Maerl Habitats Research Articles

Adaptive stacked species distribution modelling: Novel approaches to large scale quantification of blue carbon to support marine management

This study introduces a novel concept of ‘Adaptively Stacked’ Species Distribution Models (AS-SDMs) to predict blue carbon habitat distribution, abundance, carbon stocks, and carbon sequestration potential in Orkney. AS-SDMs are built from Weighted Boosted Regression Trees (WBRTs) that adaptively stack blue carbon sediment thickness, sediment carbon content, and sequestration potential to predicted abundance. A novel method to describe substrate types by relative inputs of mud, sand, and gravel is detailed that better characterises the determining factors of seagrass, maerl, and horse mussel abundance. This study also introduces a novel use of indexes to mitigate double counting issues of mixed species distribution models.Seagrass, maerl, horse mussel, and mixed seagrass and maerl (SGM) habitats are estimated to cover a maximum area of 657 km2 in Orkney, have a total sediment carbon stock of 16 Mt. C, and sequester 6000 t C yr−1. Applying a conservative threshold of 50 % abundance to habitat predictions, six key potential areas of blue carbon offset projects are identified. These areas cover just over 9 km2, have a total carbon stock of 330,000 t C, and sequester 330 t C yr−1. When applied to UK carbon credit value, assuming integration with voluntary markets and compliance with accreditation criteria, the habitats in these areas have a potential value of £24.5 million. If applied as annual values, these areas have carbon stocks with a potential value of £0.93 million yr−1 and a carbon sequestration potential value of £24,000 yr−1.

Sounding out maerl sediment thickness: an integrated data approach

Maerl beds are listed as a priority marine feature in Scotland. They are noted for creating suitable benthic habitat for diverse communities of fauna and flora and in supporting a wide array of ecosystem services. Within the context of climate change, they are also recognised as a potential blue carbon habitat through sequestration of carbon in living biomass and underlying sediment. There are, however, significant data gaps on the potential of maerl carbon sequestration which impede inclusion in blue carbon policy frameworks. Key data gaps include sediment thickness, from which carbon content is extrapolated. There are additional logistical and financial barriers associated with quantification methods that aim to address these data gaps. This study investigates the use of sub-bottom profiling (SBP) to lessen financial and logistical constraints of maerl bed sediment thickness estimation and regional blue carbon quantification. SBP data were cross validated with cores, other SBP data on blue carbon sediments, and analysed with expert input. Combining SBP data with estimates of habitat health (as % cover) from drop-down video (DDV) data, and regional abiotic data, this study also elucidates links between abiotic and biotic factors in determining maerl habitat health and maerl sediment thickness through pathway analysis in structural equation modelling (SEM). SBP data were proved to be sufficiently robust for identification of maerl sediments when corroborated with core data. SBP and DDV data of maerl bed habitats in Orkney exhibited some positive correlations of sediment thickness with maerl % cover. The average maerl bed sediment thickness was 1.08 m across all ranges of habitat health. SEM analysis revealed maerl bed habitat health was strongly determined by abiotic factors. Maerl habitat health had a separate positive effect on maerl bed sediment thickness.

Quantifying maerl (rhodolith) habitat complexity along an environmental gradient at regional scale in the Northeast Atlantic

Maerl beds are ecologically important marine biogenic habitats founded on a few species of free-living coralline algae that aggregate and form highly complex rhodoliths. The high biodiversity found in these habitats have been mainly justified by the structural complexity that they provide. However, few attempts to quantify this complexity have been made. Maerl species distribution, density, rhodolith growth forms, and shapes vary with environmental conditions. Hydrodynamics and depth have been shown to drive morphology. Using species-specific metrics such as sphericity and branching density, as well as diameter and fractal dimension at the rhodolith level, and maerl density at the habitat level, we quantified the habitat complexity within ten maerl beds at a regional scale (along ∼400 km of the coastline of Brittany in Western France). Using both long-term monitoring data and environmental models, we investigated how maerl habitat complexity varies among beds and which environmental conditions drive those differences. The effects of currents, exposure to wind-generated waves, temperature and sediment granulometry were evaluated. We confirmed variations in complexity in maerl beds at the habitat and rhodolith levels at local and regional scales, which might have ecological and conservational implications for their associated biodiversity. The analysed environmental conditions drive around a third of the variance in habitat complexity. Sediment granulometry is the main driver of maerl habitat complexity in Brittany, while the isolated effects of depth and hydrodynamics accounted for less than 5% of the variability each. Our results have important implications for paleoecology, and we suggest that maerl facies should be interpreted carefully. Our study provides a first attempt at explicitly quantifying maerl habitat complexity, and further contributes to the understanding of this fundamental ecological question.

