DOI: 10.5061/dryad.7sqv9s4wr
title: Biotic and abiotic data for: European Benthic survey of seagrass-lucinid mutualism

description: 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.
description: Sampling was done in 14 <em>Zostera</em>
 <em>noltii</em> and 17 <em>Zostera</em>
 <em>marina</em> 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 (δ<sup>34</sup>S 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.