No detectable impact of spat mussel collectors on sediment composition in high-energy Wadden Sea environments

Mussel aquaculture is generally considered sustainable, but benthic impacts from biodeposition remain a concern. Mussel culture in The Netherlands is in transition from wild seed harvesting to spat mussel collectors (SMCs) for seed production. While the environmental impacts of conventional mussel culture systems have been widely studied, those of SMCs remain largely unknown. The use of SMCs differs from conventional submerged culture because they are deployed during different periods and in more high-energy environments. We assessed sediment characteristics at 3 high-energy Dutch Wadden Sea sites before (May) and after (September) SMC deployment, comparing areas inside and outside SMC zones. No significant differences in grain size, organic carbon, nitrogen, or stable isotopes (δ 13 C, δ 15 N) were detected between positions. Effect sizes for SMC proximity were small (Cohen’s d = 0.47-0.54), contrasting with the large effects ( d > 1.6) that are reported from conventional submerged culture in sheltered waters. Stable isotope analysis revealed strong system-wide seasonal changes (δ 15 N declined 3-9‰; d = 0.95-2.18) that overwhelmed any potential localized SMC signals. Natural environmental gradients (depth, wave exposure, tidal currents) dominated spatial variability in all parameters. Strong tidal currents (0.8-1.4 m s -1 ) probably promoted continuous sediment redistribution, preventing biodeposit accumulation despite substantial operations (2.0-4.4 million kg harvest per site annually). High-energy environments effectively disperse biodeposits, supporting SMC deployment as a low-impact seed source when properly sited.