Coastal ecosystem functioning often hinges on habitat-forming foundation species that engage in positive interactions (e.g. facilitation and mutualism) to reduce environmental stress. Seagrasses are important foundation species in coastal zones but are rapidly declining with losses typically linked to intensifying global change-related environmental stress. There is growing evidence that loss or disruption of positive interactions can amplify coastal ecosystem degradation as it compromises its stress-mitigating capacity. Multiple recent studies highlight that seagrass can engage in a facultative mutualistic relationship with lucinid bivalves that alleviate sulphide toxicity. So far, however, the generality of this mutualism, and how its strength and relative importance depend on environmental conditions, remains to be investigated. Here we study the importance of the seagrass-lucinid mutualistic interaction on a continental scale using a field survey across Europe. We found that the lucinid bivalve Loripes orbiculatus is associated with the seagrasses Zostera noltii and Zostera marina across a large latitudinal range. At locations where the average minimum temperature was above 1°C, L. orbiculatus was present in 79% of the Zostera meadows; whereas, it was absent below this temperature. At locations above this minimum temperature threshold, mud content was the second most important determinant explaining the presence or absence of L. orbiculatus. Further analyses suggest that the presence of the lucinids have a positive effect on seagrass biomass by mitigating sulphide stress. Finally, results of a structural equation model (SEM) support the existence of a mutualistic feedback between L. orbiculatus and Z. noltii. We argue that this seagrass-lucinid mutualism should be more solidly integrated into management practices to improve seagrass ecosystem resilience to global change as well as the success of restoration efforts.

Sampling was done in 14 Zostera noltii and 17 Zostera marina meadows across western Europe during the growing season (August – October) in 2018–2019.  

Lucinid bivalves and seagrasses were sampled with sediment cores (ø: 10-15 cm) to a depth of about 20 cm and then sieved over a 1-mm mesh. Live seagrass and lucinids where sorted and frozen for further determination in the laboratory. In the laboratory, lucinids were counted, and total dry weight (above- and belowground) of the seagrass was determined after drying (48 hours at 60ºC). Sediment samples were taken with a small core (ø: 1.5–3.6 cm) to a depth of 5 cm, frozen after collection and weighed in the laboratory.

Dried above-ground seagrass tissue was analysed for sulphur isotope ratio (δ³⁴S and total sulphur (TS)) using dynamic flash combustion ratio mass spectroscopy (Thermo Scientific Delta V Advantage plus EA 1110, Thermo Fisher Scientific Inc., Waltham, MA, USA). 

Total iron (Fe) and sulphur (S) was determined after nitric acid digestion of dried sediment and analysed using an inductively coupled plasma emission spectrophotometer (ICP-OES iCAP 6000; Thermo Fisher Scientific, Waltham, MA, USA).

DOC was determined in a two-step water extraction procedure following Ghani et al. (2003) on 3 mg of dried sediment in 50 ml polypropylene centrifuge tubes by adding 30 ml distilled water.