A case study on the growth of Lanice conchilega (Pallas, 1766) aggregations and their ecosystem engineering impact on sedimentary processes

Ecosystem engineers are organisms that modulate natural resources enabling the survival of other species. They drive environmental change and contribute to several coastal functional attributes such as landscape heterogeneity, sedimentary processes, and coastal protection. Our study focuses on the case of Lanice conchilega, a tube-building ecosystem engineer whose aggregations impact sedimentary processes. This polychaete forms biogenic tube aggregations distributed on the coasts of the northern hemisphere from the shallow intertidal to depths of 1900 m. The aggregations engineer sedimentary processes autogenically by altering water flow at the benthic-boundary layer, and harbor highly diverse infaunal communities as a consequence. This study evaluates the relationships between intertidal L. conchilega aggregations and sedimentary processes at the intertidal zone of a sandy beach in northern France. Three experiments were executed to investigate (1) the effects of L. conchilega presence on sedimentary processes, as well as (2) the impacts of sedimentation on L. conchilega survival and patch growth, and (3) assess small-scale spatial heterogeneity in density and ecosystem engineering in L. conchilega aggregations. Weekly estimations of sedimentary properties in-situ showed that net deposition is significantly higher inside L. conchilega aggregations than in bare sand; whereas sediment mixing depth is noticeably reduced in comparison and regardless of patch tidal height. Variations in tube density above 3200 ind·m^− 2 did not significantly impact sedimentary properties suggesting that the relationship between flow attenuation and tube density is nonlinear. In-situ monitoring of L. conchilega aggregations revealed different temporal trends for tube density and EPS content at the sediment surface between the center and edges of patches. This hints at the presence of environmental gradients within aggregations that may cause small-scale spatial heterogeneity. Finally, laboratory experiments showed significantly higher mortality rates and tube building activity in the presence of sediment deposition between 5 cm and 12 cm in column height. Results are in agreement with previous research suggesting that a positive feedback between sedimentation and tube-building activity drives the vertical expansion of patches. However, vertical expansion may be limited by deposition-induced mortality, thereby controlling population abundance.