The inland deep sea—benthic biotopes in the Sognefjord

CHAPTER 19

The inland deep sea—benthic biotopes in the Sognefjord ˚l Buhl-Mortensen1, Henrik Glenner2, Ulf Ba˚mstedt3 Lene Buhl-Mortensen1, Pa 1 and Kjell Bakkeplass 1

Institute of Marine Research, Bergen, Norway 2University of Bergen, Bergen, Norway 3 University of Umea˚, Umea˚, Sweden

Abstract The Sognefjord is a geologically unique, long, and deep glacial valley, stretching more than 200 km inland with a maximum depth of 1300 m. Surprisingly little is known about the seafloor environment and bottom communities of this, in a global perspective, spectacularly long and deep fjord. Megafaunal species richness, seabed substrates, and biotopes of the fjord were studied in 2000, 2001, 2014, and 2015 using underwater video as part of a joint project between the University of Bergen and the Institute of Marine Research with funds from the Norwegian Biodiversity Information Centre. Clear gradients in species richness and composition were found related to distance into the fjord, depth, and landscape features (fjord sides, basin plain, shallower side fjords). Detrended Correspondence Analyses of results from detailed video annotations indicated the presence of six biotopes with characteristic species composition and environment. Earlier fjord studies have shown that deep-sea species often occur shallower in fjord basins than in adjacent offshore areas. Furthermore, the isolation of fjord basins behind one or more sills can lead to mass occurrence of species that by chance have been introduced and been able to establish dense populations. The limited contact between fjord basins in relation to the open ocean and between side fjords makes the extremely large and branched Sognefjord particularly interesting for studies of the effects of connectivity on bottom communities. Similarities to communities outside the fjord are discussed.

Keywords: Norway; fjord; habitat; biotope; deep sea; Sognefjord

Introduction Geomorphic feature type and depth range The Sognefjord is a geologically unique, long, and deep glacial valley, stretching more than 200 km inland with a maximum depth of 1300 m (Fig. 19.1), surrounded by steep fjord sides (Fig. 19.2). The Sognefjord has been created by the erosion of four major Pleistocene glacial events. The fjord has one main basin with a relatively flat bottom bounded to the Seafloor Geomorphology as Benthic Habitat. DOI: https://doi.org/10.1016/B978-0-12-814960-7.00019-1 © 2020 Elsevier Inc. All rights reserved.

355

356 Chapter 19

Figure 19.1 General bathymetry of Sognefjorden from the sill area in the west to the inner head of the fjord. The bathymetric dataset is a 50 3 50 m grid from the Norwegian Hydrographic Services.

Figure 19.2 Seabed sloping (in degrees) calculated from the bathymetric data presented in Fig. 19.1, using the ArcGIS Spatial Analyst toolbar (Slope 2D).

Benthic biotopes in the Sognefjord 357

Figure 19.3 Detailed map of the sill area in outer Sognefjorden. The bathymetric dataset is a 50 3 50 m grid from the Norwegian Hydrographic Services.

west by a high threshold. The only sill in Sognefjorden is at the mouth of the fjord (Fig. 19.3), with a sill depth of 165 m. The main fjord, starting in the eastern part, becomes abruptly deeper westwards, to reach depths of about 800 m. The maximum depth (1308 m) is further out to the west. The outer Sognefjord has few side fjords, while the inner part has five branches. These side fjords are all shallower than the bottom of the main fjord. Some of the side fjords have minor basins and thresholds.

Oceanography Estuarine circulation is a common feature of fjord systems. Several of the rivers entering Sognefjorden are regulated as part of hydroelectricity production, resulting in a stable supply of freshwater. Without this regulation, the freshwater discharge would exhibit stronger seasonal variability, with most of the water entering during summer months. Fjords with sills usually contain three different water masses: brackish/estuarine water, intermediate water, and basin water. Close to the sill the thermocline is found between 50 and 100 m, while towards the head of the fjord the depth decreases and the upper 30 m becomes more stratified (Svendsen, 2006). The brackish water is a mixture of freshwater from river discharge and the intermediate water. In wintertime the runoff is relatively low, and salinity of the brackish water is quite high. The intermediate water is

