Longhurst-Hardy Plankton Recorder Research Articles

Carbon export through zooplankton active flux in the Canary Current

Abstract Zooplankton vertical distribution, migrant biomass, and respiratory flux were studied in two transects performed in the Canary Current at 21°N and 26°N from the eu- and mesotrophic upwelling zone off Northwest Africa to the oligotrophic central gyre waters. Migrant biomass was estimated by sampling during day and night using a Longhurst-Hardy Plankton Recorder (LHPR) net in the 0–900 m depth water column. The electron transfer system (ETS) enzymatic activity was measured as a proxy for zooplankton respiration in the 0–900 m depth water column. Respiratory flux was obtained from the migrant biomass and the daytime respiration in the 200–800 m layer, assumed as the residence depth of migrant zooplankton. The southern transect at 21°N showed higher primary production, zooplankton biomass, migrant biomass, and lower passive flux. Zooplankton respiratory flux varied from 2.8 to 88.3% of the POC flux, being lower in the northern (5.3% ±2.7) than in the southern transect (46.1% ±32.9). Migrant biomass was significantly correlated to primary production (r2 = 0.570, p

Vertical and horizontal larval distribution of an offshore brachyuran crab, <em>Monodaeus couchii</em>, off the south coast of Portugal

A strong understanding of larval distribution and abundance is of major value in delineating the location and size of a breeding population of deep-sea species such as Monodaeus couchii . In this study, vertical distribution of the larvae of a brachyuran crab, M. couchii , was assessed during two week-long cruises conducted at the end of January 2006 and 2007 off the South Coast of Portugal. Larvae were collected by oblique plankton hauls with a Longhurst-Hardy Plankton Recorder from the surface to 300 m. Abundance and distribution of zoeae I and II were correlated during both years. For all stages, abundance decreased with depth during the day while it increased with depth at night; the larvae thus displayed reverse diel vertical migration. Abundance of zoeae I and II was correlated with chlorophyll a levels, whereas those of later stages were correlated with neither physical parameters (chlorophyll a , temperature or salinity) nor each other. An ontogenic shift in vertical distribution explained the results; earlier zoeal stages remain in the food-rich upper water column while later stages were not correlated with any physical parameters (i.e. chlorophyll a , salinity or temperature) and migrated to the bottom for settlement.

An empirical assessment of the consistency of taxonomic identifications

Plankton counting and analysis is essential in ecological study, yet scant literature exists as to the reliability of those counts and the consistency of the experts who make the counts. To assess how variable expert taxonomic identifications are, a set of six archived mesozooplankton samples from a series of Longhurst Hardy Plankton Recorder net hauls were counted by expert zooplankton analysts located at six marine laboratories. Sample identifications were repeated on two separate days with over 700 target specimens counted and identified on each day across the samples. Twenty percent of the analysts returned counts that varied by more than 10%. Thirty-three percent of analysts exhibited low identification consistencies, returning Intraclass Correlation Coefficient scores of less than 0.80. Statistical analyses of these data suggest that over 83% of the observed categorical count variance can be attributed to inconsistencies within analysts. We suggest this is the root cause of variation in expert specimen labelling consistency.

Physical control of zooplankton distribution at the Strait of Gibraltar during an episode of internal wave generation

