D. V. P. Conway

A blooming jellyfish in the northeast Atlantic and Mediterranean

A long-term time series of plankton records collected by the continuous plankton recorder (CPR) Survey in the northeast Atlantic indicates an increased occurrence of Cnidaria since 2002. In the years 2007 and 2008, outbreaks of the warm-temperate scyphomedusa, Pelagia noctiluca, appeared in CPR samples between 45° N to 58° N and 1° W to 26° W. Knowing the biology of this species and its occurrence in the adjacent Mediterranean Sea, we suggest that P. noctiluca may be exploiting recent hydroclimatic changes in the northeast Atlantic to increase its extent and intensity of outbreaks. In pelagic ecosystems, Cnidaria can affect fish recruitment negatively. Since P. noctiluca is a highly venomous species, outbreaks can also be detrimental to aquaculture and make bathing waters unusable, thus having profound ecological and socio-economic consequences.

The double LHPR system, a high speed micro- and macroplankton sampler

Abstract A double-net sampling system, consisting of two separate Longhurst-Hardy Plankton Recorders, capable of being towed at speeds up to 3 m s −1 and capturing plankton organisms such as copepod nauplii, with the minimum of damage is described. The systems are mounted in a Lowestoft sampler and connected to 53 and 280-μm mesh nets; both nets are fitted with doors opened by a variable timer unit. The system is suitable for neritic and oceanic deployment. The ratio of the area of the net mouth to the net open area ( R ) for the 280-μm net is 9 and for the 53-μm net it is 14 to 121 depending on the nose cone used. The R values are considerably better than those of previous described systems. Examples are given showing how the instrument has been used to resolve spatially the vertical distribution of the nauplii and copepodite stages of Calanus helgolandicus .

The role of copepods in the planktonic ecosystems of mixed and stratified waters of the European shelf seas

The European shelf seas can be divided into regions which have tidally mixed waters and thermally stratified waters. The tidally mixed near shore environments support zooplankton communities dominated by smaller copepods and having large meroplankton contributions. These small copepods (Centropages spp., Temora spp., Acartia spp., Para/ Pseudo/Microcalanus spp.) together with the microzooplankton component form a different and more complex food web than the larger copepod/diatom link associated with thermally stratified waters. The copepods Calanus finmarchicus and C. helgolandicus account for over 90% of the copepod dry weight biomass in stratified waters. Although occurring in lower numbers in mixed waters they can still make significant contributions to the biomass. A 31 year time series from the European shelf shows the inter- and intra-annual variability of these species. The basic biology and food web that these two systems support, and the transfer of energy, can result in marked differences in quantity and quality of particulates available as food for fish larvae. Calanus dominated systems allow the primary production to be directed straight through the trophic food chain (diatoms/Calanus/fish larvae) while the near shore communities of smaller copepods limit the amount of energy being transferred to the higher trophic levels. Eighty-two Longhurst Hardy Plankton Recorder hauls were used as the data base for this study. In all cases the zooplankton was dominated by copepods both in numbers and biomass accounting for > 80% of total zooplankton dry weight in the Irish Sea, Celtic Sea, shelf edge of the Celtic Sea and the northern and southern North Sea in Spring.

Spawning season and temperature relationships for sardine (Sardina pilchardus) in the eastern North Atlantic

Spawning temperature preferences for sardine (Sardina pilchardus) in the eastern North Atlantic were determined from egg survey data. These were compared with climatological temperature cycles (1986 2002) derived from satellite observations, by geographical region, to predict spawning seasons. Optimum spawning temperatures were determined as 14.0-15.0°C from the English Channel to Portugal and 16.0-18.0°C for all north-west African regions. Spawning seasons were closely related to the general latitudinal trend of the annual temperature cycle, with modification by upwelling in the western Iberian and north-west African regions. Some differences between temperature-based spawning season predictions and field observations were related to variations in seasonal plankton production. Correlations in the annual time-series of favourable spawning temperatures suggested relatively strong linkages between the southern areas from Portugal to Senegal. There was no consistent relationship between annual variations in extent of temperature-predicted spawning seasons and observed field abundance of eggs.

