*Corresponding author. Email: katie.sambrook@hotmail.co.uk 1

Running head: Monitoring of maerl beds 1

Mapping and monitoring the maerl beds in the Fal & Helford Special Area of Conservation: 2

a review of techniques 3

Katie Sambrook*, Trudy Russell 4

Falmouth Marine School, Killigrew Street, Falmouth, Cornwall, TR11 3QS 5

Abstract 6

Maerl is a generic term for several species of unattached, non-geniculated coralline red 7

algae. Maerl beds are considered to be of conservation importance due to their rarity, 8

environmental sensitivities and high biodiversity and are subject to both UK and European 9

legislation. The EU Habitats Directive resulted in the creation of a network of Special Areas 10

of Conservation, established to provide a degree of governance for key habitats and species. 11

The Fal & Helford Special Area of Conservation contains the largest live maerl bed in 12

southwest Britain. Conservation targets for maintaining the favourable condition of these 13

maerl beds include monitoring the extent, distribution, percentage of live maerl and species 14

composition. It is a requirement that these attributes are assessed every six years. This 15

review evaluates existing techniques used for subtidal benthic habitat mapping and 16

biodiversity surveys recommending suitable methods for monitoring maerl in the Fal & 17

Helford Special Area of Conservation. 18

19

Keywords 20

Maerl, habitat mapping, special area of conservation, biodiversity, monitoring, surveys 21

2

22

Introduction 23

24

Maerl is the generic term for unattached, non-geniculated coralline red algae (Birkett et al. 25

1998, Foster 2001). It has a hard calcium carbonate skeleton which research indicates 26

makes a significant contribution to carbon sequestration in the oceans (Canals & Ballesteros 27

1997, Birkett et al. 1998). Maerl beds form when living and dead maerl thalli accumulate, 28

with accretion occurring over long periods due to the slow growth rate of maerl, usually 29

between 0.05 and 1.0mm y-1 (Birkett et al. 1998, Foster 2001, Bosence & Wilson 2003, Grall 30

& Hall-Spencer 2003). Found in polar, temperate and tropical waters, maerl-forming species 31

are patchily distributed due to narrow environmental tolerances (Barbera et al. 2003, Grall 32

& Hall-Spencer 2003). Current flow is the primary abiotic factor that influences the 33

distribution of maerl as it is intolerant to smothering and burial (Birkett et al. 1998, Barbera 34

et al. 2003). Light, temperature and salinity also contribute to its spatiotemporal presence 35

(Birkett et al. 1998, Barbera et al. 2003, Wilson et al. 2004, Sciberras et al. 2009). Three 36

species of maerl are found to contribute to the majority of maerl beds in the UK, 37

Lithothamnion corallioides, Lithothamnion glaciale and Phymatolithon calcareum, the latter 38

being predominant (Birkett et al. 1998). 39

40

The nodular structure of maerl creates an interlocking matrix providing a habitat for a wide 41

range of infauna and epifauna (Birkett et al. 1998, Barbera et al. 2003). This lattice 42

formation is responsible for the high biodiversity found on maerl beds which is comparable 43

to other algal biotopes such as sea grass beds and kelp forests (Birkett et al. 1998, Kamenos 44

3

et al. 2004a). As well as exhibiting high biodiversity, maerl beds have been found to harbour 45

juveniles of commercially important species such as gadoids and the queen scallop 46

Aequipecten opercularis (Kamenos et al. 2004b, c, d). Studies in Scotland found that the 47

highest densities of juvenile cod (Gadus morhua), saithe (Pollachius virens) and pollack 48

(Pollachius pollachius) were observed between September and November (Kamenos et al. 49

2004b); and for A. opercularis between October and December (Kamenos et al. 2004b). 50

Although pristine live maerl beds (PLM) exhibit higher biodiversity, the long-term 51

accumulation of dead maerl deposits also represent an important habitat, particularly for 52

burrowing species (Birkett et al. 1998). Functional diversity on maerl beds is high with Grall 53

& Glémarec (1997) identifying eight trophic groups on a maerl bed in France. 54

55

Recognising the ecological importance of maerl, both L. corallioides and P. calcareum are 56

included in Annex V of the European Union’s Habitats Directive as species whose 57

exploitation is subject to management and are listed under the UK Biodiversity Action Plan 58

as priority species (Council Directive 92/43/EEC 1992). Maerl beds ares also protected under 59

Annex I of the Habitats Directive and appears on the Convention for the Protection of the 60

Marine Environment of the North-East Atlantic (OSPAR) list for threatened or declining 61

habitats and species. The Habitats Directive resulted in the creation of a number of Special 62

Areas of Conservation (SACs) established to provide a degree of governance and protection 63

to key habitats and species. 64

65

The Fal & Helford SAC is located in Cornwall and covers 6387.8 hectares (JNCC 2011). It 66

includes a range of Annex I habitats including ‘sandbanks which are slightly covered by 67

seawater all the time’ which contains the sub feature of maerl beds (JNCC 2011). The SAC 68

