Benthic Macrofauna Community Bioirrigation Potential (BIPc): Regional Map and Utility Validation for the South-Western Baltic Sea

SIMPLE SUMMARY: The sediments on the seafloor are inhabited by multiple macroscopic organisms such as shells and worms, which, among other things, influence the biogeochemical cycling by flushing the near-bottom water through their gangways. This is called bioirrigation, one of key processes in the functioning of marine sediments. The density of animals, in addition to the features (or traits) of each species, define their specific contributions to this process. Measuring the intensity of this rather dynamic process in nature is difficult and costly; therefore, the available direct observations are too scarce for large-scale assessments. However, such assessments are essential for broadening our understanding of ecosystem functioning, and of the role that biodiversity plays in it. To address this shortage of observational data, a traits-based index “BIPc” that expresses the bioirrigation potential, based on available data on sediment-dwelling animals, comes into play. In this paper, we focus on the performance of the BIPc index in the south-western Baltic Sea, and on how it changes in space and time. The results support the usefulness of this index, but also highlight its existing limitations. Modelled distribution map layers of the bioirrigation potential and scores for 120 key species required for index calculation are made available for reuse. ABSTRACT: Benthic community bioirrigation potential (BIPc), an index developed to quantify the anticipated capacity of macrofauna to influence the solute exchange at the sediment–water interface, was calculated for the south-western Baltic Sea. This index can be regarded as an effect trait that is useful for predicting ecosystem processes impacted by animal burrow ventilation. The special feature, and presumably an advantage, of BIPc, compared to alternative recently developed benthic macrofauna-based bioirrigation indices, lies in its ability to distinguish the taxa-specific score values between diffusion- and advection-dominated sediment systems. The usefulness of the BIPc index was compared against the estimates of the well-established community bioturbation potential index (BPc). The BIPc index displayed a moderately but significantly stronger correlation with estimates of irrigation rates derived from tracer experiments. Using a random forest machine learning approach and a number of available relevant environmental predictor layers, we have modelled and mapped the spatial differences in this ecosystem functioning expression. The key species contributing to bioirrigation potential in the study area were identified. The interannual variation in BIPc was assessed on a small exemplary dataset. The scores required to calculate the index, that were assigned to 120 taxa dominating abundance and biomass in the region, are provided for reuse. The utility, temporal variability and uncertainty of the distribution estimate are discussed.