Marijn Rabaut

Do Lanice conchilega (sandmason) aggregations classify as reefs? Quantifying habitat modifying effects

The positive effects of the tube dwelling polychaete Lanice conchilega for the associated benthic community emphasizes this bio-engineer’s habitat structuring capacity (Rabaut et al. in Estuar Coastal Shelf Sci, 2007). Therefore, L. conchilega aggregations are often referred to as reefs. The reef building capacity of ecosystem engineers is important for marine management as the recognition as reef builder will increase the protected status the concerned species. To classify as reefs however, bio-engineering activities need to significantly alter several habitat characteristics: elevation, sediment consolidation, spatial extent, patchiness, reef builder density, biodiversity, community structure, longevity and stability [guidelines to apply the EU reef-definition by Hendrick and Foster-Smith (J Mar Biol Assoc UK 86:665–677, 2006)]. This study investigates the physical and temporal characteristics of high density aggregations of L. conchilega. Results show that the elevation and sediment consolidation of the biogenic mounds was significantly higher compared to the surrounding unstructured sediment. Areas with L. conchilega aggregations tend to be extensive and patchiness is high (coverage 5–18%). The discussion of present study evaluates whether L. conchilega aggregations can be considered as reefs (discussing physical, biological and temporal characteristics). Individual aggregations were found to persist for several years if yearly renewal of existing aggregations through juvenile settlement occurred. This renewal is enhanced by local hydrodynamic changes and availability of attaching structures (adult tubes). We conclude that the application of the EU definition for reefs provides evidence that all physical and biological characteristics are present to classify L. conchilega as a reef builder. For temporal characteristics, this study shows several mechanisms exist for reefs to persist for a longer period of time. However, a direct evidence of long-lived individual reefs does not exist. As a range of aggregation development exists, ‘reefiness’ is not equal for all aggregations and a scoring table to quantify L. conchilega reefiness is presented.

Ephemeral Bio-engineers or Reef-building Polychaetes: How Stable are Aggregations of the Tube Worm Lanice conchilega (Pallas, 1766)?

Dense aggregations of tube-worms can stabilize sediments and generate oases for benthic communities that are different and often more diverse and abundant than those of the surroundings. If these features are to qualify as biogenic reefs under nature-conservation legislation such as the EC Habitats Directive, a level of stability and longevity is desirable aside from physical and biological attributes. Lanice conchilega (Pallas, 1766) is widely distributed around the European coast and aggregations of this tube-dwelling polychaete are known to have a positive effect on the biodiversity of associated species in inter- and sub-tidal areas. This increases the value of L. conchilega-rich habitats for higher trophic levels such as birds and fish. However, L. conchilega is currently not recognized as a reef builder primarily due to uncertainty about the stability of their aggregations. We carried out three studies on different spatial and temporal scales to explore a number of properties relating to stability: (1) Individual aggregations of L. conchilega of ∼1 m(2) were monitored for up to 1 year, (2) records of L. conchilega from a 258-ha area over a 35-year period were analyzed, (3) the recovery of a population of L. conchilega subjected to disturbances by cultivation of Manila clams (Ruditapes philippinarum) was followed over 3 years. The studies provided evidence about the longevity of L. conchilega aggregations, their resistance to disturbance, their resilience in recovering from negative impact and their large-scale persistence. The results showed that populations of L. conchilega were prone to considerable fluctuation and the stability of aggregations depended on environmental factors and on recruitment. The tube-worms proved to be susceptible to disturbance by cultivation of Manila clams but demonstrated the potential to recover from that impact. The long-term monitoring of a large L. conchilega population in the Bay of Mont Saint Michel (France) indicated that aggregations can persist over many decades with a constant, densely populated core area and an expanding and contracting more thinly populated fringe zone. The stability of aggregations of L. conchilega and the structures they form do not unequivocally fit the currently accepted definition of a reef. However, given L. conchilegas accepted reef-like potential to influence diversity and abundance in benthic communities, we suggest clarifying and expanding the definition of reefs so that species with records of significant persistence in particular areas and which otherwise meet expectations of reefs are included within the definition.

Fine-scale geomorphological mapping of sandbank environments for the prediction of macrobenthic occurrences, Belgian Part of the North Sea

Publisher Summary Fine-scale geomorphological mapping is demonstrated to be able to predict macrobenthos occurrences within shallow sandbank areas of the Belgian part of the North Sea. The Belgian part of the North Sea (BPNS) is a siliciclastic macrotidal environment comprising several groups of sandbanks. The sandbanks represent a thin and patchy Holocene cover, which overlies Tertiary clayey sediments that outcrop locally in troughs. Sediment transport is mainly driven by tidal currents, though wind-induced currents and waves may have a direct effect on sediment resuspension and bedform morphology. Human activities are widespread and relate mainly to harbor infrastructure works, dredging and disposal of dredged material, marine aggregate extraction, and windmill farm construction. The statistical habitat suitability model predicts the occurrence of the four macrobenthic communities based only on median grain size and silt–clay content. On the scale of the BPNS, marine landscapes have been mapped and translated into macrobenthic community preferences. The emphasis is placed upon relationships between the macrobenthos and the fine-scale geomorphological and sediment nature, for which sonar imagery is needed. Ideally, seafloor classification techniques would allow automated discrimination of acoustic facies, but results did not depict the fine-scale variation, as seen from the imagery. Interpretation in terms of bedforms and sedimentary characteristics led to 15 acoustic facies, which were further related to macrobenthic community occurrences. Successful validation over the entire Belgian coastal zone promotes its use for classifying sandbank environments. Furthermore, it is demonstrates that sidescan sonar and multibeam technology can be used to directly map reefs of tube-building polychaetes. These ecosystem engineers are key species within hotspots of biodiversity among inter- and subtidal soft sediment environments of the North Sea.