Spatial distribution of heavy metal concentrations and biomass indices in Cerastoderma edule Linnaeus (1758) from the German Wadden Sea: An integrated biomonitoring approach

Abstract

The spatial distribution of heavy metal concentrations (Cu, Pb, Cd, Ni and Zn) and biomass indices (total, soft-body and standardised soft-body weight, TW, SBW and SSBW) in the cockle Cerastoderma edule from the German Wadden Sea has been investigated at an intermediate ecological scale of 300 × 300 m to evaluate the range of spatial autocorrelations for the different variables under study and to assess the extent to which biological properties of the organisms may be related to the spatial distribution of metal concentrations in C. edule from the sampling area. Semivariogram models obtained by geostatistical procedures indicate a distinct increase in variability for most variables with sampling distance: exceptions are Cd, Pb and Ni, for which a pure nugget-effect model is found. Only if samples are taken at distances above the estimated values for the practical range of the semivariogram, can stochastic independence of the data be assumed. These distances are 30 m for Cu, 122 m for Zn, 132 m for SBW, 156 m for SSBW and 170 m for TW. The contour plots of the mapped spatial distribution show a clear coincidence of increasing values for TW, SBW and SSBW with increasing relative height above sea level (RH) from shoreline to the sea ( viz. from south to north). The spatial distribution of SSBW indicates the cumulation of larger-sized cockles in the northern part of the sampling area, while Cu concentrations in cockles decreased with increasing distance from shoreline. This spatial correlation is supported by results of Mantel's test, taking the relative distance among sample locations into account. In future studies such an approach might be useful when addressing the problem of trophic transfer of energy or pollutants from food (e.g. cockles) to higher trophic levels like fish and seabirds.