Fatty acids as trophic markers and indicators of the quality of benthic habitats: The example of maerl and crinoid beds in the Northwestern Mediterranean

Highly productive and vulnerable habitats such as maerl and crinoid beds are subject to severe impacts from trawling in Mediterranean Sea. One of the most valuable target species that inhabit these habitats is the red mullet, Mullus barbatus, which can achieve there optimum feeding conditions based on infauna, which can provide some Fatty Acids (FAs) such as Polyunsaturated Fatty Acids (PUFAs) that are essential in shaping life history traits such as growth and reproduction. This study is the first one evaluating how habitat type can affect the FA profile of red mullet and that of their prey (infauna). The analysis was carried out in both habitats (maerl and crinoid beds) and compared to muddy bottom, the most common trawl fishing ground. In red mullet muscle from maerl or crinoid beds, we observed a higher concentration of PUFAs omega 3 and omega 6 such as C22:6n-3 (docosahexaenoic acid, DHA), C18:3n-3 (alfa-linolenic acid, ALA) or C18:2n-6 (linoleic acid) than in muddy bottoms. On the other hand, MUFAs such as C18:1cn-9 (oleic acid) presented higher concentrations in muddy bottoms compared to maerl habitat. Regarding infauna's FAs, there was between 30% and 35% of dissimilarity between maerl and crinoid habitats and muddy habitat respectively. In addition, differences found in infauna's PUFAs concentration were very similar to the differences found in M. barbatus PUFAs when maerl and crinoid were compared with muddy habitats. Our results support the idea of using FAs as an indicator of the quality and trophic marker of productive and sensitive benthic habitats, which can contribute to assess the good environmental status (GES) of these habitats and the goods and services they provide.

The Impossible Sustainability of the Bay of Brest? Fifty Years of Ecosystem Changes, Interdisciplinary Knowledge Construction and Key Questions at the Science-Policy-Community Interface

In this contribution, the study of the Bay of Brest ecosystem changes over the past 50 years is used to explore the construction of interdisciplinary knowledge and raise key questions that now need to be tackled at the science-policy-communities interface. The Bay of Brest is subject to a combination of several aspects of global change, including excessive nutrient inputs from watersheds and the proliferation of invasive species. These perturbations strongly interact, affecting positively or negatively the ecosystem functioning, with important impacts on human activities. We first relate a cascade of events over these five decades, linking farming activities, nitrogen and silicon biogeochemical cycles, hydrodynamics of the Bay, the proliferation of an exotic benthic suspension feeder, the development of the Great scallop fisheries and the high biodiversity in maerl beds. The cascade leads to today’s situation where toxic phytoplankton blooms become recurrent in the Bay, preventing the fishery of the great scallop and forcing the fishermen community to switch pray and alter the maerl habitat and the benthic biodiversity it hosts, despite the many scientific alerts and the protection of this habitat. In the second section, we relate the construction of the interdisciplinary knowledge without which scientists would never have been able to describe these changes in the Bay. Interdisciplinarity, first among natural sciences (NS) and then, between natural sciences and human and social sciences (HSS). We finally ask key questions at the science-policy interface regarding this unsustainable trend of the Bay: How is this possible, despite decades of joint work between scientists and fishermen? Is adaptive co-management a sufficient condition for a sustainable management of an ecosystem? How do the different groups (i.e. farmers, fishermen, scientists, environmentalists), with their diverse interests, take charge of this situation? What is the role of power in this difficult transformation to sustainability? Combining natural sciences with political science, anthropology and the political sociology of science, we hope to improve the contribution of HSS to integrated studies of social-ecological systems, creating the conditions to address these key questions at the science-policy interface to facilitate the transformation of the Bay of Brest ecosystem towards sustainability.

