Benthic litter distribution on circalittoral and deep sea bottoms of the southern Bay of Biscay: Analysis of potential drivers

- Annual average benthic litter densities in Northwestern Spain were 43 + 33 kg km2 and 74 + 28 items km2.
- Population density was the main explanatory variable for the distribution of benthic macrolitter.
- Geographical sector, fishing effort and ports density also drove the benthic litter distribution.
- Fishing effort had a non-linear negative effect on microlitter.
- Benthic macrolitter delocalization by the fishing fleet could explain the observed patterns.

We analyze marine litter densities in soft bottoms of the southern Bay of Biscay using five years of demersal trawling data (2006-2010). Marine litter densities amounted to 43 ± 33 kg km−2 and 74 ± 28 items km−2, with plastics and fisheries derived litter being the most widespread categories. Litter densities generally decreased along the water depth axis. To identify possible drivers for the observed litter distribution we performed a generalised additive model, which explained 14.8% of the variance and pointed to densely populated areas, number of fishing ports, geographical sector and fishing activity as the main explanatory factors. The most important driver for the benthic litter distribution was human population, as litter density linearly increased along this variable. Similarly, the number of ports in neighbouring areas had a positive effect on litter densities. Fishing effort had a negative and non-linear effect on benthic litter density which could be explained by litter delocalization during fishing operations. We hypothesise that litter might accumulate preferentially on the periphery of rocky bottoms, out of reach for our sampling methodology. Litter distribution differed among geographical sectors, pointing to other variables such as shipping traffic and oceanographic currents, which were not explicitly considered in the analysis. Our study sets a reference level for benthic macro-litter in the southern Bay of Biscay and identifies factors driving its distribution, which can be extrapolated to other continental shelf seas. Our findings lay the foundations to develop measures aiming to reduce macro-litter densities on the seafloor.