Does Manila clam cultivation affect habitats of the engineer species Lanice conchilega (Pallas, 1766)?

Role of Reef-Building, Ecosystem Engineering Polychaetes in Shallow Water Ecosystems

Although the effect of ecosystem engineers in structuring communities is common in several systems, it is seldom as evident as in shallow marine soft-bottoms. These systems lack abiotic three-dimensional structures but host biogenic structures that play critical roles in controlling abiotic conditions and resources. Here I review how reef-building polychaetes (RBP) engineer their environment and affect habitat quality, thus regulating community structure, ecosystem functioning, and the provision of ecosystem services in shallow waters. The analysis focuses on different engineering mechanisms, such as hard substrate production, effects on hydrodynamics, and sediment transport, and impacts mediated by filter feeding and biodeposition. Finally, I deal with landscape-level topographic alteration by RBP. In conclusion, RBP have positive impacts on diversity and abundance of many species mediated by the structure of the reef. Additionally, by feeding on phytoplankton and decreasing water turbidity, RBP can control primary production, increase light penetration, and might alleviate the effects of eutrophication affecting supporting ecosystem services, such as nutrient cycling. They can also modulate cultural ecosystem services by affecting recreational activities (e.g., negative impacts on boating and angling, increased value of sites as birdwatching sites). Acknowledging the multiplicity of synergistic and antagonistic effects of RBP on ecosystems and linking changes in habitat structure, filter-feeding activities, and biodeposition to ecosystem services are essential for effective decision-making regarding their management and restoration.

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. conchilega's 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.

Clam farmers and Oystercatchers: effects of the degradation of Lanice conchilega beds by shellfish farming on the spatial distribution of shorebirds

The Manila clam Ruditapes philippinarum cultivation is an original shellfish farming activity strongly mechanized. In the Chausey archipelago (France) this activity settles on the Lanice conchilega beds, habitat known to host a rich and diversified benthic macrofauna and which is an attractive feeding ground for birds. A first study highlighted that this activity had strong negative effects on the L. conchilega beds and their associated benthic macrofauna. Here we assess the impacts of such an activity on the Eurasian Oystercatcher Haematopus ostralegus for which Chausey is one of the most important national breeding sites and which is also a common species in winter, spring and autumn migrations. We found that Oystercatchers significantly selected the L. conchilega beds to feed and that their spatial distribution was significantly modified after the creation of new clam concessions. In a context of a growing disappearance of pristine coastal ecosystems for the benefit of anthropo-ecosystems, we discuss the problem of the degradation of such benthic habitats with a low resilience which may loose their high functional value.