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 408:79-95 (2010) - DOI: https://doi.org/10.3354/meps08566 Physical control of zooplankton distribution at the Strait of Gibraltar during an episode of internal wave generation D. Macías1,2,*, R. Somavilla3, J. I. González-Gordillo4,5, F. Echevarría1,5 1Departamento de Biología, Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz, 1150 Cádiz, Spain 2Integrative Oceanography Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92037, USA 3Centro Oceanográfico de Santander, Instituto Español de Oceanografía, Promontorio de San Martin s/n, 39080 Santander, Spain 4Centro Andaluz de Ciencia y Tecnología Marina (CACYTMAR), Universidad de Cádiz, 11510 Cádiz, Spain 5Unidad Asociada de Oceanografía Interdisciplinar UCA-CSIC, 11510 Cádiz, Spain *Email: diego.macias@uca.es ABSTRACT: We analyzed the effect of physical forcing on zooplankton biomass distribution in the Strait of Gibraltar as a function of tidal phase. A set of 5 Longhurst-Hardy-Plankton-Recorder (LHPR) casts were made along the main channel of the Strait during different phases of the tidal cycle to determine how the hydrological processes taking place in the strait (mainly internal wave generation) influenced the distribution and taxonomic composition of zooplankton biomass. A CTD coupled to the LHPR net allowed a detailed study of the hydrological patterns created by the generation and development of the internal waves. Plankton samples were analyzed using a semi-automatic method for zooplankton discrimination (plankton visual analyzer) which is able to differentiate each individual within a sample into specific taxonomic groups. The results show that the internal wave generation on the main channel of the strait induces divergent processes causing a reduction of the zooplankton biomass. Flow patterns within the internal waves lead to alternating bands of high and low zooplankton biomass with varying taxonomic composition. Changes in taxonomic composition appear to be the result of spatial differentiation between smaller (weak-swimming) and larger (strong-swimming) taxa. This work demonstrates the necessity to consider and understand physical processes when addressing biological patterns, especially in regions with intense hydrodynamic regimes. KEY WORDS: Strait of Gibraltar · Internal waves · Physical forcing · Zooplankton distribution Full text in pdf format PreviousNextCite this article as: Macías D, Somavilla R, González-Gordillo JI, Echevarría F (2010) Physical control of zooplankton distribution at the Strait of Gibraltar during an episode of internal wave generation. Mar Ecol Prog Ser 408:79-95. https://doi.org/10.3354/meps08566 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 408. Online publication date: June 03, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Diel vertical migration of decapod larvae in the Portuguese coastal upwelling ecosystem: implications for offshore transport

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 359:171-183 (2008) - DOI: https://doi.org/10.3354/meps07341 Diel vertical migration of decapod larvae in the Portuguese coastal upwelling ecosystem: implications for offshore transport Antonina dos Santos1,*, A. Miguel P. Santos1, David V. P. Conway2, Cátia Bartilotti1, Patrícia Lourenço1,3, Henrique Queiroga3 1Instituto Nacional de Recursos Biológicos – IPIMAR, Av. Brasília, s/n, 1449-006 Lisboa, Portugal 2Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth, PL1 2PB, UK 3Departamento de Biologia, Centro de Estudos do Ambiente e do Mar, Universidade de Aveiro, 3810-193 Aveiro, Portugal *Email: antonina@ipimar.pt ABSTRACT: The vertical distribution of decapod larvae off the northwest Portuguese coast was analysed in relation to associated environmental conditions from sampling during a 69 h period around a current meter mooring located on the shelf, approximately 21 km off the coast. Plankton samples were collected every 2 h at the surface with a neuston net and through the water column with a Longhurst Hardy Plankton Recorder (Pro-LHPR), allowing a very detailed resolution of larval vertical distribution. Environmental data (temperature, salinity, and chlorophyll a) were obtained every hour. To investigate the horizontal distribution of decapod larvae in relation to the coast, a plankton-sampling grid was carried out before the 69 h fixed station. Larvae of shelf decapod species were widely distributed over the shelf, while those of inshore species were found much closer to the coast. Decapod larvae (zoeae and megalopae) showed clear diel vertical migrations, only appearing in the upper 20 m at night, a migration that did not appear to be affected by physical conditions in the water column. Larval densities were highly variable, 0.01 to 215 ind. m–3 for zoeae and 0 to 93 ind. m–3 for megalopae, the zoeae being generally more abundant. The results indicated that during the day larvae accumulate very close to the bottom. The diel vertical migration behaviour is discussed as one of the contributing mechanisms for larval retention over the shelf, even with offshore transport conditions promoted by coastal upwelling, and is hence of major relevance for the recruitment success of decapod species that inhabit inshore and shelf zones of coastal upwelling systems. KEY WORDS: Decapod larvae · Diel vertical distribution · Portuguese shelf · Transport processes Full text in pdf format PreviousNextCite this article as: dos Santos A, Santos AMP, Conway DVP, Bartilotti C, Lourenço P, Queiroga H (2008) Diel vertical migration of decapod larvae in the Portuguese coastal upwelling ecosystem: implications for offshore transport. Mar Ecol Prog Ser 359:171-183. https://doi.org/10.3354/meps07341 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 359. Online publication date: May 05, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Comparison of zooplankton sampling performance of Longhurst-Hardy Plankton Recorder and Bongo nets