358 Chapter 19 coastal water and is found in the fjord between the brackish water and sill depth. Below the thermocline there is a layer of warm water. In February the temperature in this layer is around 8.2 C in the outer parts of the fjord, while in the inner parts the temperature is around 9.1 C. Toward the head of the fjord the depth of the warm layer decreases. Below the warm layer the temperature decreases and this decrease is quite sharp: from 8.0 C to 7.6 C. Below 500 m the temperature is almost constant at 7.1 C. The halocline is located at about the same depth as the thermocline and below the halocline the salinity increases slowly with depth. There is a thin layer with salinity greater than 35.0 psu at around 200 m depth in the outer part of the fjord, but below 300 m the salinity is constantly 35.0 psu. The basin water has high density and is found below sill depth. This water is only replaced or ventilated when water with greater density crosses the sill. Such replacement of basin water does not happen very often—occurring approximately every 8th year in Sognefjorden (Hermansen, 1974). The current systems of the fjord are largely controlled by tidal forces and the Coriolis effect (Svensen, 1981; Svendsen, 2006), with main transport above the basin water following the depth contours into the fjord on the southern side and outwards on the northern.

Naturalness, condition, and trend Alterations of natural cycles of freshwater discharge due to hydroelectric regulation of power plants have changed the physical conditions of the pelagic ecosystem. The degree to which this has influenced the benthic ecosystems is less known. The Sognefjord is utilized for aquaculture, with several fish farms in the outer part of the fjord. The habitats in the fjord should thus be scored as Good (6). There is some alteration of habitat (mainly pelagic) due to the anthropogenic activities described previously. However, this is difficult to assess since there are very few previous studies from Sognefjorden focusing on habitat aspects.

General information on data reported in the case study Using high-resolution bathymetry, ecosounder, underwater positioning, and HD video, it is today possible to make scientific investigations of the seafloor without negative impact on the vulnerable communities there. Tethered video platforms and an ROV were used in this study to characterize the seabed and biological communities in Sognefjorden, covering both steep and rocky sides of the main fjord and the deep soft bottom at several sites along the main fjord and in five side fjords (Fig. 19.4). Videos of the seabed were recorded at 32 locations investigated with the video platform Campod and the ROV Aglantha in 2014 (cruise # 2014624 and 2014625) and one in 2015

Benthic biotopes in the Sognefjord 359

Figure 19.4 Benthic biotopes were surveyed with video recording at 57 locations, including five side fjords. The deeper parts and some side fjords were mapped using the video platform Chimaera (23 transects conducted in 2014 15) while shallower parts of the bedrock dominated fjord sides was mapped using the ROV “Aglantha” (25 transects in 2001/2002 and nine in 2014).

(cruise # 2015624) (Fig. 19.4, Table 19.1). All these surveys were carried out with RV Ha˚kon Mosby. During the first cruise in March 2014, the video platform Chimaera was deployed, at eight locations at depths between 354 and 1257 m. In November 2014 the ROV Aglantha was used on 9 locations at depths between 120 and 558 m. During the last cruise in 2015, 15 locations were surveyed with the video platform CAMPOD at depths between 1200 and 72 m.

360 Chapter 19 Table 19.1: General information for locations surveyed with ROV (2000 and 2001), and for locations surveyed with the video platforms Campod and Chimaera. Station

Lon

Lat

Maks depth

Min depth

Mean depth

Main bottom type

ROV surveys (2000 01) B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23 B24

53 755 53 755 56 783 56 783 54 753 54 753 54 753 54 813 53 992 54 033 72 460 72 463 72 463 72 463 72 463 72 463 54 113 58 974 51 828 51 828 51 828 51 818 65 603 70 893

610 610 611 611 610 610 610 610 610 610 611 611 611 611 611 611 610 611 611 611 611 611 612 612

735 735 411 411 574 574 574 593 561 561 597 622 622 622 622 622 616 324 202 202 202 204 417 142

1205 1205 606 781 1231 1170 199 1249 1241 1240 898 883 840 689 309 280 600 573 564 470 446 662 272 257

1205 1205 450 781 1170 1080 690 1249 1241 1180 898 851 840 689 309 238 600 422 564 470 395 662 210 257