4

has the largest live maerl bed in southwest Britain, found on St Mawes Bank, and also 69

contains deep deposits of dead maerl gravel indicating that during the past live maerl was 70

much more prevalent than today (Birkett et al. 1998). 71

72

A number of studies have been conducted on the extent and biodiversity of maerl in the Fal 73

& Helford SAC. Blundel et al. (1981) recorded that live maerl was found at depths of 0-10m 74

in the Fal & Helford SAC; Davies & Sotheran (1995) mapped the extent of live and dead 75

maerl as part of the BioMar project; Perrins et al. (1995) compared the percentage of live 76

and dead maerl on St Mawes Bank between 1982 and 1992; Frau-Ruiz et al. (2007) surveyed 77

Falmouth Bay to establish the presence of maerl to inform decision-making on scallop 78

dredging and ship anchoring and Axelsson et al. (2008) undertook an ecological survey as 79

part of an appropriate assessment for the Port of Falmouth Development Initiative which is 80

currently seeking to dredge a part of the SAC in order to deepen the channel and increase 81

ship access. 82

83

The morphology and community structure of the maerl is likely to have changed since 84

Davies & Sotheran (1995) mapped the biotopes in the estuary. At this time both aggregate 85

dredging and scallop dredging were allowed within the site, although both activities were 86

banned in 2005, with the scallop dredging ban extending into Falmouth Bay in 2008 (Hall-87

Spencer 2005, legislation.gov.uk 2008). Both practices are known to be destructive with 88

research into maerl beds in other regions showing that scallop dredging can lead to >70% 89

reduction in live maerl coverage with no indication of recovery during the subsequent four 90

years (Hall-Spencer & Moore 2000). Due to its slow growth rates and intolerance to 91

5

activities that create sediment perturbation and siltation, maerl is now recognised as a non-92

renewable resource (Barbera et al. 2003). 93

94

In the UK to date, most surveys on maerl have been in relation to disturbance activities with 95

limited time-series data collected (OSPAR 2010). However, in order to maintain the integrity 96

of the SAC designation, it is fundamental that a monitoring programme is implemented to 97

conduct regular assessments of the protected habitats or species enabling early detection of 98

changes such as a decline in live maerl, issues with invasives, effects of climate change or 99

regime shifts (Birkett et al. 1998, Hiscock 1998, Sciberras et al. 2009). Conservation targets 100

under OSPAR and the Habitats Directive for maintaining the favourable condition of maerl 101

beds include monitoring the extent, distribution, percentage of live maerl, species 102

composition as well as recording the abiotic factors that could impact the health of the 103

maerl (OSPAR 2010). These checks are required at six-yearly intervals (Malthus & Karpouzli 104

2003, OSPAR 2010). 105

106

This paper seeks to evaluate current methodologies for obtaining this information, review 107

examples of good practice and make recommendations on the techniques that would work 108

best for the collection of data pertaining to the maerl in the Fal & Helford SAC. It does not 109

aim to provide detailed methodologies on how the surveys should be carried out. While 110

each site should be reviewed separately to assess potential threats specific to its location 111

and activity levels, it is a long-term aim that a standard protocol could be implemented 112

across other SACs containing maerl to establish a comprehensive network of data 113

contributing to its long-term sustainability. 114

115

6

Methods 116

117

Mapping the extent and distribution of maerl 118

The Fal & Helford SAC contains a busy port, has many recreational users and is home to the 119

only oyster fishery worldwide still fished under sail using traditional methods of dredging 120

(Challinor et al. 2009). An understanding of the extent and distribution of the live and dead 121

maerl is essential for informing decision-making surrounding the management and 122

conservation of this site. 123

124

Remote sensing 125

Recent technological advances have seen the development of remote sensing techniques 126

such as airborne sensing (aerial photography or video and hyperspectral data), satellite 127

imagery and acoustic sensing to support biotope mapping (Held et al. 2003, Diaz et al. 2004, 128

Godet et al. 2009). The use of remote sensing in the marine environment is advantageous 129

over traditional approaches such as grab sampling or dredging as it allows large areas to be 130

mapped over short timescales (Brown et al. 2002, Sciberras et al. 2009, Simons & Snellen 131

2009; Brown et al. 2011); provides more accurate spatial discrimination due to virtually 132

continuous sampling (Brown et al. 2002, Godet et al. 2009) and is non-destructive which is 133

important when surveying protected habitats and species (De Backer et al. 2009). 134

High costs and limited depth range restrict the use of both airborne sensing and satellite 135

imagery in marine habitat mapping at present although advances in hyperspectral mapping 136

7

should be reviewed in the future (Kenny et al. 2003, Brown et al. 2011). Acoustic sensing 137

has been used extensively in subtidal environments as a predictive tool, validated by 138

rigorous groundtruthing to confirm the results (Birkett et al. 1998, Hiscock 1998, Brown et 139

al. 2005, Ehrhold et al. 2006). There are four broad categories of acoustic mapping device: 140