Testing the resilience of dead maerl infaunal assemblages to the experimental removal and re-lay of habitat

An experimental trial to mitigate dredging impact was undertaken within Falmouth Harbour, UK, removing a surface layer of dead maerl for storage on a barge and allowing the channel to be deepened before re-laying the maerl. The resilience (resistance and recovery) of the habitat and faunal assemblage to this disturbance was assessed. Six sites each had 2 conditions — a manipulated treatment area where maerl (25 m 2 plots, top 0.3 m) was removed, stored on a barge and re-laid by backhoe dredger and a control area — which were cored at 0 (before), 5 and 44 wk after re-lay. PERMANOVA was used to test for differences between condition and time using a 2-factor design. Results should not be extrapolated to live maerl habitats or to large, long- lived fauna that may live within them. Following the mechanical disturbance, the maerl matrix structure was altered through loss of fine sediment from the lower half of cores (>10 cm). There was also a significant reduction in the number of taxa and abundance of infauna and a change in the assemblage composition. By Week 44, however, no such significant differences were evident, indicating that the infauna was in a state of recovery. The only response variable showing recov- ery was annelid biomass. The trial demonstrated that removing and re-laying the top 0.3 m of maerl habitat is technically feasible, and whilst some differences in the habitat structure following re-laying were evident, this did not affect the habitat quality enough to prevent recolonisation of infauna.

The ecosystem service value of living versus dead biogenic reef

Mixed maerl beds (corralline red algae) comprise dead thalli with varying amounts of live maerl fragments, but previously it was not known whether the presence of the live maerl increases the ecosystem service ‘habitat provision’ of the dead maerl for the associated epibenthos. A ‘flying array’ towed sled with high definition video was used to film transects of the epibenthos in dead maerl and mixed maerl beds in two locations to the north and south of the English Channel (Falmouth and Jersey). Mixed maerl beds supported greater number of taxa and abundance than dead beds in Falmouth, while in Jersey, mixed and dead beds supported similar number of taxa and dead beds had a greater abundance of epifauna. Scallops tended to be more abundant on mixed beds than dead beds. Tube worms were more abundant on mixed beds in Falmouth and dead beds in Jersey. An increasing percentage occurrence of live maerl thalli correlated with increasing number of taxa in Falmouth but not Jersey. It was concluded that while live thalli can increase the functional role of dead maerl beds for the epibenthos, this is dependent on location and response variable. As a result of this work, maerl habitat in SE Jersey has been protected from towed demersal fishing gear.

Correction: Corrigendum: Coralligenous and maërl habitats: predictive modelling to identify their spatial distributions across the Mediterranean Sea

CORRIGENDUM: Coralligenous and maerl habitats: predictive modelling to identify their spatial distributions across the Mediterranean Sea

The future of the northeast Atlantic benthic flora in a high CO2 world.

Seaweed and seagrass communities in the northeast Atlantic have been profoundly impacted by humans, and the rate of change is accelerating rapidly due to runaway CO2 emissions and mounting pressures on coastlines associated with human population growth and increased consumption of finite resources. Here, we predict how rapid warming and acidification are likely to affect benthic flora and coastal ecosystems of the northeast Atlantic in this century, based on global evidence from the literature as interpreted by the collective knowledge of the authorship. We predict that warming will kill off kelp forests in the south and that ocean acidification will remove maerl habitat in the north. Seagrasses will proliferate, and associated epiphytes switch from calcified algae to diatoms and filamentous species. Invasive species will thrive in niches liberated by loss of native species and spread via exponential development of artificial marine structures. Combined impacts of seawater warming, ocean acidification, and increased storminess may replace structurally diverse seaweed canopies, with associated calcified and noncalcified flora, with simple habitats dominated by noncalcified, turf-forming seaweeds.