1205 1205 528 781 1201 1125 445 1249 1241 1210 898 867 840 689 309 259 600 498 564 470 421 662 241 257

Mud Bedrock with detritus Bedrock Shell sand/hard Bedrock with detritus Bedrock with detritus Bedrock with detritus Mud Mud Bedrock with detritus Mud Soft/hard Soft/hard Soft/hard Soft/hard Soft/hard Bedrock with soft-bottom shelfs Bedrock with detritus Bedrock with detritus Sand/bedrock/soft Mud Bedrock with detritus Mud

Video platform surveys (2014 15) Cruise 2014625 S2VL3 S3VL4 S4VL5 S5VL6 S6VL7 S7VL8 S8VL9 S9VL10

51 642 51 932 53 512 59 350 64 787 65 982 69 079 72 511

611 610 610 611 610 611 611 611

090 971 641 436 974 908 238 548

382 709 1206 1257 1167 1089 851 891

354 652 1164 1254 1163 896 847 883

371 703 1195 1256 1165 1023 849 887

Mud/bedrock with detritus Mud/bedrock with detritus Mud Mud Mud Mud Mud Mud

Muddy sand/bedrock with detritus Mud/bedrock with detritus Bedrock with detritus Mud Bedrock with detritus Bedrock with detritus Bedrock with detritus

Cruise 2014624 S10VL11

51 776

611 064

382

372

377

S11VL12 S12VL13 S13VL14 S14VL15 S15VL16 S16VL17

72 515 72 518 69 294 68 827 64 705 64 876

611 611 611 611 611 610

448 558 506 517 494 548

415 479 495 156 180 210

428 490 499 296 350 424

662 475 270 192 120 852

(Continued)

Benthic biotopes in the Sognefjord 361 Table 19.1: (Continued) Station S17VL18 S18VL19

Lon

Lat

Maks depth

Min depth

Mean depth

Main bottom type

59 337 52 077

611 570 611 048

535 553

120 431

328 496

Bedrock with detritus Mud

1121 137 1152 867 618 644 664 770 746 374 535 355 348 180 274

Mud Muddy sand Mud Sandy mud Bedrock with detritus Mud Sandy mud Mud/bedrock with detritus Mud/bedrock with detritus Mud Mud Mud Sandy mud Mud Mud

Cruise 2015624 S19VL20 S20VL21 S21VL22 S22VL23 S23VL24 S24VL25 S25VL26 S26VL27 S27VL28 S28VL29 S29VL30 S30VL31 S31VL32 S32VL33 S33VL34

52 54 59 64 64 69 68 72 72 73 73 70 71 66 52

765 747 463 756 878 253 935 900 851 784 570 514 663 750 004

610 610 611 610 611 611 610 611 611 613 612 609 609 612 611

905 264 523 875 114 228 985 555 467 648 287 902 331 998 279

1128 146 1274 1172 908 834 845 864 1203 376 649 494 405 220 620

1115 130 849 548 425 498 472 280 308 370 265 150 260 54 72

The locations for video investigation were planned as 700 m long lines, with a defined start and end location. At some locations the inspected distance was shorter than planned because of technical problems or occurrence of great vertical walls that could not be inspected in a controlled way with Chimaera or CAMPOD. The total distance investigated was 21.57 km, and the mean distance for video lines (not including the stationary deployment at S19) was 695 m. The total duration of the new video inspections was 14 hours and 5 minutes, with an average of 1 hour and 11minutes per location. The shortest inspection was at S19 where the inspection had to be aborted due to technical problems. At this station the CAMPOD was only kept stationary at the start position. The longest inspection (1 hour and 46 minutes) was at S24. For a detailed description of video recording see Buhl-Mortensen et al. (2015). Geopositioning of the video data was provided by a hydroacoustic positioning system (Simrad HIPAP and Eiva Navipac software) with a transponder mounted on CAMPOD, giving a position accuracy of c.2% of water depth. Navigational data (date, UTC time, positions, and depth) were recorded automatically at 10-second intervals using the software CampodLogger (IMR), and also used to annotate fauna, bottom types, signs of fishing impact, occurrence of litter, and local geological seabed features during video recording. Video records from surveys using Algantha, carried out in 2000 01 were also included in this study covering 24 stations, at depths between 238 and 1240 m.

