Biodeposition by an Invasive Suspension Feeder Impacts the Biogeochemical Cycle of Si in a Coastal Ecosystem (Bay of Brest, France)

Biodeposition is a process that strongly affects physical, chemical and biological properties near the sediment–water interface, and mollusks have been shown to influence the cycling of many biogenic elements. In the Bay of Brest ecosystem, the invasive benthic suspension feeder Crepidula fornicata has been proliferating for 50 years, and its influence on phytoplankton dynamics at a seasonal scale, via its role on a coastal silicate pump, has been suggested by Chauvaud et al. (2000) [Chauvaud L., Jean F., Ragueneau O. and Thouzeau G. 2000. Mar. Ecol. Prog. Ser. 200: 35–48] and tested by Ragueneau et al. (2002a) [Ragueneau O., Chauvaud L., Leynaert A., Thouzeau G., Paulet Y.-M., Bonnet S., Lorrain A., Grall J., Corvaisier R., Le Hir M., Jean F. and Clavier J. (2002a). Limnol. Oceanogr. 47(6): 1849–1854]. In the present manuscript, we explore further, at an annual scale, the role of biodeposition in the Si cycle of the Bay of Brest. We show that annual biodeposition Si can represent up to 84% of river silicic acid inputs. We also show that preservation of biogenic silica in C. fornicata mats is very good (27%), leading to an annual retention of Si of 23% of river inputs. We suggest that this high retention is essentially controlled by the biodeposition mechanism, which is directly under the control of the proliferation of the exotic suspension feeder. We conclude that this mechanism might be very important in controlling the Si retention along the land–ocean continuum and should be tested in the many areas of the coastal ocean submitted to the proliferation of such exotic species that modify sedimentation of biogenic matter.