English Channel towed sledge seabed images. Phase 2: Analysis of selected tow images

During the 1970s and 1980s, 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of the resulting images were analysed and reported before his death in 1989 but logbooks, video and film material has been archived in the National Marine Biological Library (NMBL) in Plymouth.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 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 (Phase 1 of the study reported here: Oakley & Hiscock, 2005). Some image analysis was undertaken as a part of Phase 1. Phase 2 (this report) was to further analyse selected images. Having determined in Phase 1 that only the 35 mm photographic transparencies provided sufficient clarity to identify species and biotopes, the tows selected for analysis were ones where 35mm images had been taken. The tows selected for analysis of images were mainly in the vicinity of Plymouth and especially along the area between Rame Head and the region of the Eddystone. The 35 mm films were viewed under a binocular microscope and the taxa that could be recognised recorded in note form. Twenty-five images were selected for inclusion in the report. Almost all of the images were of level sediment seabed. Where rocks were included, it was usually unplanned and the sled was hauled before being caught or damaged. The main biotopes or biotope complexes identified were: SS.SMU.CSaMu. Circalittoral sandy mud. Extensively present between the shore and the Eddystone Reef complex and at depths of about 48 to 52 m. At one site offshore of Plymouth Sound, the turret shell Turritella communis was abundant. In some areas, this biotope had dense anemones, Mesacmaea mitchelli and (more rarely) Cerianthus lloydii. Queen scallops, Aequipecten opercularis and king scallops, Pecten maximus, were sometimes present in small numbers. Hard substratum species such as hydroids, dead mens fingers Alcyonium digitatum and the cup coral Caryophyllia smithii occurred in a few places, probably attached to shells or stones beneath the surface. South of the spoil ground off Hilsea Point at 57m depth, the sediment was muddier but is still assigned to this biotope complex. It is notable that three small sea pens, most likely Virgularia mirabilis, were seen here. SS.SMx.CMx. Circalittoral mixed sediment. Further offshore but at about the same depth as SS.SMU.CSaMu occurred, coarse gravel with some silt was present. The sediment was characterised must conspicuously by small queen scallops, Aequipecten opercularis. Peculiarly, there were ‘bundles’ of the branching bryozoan Cellaria sp. – a species normally found attached to rock. It could not be seen whether these bundles of Cellaria had been brought-together by terebellid worms but it is notable that Cellaria is recorded in historical surveys. As with many other sediments, there were occasional brittle stars, Ophiocomina nigra and Ophiura ophiura. Where sediments were muddy, the burrowing anemone Mesacmaea mitchelli was common. Where pebbles or cobbles occurred, there were attached species such as Alcyonium digitatum, Caryophyllia smithii and the fleshy bryozoan Alcyonidium diaphanum. Undescribed biotope. Although most likely a part of SS.SMx.CMx, the biotope visually dominated by a terebellid worm believed to be Thelepus cincinnatua, is worth special attention as it may be an undescribed biotope. The biotope occurred about 22 nautical miles south of the latitude of the Eddystone and in depths in excess of 70 m. SS.SCS.CCS.Blan. Branchiostoma lanceolatum in circalittoral coarse sand with shell gravel at about 48m depth and less. This habitat was the ‘classic’ ‘Eddystone Shell Gravel’ which is sampled for Branchiostoma lanceolatum. However, no Branchiostoma lanceolatum could be seen. The gravel was almost entirely bare of epibiota. There were occasional rock outcrops or cobbles which had epibiota including encrusting calcareous algae, the sea fan Eunicella verrucosa, cup corals, Caryophyllia smithii, hydroids and a sea urchin Echinus esculentus. The variety of species visible on the surface is small and therefore identification to biotope not usually possible. Historical records from sampling surveys that used grabs and dredges at the end of the 19th century and early 20th century suggest similar species present then. Illustrations of some of the infaunal communities from work in the 1920’s is included in this report to provide a context to the epifaunal photographs.

Is the Russell Cycle a true cycle? Multidecadal zooplankton and climate trends in the western English Channel

The Russell Cycle is one of the classical examples of climate influence on biological oceanography, represented as shifts in the marine plankton over several decades with warm and cool conditions. While the time-series data associated with the phenomenon indicate cyclical patterns, the question remains whether or not the Russell Cycle should be considered a “true cycle”. Zooplankton time-series data from 1924 to 2011 from the western English Channel were analysed with principal component (PC), correlation and spectral analyses to determine the dominant trends, and cyclic frequencies of the Russell Cycle indicators in relation to long-term hydroclimatic indices. PC1 accounted for 37.4% of the variability in the zooplankton data with the main contributions from non-clupeid fish larvae, southwestern zooplankton, and overall zooplankton biovolume. For PC2 (14.6% of data variance), the dominant groups were northern fish larvae, non-sardine eggs, and southern fish larvae. Sardine eggs were the major contributors to PC3 (representing 12.1% of data variance). No significant correlations were observed between the above three components and climate indices: Atlantic Multidecadal Oscillation, North Atlantic Oscillation, and local seawater temperature. Significant 44- and 29-year frequencies were observed for PC3, but the physical mechanisms driving the cycles are unclear. Harmonic analysis did not reveal any significant frequencies in the physical variables or in PCs 1 and 2. To a large extent, this is due to the dominant cycles in all datasets generally being long term (>50 years or so) and not readily resolved in the examined time frame of 88 years, hence restricting the ability to draw firm conclusions on the multidecadal relationship between zooplankton community dynamics in the western English Channel and environmental indices. Thus, the zooplankton time-series often associated and represented as the Russell Cycle cannot be concluded as being truly cyclical.

Is the Russell Cycle a true cycle? Multidecadal zooplankton and climate trends in the western English Channel

The Russell Cycle is one of the classical examples of climate influence on biological oceanography, represented as shifts in the marine plankton over several decades with warm and cool conditions. While the time-series data associated with the phenomenon indicate cyclical patterns, the question remains whether or not the Russell Cycle should be considered a “true cycle”. Zooplankton time-series data from 1924 to 2011 from the western English Channel were analysed with principal component (PC), correlation and spectral analyses to determine the dominant trends, and cyclic frequencies of the Russell Cycle indicators in relation to long-term hydroclimatic indices. PC1 accounted for 37.4% of the variability in the zooplankton data with the main contributions from non-clupeid fish larvae, southwestern zooplankton, and overall zooplankton biovolume. For PC2 (14.6% of data variance), the dominant groups were northern fish larvae, non-sardine eggs, and southern fish larvae. Sardine eggs were the major contributors to PC3 (representing 12.1% of data variance). No significant correlations were observed between the above three components and climate indices: Atlantic Multidecadal Oscillation, North Atlantic Oscillation, and local seawater temperature. Significant 44- and 29-year frequencies were observed for PC3, but the physical mechanisms driving the cycles are unclear. Harmonic analysis did not reveal any significant frequencies in the physical variables or in PCs 1 and 2. To a large extent, this is due to the dominant cycles in all datasets generally being long term (>50 years or so) and not readily resolved in the examined time frame of 88 years, hence restricting the ability to draw firm conclusions on the multidecadal relationship between zooplankton community dynamics in the western English Channel and environmental indices. Thus, the zooplankton time-series often associated and represented as the Russell Cycle cannot be concluded as being truly cyclical.