362 Chapter 19 Detailed video analysis was undertaken, where all organisms were identified to the lowest possible taxonomic level and counted, or quantified as percentage seabed coverage for encrusting organisms (sponges, bryozoans, and colonial tunicates), using the custom-made software, VideoNavigator (IMR). The percentage cover of eight classes of bottom substrata (clay, silt, sandy mud, muddy sand, sand, pebbles, cobbles, boulders, bedrock, and bedrock with detritus cover) was estimated subjectively on a scale of 5% intervals at regular intervals within video sequences. These estimates were converted to bottom type classes following the Folk scale (Folk, 1954), and later grouped into five classes dominated by: (1) mud, (2) sandy mud, (3) muddy sand, (4) cobble, and (5) bedrock with detritus cover. The area covered by a video transect was calculated based on traveled distance (calculated from “cleaned” geographical positions where noise was removed) and average field width (estimated from two laser scales, 10 cm apart). Density data (the number of organisms counted divided by the area observed) for solitary organisms were standardized as the number of individuals per 100 m2. Fig. 19.5 shows the difference in number of species observed on video at different depth intervals, for different habitat types. Table 19.1 presents details of sampling locations, depths, and main bottom types encountered. Statistical analysis: To investigate similarities in fauna composition and to relate patterns with environmental factors, detrended correspondence analysis (DCA; Hill and Gauch, 1980) was conducted. DCA is an ordination technique based on reciprocal averaging (Hill, 1973). It is an indirect gradient analysis, where environmental data are overlain on the ordination plot based on the species data. The environmental variables can then be correlated with variation covered by the axes in a multidimensional space. The environmental variables contained 16 numerical variables (depth, percentage cover of 10 different substrate types, percentage cover of terrestrial leaf, percentage cover of seaweed fragments, and distance to the inner part of the fjord), and two categorical variables (fjord location referring to side fjord or main fjord, and bottom type). Only the 90 taxa occurring in at least three video sequences were used for the analyses. The dataset contained 124 video sequences with observations of organisms. DCA was performed using the software PC-Ord, with rescaling of axes and downscaling of rare species (McCune and Mefford, 2006).

Geomorphic features and fjord habitats Sognefjorden is an atypical fjord, with a long, connected deep basin. Most other Norwegian fjords have several sills. At a broad (landscape element) scale, the main fjord and side fjords can be divided into sill area, basin plains, and steep fjord sides. The topography of the fjord sets constraints for the oceanography, influencing both the connection to open water and internal circulation. There was a clear trend of changing bottom types with depth,

Benthic biotopes in the Sognefjord 363

Figure 19.5 (A) The relationship between species richness and depth for three main types of habitat/settings. (B) The relationship between species richness and the gradient from outer to inner fjord for three major depth ranges.

where mud became more common with increasing depth. At the deepest parts of the fjord mud covers 100% of the seabed. Most of the rocky seabed consists of bedrock covered with a layer of detritus. Clean bedrock without a detritus cover was not common. The relative composition of this substrate decreased with depth and did not occur below 1000 m.

364 Chapter 19 Shelves with accumulated sediment were common on the steep fjord sides. Sand, muddy sand, and sandy mud were most common above 200 m depth, and did not occur below 500 m. Sand and shell sand were only observed at depths shallower than 200 m.

Biological communities Sognefjorden differs from other large Norwegian fjords, such as Hardangerfjorden, both with respect to environment and communities. Similar to many fjords in western Norway Hardangerfjorden has several sills isolating the inner fjord basin. Several species that occur in the Hardangerfjorden are not present in Sognefjorden (Buhl-Mortensen and BuhlMortensen, 2014), and vice versa. Even though western Norwegian fjords have many common attributes, with sills and basins, differences in topography, terrestrial runoff may contribute to faunal differences.

