Renewable energy homes for marine life: Habitat potential of a tidal energy project for benthic megafauna
Introduction
Artificial reefs are man-made structures placed on the sea bed in aquatic habitats for different purposes, for instance to mimic characteristics of natural reefs such as substrate and/or shelter provision to associated organisms (Bohnsack et al., 1991; Jensen et al., 2000a; Thierry, 1988). Development of artificial reefs may locally increase both hard substratum availability and habitat heterogeneity (especially when deployed on soft-sediment bottoms), which can consequently lead to higher densities and biomass of fish and decapods (Bohnsack et al., 1994; Bombace et al., 1994; Langhamer and Wilhelmsson, 2009).
Enhancement of associated benthic diversity by artificial reefs depends both on reef properties and on local environmental characteristics. Colonisation success depends on artificial reef shape and size, constitutive material, orientation and degree of complexity, that directly determine habitat, and refuge availability (Charbonnel et al., 2002; Ferreira and Coutinho, 2001; Hackradt et al., 2011; Sherman et al., 2002). A range of local environmental factors (e.g. neighbouring habitat type, hydrological features, amplitude of seasonal variation) can significantly influence the amount and the diversity of colonising organisms (Bohnsack et al., 1991; Bombace et al., 1994; Godoy et al., 2002; Noh et al., 2017). A long-standing scientific debate persists between two dominant theories regarding the role of artificial reefs for mobile fauna: (i) the “attraction hypothesis” and (ii) the “production hypothesis” (Lima et al., 2019). The first assumes that artificial reefs only attract specimens from nearby ecological communities, without increasing overall biomass production (Bohnsack, 1989) while the latter advocates that artificial reefs increase abundance and biomass of associated species by enhancing habitat and food availability (Pickering and Whitmarsh, 1997; Polovina and Sakai, 1989). Literature shows that the two processes exist, the productive potential of artificial reef is indeed reef-dependant and varies according to an important number of factors (e.g. number and design of reef units, distance to natural reef, association with protected area etc.; Pickering and Whitmarsh, 1997). Nevertheless, Lima et al. (2019) highlight that, despite several decades of scientific observations and experiments on the subject, separating the reef effect and the effects of changing environmental and socioeconomic conditions remains complex, impacting the assessment of artificial reefs performance.
Artificial reefs can be divided into two types: i) structures designed and installed specifically for their reef properties (for a variety of reasons e.g. ecosystems conservation/restoration, fish stocks enhancement, fisheries management etc.; Jensen, 2002) and ii) structures deployed for other purposes, such as oil platforms, breakwaters, or marine renewable energy (MRE) facilities (Langhamer, 2012; Lima et al., 2019; Wilson and Elliott, 2009). MRE facilities and associated structures (e.g. protection structures, submarine power cables, foundations, turbines etc.) are not only colonised by a variety of benthic organisms including algae, sessile epifauna and mobile macrofauna but also mobile megafauna (i.e. fish and decapods). A diversity of fish and large crustaceans can settle on artificial reefs deployed as part of MRE facilities (see Wilhelmsson and Langhamer, 2014 for a review). For example, commercially valuable crustacean species such as the European lobster (Homarus gammarus) or the edible crab (Cancer pagurus) can shelter around the foundations of offshore wind (Hooper and Austen, 2014; Krone et al., 2017) or wave farms (Langhamer and Wilhelmsson, 2009). Thus, such reef effects can represent an ecological benefit of MRE, since artificial structures generally host higher diversity, densities and biomass of benthic organisms than the surrounding soft bottoms (Broadhurst and Orme, 2014; Dannheim et al., 2020; Langhamer and Wilhelmsson, 2009). Wilson and Elliott (2009) estimated that in the long term, a wind-turbine facility provides 2.5 times the amount of habitat relative to the initial loss during the installation process, even though this new habitat may be of a different character to the initial one. When their deployment requires the implementation of new exclusion areas for fishing, MRE may thus act as a refuge for commercially-exploited populations, with potential spill-over benefits for adjacent stocks and fisheries (Lindeboom et al., 2015, 2011). However, the long-term reef effect associated with MRE facilities remains poorly characterised (Copping et al., 2016; Langhamer and Wilhelmsson, 2009; Lindeboom et al., 2015), especially within high hydrodynamic energy areas (as tidal energy sites; Copping et al., 2016).
The purpose of this study is to assess the role of habitat associated with MRE facilities using a French tidal energy test site as a case study. We specifically examined the habitat capacity of concrete mattresses that stabilise an unburied submarine power cable that connects the test site to the mainland. Based on a 4-year monitoring of fish and crustacean abundance on these mattresses, we (1) characterise the reef effect associated with MRE structures, and more specifically (2) how interactions between artificial reefs and natural seafloor characteristics can determine diversity and abundance of associated megafauna.
Section snippets
Study site
The study area consists of a 15 km-long submarine power cable (8 MVA - 10 kVDC) laid in 2012 by Electricité de France (EDF) to connect the tidal test site of Paimpol-Bréhat to the mainland (Brittany, France; Fig. 1). Due to several setbacks in the project development, no electric current transited through the cable during the course of this study. An 11 km cable portion is unburied due to local seafloor characteristics (dominance of pebbles and presence of boulders; Fig. 2A) and stabilised by
Temporal variation
Although occupancy of individual mattresses varied slightly during the different campaigns (SI 2), mean abundance estimates across all mattresses did not significantly change for H. gammarus (χ2 = 0.44, df = 4, p = 0.98), C. pagurus (χ2 = 0.6, df = 4, p = 0.96), C. conger (χ2 = 5.42, df = 4, p = 0.25) and L. bergylta (χ2 = 5.46, df = 4, p = 0.24, Fig. 3). Only Trisopterus spp. displayed significant abundance changes between campaigns (χ2 = 26.42, df = 4, p < 0.001; Fig. 3) due to significantly
Discussion
By combining in situ visual census by divers and video analysis, our results help to characterise how MRE facilities can enhance benthic megafauna diversity by providing artificial reefs. Specifically, our findings help: (i) characterise the habitat potential of concrete mattresses deployed to anchor an unburied power cable; (ii) disentangle how interactions between artificial reef and natural substrate determine the effectiveness of the ‘reef effect’ and (iii) to a lesser extent identify
Conclusion
Although the concrete mattresses deployed to anchor the submarine power cable were not specifically designed to act as a refuge for marine fauna, a five-year monitoring study (both in situ and using videos) shows that they offer a suitable and stable habitat for at least 5 benthic megafauna species. Interactions between local seafloor and hydrodynamic characteristics (substratum type, topography, exposition to current etc.) and artificial reef units directly condition the variety and the
Author statement
Bastien Taormina, Antoine Carlier and Martial Laurans conceived the project. Didier Leroy, Martial Laurans, Bastien Taormina and Stéphane Martin took part in diving activities. Noémie Dufournaud and Bastien Taormina scored the images. Bastien Taormina and Martin P. Marzloff analysed the data. Finally, all authors contributed to writing the manuscript.
Funding
This work is sponsored by the Région Bretagne, France Energies Marines and the National Research Agency within the framework of Investments for the Future program under reference ANR-10-IED-0006-17.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank Laura Taormina, Fernando Tempera, Natacha Go, Olivier Dugornay, Xavier Caisey and Fabrice Pernet for their kind assistance.
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