diver-operated suction sampling in … · diver-operated suction sampling in norwegian cobble...

Crustaceana 88 (2) 184-202

DIVER-OPERATED SUCTION SAMPLING IN NORWEGIAN COBBLEGROUNDS: TECHNIQUE AND ASSOCIATED FAUNA

BY

HALLDIS RINGVOLD1), JOHN-ARVID GRYTNES2) and GRO I. VAN DER MEEREN3,4)1) Sea Snack Norway, NO-5055 Bergen, Norway

2) Institute of Biology, University of Bergen, NO-5020 Bergen, Norway3) Institute of Marine Research, Austevoll Research Station, NO-5392 Storebø, Norway

ABSTRACT

Marine cobble habitats in shallow waters are rich in faunal assemblages and known settlinggrounds for valuable fishery resources such as lobsters and crabs. Sampling these grounds ischallenging as traditional techniques do not efficiently collect fast-moving benthic invertebrates.Typically, fast moving crustaceans are not sampled according to actual densities. This study usedairlift suction sampling, pioneered in North America, to quantify benthic faunal assemblages incobble grounds across 68 sampling locations in south-western Norway. In total, 72 species of benthicinvertebrates (5276 individual specimens) were identified, with an overall sampling efficiency of76.4%. Polychaeta and decapod crustaceans dominated the samples, with species diversity (ShannonIndex, H ′) highest in Location 3. Cluster and Ordination analyses were further used to relateassemblages to a number of selected variables. Overall, the study highlights that suction samplingprovides a low-cost and efficient method for quantifying mobile benthic fauna in structurallycomplex marine habitats (i.e., cobble).

RÉSUMÉ

Les habitats marins à galets en eau peu profonde sont riches en assemblages fauniques et sontdes fonds de recrutement connus pour les ressources halieutiques de valeur comme les homards etles crabes. L’échantillonnage des ces fonds est un défi car les techniques traditionnelles ne récoltentpas efficacement les invertébrés benthiques se déplaçant rapidement. Typiquement, vu les densitésactuelles, les Crustacés à déplacement rapide ne sont pas échantillonnés. Cette étude utilise unéchantillonnage par aspiration, pionnier en Amérique du Nord, pour quantifier les assemblagesbenthiques sur les fonds de galets au niveau de 68 points d’échantillonnage dans le sud-ouestde la Norvège. Au total, 72 espèces d’invertébrés benthiques ont été identifiées (5276 spécimensindividuels) avec une efficacité d’échantillonnage totale de 76,4%. Les Polychètes et les CrustacésDécapodes dominent dans les échantillons, avec une diversité spécifique (Index de Shannon, H ′) laplus élevée pour Location 3. Des analyses typologiques et de classification étaient utilisées par lasuite pour relier les assemblages à un nombre sélectionné de variables. Au total, l’étude souligneque l’échantillonnage par aspiration fournit une méthode efficace et peu coûteuse pour quantifier lafaune mobile benthique dans les habitats marins structurellement complexes (i.e., galets).

4) Corresponding author; e-mail: [email protected]

© Koninklijke Brill NV, Leiden, 2015 DOI 10.1163/15685403-00003406

mailto:[email protected]

http://dx.doi.org/10.1163/15685403-00003406

SUCTION SAMPLING IN NORWAY 185

INTRODUCTION

While sampling techniques such as cores, dredges, grab shots, and bottomnets have been successful at sampling soft bottom habitats (Tunberg, 1981;Josefson, 1985; Dahle et al., 1998), quantitative sampling of epifauna in cobble orboulder substrata has traditionally been problematic, especially when interstitial,fast swimming species are being targeted. The development of the airlift suctionsampler in the 1980s (Incze & Wahle, 1991), provided researchers with theopportunity to address this issue. Initially pioneered to investigate the spatial andtemporal patterns of benthic recruitment within the American lobster Homarusamericanus (H. Milne Edwards, 1837) (cf. Incze & Wahle, 1991), the techniqueprovided quantitative estimates for a range of mobile species in this previouslyinaccessible habitat (fig. 1). This lead to an increased biological understandingin a wide range of benthic animals, especially highly mobile invertebrates, thatpreviously avoided traditional sampling techniques.