Vertical distribution and seasonal numerical abundance of the Calanidae in oceanic waters to the south-west of the British Isles

The vertical distribution and seasonal abundance of the copepodite and adult stages of Calanus finmarchicus, C. helgolandicus, C. tenuicornis, Neocalanus gracilis, Nannocalanus minor and Calanoides carinatus from a series of Longhurst Hardy Plankton Recorder hauls taken in the oceanic waters, off the continental shelf, to the south-west of the British Isles are described. The sampling area was selected because the geographical distributions of the major Calanidae copepods of the north-east Atlantic Ocean are shown to overlap in this region. It marks the southern boundary of the distribution of C. finmarchicus, the central area of C. helgolandicus and the approximate northern limit of distribution of C. tenuicornis, N. gracilis, N. minor and C. carinatus. These four southern species occasionally penetrate further north (60° N) in the open ocean but do not breed at these northern latitudes. In autumn and winter, when C. finmarchicus and C. helgolandicus were overwintering below 400 m primarily as Stage V copepodites, N. gracilis occurred in the upper 200 m of the water column in a breeding condition; all copepodite stages were present. This copepod reproduced throughout the year in this regions while C. tenuicornis was observed to breed primarily in spring and summer. The geographical and vertical distributions of the Calanidae are related to the observed seasonal temperatures of the North Atlantic and the breeding strategies of species are compared.

Vertical distribution, and seasonal and diurnal migration of Calanus helgolandicus in the Celtic Sea

The vertical distribution and migration (seasonal, diel and ontogenetic) of Calanus helgolandicus are described from the shallow (100 m) shelf-seas to the south-west of the British Isles. In 1978 and 1979, the overwintering population of C. helgolandicus consisted primarily of Stage V copepodites and adults. By late winter/early spring the copepodites had moulted to adult females (>90%), which matured and bred the first cohorts of the year, prior to onset of the spring phytoplankton bloom in April/May. C. helgolandicus reached a peak of numerical abundance in August of 20x103 copepodites m-2 (over the depth range sampled -0 to 70 m), which was 200 times the population in winter. The seasonal peak of abundance occurred 4 mo after the peak of the bloom of phytoplankton in spring. The yearly development of the copepod was not always out of phase with the diatom bloom, as seen when the data from 1978 was placed in the context of a longer time-series collected at 10 m over 22 yr (1960–1981, inclusive). Large vertical migrations were observed in the younger copepodites (CI and II) in May from below to above the thermocline. In the remainder of the year, the CI and CII stages behaved differently and were located above the thermocline within the euphotic zone. The largest vertical displacements of biomass were seen in the summer months due to the migrations of the CV stages and adults, which had developed from the spring cohorts. It was contended that the seasonal and vertical migrations of C. helgolandicus are part of a more complex pattern of inherent behavior than has been reported previously and that, however difficult this is to discern in the natural populations, it always expresses itself.

Seasonal population structure, vertical distribution and migration of the chaetognath Sagitta elegans in the Celtic Sea

The biology of the chaetognath Sagitta elegans Verrill has been much researched, but detailed studies of population structure have generally been conducted in coastal water where dynamic tidal conditions may cause difficulty in interpretation of data. The resolution of sampling examining vertical distribution and diurnal migration has also been rather coarse. During a series of eight cruises to a seasonally thermally stratified sampling site in the Celtic Sea in 1978 and 1979, detailed vertical zooplankton profiles were taken to study the seasonal population structure, vertical distribution and migration of this species. The overwintering stock of S. elegans (22 to 52 individuals m-2, 0 to 90 m) had a wide range of lengths (5 to 20 mm) and matured in 1978 from early March, spawning several times before dying out by late July. Young produced by the overwintering stock started to mature in July and population numbers reached their highest in August (2483 m-2, ≡ 132.8 mg C m-2) when sea temperature peaked (17.1°C). By October, the population of S. elegans declined (284 m-2), which was thought to be due to a combination of lower sea-water temperature, competition for and availability of food, and predation. Because of the length range of the overwintering population (5 to 20 mm), it is assumed that reproduction continued at a low level over the winter, although eggs were not found in January and February, the coldest months of the year. In summer, the smallest S. elegans (2 to 6 mm) were found in the near-surface waters and did not migrate, but as their lengths increased they occupied deeper depth ranges and a portion of the population started to migrate diurnally. Individuals which did not migrate and stayed in the warmer surface waters, or those which migrated into it, matured faster than those remaining in the colder water below the thermocline. Migration to surface waters by mature individuals seemed to be stopped by high surface temperatures (17°C) and a sharp thermocline (3 C°). As sea temperature increased during the year from the winter minimum of 7.7°C, S. elegans matured at a progressively shorter length (14 mm in March 1978 to 10 mm in August). There are probably only three generations of S. elegans a year in the Celtic Sea.

