THE SHETLAND ISLES: LONG-TERM OBSERVATIONS ON THE SUBTIDAL MARINE FLORA IIAN TITTLEY & WILLIAM F. FARNHAM T~Y,I. & W.F. FARNHAM.2001. The Shetland Isles: Long-term observations on the subtidal marine flora. Arquipklago. Life and Marine Sciences. Supplement 2 (Part B): 1-17. Ponta Delgada. ISSN 0873-4740. The marine algal flora of Sullom Voe, Shetland, has been surveyed on three occasions in twenty years (1973, 1983, 1993). To date 281 species have been recorded in Shetland, of which 175 species occur in Sullom Voe. The subtidal vegetation of the outer part of the voe is characterised by forest formations of the perennial kelp Lantinaria hyperborea on bedrock and a species-rich epinora on stipes and holdfasts. This community was stable with time despite the wave-exposed conditions there. The subtidal vegetation of the inner, sheltered, voe with a sea-bed of sediment, stone and shell, was characterised by the canopy- forming annual Lantinaria saccharina / longicruris. Comparison of survey results revealed this canopy and associated subflora communities to vary in abundance and extent with time. Ian Tittley (e-mail: [email protected]) Department of Botany, The Natural History Museum, London SW7 5BD. UK - Williant F. Farnham, Institute of Marine Sciences, University of Portsmouth, Porrsntouth P049 9LY, UK INTRODUCTION Although accounts of the algal flora of the Shetlands go back almost two centuries (e.g. Shetland is an archipelago of about 100 islands in EDMONSTON1809), studies have been sporadic the Atlantic Ocean lying between 60" and 61°N, and key works are those of B~RGESEN(1903a, b), 170 km north of mainland Scotland and 300 km B~RGESEN& J~NSSON(1905), DIXON (1963). southeast of the Faroe Islands. Although the IRVINE(1962, 1974, 1980) and IRVINEet al. archipelago lies within the cold-temperate region, (1975). The marine algal flora of Shetland is the sea does not freeze due to the warming typical of the species poorer cold temperate North influence of the Gulf Stream (average surface Atlantic Ocean with 281 species recorded (50 temperatures are 4OC in winter and 12°C in Chlorophyta, 101 Phaeophyta, 130 Rhodophyta). summer). The Shetlands contain a mainland Sullom Voe had been scantly studied until the which is 85 km long and incised by deeply construction of Europe's largest oil terminal in the penetrating, fjordic arms (sea-lochs or voes); one 1970s (DUNNET& MCINTYRE1995). Long-term of these, Sullom Voe (Fig. l),is the subject of this surveillance of the intertidal biota of Sullom Voe paper. has been undertaken regularly since 1981 Sullom Voe is a 15 km long, narrow inlet of (MOORE et al. 1995). Studies on the subtidal the sea where there is a gradient of wave-exposure vegetation were initiated in 1973 (TI'ITLEYet al. from extremely wave-exposed to sheltered. A few 1977) as part of a wider study of the algae of rivulets drain into the voe but cause no significant Shetland (IRVINE1974), and repeated in 1983 reduction in salinity. Sullom Voe has a range of (TITTLEYet al. 1985) and 1993 (T~T~LEY& subtidal habitat types, including rocky outcrops, Fr\RNHAM 1997 - unpublished report). extensive sediment areas of shell, stone, sand and The 1993 algal survey of the Sullam Voe silt, and biogenic surfaces such as beds of provided an opportunity to revisit previous study Modiolus (horse-mussel). sites, assess the current status .of the subtidal vegetation, and make comparisons with previous dimensional graphs in which each point represents observations; some results are presented in this a vegetation sample or quadrat. The distances paper. between the points in the graph are taken as an indication of their similarity or difference. METHODS The surveys were carried out in July / August 1973, July 1983 and August 1993. In the original 1973 survey, the principal features of the algal vegetation of Sullom Voe were recorded by sampling from quadrats along transects; these transect studies were complemented by sampling from 'spot-dive' study sites at pre-selected depths (T~LEYet al. 1977). As the original survey was essentially semi-quantitative and descriptive, transect sampling was not replicated; subsequent reassessment was undertaken in the same manner. Transects were relocated as accurately as previous field data allowed. At each site a rope line was laid from an accessible reference point at high water mark to the subtidal region. The transects were 200 m long and the rope forming the transect line was weighted and marked at 10 m intervals. Depths at these points were recorded at noted times and adjusted to Chart Datum. At each marked 10 m interval, vegetation was cleared from a 1 m X 1 m quadrat beside the transect line and brought back to the laboratory for identification. Where possible, whole stones and cobbles were collected for examination of encrusting algae. Laminarians were weighed wet Fig. 