In 1997 this method was introduced to Europe for a collaborative-Europeanproject aimed at sampling newly settled, shelter-restricted European lobster Homa-rus gammarus (Linnaeus, 1758) (FAIR CT-96-1775; see Mercer et al., 2001).The study identified a rich variety of interstitial benthos in cobble ground, andprovided the first quantitative estimates for many species in such habitats (Linnaneet al., 2001). However, in addition to substrate, a number of environmentalvariables such as depth, temperature, salinity, currents, and waves are also knownto influence benthic biodiversity (Tunberg, 1981, 1982; Persson, 1983; Aschan,1988; Boudreau et al., 1992), even at reduced spatial scales (Tunberg, 1982). Theaim of this study was to analyse the suitability of airlift suction sampling as amethod for describing the biodiversity of both a mixed- and hard bottom substratesin Norwegian waters. This was undertaken by describing the associated fauna andspecies diversity within six separate dive locations. The study also examined theinfluence of environmental factors in relation to species diversity within these sites.

MATERIAL AND METHODS

Suction sampler

The airlift consisted of a black PVC tube (170 cm length, 7.0 cm diameter) witha SCUBA cylinder supplying air fitted 10 cm above the mouth. The other end ofthe tube was affixed to a 1 mm mesh nylon bag that could be removed, closedand replaced underwater. The suction sampler was operated by two divers and wasused in conjunction with a 0.5 m2 quadrat constructed from buoyant PVC piping,a 30 cm high sheet of 1 mm mesh curtain, and a heavy chain base. This designprevented the escape of mobile decapods from the quadrat (fig. 1).

186 HALLDIS RINGVOLD, JOHN-ARVID GRYTNES & GRO I. VAN DER MEEREN

On sampling, the quadrat was placed haphazardly on a suitable cobble patch.The cobble stones were then removed by hand by one diver while the secondoperated the airlift, ensuring that the mouth of the tube reached into the interstitialspaces of the substratum. Divers worked slowly until all of the interstitial spaceswere exposed and sampled. Periodically, sample bags were returned to the surface.They were not lifted out of the water until all sampling at the sample locationwas done. During transportation to the lab they were kept in tanks filled withambient temperature sea water. Transportation time never exceeded 1 h. If notanalysed immediately at the arrival at the lab, the tanks were fitted with run-throughsea water overnight and then preserved for future examination. Sampling at eachlocation lasted for a maximum of 90 min.

To test for sampling efficiency, two different sample stations per locations weresampled twice in rapid succession. Based on the combined number of specimensfrom each sample, the fractions (%) of specimens in the first effort were calculatedand noted as sampling efficiency at that location. This was done in all but onelocation (Loc 2).

After sampling all collected animals, grouped by order, were preserved in 4%buffered Formaldehyde for a week, before being washed in freshwater and furtherpreserved in 70% ethanol. All nomenclature was checked at the website of theWorld Register of Marine Species (WoRMS). As the LEAR project was focusedon decapod crustaceans and possible prey species, only Crustacea, Polychaeta,Mollusca, and Echinodermata were identified to species level.

Sampling sites

The sampling sites were located at Bjørnafjorden and Korsfjorden (fig. 2) at thefollowing co-ordinates:

Latitude (N)/Longitude (E)Location 1 Vinnes 60°09′00′′/05°34′48′′

Location 2 Os 60°10′54′′/05°28′30′′

Location 3 Turtelsvik 60°13′18′′/05°19′18′′

Location 4 Langøy 60°13′00′′/05°18′01′′

Location 5 Eidholmen 60°06′09′′/05°15′45′′

Location 6 Skårøy 60°08′36′′/05°09′54′′

Locations 1, 4, 5, and 6 were all sampled from August-October 1997, whileLocations 2 and 3 were sampled from August-September 1998. Locations 1, 2,3, and 5 were at Bjørnafjorden, and Locations 4 and 6 were at Korsfjorden. Eachsite location ranged from 2 to 13 m with an average depth of approximately 7-8 mdepth. The substrata found in Locations 1, 2, 3, and 5 consisted of cobble in/on


Fig. 1. A, Schematic drawings of the i, diver-operated airlift suction sampler rigged with samplingbag; ii, valve to air-connection from a scuba tank; iii, handle for support; iv, elastic band forattachment of empty sample bags; v, 1 mm mesh sampling bags (three rolled together, one fullview); vi, band for attachment of the bag to the top of the suction tube and for closing the filledbag; vii, 0.5 m2 sample quadrant constructed with buoyant PVC pipe line; viii, 0.3 m high curtainof 1 mm mesh, weighted down by a heavy chain and lifted by the PVC pipe line fitted inside. B,

Demonstration of the use of the equipment by two divers. Drawing by H. Ringvold.