(i) broad acoustic beam systems such as side scan sonar (SSS); (ii) acoustic ground-141

discrimination systems such as RoxAnnTM and QTC-ViewTM (iii) multiple narrow-beam swath 142

bathymetric systems and (iv) multiple-beam side scan sonar (Kenny et al. 2003). 143

144

Main types of acoustic survey method for benthic habitat mapping 145

146

Side scan sonar 147

Broad beam swath systems such as side scan sonar can produce an almost photographic 148

representation of the seabed (JNCC 2001, Kenny et al. 2003, Georgiadis et al. 2009). 149

Comprising of an underwater transducer connected by a cable to the towing vessel where 150

data is recorded, an acoustic signal is emitted from a single beam or multiple beams rapidly 151

returning echoes which are transmitted to the recording device for later analysis (JNCC 152

2001). As the sonar is towed at a fixed height above the seabed, it casts relatively large 153

acoustic shadows enabling the detection of variations in sediment structure (Kenny et al. 154

2003). Side scan sonar provides continuous coverage of the area being surveyed (Brown et 155

al. 2011). The introduction of digital side scan sonar devices has increased mapping 156

capabilities with object detection possible down to tens of centimetres (JNCC 2001). To 157

some extent, the quality of the images generated can be evaluated manually with features 158

such as sand ripples identifiable by eye (JNCC 2001). The REBENT monitoring network in 159

8

France has adopted side scan sonar as part of its benthic mapping programme which 160

includes maerl beds (Ehrhold et al. 2006). Georgiadis et al. (2009) used side scan sonar to 161

map coralline algae in the southern Aegean Sea. Side scan sonar has also been used in 162

studies by Brown et al. (2002, 2005) and Freitas et al. (2006) to identify different seabed 163

assemblages alongside groundtruthing exercises and OSPAR recommends its application 164

where maerl beds are thick and extensive (OSPAR 2010). 165

166

Acoustic ground-discrimination systems (AGDS) 167

Acoustic ground-discrimination systems (AGDS) operate using a hull-mounted single-beam 168

echosounder which detects differences in the seabed using acoustic reflection properties 169

(Greenstreet et al. 1997, Kenny et al. 2003). A single pulse is emitted from the device which 170

is directed straight down towards the seabed producing a footprint of the area directly 171

below the vessel (Brown et al. 2005). On reflection, the echo is returned to a transducer 172

(JNCC 2001). Due to the focused pulse emitted from the sounder, acoustic ground-173

discrimination systems do not produce continuous coverage maps like side scan sonar, but 174

instead produce a track covering the area beneath the vessel (Wilding et al. 2003). Gaps 175

between the tracks must be interpolated which can result in inaccurate assumptions about 176

the seabed habitat (Kenny et al. 2003, Brown et al. 2005). This means that AGDS should be 177

considered a predictive tool rather than a definitive view of the seabed (JNCC 2001, Wilding 178

et al. 2003). There are two systems commonly in use in the UK - RoxAnnTM and QTC-ViewTM 179

(JNCC 2001, Foster-Smith & Sotheran 2003). RoxAnnTM derives its values from the 180

interpretation of two elements of the returning echo: the first echo (E1) is related to 181

seafloor roughness, while the second multiple return echo (E2) is related to the hardness of 182

the seabed (Kenny et al. 2003, Wilding et al. 2003). Supplemented by groundtruthing, the 183

9

returned responses are classified into seabed characteristics (Kenny et al. 2003, Wilding et 184

al. 2003, Brown et al. 2005). QTC-ViewTM converts the echo into a digital form using the first 185

returning signal only and organises the seabed into acoustic classes (JNCC 2001, Ellingsen et 186

al. 2002). It can used in ‘supervised’ or ‘unsupervised’ modes, the first requiring 187

considerable groundtruthing using calibration sites, the second relying on post-processing 188

analysis (Ellingsen et al. 2002). The use of AGDS has become widespread in SACs in the UK 189

(Brown et al. 2005) with Davies & Sotheran (1998) adopting this approach when evaluating 190

biotopes in Falmouth Bay and the lower Fal Ruan Estuary. The JNCC Marine Monitoring 191

Handbook (2001) recommends the use of AGDS where broad-scale surveys are required and 192

as a tool for locating sites of particular interest, thus reducing survey time. The cost of AGDS 193

in comparison to other acoustic devices is relatively cheap and integration with other 194

onboard systems such as GPS is straightforward (Birkett et al. 1998, Kenny et al. 2003, 195

Wilding et al. 2003). However, concerns have been raised around the unpredictability of 196

responses, resolution at varying depths, levels of interpolation required and accuracy when 197

determining habitat boundaries (Kenny et al. 2003, Wilding et al. 2003, Brown et al. 2005; 198