Data on the abundance and biomass of zooplankton off the northwestern Portuguese coast, separately estimated with a Longhurst-Hardy Plankton Recorder (LHPR) and a Bongo net, were analysed to assess the comparative performance of the samplers. Zooplankton was collected along four transects perpendicular to the coast, deployments alternating between samplers. Total zooplankton biomass measured using the LHPR was significantly higher than that using the Bongo net. Apart from Appendicularia and Cladocera, abundances of other taxa (Copepoda, Mysidacea, Euphausiacea, Decapoda larvae, Amphipoda, Siphonophora, Hydromedusae, Chaetognatha and Fish eggs) were also consistently higher in the LHPR. Some of these differences were probably due to avoidance by the zooplankton of the Bongo net. This was supported by a comparative analysis of prosome length of the copepod Calanus helgolandicus sampled by the two nets that showed that Calanus in the LHPR samples were on average significantly larger, particularly in day samples. A ratio estimator was used to produce a factor to convert Bongo net biomass and abundance estimates to equate them with those taken with the LHPR. This method demonstrates how results from complementary zooplankton sampling strategies can be made more equivalent.

The autumn mesozooplankton community at South Georgia: biomass, population structure and vertical distribution

Mesozooplankton were sampled at shelf and oceanic stations close to South Georgia, South Atlantic during austral autumn 2004 with a Longhurst Hardy Plankton Recorder. Onshelf biomass ranged from 2.18 to 5.75 g DM m−2 (0–200 m) and was dominated by the small euphausiid Thysanoessa spp. At the oceanic stations (10.57–14.71 g DM m−2, 0–1,000 m) large calanoids, principally Rhincalanus gigas comprised ∼47–52% of biomass. Here Calanus simillimus was still active and reproducing in surface waters (0–11.2 eggs fem day−1) but R. gigas and Calanoides acutus were largely resident in the warm deep water and undergoing their seasonal descent. A comparison with spring and summer data indicated increased abundance and biomass from spring through to summer followed by a decline towards autumn particularly over the shelf. Autumn values in oceanic waters differed little from summer. Mesozooplankton biomass in the surface 200 m of the oceanic stations as a proportion of that found in the top 1,000 m ranged from 63 to 78% of the total in spring and 62–73% in summer, but was only 23–29% of the total in this study, following redistribution down the water column.

Vertical distribution of the European sardine (Sardina pilchardus) larvae and its implications for their survival

This study presents results of the vertical behaviour of the European sardine (Sardina pilchardus) larvae as observed at sea off the NW Iberian coast during an oceanographic cruise conducted in May 2002. Samples were taken in a grid of 38 stations (conductivity-temperature-depth [CTD] measurements and Longhurst Hardy Plankton Recorder [LHPR] plankton hauls); a 69-h fixed station study was also performed (hourly CTD measurements and LHPR/neuston hauls every 2 h). The horizontal distribution of larvae is closely related to the circulation patterns measured by a current metre-mooring array deployed during the cruise. Larvae were mainly distributed in the upper 20-25 m of the water column, in evident association with the waters of the Western Iberia Buoyant Plume (WIBP). Large (older) larvae are found mainly in the surface layers, and larval size decreases with depth. A diel rhythm of migration to the neuston layer was observed, correlated with the inflation/deflation activity of the swim bladder. Larvae with lengths greater than 12.5 mm and inflated swim bladders were only found in this layer. Considering the near surface stratification conditions for food availability and Ekman transport in the upper few metres, even small vertical migrations of larvae can be very important for their survival and subsequent recruitment success.

How well does the Continuous Plankton Recorder (CPR) sample zooplankton? A comparison with the Longhurst Hardy Plankton Recorder (LHPR) in the northeast Atlantic

The Continuous Plankton Recorder (CPR) survey has collected data on basin-scale zooplankton abundance in the North Atlantic since the 1930s. These data have been used in many studies to elucidate seasonal patterns and long-term change in plankton populations, as well as more recently to validate ecosystem models. There has, however, been relatively little comparison of the data from the CPR with that from other samplers. In this study we compare zooplankton abundance estimated from the CPR in the northeast Atlantic with near-surface samples collected by a Longhurst-Hardy Plankton Recorder (LHPR) at Ocean Weather Station India (59°N, 19°W) between 1971 and 1975. Comparisons were made for six common copepods in the region: Acartia clausi, Calanus finmarchicus, Euchaeta norvegica, Metridia lucens, Oithona sp., and Pleuromamma robusta. Seasonal cycles based on CPR data were similar to those recorded by the LHPR. Differences in absolute abundances were apparent, however, with the CPR underestimating abundances by a factor of between 5 and 40, with the exception of A. clausi. Active avoidance by zooplankton is thought to be responsible. This avoidance is species specific, so that care must be taken describing communities, as the CPR emphasises those species that are preferentially caught, a problem common to many plankton samplers.