Fig. 2. Map showing the six locations in the Korsfjord/Bjørnafjord area in southern Norway (blackstars). The six locations are: Loc 1, Vinnes; Loc 2, Os; Loc 3, Turtelsvik; Loc 4, Langøy; Loc 5,

Eidholmen; Loc 6, Skårøy.


softer sediments (coarse sand (Loc 2)) or sand/silt mixtures (Locations 1, 3, 5),while at Locations 4 and 6 small boulders and cobble dominated among pocketsof sand. The variable “Location”, which explained most of the variation in thedata set, possibly included other, non-measured but important variables for speciescomposition, such as current, wave exposure, algal cover, seasonal variations intemperature and salinity, as well as sediment type. The sea temperature and salinityat the surface, 1 m depth, 5 m depth, and 10 m depth was measured by the use ofa standard Ruttner water sampler at each location once a month through 1998.Salinity was measured using a WTW™ electrode.

Ordination and diversity analyses

A two-way cluster analysis using PC-Ord (Version 6, Bray-Curtis Index andGroup average Linkage) was performed that factored in both stations and speciesbased on presence-absence data. Ordination analyses were then used to identifypossible patterns in species distribution in relation to environmental variables.A preliminary Detrended Correspondence Analysis (DCA) revealed a gradientlength of 3.53, and a unimodal method was chosen (Lepš & Šmilauer, 2003).Correspondence Analyses (CA; Lepš & Šmilauer, 2003) was used to find the maingradient in species composition, and a Canonical Correspondence Analyses (CCA;Lepš & Šmilauer, 2003) was chosen to investigate the relationship between speciescomposition variation and different explanatory variables (location, depth, salinity,and temperature). All species abundances were log-transformed prior to analysesand rare species down-weighted.

To evaluate the significance of the different explanatory variables, permutationtests were performed. This analysis used the default options in the package “vegan”(Oksanen et al., 2009) in R (R Development Core Team, 2010), if not otherwisestated. Temperature and salinity at sampling depth were estimated through linearinterpolation of measured environmental parameters at each location. At Loca-tion 5, as some sample stations depths were missing, these were set at 10 m, sincethe remaining depths were within a narrow depth range of 9.5 m to 10.7 m.

Since the primary focus of the study was to examine mobile crustaceans, allpolychaeta species samples were grouped, and the Shannon Index (H ′) (Shannon& Weaver, 1949) and Evenness (J ) Index (Pielou, 1966) were calculated for eachlocation.

RESULTS

Physical variables

The two innermost Locations (1 and 2) had more marked thermoclines andhigher temperature and salinity variations throughout the year compared to the


other locations. According to the monthly measurements in 1998, the temperatureat the sampled locations varied between 1.8°C at 1 m depth and 1.1°C at 10 mdepth among the locations, both above and below the thermocline. Salinities at 1to 10 m depth for all locations were between 29.4 (Location 1) and 32.4 (Location6) in August, and above 31.3 in September. At Location 2, typical with less salinityand higher inter-location variety in surface water, surface salinity was down to 19.2after rainfall in August. The surface salinity at all other locations was above 24.8in August and all locations had a surface salinity above 27.4 in September. Thetemperature at 1 m depth went from approx. 13°C in July and September to apeak at 16.4°C in September. At 10 m depth the variations were from 11.7°C to14.7°C in the same time period. The thermocline moved from about 6 m to 10-11 m depth from July to September. Salinities in the surface varied. In both yearsthe majority of the samples was collected above the thermocline, in the warmerand less saline layer of the littoral zone. Even with annual and seasonal variations,the water temperature and salinity were within the normal range (Bakke & Sands,1977).

Suction sampling

A total of 68 sites (34 m2) were sampled with an average sampling efficiency of76.4% among all locations (table I).