Freitas et al. 2011). 199

200

Multibeam echosounders 201

Multibeam echosounders (MBES) were originally an extension of single-beam 202

echosounders, transmitting multiple beams which are capable of covering a broad swath 203

either side of the vessels track (Brown & Blondel 2009, Simons & Snellen 2009). Multibeam 204

echosounding is becoming increasingly utilised within the field of seabed habitat mapping 205

due to its collective ability to obtain bathymetry and backscatter data concurrently (Brown 206

& Blondel 2009, Brown et al. 2011). The return signal can provide details of the geoacoustic 207

10

properties of the sea bed including grain size and porosity, both of which could be useful 208

features for identifying maerl beds (Brown & Blondel 2009). With the introduction of faster 209

processing capabilities and quality improvements to the backscatter images which are now 210

comparable to those collected through side scan sonar, studies using multibeam 211

echosounders have rapidly increased (Brown et al. 2011). Like side scan sonar, continuous 212

coverage maps can be created in conjunction with groundtruthing (Simons & Snellen 2009). 213

Brown & Blondel (2009) observed that multibeam echosounders are now able to provide as 214

much if not more information than side scan sonar. However, a high level of understanding 215

is required to establish the most appropriate analytical approach and significant expertise is 216

needed to interpret the data (Brown et al. 2011). To date no studies have been published 217

demonstrating the successful application of multibeam echosounders to identify maerl. 218

A more comprehensive review of acoustic techniques used for seabed habitat mapping can 219

be found in Brown et al. (2011). 220

221

Groundtruthing 222

While acoustic surveys are increasingly used to map and classify seabed habitats, 223

groundtruthing is still necessary either for calibration with acoustic analytical tools or to 224

enable classification of the discrete characteristics identified through acoustic imaging (JNCC 225

2001, Brown et al. 2011). Grab samples, towed or drop-down video and remote operated 226

vehicles (ROVs) are common methods of groundtruthing. 227

228

Grab samples 229

11

Grab sampling involves the use of a two shelled steel device that is lowered open to the 230

seabed and digs into the sediment, bringing the two shells together to retain a sediment 231

sample which can be analysed at the surface. There are a number of grab samples in 232

standard use: the van Veen which is most appropriate for soft sediments; the Day Grab 233

which can be used on a variety of sediment types and the Hamon Grab which can be used to 234

collect samples on coarser sediments including cobbles (JNCC 2001). Both the van Veen and 235

Day Grab devices can be used from a small vessel with two operators where the Hamon 236

Grab sampler can only be launched from a larger vessel and requires a minimum of three 237

operators (JNCC 2001). When using grab sampling to groundtruth acoustic data, 238

consideration should be given to the method of sampling arrays. The JNCC Marine 239

Monitoring Handbook (2001) has details on random, stratified random and systematic 240

approaches to sampling. Analysis of the contents of the grab can be used to assist with the 241

classification of seabed characteristics found during acoustic surveys (Greenstreet et al. 242

1997, Davies & Sotheran 1998, Brown et al. 2002, Foster-Smith & Sotheran 2003, Freitas et 243

al. 2003, Ehrhold et al. 2006, Georgiadis et al. 2009). The number of samples and replicates 244

collected should take into consideration the area of the survey and the variations detected 245

from the acoustic mapping exercise (JNCC 2001). 246

247

Underwater video imaging 248

Boat operated underwater video imaging systems include drop-down video, towed sledge 249

video and remotely operated vehicles. Drop-down video involves the use of a video camera 250

attached to an umbilical cord being lowered over the side of the survey vessel either when 251

stationary or at low speed (JNCC 2001). This method gives the operator a limited degree of 252

control about the location studied and can be easily deployed to survey as many areas as 253

12

required (JNCC 2001). Brown et al. (2002) and Ehrhold et al. (2006) used drop-down video 254

to validate acoustic survey data collected for biotope mapping. Towed sledge video involves 255

a camera and lights mounted onto a rigid sled supported by buoyancy devices to keep the 256

image stable as the gear is towed (JNCC 2001). Large areas can be surveyed rapidly using 257

this approach and the clarity of the image enables accurate identification of biotopes (JNCC 258

2001). To reduce positional errors when comparing acoustic data to the video footage, a 259

transponder should be ideally fitted to the sledge to pinpoint its location (JNCC 2001). 260

Without a transponder, manual calculations will have to be performed prior to comparison. 261

Davies & Sotheran (1998) and Ruiz-Frau et al. (2007) used towed video when mapping 262

biotopes in the Fal. Remotely operated vehicles (ROVs) are also linked to the research vessel 263

by an umbilical cord but can be manoeuvred remotely by an operator onboard. ROVs can 264

survey large areas and focus on precise points of interest and are useful at sites with rapid 265

depth changes (Moore & Bunker 2001). However they are expensive and difficulties can be 266

encountered with recording the devices position. Georgiadis et al. (2009) used ROV drops in 267

conjunction with side scan sonar to map coralligène formations in the eastern 268

