The burrowing mud shrimp Callianassa subterranea (Decapoda) and bioturbation in the North Sea
A muddy-sand site in the North Sea was studied to identify the role of the benthic macrofauna in the resuspension of sediment. The macrobenthic community contained species capable of significant bioturbation. In particular, the presence and the temporal occurrence of the brittle star Amphiura filiformis was correlated with seasonal geotechnical and geophysical properties of the seabed. In addition, the extensive burrowing habit of the mud shrimp, Callianassa subterranea (Montagu), make this species potentially the most important contributor to the degree of bioturbation experienced at the site. Mud shrimps were an abundant and stable member of the study site's macrobenthic community [mean= 11.4 indiv./0.25m²(1SD±2.1)]. The sexually dichotomous individuals live between 2-3 years, and reproduction and recruitment primarily occurred in the summer (though an additional late winter/early spring period was hypothesised). Resin casting of burrows constructed by C. subterranea in the laboratory revealed a consistent morphological pattern with particular size- and sex-specific details of dimension and design. Mud shrimps, recovered from the site, were returned to the laboratory to investigate the influence of body size and temperature upon the amount of sediment expelled. A clear relationship between these variables and the quantity of expelled sediment was identified, and a well-defined temporal pattern of expulsion activity and inactivity was demonstrated. These experimental data, together with field information on seawater temperatures and mud shrimp population dynamics from the site, allow the construction of an annual sediment turnover budget [11 kg(dry)/m²/yr] with a confidence to date unrealised. Field observations at the North Sea site show that the sediment expelled by the mud shrimp occasionally forms a multitude of unconsolidated volcano-like mounds, which significantly modify seabed surface topography. The dimensions of these surface features were measured from bottom photographs and used to determine values of boundary roughness length (Zo) for the site (eg, September Zo= 0.79cm). The mud shrimps' contribution to resuspension was estimated by calculating the derived lateral sediment transport rate of 7 kg/m/month (from values of the site's near-bed current velocity, modified boundary roughness length and predicted sediment turnover rates). The links between sediment resuspension and the fluxes of trace metals, carbon and nutrients established by associated studies, demand that the bioturbatory activities of Callianassa subterranea must be included in any discusssion of the fate of contaminants and the future modelling of associated water quality in the North Sea.