Fauna samples

A total of 5276 animals across 72 species were sampled from the six locations,ranging from 1593 animals (245.1 ind/m2) to 229 animals (45.8 ind/m2) (Loca-tions 5 and 3, respectively) (table I). High numbers of Turritella sp. were found inhigh density at Location 5 and sporadic in Loc 6, and not any other locations. Thetotal density of specimens per m2 ranged from 6.0 at Location 5 to 9.5 at Loca-tion 2 (table I). Sixteen species (21%), including seven crustacean species (10%),were found at all locations (table II). Maximum richness was found at Location 4(46 specimens, table I). Across all locations the crustaceans dominated by numbers(2685), followed by polychaetes (873), echinoderms (367), and molluscs (263).

The decapods Athanas nitescens (Leach, 1813), Upogebia deltaura (Leach,1815), Galathea squamifera Leach, 1814, Pisidia longicornis (Linnaeus, 1767),Liocarcinus navigator (Herbst, 1794), Pandalina brevirostris (Rathke, 1843),and Pagurus bernhardus (Linnaeus, 1758) were found to be the most commoncrustaceans at all locations. The polychaetes Pherusa plumosa (O. F. Müller,1776), Glycera lapidum Quatrefages, 1866, Terebellides stroemi M. Sars, 1835,and Hilbigneris gracilis (Ehlers, 1868) were also found at all locations, while therarely seen polychaete Arenicolides ecaudata (Johnston, 1835) was sampled at


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two locations (2 and 3). Both Sedentaria and Errantia polychaetes were sampled,including highly mobile phyllodicidan species like Nereis pelagica Linnaeus, 1758and Phyllodoce cf. groenlandica Örsted, 1842 (table II). Echinoderms, especiallyOphiuroidea, but also Asteroidea like Asterias rubens (Linnaeus, 1758) were foundin relatively high amounts at all stations (table II).

The Shannon Index (H ′) varied from 1.47 (Location 5) to 3.05 (Location 3). TheEvenness (J ) values were high at all locations (0.71 to 0.85) with the exception ofLocation 5 (0.40) (table I).


Dendrogramme outputs, based on the presence-absence data of species andstations, resulted in three main species clusters (clusters 1, 2, and 3) (fig. 3).Cluster 1 (Location 5) showed no similarity with the other clusters. Cluster 2and 3 were distinguished at approx. 30% similarity. Cluster 1 was dominated byTurritella sp. and Lima sp., while cluster 2 (Locations 1 and 3) was dominated byOphiura albida Forbes, 1839 and Amphipoda indet. Cluster 3 (Locations 2, 4, and6) was dominated by P. longicornis, Pandalina brevirostris (Rathke, 1843), and G.squamifera. Locations 2, 4, and 6 showed the highest in-site similarity (approx.85%), whereas Locations 1 and 3 showed approx. 60% similarity. A sequenceof four main species clusters (W, X, Y, and Z) were distinguished at approx.30% similarity, with W representing hyperbenthos and Crustacea, X Mollusca andCrustacea, Y a mix of taxa, and Z Mollusca and Crustacea, respectively.

A CA ordination plot indicated that the first axis explained 16% of the totalvariation in the species data set, while abiotic factors represented by the secondaxis explained 12% (fig. 4A). The CA clearly separated all sampling stations atLocation 5 along the first axis, indicating that Location 5 differs from the restdue to lower species variation. The other localities were less clearly separatedalong the second axis, indicating that although they do differ in several abioticvariables, these differences are not clear-cut. Regarding the CCA-plot, the variable“Location” alone explained 34% of the total inertia (2.17), i.e., the total variationin the data set (fig. 4B). CCA1 and CCA2 were not much lower than CA1 andCA2, indicating that the variables (location, depth, salinity, and temperature)included in the CCA explained a large part of the main variation in the speciescomposition. Maximum explained variation with all variables is 38%. Each of thevariables temperature, depth, and salinity explained 7, 8, and 8%, respectively.Total variation explained by salinity, depth, and temperature together was 16%.


Fig. 3. A two-way cluster analysis using PC-Ord (Version 6, Bray-Curtis Index and Group averageLinkage) was performed factoring in both stations and species based on presence-absence data. Allsamples were above 14 m, on cobble grounds with variation in currents, temperatures, and waveexposure, off the western Norwegian coast (Bray-Curtis with Group Average Linkage). The speciesand groups are listed in full in table II. Loc 5 is not clustered with any other locations with lowestspecies variation. Locations 2, 4, and 6 show highest in-site similarity (approx. 85%), whereaslocations 1 and 3 show approx. 60% similarity. The six locations are: Loc 1, Vinnes; Loc 2, Os;

Loc 3, Turtelsvik; Loc 4, Langøy; Loc 5, Eidholmen; Loc 6, Skårøy.


