English Channel towed sledge seabed images. Phase 1: scoping study and example analysis

During the 1970’s and 1980’s, the late Dr Norman Holme undertook extensive towed sledge surveys in the English Channel and some in the Irish Sea. Only a minority of the resulting images were analysed and reported before his death in 1989 but logbooks, video and film material has been archived in the National Marine Biological Library (NMBL) in Plymouth. A scoping study was therefore commissioned by the Joint Nature Conservation Committee and as a part of the Mapping European Seabed Habitats (MESH) project to identify the value of the material archived and the procedure and cost to undertake further work. The results of the scoping study are: 1. NMBL archives hold 106 videotapes (reel-to-reel Sony HD format) and 59 video cassettes (including 15 from the Irish Sea) in VHS format together with 90 rolls of 35 mm colour transparency film (various lengths up to about 240 frames per film). These are stored in the Archive Room, either in a storage cabinet or in original film canisters. 2. Reel-to-reel material is extensive and had already been selectively copied to VHS cassettes. The cost of transferring it to an accepted ‘long-life’ medium (Betamax) would be approximately £15,000. It was not possible to view the tapes as a suitable machine was not located. The value of the tapes is uncertain but they are likely to become beyond salvation within one to two years. 3. Video cassette material is in good condition and is expected to remain so for several more years at least. Images viewed were generally of poor quality and the speed of tow often makes pictures blurred. No immediate action is required. 4. Colour transparency films are in good condition and the images are very clear. They provide the best source of information for mapping seabed biotopes. They should be scanned to digital format but inexpensive fast copying is problematic as there are no between-frame breaks between images and machines need to centre the image based on between-frame breaks. The minimum cost to scan all of the images commercially is approximately £6,000 and could be as much as £40,000 on some quotations. There is a further cost in coding and databasing each image and, all-in-all it would seem most economic to purchase a ‘continuous film’ scanner and undertake the work in-house. 5. Positional information in ships logs has been matched to films and to video tapes. Decca Chain co-ordinates recorded in the logbooks have been converted to latitude and longitude (degrees, minutes and seconds) and a further routine developed to convert to degrees and decimal degrees required for GIS mapping. However, it is unclear whether corrections to Decca positions were applied at the time the position was noted. Tow tracks have been mapped onto an electronic copy of a Hydrographic Office chart. 6. The positions of start and end of each tow were entered to a spread sheet so that they can be displayed on GIS or on a Hydrographic Office Chart backdrop. The cost of the Hydrographic Office chart backdrop at a scale of 1:75,000 for the whole area was £458 incl. VAT. 7. Viewing all of the video cassettes to note habitats and biological communities, even by an experienced marine biologist, would take at least in the order of 200 hours and is not recommended. English Channel towed sledge seabed images. Phase 1: scoping study and example analysis. 6 8. Once colour transparencies are scanned and indexed, viewing to identify seabed habitats and biological communities would probably take about 100 hours for an experienced marine biologist and is recommended. 9. It is expected that identifying biotopes along approximately 1 km lengths of each tow would be feasible although uncertainties about Decca co-ordinate corrections and exact positions of images most likely gives a ±250 m position error. More work to locate each image accurately and solve the Decca correction question would improve accuracy of image location. 10. Using codings (produced by Holme to identify different seabed types), and some viewing of video and transparency material, 10 biotopes have been identified, although more would be added as a result of full analysis. 11. Using the data available from the Holme archive, it is possible to populate various fields within the Marine Recorder database. The overall ‘survey’ will be ‘English Channel towed video sled survey’. The ‘events’ become the 104 tows. Each tow could be described as four samples, i.e. the start and end of the tow and two areas in the middle to give examples along the length of the tow. These samples would have their own latitude/longitude co-ordinates. The four samples would link to a GIS map. 12. Stills and video clips together with text information could be incorporated into a multimedia presentation, to demonstrate the range of level seabed types found along a part of the northern English Channel. More recent images taken during SCUBA diving of reef habitats in the same area as the towed sledge surveys could be added to the Holme images.

Basin scale distribution of zooplankton in the Ligurian Sea (north-western Mediterranean) in late autumn

Mesozooplankton biomass and abundance were evaluated in epipelagic waters at 59 stations covering the Italian sector of the Ligurian Sea (north-western Mediterranean) in December 1990. This region is characterised by a cyclonic circulation which encloses a central divergence zone and is associated with a main thermohaline front offshore the western Ligurian coast. At the end of autumn, mesozooplankton biomass (range: 0.80–4.24 mg DW m−3) and the abundance (range: 83.8–932 ind. m−3) were lower in the divergence zone. On the contrary, in the Ligurian frontal zone at the periphery of the divergence and on the eastern continental shelf the greatest values of biomass and abundance were recorded. Copepods and appendicularians dominated the mesozooplankton community, the main taxa being the copepods Clausocalanus spp. (46% of total zooplankton) and Oithona spp. (15%) and the appendicularian Fritillaria spp. (12%). Three hydrological sub-regions, i.e. the divergence, the eastern continental shelf and the periphery of the divergence, were characterised by different zooplankton communities and characteristic species. Environmental differences between the three zones were mainly related to changes in bottom topography, sea surface temperatures and quantity of particulate organic matter. Vertical mesozooplankton abundance and taxa distribution from the surface to 1,900 m depth were also examined in one station. The results showed that the bulk of the community was concentrated in the upper 200 m, small copepods being dominant particularly in the upper 50 m. The copepod community was more diversified in sub-superficial waters, with a maximum observed in the 200–400 m layer. The distributions of main zooplankton taxa described in epipelagic waters in the eastern Ligurian Sea in autumn were compared with their distribution at surface in the north-western Mediterranean obtained by sampling performed with the Continuous Plankton Recorder in 1997–1999. The analysis of the zooplankton community in CPR samples confirms the dominance of small copepods (Paracalanus spp., Clausocalanus spp., Oithona spp.) and appendicularians in the north-western Mediterranean in late autumn-winter and shows that their distribution is mainly related to the main mesoscale hydrographic features characterising this basin.