Coralligenous and maërl habitats: predictive modelling to identify their spatial distributions across the Mediterranean Sea

Bioconstructions such as coralligenous outcrops and maerl beds are typical Mediterranean underwater seascapes. Fine-scale knowledge on the distribution of these sensitive habitats is crucial for their effective management and conservation. In the present study, a thorough review of existing spatial datasets showing the distribution of coralligenous and maerl habitats across the Mediterranean Sea was undertaken, highlighting current gaps in knowledge. Predictive modelling was then carried out, based on environmental predictors, to produce the first continuous maps of these two habitats across the entire basin. These predicted occurrence maps for coralligenous outcrops and maerl beds provide critical information about where the two habitats are most likely to occur. The collated occurrence data and derived distribution model outputs can help addressing the challenge of developing basin-wide spatial plans and to guide cost-effective future surveys and monitoring efforts towards areas that are presently poorly-sampled.

Uncommon sponges associated with deep coral bank and maerl habitats in the Strait of Sicily (Mediterranean Sea)

The ongoing exploration of deep-water coral habitats in the Mediterranean Sea provides continuously new and original information on such bathyal habitats and their biodiversity. The MARCOS (Malta StRait of Sicily CORals) cruise conducted in the Strait of Sicily in springtime 2007 sampled, among other things, a number of sponges associated with coral banks off Linosa Island and south of Malta and, subordinately, with maerl beds at shallower depths. Twenty species belonging to Demospongiae (19) and Hexactinellida (one) are here listed. Two species are first records for the Mediterranean Sea while two are new additions to the Italian sponge fauna. Finally, three taxa are recorded for the first time associated with Mediterranean deep-water corals.

Dysponetus joeli sp. nov. (Polychaeta: Chrysopetalidae) from the north-east Atlantic, with a cladistic analysis of the genus and a key to species

We describe Dysponetus joeli sp. nov. from shallow maerl habitats in the north-east Atlantic (English Channel and Bay of Biscay). Dysponetus joeli differs from congeneric species by a unique combination of characters, including a large syllid-like pharynx, 2–4 simple serrated neurochaetae (closely similar to notochaetae, but much smaller and more delicate), D-shaped chaetal spines and ventral cirri on the third segment. A phylogenetic parsimony analysis based on morphological traits suggests that Dysponetus is not monophyletic unless it includes the closely related genera Vigtorniella and Pseudodysponetus, which are well delineated inside the dysponetid clade. Chaetal spines seem to be secondarily derived from paleae and to have originated in infaunal dysponetid forms. They should not be considered as plesiomorphic, but as evidence to support the clade made up by Dysponetus–Vigtorniella and Pseudodysponetus as delineated by a phylogenetic analysis.

An experimental study of the ecological impacts of hydraulic bivalve dredging on maerl

AbstractA short-term experiment to assess the ecological impact of a hydraulic blade dredge on a maerl community was carried out during November 2001 in the Clyde Sea area on the west coast of Scotland. A fluorescent sediment tracer was used to label dead maerl, which was then spread out on the surface of sediment to act as a proxy for living maerl. The fauna collected by the dredge was dominated by the bivalves Dosinia exoleta and Tapes rhomboides, which were found to be intact. The target razor clams Ensis spp. were caught in low numbers, which reflected the low abundance of this genus within the maerl habitat. The hydraulic dredge removed, dispersed and buried the fluorescent maerl at a rate of 5.2 kg m−2 and suspended a large cloud of sediment into the water column, which settled out and blanketed the seabed to a distance of at least 8 m either side of the dredge track. The likely ecological consequences of hydraulic dredging on maerl grounds are discussed, and a case is made for protecting all maerl grounds from hydraulic dredging and establishing them as reservoirs to allow for the recruitment of commercial bivalve populations at adjacent fished sites.

Problems facing maerl conservation in Brittany

1. Brittany has the majority of French maerl beds and some of the most extensive maerl grounds in Europe. 2. Breton maerl forms ancient (>5500 yr) but threatened habitats; several beds have been destroyed in the past 30 years. 3. Maerl extraction, eutrophication, mariculture and fishing are all major threats to Breton maerl beds, as is a continuing uncontrolled spread of the invasive gastropod Crepidula fornicata. 4. Maerl beds occur in 14 of the 28 Natura 2000 sites proposed along the Breton coast. 5. Maerl habitats are currently impacted by human activities in nearly all of the Breton proposed Natura 2000 sites which were mainly selected for terrestrial nature conservation features. Copyright © 2003 John Wiley & Sons, Ltd.