Species richness In the fjord there are several specific habitats that affect the species composition when progressing deeper inland in the fjord system. Earlier fjord studies have shown that deep-sea species often occur at shallower depths in fjord basins than in adjacent offshore areas (Brattegard, 1980; Buhl-Mortensen and Buhl-Mortensen, 2014). Species richness of the benthic communities often decreases from outer to inner parts of fjord system, and inner parts are in general dominated by the terrestrial input from land and rivers, for example, leaves and branches (Buhl-Mortensen and Høisæter, 1993; BuhlMortensen, 1996; Klitgaard-Kristensen and Buhl-Mortensen, 1999). The number of observed species decreases with depth, both in muddy and bedrock habitats (Fig. 19.5A). In the deeper parts the diversity is highest in the middle of the fjord, while shallower than 400 m the number of species decreases from outer fjord to inner fjord (Fig. 19.5B). The side fjords have fewer species than the main branch of the fjord. The sides of fjords often consist of steep bedrock intercepted with mud covered shelves. A specific fjord habitat is sloping bedrock with a thin layer of organic matter (detritus). This results from a combination of high productivity and weak bottom currents. The steep sloping bedrock only accumulates a thin cover of detritus, allowing for the presence of both sessile hard-bottom suspension feeders and motile detritivores. A fauna change within this bottom type appears to occur at around 600 m deep. The shelves along the fjord sides support soft-bottom communities, characterized by sea pens and a variety of holothurians. At shallow shelves (200 and 400 m depth) the community is characterized by the seapen Virgularia mirabilis, burrowing cerianthid anemones, and the holothurian Parastichopus tremulus. Further into the fjord, shelves at middle depths (400 600 m) (biotope 4) the

Benthic biotopes in the Sognefjord 365 holothurian Mesothuria intestinalis and the seapen Funiculina quadrangularis are more common. M. intestinalis often covers its body with leaves and other terrestrial material, whereas in offshore habitats it normally uses other types of debris, such as shells. In total 107 taxa were observed (Table 19.2). Forty-eight taxa were identified to species level (including two uncertain (cf.)) and 19 were identified to genus level only, whereas the Table 19.2: Species and unidentified taxa observed during visual surveys with ROV and tethered video platforms at different broad bottom types. XX: abundant, X: common, x: occurring. Main bottom type

Mud

Sandy mud

Muddy sand

Mixed sediments

Bedrock/detritus covered

Algae Lithothamnion sp.

X Foraminifera

Pelosina arborescens

XX

X Porifera

Asconema sp. Axinella infundibuliformes Chelonaplysilla sp. Hexactinellida Indet Hymedesmia cf. paupertas Polymastiidae Indet Phakellia sp. Porifera Indet Porifera branched white Porifera Encrusting Sycon/Grantia compressa Thenea abyssorum Stylocordyla borealis

XX

X

XX X

x X

X XX

X

XX

X X

XX X

X X

XX X

X X X X X

x X

X

X

X X

X

X Cnidaria

Actiniaria cf. Hormathiidae Anthomastus sp. Bolocera tuediae Cerianthus vogti Ceriantidae Indet Corymorpha sp. Gorgonacea Indet Halipteris sp. Funiculina quadrangularis Kophobelemnon stelliferum Pennatulacea Indet Paragorgia arborea Paramuricea placomus Virgularia mirabilis

X X X XX X X XX XX X

X X

x X

X X

XX

X

XX

X

X

X

X X

X X X

X x x

XX

X

X XX X XX X X X X X X X X X (Continued)

366 Chapter 19 Table 19.2: (Continued) Main bottom type

Mud

Sandy mud

Muddy sand

Mixed sediments

Bedrock/detritus covered

X

X

X

XX

x

X X X

Nemertea Nemertea

X Plathyhelminthes

Plathyhelmintes

X Mollusca

Acesta excavate Buccinidae Indet Cephalopoda Indet Gastropoda Indet Octopoda cf. Rossia Pectinidae Indet

X X X

X X Polychaeta

Chaetopterus sp. Polychaeta tube Polynoidae Indet Sabellidae Indet Serpulidae Indet

x XX X X x

X X x

x

XX X

X

X XX

X

XX

Echiura Bonellia viridis Maxmuelleria faex

X X

XX X

Asteroidea Indet Asteronyx loveni Bathyplotes natans Brisingidae Indet Ceramaster granularis Crinoidea Indet Echinoidea Indet Echinus esculentus Gracilechinus acutus Henricia sp. Hippasteria phrygiana Holothuroidea Indet Hymenodiscus coronate Luidiidae Indet Mesothuria intestinalis Ophiuroidea Indet Parastichopus tremulus Poraniidae Indet Psammechinus sp. Psolus squamatus Pteraster sp. Stichastrella rosea

