A common error in calculating density of invertebrates in sediment core sections of unequal lengths

In benthic studies, an absolute measure of invertebrate density with depth in the sediment is desirable. It can be expressed as invertebrates per unit of sediment volume or invertebrates per unit of sediment surface area. Calculations are often distorted, however, by whether the core is divided into unequal or equal segments. This is demonstrated in a study conducted in the Vistula Lagoon (South Baltic Sea). We collected 30 cores, and performed an analysis of distribution of invertebrates along the sediment profiles divided into sections of different lengths, or of equal lengths. In both cases, animal density/biomass was calculated both per unit area (m2) and per unit volume (L). When cores are divided into equal sections, various ways of data computing do not affect the final results. If, for any reason, the cores are divided into sections varying in length, the only way to obtain an accurate measure of invertebrate distribution in the sediment profile is to express the data per unit volume. In many studies, calculations have been made per unit area using unequal segments, and these calculations are erroneous. The same error is made if density is expressed as the percentage contribution of the numbers/biomass or number of specimens recorded in particular unequal sections.The study shows that for sediment core sections of unequal lengths (which is frequently the case because researchers expect higher variability in the surface layers compared to the deeper ones), the only correct approach is to express all density measurements per unit volume (not area). Given the high number of studies using incorrect calculations, the study paves the way for better practices in benthic sampling