Conservation and management of northeast Atlantic and Mediterranean maerl beds

1. Maerl beds occur worldwide and are formed by an accumulation of unattached calcareous red algae (Rhodophyta). 2. Maerl-forming algae grow in a superficial living layer on sediments within the photic zone. 3. Maerl beds are spatially complex habitats with a high degree of species and trophic group diversity. 4. The European Commission's ‘Habitats Directive’ mandates the conservation management of two of the main European maerl-forming species, Phymatolithon calcareum and Lithothamnion corallioides. 5. Mediterranean maerl beds are to be considered for inclusion in national inventories of sites of conservation interest, as required by the SPABIM Protocol of the Barcelona Convention. 6. In spite of their importance, and the requirement for their conservation management, European maerl grounds suffer a variety of anthropogenic perturbations including direct exploitation through extraction, fishing impacts and chemical pollution by organic matter and excess nutrients. 7. The ecology of northeast Atlantic and Mediterranean maerl beds has received little attention, in contrast to other marine communities (e.g. kelp forests, sea-grass meadows). 8. Key conservation and management measures proposed include: the recognition that maerl beds are non-renewable resources and cannot sustain direct exploitation; prohibitions on the use of towed gear on maerl grounds; moratoria on the issue of further permits for the siting of aquaculture units above maerl grounds; monitoring of existing exploited or impacted maerl beds; the designation of ‘no-take’ reserves; measures to limit the impacts that might affect water quality above maerl beds; a programme of monitoring of the ‘health’ of European maerl beds; an awareness campaign on the biological importance of maerl beds; a higher conservation status for maerl habitats and maerl-forming species in European legislation; and further research on maerl ecosystems. Copyright © 2003 John Wiley & Sons, Ltd.

Scallop dredging has profound, long-term impacts on maerl habitats

Maerl beds are mixed sediments built by a surface layer of slow-growing, unattached coralline algae that are of international conservation significance because they create areas of high biodiversity. They are patchily distributed throughout Europe (to ∼30 m depth around the British Isles and to ∼120 m depth in the Mediterranean) and many are affected by towed demersal fishing. We report the effects of Newhaven scallop dredges on a previously unfished maerl bed compared with the effects on similar grounds that have been fished commercially in the Clyde Sea area, Scotland. Sediment cores were taken to assess the population density of live maerl thalli prior to scallop dredging on marked test and control plots. These plots were then monitored biannually over a four-year period. Live maerl thalli were sparsely distributed at the impacted site, and experimental dredging had no discernible effect on their numbers. The previously unfished ground had dense populations of live maerl and scallops (both Aequipecten opercularis and Pecten maximus). While counts of live maerl remained high on the control plot, scallop dredging led to a >70% reduction with no sign of recovery over the subsequent four years. The vulnerability of maerl and associated benthos (e.g., the delicate bivalve, Limaria hians) is discussed in relation to towed demersal fishing practices.

The Influence of Sedimentary Heterogeneity on Within Maerl Bed Differences in Infaunal Crustacean Community

A study was undertaken of the crustacean community associated with maerl habitats in Mannin Bay, Ireland. Based on a visual classification of sedimentary fractions, eight sedimentary facies could be distinguished. These ranged from live maerl banks and seagrass-covered live maerl banks in the shallow, low energy parts of the inner bay to maerl debris facies, mixed with varying proportions of sand, mud and shell gravel in the more exposed, high energy, parts of the bay. Amphipoda dominated the crustacean fauna, accounting for more than 95% of total numerical abundance. Overall, the crustacean fauna occurring in the studied maerl habitats is similar to the fauna occurring in subtidal gravel habitats, with little evidence of elective species. Only the mud-maerl debris facies and the seagrass-covered live maerl banks appear to harbour a relatively different crustacean fauna from the remainder of the sedimentary facies. These differences were mainly changes in the numerical abundance of dominant species, rather than species displacements.