestar Amphiura filiformis showed tenfold reduction and remained low throughout the observation period (1986–2000). In the same period, the burrowing mud shrimp Callianassa subterranea showed a fourfold increase in density. Since the Frisian Front is a transition area where tidal currents fall just below the erosion threshold of silt and bioturbating mud shrimps can potentially affect the sediment erosion threshold, we examined the effects of the species on sediment erodibility. Two experiments were conducted in laboratory annular flumes. In the first experiment we compared the effects of pre-shift and post-shift combinations of the two species on the erodibility of Frisian Front sediment. The results indicated that the erosion was facilitated by a combination of high densities of C. subterranea and low densities of A. filiformis, representing the situation in the late 1990s. The difference in erodibility was already apparent at velocities equal to the critical near-bed current speed measured at the Frisian Front. In the second experiment the effects of pre- and post-shift densities of A. filiformis on sediment erodibility were compared. This showed that low A. filiformis densities as found in the late 1990’s facilitate erosion, but the effect is only significant at velocities above the critical near-bed velocity in the field. At the latter velocity, no significant difference in erodibility was found between the two A. filiformis densities. This implies that the difference in turbidity in the first experiment was mainly due to the activity of C. subterranea. The experimental results indicate that resuspension at the Frisian Front may occur more frequently as a result of the shift towards a Callianassa dominated community. More frequent resuspension of bottom sediment, most likely amplified by the climate induced increase of wind strength in the North Sea, may hamper the recovery of the A. filiformis adult population.

The effect of tidal resuspension on benthic food quality in the southern North Sea

In the present study we estimated differences in shell growth rates ofChamelea gallina (Linnaeus), from relatively food poor areas with growth rates from specimens collected from a frontal area, which has been regarded as highly productive. The results have been compared with growth rates obtained for two other benthic interface feedersArctica islandica (Linnaeus) andEchinocardium cordatum (Pennant) from the same geographical area.The results suggest that skeletal growth of above three macro benthic invertebrates in organically poor sediments is faster when compared to growth rates of the same species from a food rich frontal region with fine grained sediment. This led to the idea that growth rates and food quantity are at least partly uncoupled. As an alternative hypothesis we think that food quality is one of the key factors to explain the observed differences.To test this hypothesis we tried to estimate the differences in the quantity and quality of near bottom suspended matter at two opposing but hydrodynamically representative regimes in the southern North Sea. At these sites we deployed a new sediment water interface sampler, which caught particles from approximately 10 cm above the bottom. These samples were analysed for their ratio of chlorophyll to total dry matter to give an impression of the food quality.In the shallow sandy sediments with strong tidal currents the highest ratio of chlorophyll: dry matter was found. In the silty frontal area with weaker tidal currents, which was supposed to be food rich, the food quality was poor because it was mixed with large quantities of resuspended silt. This dilution effect appeared to be related to the tidal cycle in combination with the difference in settling velocities of the mineral particles at slack tide. In the southern area the coarse sand grains quickly settle at slack tide. In the frontal area fine silt particles remain long in suspension thereby depressing the quality of suspended matter. This difference is believed to be an important mechanism behind the unexpected low growth rates in the frontal area when compared to the supposedly food deprived Southern Bight.

The mollusc fauna along a depth transect in the Faroe Shetland Channel: Is there a relationship with internal waves?

Abstract From boxcore and dredge samples taken along a depth transect across the Faroe Shetland Channel (FSC) in 1997 and 1999, molluscs were sorted and identified. On the basis of their numerical abundance, the stations could be clustered into three groups, reflecting the depth extension of the main water masses in the FSC. Between 400 and 500 m depth, both the number of taxa per sample and the number of specimens, were highest. Bivalves were mainly caught by the boxcorer and the dredge was best suited to collect the epifaunal gastropods. According to the literature, at mid-slope depths, resuspension is increased by the action of internal waves, which leads to the development of intermediate nepheloid layers. This increased concentration of suspended matter may serve as a food source of low quality. Likewise the low amounts of suspended matter with a relatively high quality of particulate organic matter, such as from ∼400 and ∼700 m may act similarly and explain the relative high abundance of filter feeders at these depths. However, in the various multivariate analyses we could not distinguish the fauna from these depths from that collected from other sites. We observed, however, a pattern in the depth distribution of feeding guilds across the slope of the FSC. Deposit feeders demonstrated a trend of decreasing numbers with depth. Filter-feeding taxa were found to have peaks at depths of approximately 350 m, between 500 and 600 m and between 700 and 800 m. This observation supports the idea that the fauna at the deep sites reflects the locally high cross-slope currents and the subsequent increased amounts or availability of food with a better quality, which seems to be related to resuspension events.

