Abstract
Marine Restoration Potential (MaRePo) project is a proof-of-concept study which explores the habitat restoration potential of some key threatened and declining (subtidal) marine habitats as defined by the OSPAR convention: kelp, maerl, native oysters, horse mussels, and sea pen and burrowing megafauna communities. These habitats were chosen as they occurred within English waters and were known to have some possibility for active or passive restoration interventions. This project uses a spatial analysis approach to investigate the current, historic, and potential future distribution of these habitats in English waters (out to 200 nautical miles (nm) from the shore). Below we outline the main characteristics of each habitat covered in the MaRePo project and summarise our findings on their distribution and restoration potential in English waters.
Kelp species are important bioengineers that modify the environment and resources available to associated species, by providing habitat structure and complexity, altering light, nutrients sediments, and reducing physical scour and water flow. Kelp habitats provide a range of ecosystem services: they support commercial and recreational fisheries through provision of nursery habitat, contribute to nutrient cycling and climate change mitigation. Kelp is also harvested for food, pharmaceutical and fertiliser use.
Kelp habitats are currently under threat from a wide range of pressures, including ecological pressures (such as overgrazing, competition and invasive non-native species) and anthropogenic pressures (e.g. elevated nutrient and sediment inputs, overfishing and abrasion by bottom towed gear). Climate change induced increase in temperature and more frequent and severe storm events may also have an impact on kelp survival and productivity. There are limited examples of successful active kelp habitat restoration, however passive restoration through the removal of pressures (e.g. exclusion of bottom towed gear) has been shown to produce rates of recovery
comparable to active restoration in the Sussex kelp restoration project. The two most common habitat-forming kelp species in English waters were included in the MaRePo mapping: Laminaria hyperborea and Saccharina latissima. The modelled predictions of L. hyperborea show significant potential for restoration in English waters, with model highlighting restoration potential areas along the northeast, southern and southwest coasts of England. The modelled predictions of S. latissima restoration potential areas in English waters were very limited, with small areas identified in the northeast, southern and southwest of England.
Maerl beds are considered as biodiversity “hotspots” because they form structurally complex habitats important for a wide range of marine species. Maerl ecosystems provide substrate stabilisation, food, and shelter for associated species. Maerl ecosystems have the highest carbonate production of all ecosystems found along European coasts and may thus have an important role in climate change mitigation through carbon capture and storage.
Maerl species form slow-growing habitats that can be found on open coasts and in the tide-swept channels of marine inlets. Although extensive areas of maerl exist in the southwest of Scotland and parts of Europe, maerl habitats are rare in English waters, with few isolated patches present predominantly in the southwest and northeast of England. Maerl habitats are affected by a range of environmental and ecological factors such as changes in sea water temperature, light availability, currents, and the presence of invasive non-native species including the slipper limpet (Credipula fornicata). Maerl is also sensitive to anthropogenic pressures such as commercial fisheries, chemical pollution and industrial extraction. The slow growth and reproductive characteristics of maerl exacerbate their susceptivity to disturbance and therefore hinder the natural recovery potential of impacted maerl populations.
As data on the abiotic factors affecting maerl is lacking and the geographic scope of this study was limited, the predictive habitat models for maerl were developed as a proof-of-concept for the MaRePo project and should therefore not be treated as a finished product. Regardless of these limitations, the draft models show a good alignment to the known distribution of maerl, although the predicted areas of maerl habitat are considerably more extensive than the current known maerl habitat extent. The results of this study identify the presence of potential areas suitable for maerl recovery in English waters; however, as maerl habitats are extremely slow growing, passive restoration through reduction of pressures within existing areas of maerl habitat should be considered as the most feasible option for assisting maerl recovery.
Native oyster (Ostrea edulis) is another keystone species that provides a range of ecological functions and ecosystem services, including provision of hard substratum for settlement by other species; biodiversity enhancement; natural protection against shoreline erosion; and potentially, sequestration of carbon. Native oyster habitats are sparsely distributed around the UK, with only a few scattered locations recorded in the southwest and east coasts of England. O. edulis beds are highly sensitive to substrate loss, smothering, and introduction of microbial pathogens/parasites and invasive non-native species. The main threat to O. edulis beds is overexploitation, as destructive harvesting and overfishing can reduce the extent, vertical relief, complexity and biodiversity of oyster bed habitat. The limited success of restoration efforts in the UK have been attributed to factors such as unregulated harvesting, bonamiosis infection, depletion of donor stocks, biased sex ratio and high mortality.