Mediterranean Sea and Thomson (2003) used an ROV as part of project to develop 269

autonomous sensors for marine resource mapping. All of these approaches provide a 270

permanent record of the seabed which can be stored, edited and reused. They are also 271

non-intrusive methods for identifying different substrate types. Successful use of any image 272

recording device is highly dependent on underwater visibility. 273

274

Evaluating proportions of live and dead maerl 275

276

13

Both live and dead maerl are considered important habitats for a diverse range of organisms 277

although the biodiversity on live maerl beds is considered greater (Birkett et al. 1998). The 278

slow growth rate of maerl means that any detrimental influences such as eutrophication, 279

invasive species, dredging or pollution is likely to have long-term implications for maerl and 280

its inhabitants (Hall-Spencer & Moore 2000, Barbera et al. 2003, Grall & Hall-Spencer 2003). 281

By establishing and monitoring the location and extent of live maerl beds, managers of the 282

Fal & Helford SAC will be able to observe any reduction in area. 283

284

There are a number of methods that could be used to collect this information. Grabs or 285

core samples (discussed above) could be used but would only provide data on a small 286

proportion of the survey area. They would also be unable to provide a quantitative 287

measurement as the grabs take a significant ‘bite’ from the seabed and the proportions of 288

sediment may not represent the actual quantity of live maerl present (Ruiz-Frau et al. 2007). 289

The use of drop-down or towed sledge video means that large areas can be surveyed 290

relatively quick to assess the extent of a live maerl bed but quantification of results would 291

again be difficult (Davies & Sotheran 1995, Ruiz-Frau et al. 2007). Thomson (2003) found, as 292

part of the SUMARE programme (Survey of Marine Resources) to establish autonomous 293

sensors for identifying maerl, that the use of a greylevel histogram applied to video footage 294

identified unique signatures for live and dead maerl. Although time consuming, direct diver 295

observation using quadrats or transect lines could be employed and would provide 296

quantifiable data on percentages of dead and live maerl (Axelsson et al. 2008). In the 297

Republic of Ireland, SCUBA divers use direct propulsion vehicles to rapidly assess 298

percentages (OSPAR 2010). All these methods could be interpreted and used in mapping. 299

300

14

Assessing community structure / biodiversity 301

302

The complex structure of maerl brings a number of challenges when surveying the 303

biodiversity of maerl beds. Steller et al. (2003) classified the type of organisms associated 304

with maerl into three main categories: 305

Epibenthic: motile or sessile organisms living on the seabed. 306

Cryptofaunal: those organisms living within the natural cavities created by the lattice 307

network of maerl. 308

Infaunal: those individuals living buried in the substrate. 309

The small size of many organisms found on maerl beds has implications for their 310

classification and any detailed studies require a high level of taxonomic expertise. Birkett et 311

al. (1998) recognised this as an issue and observed that identifying down to genus level 312

could still provide enough information to assess the health of the biotope. The size of 313

organisms also affects the types of survey that can be selected to gain sufficient quantitative 314

information on biodiversity. Diver surveys, hand-held video and still photographs can 315

provide data on conspicuous species but may miss smaller organisms. These methods 316

cannot be used to survey the cryptofauna and infauna. In order to obtain results for these, 317

samples must be collected and analysed ex situ. As maerl is slow growing, the quantity of 318

samples required when employing any intrusive survey techniques such as grab samples and 319

cores needs to be carefully considered. Maggs (1983) suggests that the minimum sub-320

sample size should be where a 10% increase in the number of species in the sub-sample is 321

derived from a 10% increase in the area. Processing of samples must be appropriately 322

15

designed to ensure the preservation of organisms for analysis and successful capture across 323

the size range of organisms. 324

325

In situ diving surveys 326

Diver quadrat surveys can be used for recording epibenthic species found on maerl. 327

Quadrats come in a variety of sizes, commonly 1m2, 0.25m2 or 0.1m2 (JNCC 2001). For maerl, 328

a 0.25m2 grid quadrat would be a sensible choice for the size of organisms under survey. A 329

minimum of two suitably qualified divers descend a weighted shot line, which can be used 330

by the survey vessel to record location, and lay a transect in a pre-agreed direction. This 331

avoids potential bias by the divers once underwater to select areas that look ‘most 332

interesting’ (Eleftheriou & McIntyre 2005). The quadrat and transect line should be slightly 333

negatively buoyant so that it can rest on the seabed while the divers complete the survey 334

(Axelsson et al. 2008). Divers should have a standard survey form to record species and 335

abundance. The JNCC Marine Monitoring Handbook (2001) recommends conducting a pilot 336

survey of the location to familiarise the divers with the species likely to be encountered 337

during the survey. Once the survey is complete, the dive team should use a delayed surface 338

marker buoy to inform the survey vessel of the end location. This will enable the results to 339

be mapped. Collecting data in this way is a non-destructive technique; provides quantitative 340

data which allows changes to species composition to be monitored over time; is a simple 341

method for recording conspicuous species without damaging the maerl and is easily 342

repeatable (Eleftheriou & McIntyre 2005). The divers need to be confident of their buoyancy 343

control to avoid kicking up sediment or causing damage to the surrounding maerl; should 344

have an understanding of the biotope and potential species and have the appropriate 345