Growth variations in the bivalve Mya truncata: a tool to trace changes in the Frisian Front macrofauna (southern North Sea)?

Annual monitoring of the benthic fauna living at the Frisian Front (southern North Sea) has shown a tenfold decrease in the dominant brittlestar Amphiura filiformis in 1993–1995. In search of evidence that this decline was caused by a change in benthic food supply, we analysed variations in the shell growth of the bivalve Mya truncata from the Frisian Front during the period of interest. For this purpose, the widths of the internal growth bands in the chondrophore of M. truncata were standardised and assigned to calendar years. Averaging the yearly band width in the period 1985–2000 among 25 individuals revealed low growth rates in 1986 and 1992. Growth of M. truncata quickly recovered after 1992, while A. filiformis densities remained at low levels. Moreover, the 1986 dip in M. truncata growth had no equivalent in A. filiformis density. We conclude that there is no direct coupling between fluctuations in density of A. filiformis and variations in growth of M. truncata. The data we collected during this study on the size and spatial distribution of M. truncata are discussed in the light of plans for the protection and conservation of long-lived benthic organisms in the North Sea.

Patterns of growth and undetectable growth lines ofAstarte sulcata (Bivalvia) in the Faroe-Shetland channel

To study the effects of increased near-bottom turbulence, individuals ofAstarte sulcata (Bivalvia) were collected along a depth gradient in the Faroe-Shetland Channel during a research cruise in April 1999. These bivalves were found at depths ranging from 250 m to 800 m. We analysed their long-term growth by means of counting growth lines in the hinge. The average shell size differed at the different depths. The largest animals were found at 300 m (average: 22–23 mm) and the smallest ones at 600 m (average: 15–16 mm). A trend was also found in the age structure at 250 m to 350 m depth.Astarte at 250 m possessed on average 45 growth lines compared to an average of 69 lines in animals obtained from 350 m depth. It was impossible to determine the relative age ofAstarte from 500 m, 600 m and 800 m depth because they did not have clearly defined growth lines. The above-mentioned differences in size, age and growth line visibility are discussed in the light of the effects of temperature and internal waves. Interestingly, unlike other large filter feeders,Astarte appears to be negatively affected by internal wave action.ZusammenfassungUm den Effekt von erhöhter Turbulenz in Bodennähe auf die Benthosfauna zu untersuchen, haben wir während einer Forschungsfahrt im April 1999Astarte sulcata (Bivalvia) in verschiedenen Tiefen des Faroe-Shetland Kanals gesammelt. Das Langzeitwachstum (Wachstumsringe) sowie Muschlegröße und Wachstumsrate in den verschiedenen Tiefen (250 m, 300 m, 350 m, 500 m, 600 m, 800 m) wurden untersucht. Die gefundenen Unterschiede zwischen den Tiefen werden anhand von Temperaturdaten und vor dem Hintergrund der Internen Wellen-Theorie diskutiert.

Environmental and biological factors influencing trace elemental and microstructural properties of Arctica islandica shells

Long-term and high-resolution environmental proxy data are crucial to contextualize current climate change. The extremely long-lived bivalve, Arctica islandica, is one of the most widely used paleoclimate archives of the northern Atlantic because of its fine temporal resolution. However, the interpretation of environmental histories from microstructures and elemental impurities of A. islandica shells is still a challenge. Vital effects (metabolic rate, ontogenetic age, and growth rate) can modify the way in which physiochemical changes of the ambient environment are recorded by the shells. To quantify the degree to which microstructural properties and element incorporation into A. islandica shells is vitally or/and environmentally affected, A. islandica specimens were reared for three months under different water temperatures (3, 8 and 13 °C) and food concentrations (low, medium and high). Concentrations of Mg, Sr, Na, and Ba were measured in the newly formed shell portions by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The microstructures of the shells were analyzed by Scanning Electron Microscopy (SEM). Shell growth and condition index of each specimen were calculated at the end of the experimental period. Findings indicate that no significant variation in the morphometric characteristics of the microstructures were formed at different water temperatures or different food concentrations. Shell carbonate that formed at lowest food concentration usually incorporated the highest amounts of Mg, Sr and Ba relative to Ca+2 (except for Na) and was consistent with the slowest shell growth and lowest condition index at the end of the experiment. These results seem to indicate that, under food limitation, the ability of A. islandica to discriminate element impurities during shell formation decreases. Moreover, all trace element-to‑calcium ratios were significantly affected by shell growth rate. Therefore, physiological processes seem to dominate the control on element incorporation into A. islandica shells.