DISCUSSION

Suction sampler: efficiency and limitations of the sampling method

While this study found that airlift suction sampling has high levels of samplingefficiency, it is limited to specific depths and substrate types. Based on the samplingundertaken in Norwegian waters we suggest that the optimal working depth rangeis 2 to 30 m. The primary advantage of this sampling technique is its ability tocatch, and thereby quantify, highly mobile crustacean species such as: Cancerpagurus Linnaeus, 1758, Liocarcinus navigator (Herbst, 1794), and Galathea sp.Notably, however, the technique was ineffective at capturing small fish speciessuch as gobiids (Gobiidae indet.) despite the fact that some species were observedat relatively high densities in some sites.

The efficiency of the sampler was highly dependent on the substrate type. Onenotable disadvantage with the suction sampler is that where the shell sand is largerthan the mesh size of the sampling bags, this substrate crushes much of the sampledepifauna in a relatively short period of time (Linnane et al., 2001). All the chosenNorwegian locations had a finer substrate and this problem was avoided. Theresults suggested that the highest sampling efficiency occurred on homogenousrock and multi layered cobble grounds, with low levels of silt and coarse particlesand few heavy rocks and boulders. Another sampling limitation was the angle ofthe substrate. Horizontal crevices were impossible to sample without modificationsof the sampling tube. The primary advantage of the technique lies in its ability tosample highly mobile invertebrates. While grabs and cores appear to be relativelyefficient on sand and gravel substrata (Flannagan, 1970), they are unlikely tocapture fast moving taxa. The results presented in this study indicate that suctionsampling was highly efficient at capturing a wide range of mobile invertebrates,especially crustaceans in mixed and cobble substrata, in habitats that wouldnormally be unsuited to traditional techniques such as cores, grabs, or nets.

Fauna samples

High densities of smaller decapod crustaceans, usually missing from grab-and core collections, were commonly collected. These included small Brachyura,various Anomura species, as well as the shrimp Athanas nitescens (Leach, 1813),along with other epifauna species. Interestingly, A. nitescens were previouslyregarded as one of the rarer species along the western Norwegian coast, with thelast record from the late 1800s (see Brattegard, 2001, for historic list). However,this study revealed that this well-camouflaged, rapid-swimming shrimp is actuallyvery common in cobble grounds within shallow coastal waters.

A total of 72 species were found, of which 71 have previously been describedin the Hordaland region (Brattegard, 2001). One polychaete species (Arenicolides


ecaudata) was positively identified for the first time in the region (Ringvold et al.,1999).

Marine benthic macrofauna has previously been described from the nearbyFanafjorden using van Veen grab shots on soft bottoms (Lie, 1978). These studiesindicated that the Annelida were the dominating phylum with Polychaeta as thedominating class, followed by the phyla Mollusca, Arthropoda, represented bythe subphylum Crustacea, and Echinodermata. The fauna collected in this presentstudy were dominated by the Crustacea. Lie (1978) identified the difference inbenthic fauna composition between soft and hard bottoms, but suggested that suchdifferences were due to variations in sampling methods.

Although all sampling locations had a high proportion of cobble as the dominantsubstrate (>30%), only 21% of species were found at all locations. In many cases,more than 30% of one species was found within one or two locations, suggestingthat the presence of cobble alone cannot explain the variation among the locations.

The data in the present study revealed largest in-site cluster similarity betweenLocations 1 and 3 (mixtures of cobble in/on softer sediments), as well as Loca-tions 4 and 6 (cobble and boulders/pockets of sand), thus indicating that substratecould be of importance regarding species distribution. Since substrate was meantto be a common quality among the locations, it was not included in the ordina-tion analysis. Nonetheless, Location 2 (mixtures of cobble in/on softer sediments)clustered with Locations 4 and 6 showed a higher presence of hyperbenthos. Loca-tion 5 did not cluster with any other location, most probably due to the quantitativedominance of Turritella sp. (fig. 2). Four main species-clusters could also be de-tected (W, X, Y, and Z). No clear abiotic factors to explain the phenomenon wasfound but different taxa dominated in each cluster: W, hyperbenthos and Crustaceadomination; X, Mollusca and Crustacea domination; Y, mix of few taxa; and Z,Mollusca and Crustacea domination, respectively.