16

qualifications for conducting underwater surveys (JNCC 2001). Percentages of live and dead 346

maerl could also be recorded as part of this survey. Steller et al. (2003) used transect 347

surveys to estimate species richness and abundance on maerl beds in the Gulf of California. 348

Hand-held video surveys carried out by qualified divers can provide information on the 349

conspicuous benthos in the chosen study site (JNCC 2001). A video camera, mounted in 350

underwater housing and with lighting to improve image quality is the standard equipment 351

required (JNCC 2001). It provides a permanent record of the site surveyed and analysis can 352

be conducted ex situ. Poor underwater visibility; scaling of the images recorded; the ability 353

to collect quantitative data that can be readily related to GPS points and consistency of 354

recording means that this method may be more appropriate for collecting footage that can 355

be used for educational purposes rather than monitoring. Axelsson et al. (2008) conducted 356

diver video surveys in the Fal Estuary in conjunction with in situ diver observations. 357

Still photography of maerl beds involves the use of an underwater high-resolution camera 358

with lighting. In order to obtain quantitative data, the camera should be mounted onto a 359

reference frame or a quadrat used to provide a scale to the photograph (JNCC 2001). In the 360

same way as video, photographs provide a permanent record and with greater resolution 361

capabilities than video, photographs can enable more detailed images to be collected. 362

Analysis can be carried out ex situ meaning that the divers do not need to be taxonomists. 363

Most commonly used for collecting fixed point data, there are currently no examples of this 364

method being used for random sampling (JNCC 2001). However, this method could be used 365

in conjunction with a diving quadrat survey to record unidentified specimens or get close up 366

shots without removing specimens. 367

368

17

Sample collection techniques 369

Hand-held cores can be used to collect cryptic fauna, infauna and sediment samples 370

(Bordehore et al. 2003, Moore et al. 2004, Axelsson et al. 2008, Sciberras et al. 2009). These 371

cores are operated by divers who manually push the core into the sediment. Axelsson et al. 372

(2008) found that the substrate in areas of the Fal made the extraction of sediment samples 373

using hand-held cores quite challenging. On reaching the appropriate depth, the top and 374

bottom of the corer are sealed with caps and it is placed into bags for secure storage and to 375

prevent any organisms escaping (Axelsson et al. 2008, Sciberras et al. 2009). Ideally on 376

board the survey vessel and within 24 hours of collection, the samples should be sieved and 377

fixed in 10% formal saline to ensure the samples are preserved for laboratory analysis (JNCC 378

2001, Axelsson et al. 2008). The small nature of some species found on maerl means that 379

the mesh size should be below 1000μm to ensure robust analysis (Moore et al. 2004, 380

Axelsson et al. 2008). 381

To collect cryptic fauna, Steller et al. (2003) used divers to collect random samples of maerl 382

thalli by hand, which were then preserved and the cryptic and burrowing species were 383

extracted in the laboratory for analysis. 384

Grab samples (discussed in the mapping section) can be used for biological sampling as well 385

as groundtruthing (Sciberras et al. 2009). A combined approach to groundtruthing and 386

infaunal sample collection can involve skimming off the top layer of the contents of the grab 387

sample for sediment analysis and fixing the rest for biological analysis in the laboratory 388

(Moore et al. 2004). Ruiz-Frau et al. (2007) used a 0.1m2 Day Grab for collecting sediment 389

and biological samples in Falmouth Bay. Grab samplers usually require deployment from a 390

research vessel with space to winch the samples in. 391

18

392

Measuring abiotic factors that could impact the health of maerl beds 393

Environmental tolerances of maerl are still poorly understood (Birkett et al. 1998). However, 394

any robust monitoring programme should include abiotic measurements so they can be 395

factored in if any unexplained degradation of the maerl beds occurs. 396

Temperature is known to affect the geographic distribution of maerl beds with L. 397

corallioides absent in Scotland due to cooler temperatures but present in southwest Britain 398

(Birkett et al. 1998, Wilson et al. 2004). The species composition of maerl beds is influenced 399

by temperature, so monitoring the water temperature on maerl beds could track changes 400

that may occur as a result of climate change (Wilson et al. 2004). Maerl beds are normally 401

found in fully saline conditions but the impact of variable salinity conditions is not 402

understood (Birkett et al. 1998). Temperature and salinity can be measured together in situ 403

using a temperature-salinity probe (Sciberras et al. 2009); through the use of a CTD 404

(conductivity-temperature-depth) package or by using a niskin bottle to collect water 405

samples for analysis on the surface. 406

Maerl is a coralline algae and therefore needs to photosynthesise in order to grow, however 407

irradiance requirements are not understood for maerl (Birkett et al. 1998). In the UK, maerl 408

beds rarely exceed 30m (Wilson et al. 2004). Turbidity can affect the amount of light 409

penetrating to the maerl bed and can be measured simply using a secchi disk (Sciberras et 410

al. 2009). 411

The nutrient requirements of maerl are not known, however with a rigid skeleton of calcium 412

carbonate maerl does have a requirement for calcium (Birkett et al. 1998). In laboratory 413

