Rare earth element cycling in high productivity coastal environments – in situ vs. benthic-pelagic coupling mediated patterns

Coastal ecosystems are biogeochemically complex, shaped by intense organic matter cycling and shifting redox gradients driven by both in-situ processes and benthic-pelagic interactions. The influence of these dynamics on the cycling of rare earth elements (REE) remains unclear, which is concerning given the growing input of potentially harmful anthropogenic REE into coastal environments. The main objective of this study was to investigate the biogeochemical cycling of natural and anthropogenic REE in a highly productive coastal system and to identify the key processes controlling their distribution and transformation. Using a controlled mesocosm experiment simulating the intertidal Wadden Sea, we show that REE dynamics are strongly mediated by the interplay of sediment geochemistry, redox gradients, and phytoplankton bloom development. Shifts from suboxic to euxinic conditions enhance REE sequestration through redox-dependent transformations, highlighting the importance of mineral phases and organic matter dynamics in controlling REE behavior. The REE fractionation was shifting significantly with respect to phytoplankton growth and associated dissolved organic matter. Sediments act as major reservoirs for both natural and anthropogenic REE, controlling their retention and mobilization. Natural REE and anthropogenic samarium and lanthanum were effectively transferred to and retained in sediments, with transfer capacities dependent on carrier phase pool properties and retention strongly depending on sediment composition and redox conditions. In contrast, anthropogenic gadolinium behaved mostly conservatively, non-conservative enrichments were only present at times of highest bloom intensities and organic matter content, suggesting that benthic-derived reactive anthropogenic Gd was stabilized by organic carrier phases inhibiting its transfer to the particulate pool. Sediment disturbances simulating storm-induced resuspension resulted in the release of natural and anthropogenic REE into the water column, temporarily altering their availability. No clear ecotoxicological effects of these pulses were observed; however, experimental constraints limit definitive conclusions. Overall, our findings highlight that coastal zones act as dynamic filters for REE, with sediment retention, biotic processes, and episodic remobilization events controlling their export to the open ocean.