Moderate Shannon Index (H ′) values were found at five locations (3.05-2.67).The two innermost Locations (1 and 2), also with slightly higher temperature andsalinity variations throughout the year, showed a slight decline compared to theouter Locations 3, 4, and 6. The lower diversity at all these three locations was

Fig. 4. Ordination and diversity analysis. A, The relationship in species composition between thesix locations as shown by Correspondence Analysis (CA); Loc 1, Vinnes (grey dot); Loc 2, Os(black dot); Loc 3, Turtelsvik (grey diamond); Loc 4, Langøy (black square); Loc 5, Eidholmen(star); Loc 6, Skårøy (grey triangle). B, The relationship between the different samples along the twofirst axes of a Constrained Correspondence Analysis (CCA) with the constraining variables, depth,salinity, temperature, and locality. The three continuous variables are shown as lines (higher values ofthe variable in question in the direction of the line away from origin), whereas the nominal variables(locality) are shown as larger symbols at their average placement along the two axes. C, The species

optima along the same two axes as used in panel B.


mainly due to few dominating species. At Location 5, a low Shannon Index wascalculated (1.468), due to the dominance of Turritella sp. Evenness (J ) was highat all locations (except Location 5) (0.711-0.845). This indicates a relatively evenoccurrence of species among the sample stations within each location.


The gradient found in DCA analysis could to a high degree be explainedby the variables measured. Nonetheless, the variable “Location” explains mostof the variation in the current dataset (34%). The CA and CCA-plot supportsthe Two-way Cluster analysis. Location 5 is clearly separated from the otherlocations (fig. 4A-C). This is most likely due to the high amounts of the gastropodTurritella sp. which is found mainly at Location 5, except from one singlequadrat at Location 6. Locations 1 and 3, both bays with influx of ocean water,but also some water retention attributes, are relatively similar regarding speciescomposition. Similarities at both locations are the absence of Galathea dispersa,and the large amounts of Ophiura albida. Locations 4 and 6, being sampleddeeper than the rest and parts of the ocean-exposed Korsfjorden, are the mostsimilar locations regarding species composition (fig. 4), e.g., Galathea dispersa,Ophiopholis aculeata (Linnaeus, 1757), and Polyplacophora indet.

CONCLUSIONS

Airlift suction sampling is highly efficient at capturing and quantifying a widerange of invertebrate epifaunal species in cobble grounds, especially highly mobilecrustaceans (Linnane et al., 2003; Evans et al., 2013). The sampled animals werein general undamaged and alive, and in excellent condition for both taxonomicanalyses and behavioural experiments (Linnane et al., 2001; Mercer et al., 2001;van der Meeren, pers. obs., 2005). Ordination Analysis, based on location, depth,salinity, and temperature, was shown to separate locations in terms of speciescomposition. Variables like quantitative data on substrate composition, algae cover,current and wave patterns, as well as seasonal variation in physical variables, werenot included but could contribute to further explaining clustering variation withinand between the locations. Fauna collections sampled by the suction samplerare well suited for statistical analyses, allowing comparisons between specificlocations. While commonly applied in many areas along the North American coast,we recommend that suction sampling should also be used for benthic ecological,taxonomic and biodiversity surveillance studies in shallow waters also outsideNorth America.


ACKNOWLEDGEMENTS

This research was funded by the European Community through the researchproject LEAR FAIR CT-1775-96 and by the Institute of Marine Research. Wewish to thank Eivind Oug at NIVA (Norwegian Institute for Water Research) whoidentified the Polychaeta, Mr. Eoin D. Browne, the University of Cork, Ireland, forassisting us through the summer of 1998, the co-divers Hannu Koponen and VidarWennevik for their efforts, the laboratory staff at Austevoll Research Station forthe salinity analyses, and senior engineer Kjell Bakkeplass at IMR for the map.We are also indebted to the two referees, whose comments and suggestions haveled to great improvements on the text and the figures. Dr. Adrian Linnane (PIRSA-SARDI, Australia) has been to tremendous help with language corrections andvaluable suggestions for the final version of the text.

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First received 11 April 2014.Final version accepted 19 January 2015.

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