Non-trophic Interactions Control Benthic Producers on Intertidal Flats
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The importance of positive effects of ecosystem engineers on associated communities is predicted to increase with environmental stress. However, incorporating such non-trophic interactions into ecological theory is not trivial because facilitation of associated species is conditional on both the type of engineer and the type of abiotic stress. We tested the influence of two allogenic ecosystem engineers (lugworms, Arenicola marina L. and cockles, Cerastoderma edule L.) on the main primary producers (microphytobenthos) of the tidal flats, under different abiotic stresses controlled by reefs of blue mussels (Mytilus edulis L.). We added 25,000 cockles or 2,000 lugworms to 5 × 5 m plots, both in a muddy site with high sedimentation rates located coastward of a mussel bed, and in a sandy site without mussels and characterized by high hydrodynamic stress. After a year, cockles increased algal biomass in the sandy area, but not in the mussel bed site, where high values were measured in all plots. However, lugworms did not affect algal biomass in any of the sites. Field measurements suggest that cockles outweighed negative effects of water currents in the site without mussels by locally increasing sediment stability, whereas mussels overruled the effects of cockles in the wake of the reefs through hydrodynamic stress alleviation and/or biodeposition. Our results suggest that non-trophic interactions by ecosystem engineering bivalves control primary production of intertidal areas, and that the sediment-stabilizing effect of cockles plays a crucial role where the overruling effects of mussel beds are not present.
Keywordsecosystem engineering facilitation hydrodynamic stress microphytobenthos Cerastoderma edule Arenicola marina Mytilus edulis Wadden Sea
We thank Imke Gerwen, Marc Bartelds, Judith Westveer, Jeroen Kuypers, Tim Ruiter, Jim de Fouw, Karin de Boer, Stefania Gemignani, Nicola Stefani, Katrin Sieben, Guus Diepenmaat, and Maria van Leeuwe for help in the field; Johan Eklöf for advise on the experimental design; and three anonymous reviewers who greatly contributed to improve our manuscript. We are grateful to Vereniging Natuurmonumenten for granting us permission for the field work on the tidal flats. This study was financed by a grant from the ZKO program of the Netherlands Organization of Scientific Research (NWO) to BKE (Grant No. 839.08.310).
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