1. Sullom Voe showing transects sites. to assess relative abundance; ages of Lclrl~i~~aria hyperborea plants were ascertained by sectioning stipes and counting annual growth rings (cf. KAIN RESULTS 1963). The results for four of the transect surveys (Fig. 1) are presented below (for other sites see Inner Voe - South Ness T~LEY& FARNHAM 1997). Detrended Correspondence Analysis (DECORANA, a numerical, ordination method) The transect traversed an intertidal area of gently was used to facilitate comparison of quadrat data sloping shingle, boulder and outcropping rocks from the three surveys. A database of species that continued subtidally and, at greater depths presence in quadrats was prepared for the purpose (10 m), gave way to a sea-bed of soft mud and (in TITTLEY & FARNHAM1997). Ordination shell debris. arranges the vegetation samples in relation to each other according to their similarity of species Vegetation description and cor~rpariso~z composition and associated environmental or temporal controls (KENT & COICER1992). The Seventy-nine species were recorded in the three results of an ordination are in this case two- surveys (32 species in 1993,55 in 1983 and 36 in 1973); only a small component (l l species) of the suggests a depth gradient; quadrats at depths of total flora was found on all occasions. less than 10 m (characterised by Lntninaria At shallow sublittoral levels the transect sacharimz / lo~zgicruris)are positioned to the left crossed a canopy of mixed of Fucus serratlcs, of 200, quadrats from deeper waters (often Lniirinaria digitata, L. hyperborea and L. containing Phyllophora crispa) are positioned to saccAnri~~a/lorzgicr~cris(L. saccharina and L. the right. The second axis suggests temporal lotzgicruris may be conspecific cf. SOUTH& differences with most 1973 and 1983 quadrats in T~LEY1986). From 30 m offshore (2-3 m the lower part of the graph (below 200), and 1993 depth) the canopy was solely of L. saccharina / quadrats in the upper part. The ordination does loitgicruris (Fig. 2). In 1993 the Lnrt~inaria not show the quadrats that in 1993 lacked algae. canopy extended to only 90 m offshore (9 m depth), contrasting with 1973 when it was present in all quadrats to 140 m offshore (14 m depth), and 1983 when it was patchily present 80-130 m offshore. The subflora (underflora and epiflora) recorded in shallow waters comprised a diverse species-assemblage but differed in composition in each survey. At 30-80 m offshore (2-8 m depth) only a depauperate underflora was recorded in 1993 compared with species-richer assemblages of 1973 and 1983 (cf. TI'ITLEYet al. 1977, 1985). The principal algae in 1993 were, epiphytes on Lnrminaria blades (e.g. Ectocarplw siliculosus, Myrionenm corurlrtae), and crustose forms such as the Aglaozonia stage of Cutleria nrultifida, Litltotl~ar~zr~ioriglaciale and Pseudolitltodelnm exteizsurtt on stones and shells. At 140-150 m offshore only crustose species were recorded in 1993 in contrast to the foliose and filamentous Fig. 2. Transect 1, South Ness. Kite diagram showing species found previously; in deepest waters at 25- hmirtaria saccharina / longicniris extent and 28 m (160-200 m offshore) vegetation was absent abundance along the transect in the three survey years. in 1993 contrasting with previous occasions (cf. T~LEYet al. 1977, 1985). The red alga Inner Voe - Scarva Taing Pliyllophora crispa that in 1973 and 1983 was commonly present as extensive, detached, mats The transect crossed an intertidal area of shingle 120-200 m offshore, was not recorded in 1993. with rocky outcrops that gave way at shallow sublittoral levels (3 m depth) to a sea bed of gravel, coarse sand and cobble. At 4-5 m depth Ordination (Fig. 3) produced a single cluster of the sea-bed was of mud and sand which continued points. The lack of a clear linear pattern to 100 m offshore. Beyond this point the sea-bed successive points (quadrats along the transect) changed to mud-shell debris and Modiolrcs beds, reflects the patchy mosaic of vegetation crossed and soft mud and shell debris at the greatest by the transect. The first axis of the ordination depths investigated (25-30 m). 026 025 Axis 1 Eigenvalue 0.5506 Fig. 3. Transect 1, ordination of quadrat data. Numbered points represent quadrats (successively joined for 1973 and 1993 transects). 1973, hatched line 35 (inshore) - 49 (offshore); 1983, points unjoined 16 (inshore) - 34 (offshore); 1993, continuous line l (inshore) - 15 (offshore). Vegetation description and comparison in deepest waters (Fig. 4). Differences in extent of L. saccharina / longicruris canopy were observed Eighty species were recorded during the surveys in each survey occasion (1973 to 110 m offshore; (42 in 1973; 64 in 1983; 18 in 1993) but only 1983 to 50 m offshore; 1993 to 40 m offshore). eight on all oceasions. A diverse subflora of mainly red algae (e.g.