Magda J.N. Bergman

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.

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.