Variability in dry weight and vertical distributions of decapod larvae in the Irish Sea and North Sea during the spring

Decapod larvae and early post-larvae were extracted from detailed vertically stratified samples taken during research cruises in the Irish Sea from April to early June 1988 and in the North Sea in June 1989. The total dry weight of decapods was 4 to 106 mg m-2 which represented 0.5 to 8.6% of the total dry weight of zooplankton. The vertical distributions of abundant species were examined in relation to physical and biological variables.Nephrops norvegicus andPagurus bernhardus were found nearer to the surface during the spring bloom in April than after the bloom in May. In the near-surface waters of the North Sea in June, larvae ofLiocarcinus spp. were more abundant than those of other decapods. Diel migrations varied, apparently due to differences in the physical structure of the water column and the distribution of potential food. Conventional migrations were modified or absent in stratified areas when near-surface concentrations of chlorophyll were present and in well-mixed water. Few significant ontogenetic variations were found. The most striking case was ofLiocarcinus spp. larvae during the day in stratified water in the North Sea, where the early zoeal stages (at a mean depth of <6 m), the fifth zoeae (15.5 m mean) and the megalopas (26.7 m mean) occupied different positions in the water column.

Co-occurrence of copepods and dissolved free amino acids in shelf sea waters

The vertical distribution of chlorophylla, copepods, dissolved free amino acid concentration and the fixation of14C by phytoplankton were monitored in the springs of 1983, 1987 and 1988 in the Ushant front region, shelf edge of the Celtic Sea and central Irish Sea, respectively. In each area, two stations characterized by mixed and stratified water conditions were compared. Vertical distributions of amino acids coincided with the distribution of copepods. A positive and significant correlation was found between the abudance of copepods and the concentration of amino acids dissolved in seawater. A negative and significant correlation was found between chlorophylla and the concentration of amino acids. Enrichment of amino acids (≥ 20 to 500 nM l−1 at specific depths) due to aspartic and glutamic acids, glutamine and ornithine, was assumed to reflect copepod feeding activity and faecal production. At these depths, the natural concentration and diversity of amino acids, including aspartic acid, glutamic acid, asparagine, serine, histidine, glutamine, arginine, threonine, glycine, alanine, tyrosine, valine, phenylalanine, ornithine and lysine, were high enough and in the correct proportions for triggering feeding and swimming and swarming behavior of copepods, as well as their remote detection of food at the micro- and meso-scales (1 to 10 m). This accumulation of amino acids also constitutes a potential additional source of organic nitrogen for bacteria and phytoplankton.

Suspended particulates in the Irish Sea and feeding conditions for fish larvae

The size distributions (2 to 160 μm equivalent spherical diameter) of suspended particulate material sampled on two cruises along a transect in the Irish Sea in 1988 are described in relation to hydrographic conditions, chlorophylla concentration and carbon to nitrogen ratios. Particulates were more abundant and larger size modes in the distribution were more evident, in the upper mixed layer of stratified water than in areas where the water column was fully mixed. The detrital content was estimated at 52% of total particulate matter above the thermocline in stratified regions and at around 97% at mixed water sites. In stratified regions the predominance of larger sized phytoplankton and lower levels of detritus is argued to support a more direct and efficient transfer of energy to fish larvae via larger sizes of copepods. Conversely, in mixed areas of high detrital loading the smaller size spectrum of particulates incorporates a less efficient transfer of energy through bacterial cycling and smaller copepods.