Here we have produced a restoration potential map to provide a national ‘high level’ indication of where native oyster beds could potentially be restored in English waters based on three key environmental variables: current speed, presence of subtidal mixed sediment habitat, and depth. Our restoration potential model closely aligns with the current and historic distribution maps, and highlights restoration potential areas for O. edulis within all inshore Marine Plan Areas.
Horse mussel (Modiolus modiolus) beds play an important role in productivity, habitat complexity and ecosystem functioning. They form physically complex structures which provide favourable feeding environments and important refugia from predation, grazing and physical disturbance for other marine organisms. M. modiolus beds also contribute to several ecosystem services including water filtration, nutrient cycling, sediment stabilisation, and potential carbon storage. M. modiolus beds are highly sensitive to pressures associated with anthropogenic activities such as bottom towed fishing activities, aggregate extraction, and climate change – induced increase in temperature. Physical pressures are particularly damaging as they can substantially reduce the extent and complexity of the horse mussel beds or remove them entirely. When combined with the slow growth rates and fragmentation of habitats, the recovery rates of M. modiolus beds are thought to be very slow. Restoration of M. modiolus beds requires a sufficient larval population and suitable habitat. Case studies have demonstrated the possibility of accelerated recovery of horse mussel beds using translocation and restocking, however long-term studies are lacking.
Sporadic records of horse mussel individuals are common throughout UK waters, particularly in inshore areas. However, the records of M. modiolus beds are sparser, with the majority located to the West of the UK, particularly in waters off Scotland and Wales, with just a few in English waters (See page 114 for the definition of M. modiolus bed). The habitat suitability model presented here represents the ecological niche occupied by horse mussel beds in the UK, however it has not proven successful in identifying areas for restoration. Our understanding of current population distribution, larval sources/sinks, and dispersal and connectivity dynamics in English waters is still too limited. Further work is needed to develop our understanding in these areas so that appropriate sites for restoration can be determined.
Sea pen and burrowing megafauna habitat occurs on plains of fine mud from the shallow subtidal to the deep sea. These muds are heavily bioturbated by burrowing megafauna species such as decapod crustaceans including Nephrops norvegicus, Calocaris macandreae or Callianassa subterranean, with burrows and mounds typically forming a prominent feature of the sediment surface and may include conspicuous populations of sea pens. This habitat supports a rich fauna of smaller animals and thus forms an important reservoir of biodiversity. The habitat also acts as a source of food and nursery areas for many fish species. The main threats faced by sea pen and burrowing megafauna habitat are physical disturbance and organic pollution. The use of bottom towed gear is the most prominent cause of physical disturbance. Mud habitats are more sensitive to the impact of bottom trawling than sand or gravel habitats and take longer to recover.
Although records of sea pens and burrowing megafauna habitat exist along the south coast of England, the largest known occurrences are in the North Sea and the South West Approaches. Our knowledge of the full habitat distribution is still limited. Our results of distribution models for sea pens show suitable habitat in the northern North Sea and Western Approaches, with Virgularia mirabilis extending to muddy habitats in the Celtic and Irish Seas. These areas are thus likely to be the most suitable for restoration of these habitats. Restoration success however depends on the reduction of trawl fishing effort. In some areas of Scotland, Norway and Sweden, protection and restoration efforts have been made by substituting trawl fisheries with creel fisheries.
This report provides the first investigation mapping marine restoration potential in English waters. Although the modelling approaches chosen for the different habitats vary, the restoration potential for some habitats (such as kelp and native oysters) appears more significant than for others such as maerl, horse mussels and sea pens and burrowing megafauna. There is a need for further refinement of the restoration potential models to improve both the confidence of the models and detail the hard constraints to remove areas where restoration would not be possible. There is also a need to future-proof areas of potential restoration by including limitations from climate change. Further opportunities to develop this work to map areas for restoration of blue carbon habitat as well as developing pilot restoration handbooks and testing the viability of restoration zones with on the ground pilot studies would also be beneficial.