19

experiments, King & Scramm (1982) found that the calcium ionic concentration affected 414

maerl growth, with an optimum uptake of 30 ‰. Under laboratory conditions, Martin & 415

Gatuzzo (2009) conducted experiments to assess the effects of ocean acidification on 416

coralline algae and found that based on current projections, net dissolution was likely to 417

exceed net calcification in Lithothamnion cabiochae by the end of the century. pH therefore 418

should be monitored and can be tested in situ using an underwater housed pH meter or on 419

the research vessel if a water sample has been collected. 420

421

Discussion 422

423

The three-dimensional structure of maerl brings additional complexities and considerations 424

on top of those encountered during other subtidal surveys. The current deficiency in 425

standardised protocols for assessing maerl communities combined with rapid advances in 426

the technology used for mapping benthic habitats makes difficult work for conservation 427

managers when establishing a monitoring programme. There is no single method that can 428

monitor all the conservation objectives set for maerl within the Fal & Helford SAC. Instead 429

the solution requires a suite of approaches that will satisfy the requirements and provide 430

the management team with a robust dataset that can inform decision-making. The 431

recommendations discussed in the proceeding sections are based on the knowledge gained 432

through studies on maerl in other regions, the use of standardised methodologies where 433

possible and factor in resource and cost considerations. Due to the environmental 434

sensitivities of maerl, the recommendations exhibit a preference towards non-destructive 435

techniques where feasible. Table 1 summarises the recommendations discussed below. 436

20

437

Recommendations for mapping 438

While the importance and potential application of acoustic devices for seabed habitat 439

mapping is widely recognised, the difficulties with accurately differentiating maerl habitats 440

from other mixed sediments is still problematic and can currently only be used if validated 441

with groundtruthing. All three forms of acoustic sensing discussed have been used to map 442

maerl biotopes but the most recent research shows a preference towards the use of side 443

scan sonar in conjunction with a multibeam echosounder (Kenny et al. 2003, Diaz et al. 444

2004, Ehrhold et al. 2006, Brown et al. 2011). Side scan sonar has the highest definition of 445

seabed features amongst the techniques in use and this combined with the bathymetric and 446

backscatter imaging provided by multibeam echosounders will generate a robust baseline 447

for the monitoring programme (Diaz et al. 2004). The two devices are also capable of 448

providing 100% coverage unlike AGDS which needs interpolation between the tracks. From 449

previous studies of maerl in the Fal Estuary, it is apparent that maerl occurs in fairly shallow 450

waters (Blundel et al. 1981, Hiscock 1998). Acoustic surveys therefore need to be carried 451

out during high tides for maximum accuracy. Provision will need to be made within the 452

budget for a mapping expert to advise on the survey design and interpret the outputs. Due 453

to the area the Fal & Helford SAC covers, the quickest and most cost-effective option for 454

groundtruthing is drop-down video. For deeper parts like the channel in Carrick Roads, 455

towed sledge video may be required but will give comparable results. 456

457

While these surveys can be carried out at any point during the year, it is recommended that 458

mapping is carried out during the summer months when visibility, light and weather 459

21

conditions are optimum. For consistency, future surveys should be carried out across the 460

same period. Surveys lasting longer than an hour should have tidal corrections applied 461

(Brown et al. 2005). 462

463

Recommendations for evaluating proportions of live and dead maerl 464

An initial baseline assessment should focus on establishing the location and dimensions of 465

the live maerl beds. Drop-down video is a practical way to collect this data and would work 466

well with the groundtruthing exercise. With drop-down video, GPS points can be easily 467

collected when live maerl is observed and later plotted onto a GIS map. Where live maerl 468

beds are identified surveys should be conducted to identify the perimeter of the bed so that 469

a true area can be calculated. While the primary goal is to obtain a map showing the 470

locations, another important measurement is to identify the proportion of live and dead 471

maerl present within the live maerl beds. The best method for obtaining quantitative data 472

is to conduct diver quadrat surveys along a series of transects. Procedural Guideline 3-7 473

from the JNCC Marine Monitoring Handbook (2001) covers the methodology in more detail. 474

Measurements should be taken as percentages not abundance scales to enable quantitative 475

analysis. It should be noted that these survey methods will only measure the surface layer 476

of the maerl. 477

478

Recommendations for biodiversity 479

Apart from the maerl species there are no key indicator species associated with maerl at 480

present so it is necessary to ensure a robust biodiversity survey is conducted. Collecting data 481