X X XX X X

X

Echinodermata

X X XX

X

XX X

X X X

X X X X

X X X

X XX X XX XX XX X

X X X XX X X X

X X XX X

x x X X

XX X X X X X XX X X X X X XX X X

XX XX X (Continued)

Benthic biotopes in the Sognefjord 367 Table 19.2: (Continued) Main bottom type

Mud

Sandy mud

Muddy sand

Mixed sediments

Bedrock/detritus covered

X

XX

X

X X

Brachiopoda Brachiopoda

x

x Bryozoa

Kinetoskias smitti

X

X Crustacea

Brachyura Indet Calocarides coronatus Cancer pagurus Crangonidae Indet Hyas sp. Lithodes maja Munida sarsi Munida sp. Munida tenuimana Paguridae Indet Pandalidae Indet Caridea cf. Pandalidae

X X X X

X XX X X X

X

X XX

X

X X X X

x

X

X

X X X X X X X

Tunicata Ascidiacea Solitary transparent Ascidiacea Solitary brown

XX X

X X

X Teleostei

Chimaera monstrosa Coryphaenoides rupestris Cottidae Indet Cotteidae, cf. Triglops sp. Etmopterus spinax Gadiculus argenteus Gadidae Indet Galeus melastomus Hippoglossoides platessoides Zoarcidae Indet Maurolicus muelleri Melanogrammus aeglefinus Molva dypterygia Molva molva Molva sp. Myxine glutinosa Pleuronectiformes Indet Rajiformes, cf. Dipturus sp. Sebastes sp. Selachii Indet

X X X

X X

X

X

X

X X

X X X X X X X X X X X X X

X X

X X

X X

X X X X

X

X

X X X

X

X

X X (Continued)

368 Chapter 19 Table 19.2: (Continued) Main bottom type

Mud

Teleostei Indet Teleostei, juv Benthosema glaciale Teleostei, cf. Maurolicus muelleri Trisopterus sp.

X X XX

No. of taxa

75

Sandy mud

Muddy sand

X

XX

Mixed sediments

Bedrock/detritus covered X X

X X 45

35

35

77

remaining 43 were recorded as higher level taxa. There was a trend of decreasing number of taxa with increasing depth (r 5 20.33, P , .005). In the deeper parts of the fjord, in the interconnected deep basin inside the sill area, the species richness was greatest in the middle part of the fjord (Fig. 19.1). Of the environmental variables the number of taxa was strongest positively correlated with the percentage cover of bedrock (without detritus cover) (r 5 0.34, P , .005).

Biotopes Six biotopes were identified based on multivariate analysis (DCA) conducted on environment and megafauna data from the video transects (Figs. 19.6 and 19.7). The total variance (inertia) in the species data was 3.2380. Based on the DCA plot of video sequences with axis 1 and 2, three groups could be defined related to depth: (1) ,400 m, (2) 400 700 m, and (3) .700 m. Three variables were strongly correlated (r . 0.6) with the first axis (depth, percentage coverage of mud, and coverage of bedrock with detritus cover). Biotope 1. Sill and outer fjord. The biotope is dominated by bedrock and gravel with attached filter feeding organisms taking advantage of food particles supplied by the passing current. The most frequent organisms are corals and echinoderms. Some of the common species are the bivalve Acesta excavata, the gorgonian corals Paragorgia arborea and Paramuricea placomus, and the sea urchin Gracilechinus acutus. Biotope 2. Shallow soft sediments (100 400 m). This biotope is found in bays and side fjords and on some plateaus along the sides of the main fjord. Common species are anemones, sea pens, sea cucumbers, and fish. Some examples are Ceriantharia (anemone), Chimaera monstrosa (fish), the sea pens Kophobelemnon stelliferum and Virgularia mirabilis, and Parastichopus tremulus (sea cucumber).

Benthic biotopes in the Sognefjord 369

Figure 19.6 The distribution of six main biotopes in Sognefjorden, indicated from the results of video analysis.

Biotope 3. Soft sediments on intermediary depths (400 800 m). The faunal composition of this biotope is clearly different from the communities connected to shallower and deeper soft sediments. Key species of this biotope are the large sea pen Funiculina quadrangularis and the sea cucumber Mesothuria intestinalis. Biotope 4. Shallow detritus covered bedrock on depth of 400 600 m. The key species of this biotope are the attached filter feeders Psolus squamata (holothurian), and an unidentified brachiopod. Biotope 5. Deep ( . 600 m) detritus covered bedrock. In this biotope glass sponges and the roundnose grenadier are key species. Biotope 6. Deep flat basin floor, dominated by soft mud (that appears less consolidated, and contains more leaves and pine needles than muddy areas further inland). In outer parts of the deep basin, there are dense populations of the brisingid brittle star Hymenodiscus coronate. These mainly occur below 900 m deep, where the temperature and salinity are almost constant at 7.1 C and 35.0, respectively. The species belongs to the oceanic deep-sea fauna and unlike in the Sognefjord, it normally occurs as scattered, rare, single individuals. The deep-sea squat lobster Munida tenuimana, and the worm Maxmulleria faex are other key species in this biotope.

Figure 19.7 Video frame grabs with examples of different habitats and biotopes in Sognefjorden. (A and B) The sill and shallow bedrock in outer fjord area (200 400 m) (biotope 1) represents hard-bottom habitats with relatively strong currents. Various demosponges and the soft coral Anthomastus grandiflora (A), and the horny coral Paragorgia arborea (B). (C and D) Detritus covered bedrock of the sides of (400 600 m depth), densely populated with the holothurian Psolus squamata (biotope 3). The common holothurian, Parastichopus tremulus, which normally is found on level muddy bottom is here also common on steep hard bottom. (E and F) Accumulations of finer muddy sediments with detritus on shelves at the fjord sides (biotope 3 and 5)., with the holothurians Mesothuria intestinalis (E), and Bathyplotes natans (F) and the seapen Funiculina quadrangularis (F). (G and H) Deep ( . 600 m) detritus covered bedrock with roundnose grenadier (Coryphaenoides rupestris) (G) and an unidentified glass sponge (H). (I L) The deep fjord plain ( . 800 m depth) (biotope 6) with the deep-sea squat lobster, Munida tenuimana (I), the worm Maxmulleria faex (J), and the brisingid brittle star Hymenodiscus coronata (K and L). (K) In front, in the left corner there is a deep-sea octopod Bathypolypus arcticus.

Benthic biotopes in the Sognefjord 371 It is interesting that the characteristic species Hymenodiscus coronata is common in the deep fjord plain of Sognefjord, while rare outside. It appears to be very stenothermal and thrives in the stable basin water of the extremely deep and isolated fjord basin of the Sognefjord, where this large species, measuring 60 cm across, occur in densities of two to three individuals per m2. Little is known about their biology, but they have been documented both in the present study and in others (Reyss and Soney, 1965; Pawson, 1976) to frequently sit on the seafloor with their long arms (up to 30 cm) raised up into the current to capture food. In many ways communities in the Sognefjord show similarities with those of canyons in the Mediterranean. A Brisingidae community with the key species H. coronata was identified by Reyss (1971) and Reyss and Soyer (1965) on muddy substratum at 240 300 m depth in two canyons in the western part off Golfe de Lion (western Mediterranean). The Brisingidae community occurs in the Levantine Intermediate Waters (LIW), from around 200 600 m with a stable temperature and salinity of 13 C and 38.4, respectively (VargasYa´n˜eza et al., 2017), below the upper layer of Atlantic water. Although H. coronate has an Atlantic and Mediterranean distribution, Downey (1986) indicates that greatest abundances are in Norway. The temperature in the Mediterranean canyons is at present higher than in the Sognefjord. During the Pleistoscene, when the Sognefjord was created, the temperature in the Mediterranean was colder and thus more similar to present day conditions in Sognefjord.

Acknowledgments This study is part of the project “Species inventory and nature type mapping of Sognefjorden” supported by the Norwegian BiodiversityInformation Centre, the Institute of Marine Research and the University of Bergen.

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