22

on the number and type of species found on maerl beds will require the application of 482

several methodologies and is likely to be the most time-consuming part of the monitoring 483

programme. Both live and dead maerl deposits should be surveyed. Multiple diver quadrat 484

surveys along a transect line are recommended for surveying the epifauna and flora 485

following the sample size recommendations proposed by Maggs (1983). This approach will 486

require at least one diver to be a trained biologist with an understanding of the taxonomy of 487

organisms associated with maerl. Organisms should be identified down to species level 488

where possible but genus is acceptable. The second diver should have a high-resolution 489

underwater camera with a macro setting and strobe lighting. Each quadrat should be 490

photographed with a unique id. Any unknown organisms should be photographed for 491

identification ex situ. The photograph should include a scale for reference. A standard form 492

should be generated for the survey team to complete including depth, temperature and 493

percentage of live and dead maerl. Any diving surveys must comply with the Diving at Work 494

Regulations 1997 and adhere to the Scientific and Archaeological Approved Code of 495

Practice. 496

497

There are currently no non-destructive methods for surveying the infauna. The use of a 498

diver hand-held corer is considered a suitable method for obtaining samples. A number of 499

objectives can be achieved through collecting core samples. The species of maerl will be 500

easier to identify in the laboratory, the underlying sediment can be measured and cryptic 501

fauna and infauna can be surveyed at the same time. Samples should be processed and 502

fixed within 24 hours of collection for later analysis in a laboratory. Comprehensive 503

methodologies for collecting, processing and analysing cryptic fauna and infauna can be 504

found in Steller et al. (2003) and Sciberras et al. (2009) respectively. 505

23

506

Due to the seasonal variations experienced on maerl beds, two sampling seasons are 507

recommended to gain a greater understanding of community change however this will be 508

constrained by the budget allotted to the monitoring programme. Consideration should 509

also be given to avoid extensive destructive survey techniques around any peak recruitment 510

periods. If only one survey is practicable, then July is advised when species richness is at its 511

peak yet recruitment and settlement of known commercially important species is low 512

(Kamenos et al. 2004b, c, Peña & Bárbara 2010). 513

514

Recommendations for measuring abiotic factors 515

Although OSPAR (2010) states that monitoring should be conducted as a minimum at six-516

yearly intervals for SACs, it is recommended that abiotic factors should be collected twice 517

yearly during Spring and Autumn. Plankton levels are generally high in Spring while water 518

temperature is at its annual lowest, conversely in late Autumn, annual water temperature 519

reaches its maximum and plankton is on the decline (Miller 2004). Temperature, salinity, 520

pH, turbidity and dissolved oxygen levels should be recorded. These are relatively simple 521

and quick measurements to collect and may be important if any change to the extent and 522

distribution of the maerl is observed. It is important that the same equipment is used and 523

calibrated each time thus avoiding potential discrepancies between devices. 524

525

Further recommendations 526

24

Continuing advances in the technology surrounding benthic habitat mapping and our 527

increasing understanding of maerl biotopes mean that the techniques recommended in this 528

review will need to be reassessed at appropriate junctures. 529

While not within the scope of the review, it is worth emphasising the importance of the 530

supporting systems that will need to be established in order to manage and store the data 531

collected through the monitoring programme. A suitable format will need to be selected 532

which is likely to include GIS functionality and scope for statistical analysis of the data. 533

534

Conclusions 535

536

The high biodiversity associated with maerl beds has long been recognised. However for 537

many years maerl has been subjected to damaging anthropogenic activities such as 538

aggregate extraction and dredging. Investigations on maerl have largely focused on 539

assessing the implications of these destructive processes or sought to gain a greater 540

understanding of the biology and ecology associated with maerl. As such, there have been 541

few monitoring programmes established to look at the long-term dynamics of maerl bed 542

communities. 543

Valuable contributions have been made by the scientific community to expand our 544

understanding of maerl. These studies have shown that maerl grows exceptionally slowly, 545

has narrow environmental tolerances and is highly sensitive to disturbance resulting in the 546

acknowledgement that maerl is a non-renewable resource. They have also identified its role 547

in carbon sequestration and as a nursery ground for commercially important species. 548

25

As a consequence, a growing emphasis is being placed on the conservation importance of 549

maerl beds, with local and European protection designations at both species and habitat 550

level. In order to manage maerl beds effectively, it is important to establish a 551

comprehensive baseline from which to monitor long-term trends. 552

Those parties involved in the management of the Fal & Helford SAC should find this review a 553

useful summary of the techniques most suitable for surveying maerl beds. While it shows 554

that establishing a maerl monitoring programme for the Fal & Helford SAC is a complex 555

process that will involve significant time, cost and planning, it also demonstrates that there 556

are multiple methods that can be adopted in order to achieve the primary conservation 557

objectives. 558

559

Acknowledgements 560

561

This review was kindly supported by the Falmouth Harbour Commissioners. This manuscript 562

benefited from comments by C. Eatock, A. O’Brien, T. Russell, and A. Thornton. Particular 563

thanks go to thank N. Woods for his assistance with testing ROV and multibeam 564

echosounder methodologies and the use of the research vessel RV Ann Kathleen. 565

566

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