The biological validation of ADCP acoustic backscatter through direct comparison with net samples and model predictions based on acoustic-scattering models

Abstract Mean volume-backscattering strength (MVBS) data collected using a 153-kHz, narrowband Acoustic Doppler Current Profiler (ADCP) were compared with zooplankton abundance and biovolume data collected using a Longhurst–Hardy Plankton Recorder (LHPR). A direct comparison showed that there was a linear relationship between MVBS and log-transformed zooplankton dry weight. This linear relationship, determined from a mixed zooplankton-species population, was then compared with that reported in previous work from a region dominated by a single species of copepod and found to be significantly different. The scatter around the linear relationships determined between MVBS and log-transformed dry weights in regions of complex, mixed zooplankton populations results in our inability to distinguish different relationships that could be expected from different populations in varying oceanic regions. It is suggested that, without further manipulation of the data, ADCP MVBS cannot be used to determine quantitative estimates of zooplankton abundance and biomass in mixed populations. “Observed MVBS” was compared with model-predicted backscattering, calculated using acoustic models and abundance and size measurements of zooplankton from net samples. The results show that at high backscattering intensities (>−80 dB) the observed MVBS from an ADCP was generally consistent with the model predictions. Abundance, biovolume, and model-predicted backscattering contributions of six “significant acoustic-scattering” groups (amphipods, chaetognaths, copepods, euphausiids, fish, and pteropods) are shown to vary disproportionately. In particular, a rare and small but strong acoustic scatterer such as a pteropod can contribute as little as 0.1% to the total sample abundance and 0.1% to the biovolume but represent 69.5% of the total model-predicted backscattering. Model, instrumental, and methodological artefacts are identified as potential sources of inconsistencies between the observed and model-predicted backscattering. These include the effect of the orientation of zooplankton, inadequate knowledge of model parameters such as the sound-speed and density contrasts, the mismatch between sampling volumes of the net and acoustic instrument, and net avoidance by the more mobile scatterers.

A comparison of LHPR and OPC data from vertical distribution sampling of zooplankton in a Norwegian fjord

Abstract Sampling was carried out using a double Longhurst-Hardy Plankton Recorder (LHPR) system (200 µm and 53 µm mesh nets) in June 1993 in Storfjorden on the west coast of Norway to describe the vertical distribution of zooplankton. The results are compared with concurrent Optical Plankton Counter (OPC) data. Zooplankton was most abundant in a superficial (0–30 m depth) layer ofwarrn (8 to 12 °C), low salinity (23 to 34) water. Included in this shallow distribution were cladocerans, which comprised 62.7 % of all zooplankton identitied in the 200 µm LHPR samples. Most of the small zooplankton taken in the 53 µm LHPR samples was distributed in the upper 50 m of the water column|abundant amongst these small organisms were adult Microsetella norvegica together with nauplii and early copepodite stages of other copepods. Both the number and biovolume of particles recorded by the OPC were consistently about 4 times higher throughout the water column than for an equivalent size range of zooplankton identified ...

A comparison of morphometric and geometric methods for the estimation of individual zooplankton volumes

Abstract Morphometric and geometric methods were compared for the determination of volumes of individual zooplankton sampled in a Norwegian fjord, the Storfjorden, using a double Longhurst-Hardy Plankton Recorder (LHPR). Both methods of volume estimation, after correction for shrinkage during preservation, gave satisfactory results and have applications in the determination of the biomass of individual taxa and their developmental stages.

Field comparison of an LHPR net sampling system and an Optical Plankton Counter (OPC) in the Southern Ocean

A field comparison of an optical plankton counter (OPC) and a Longhurst-Hardy Plankton Recorder (LHPR) was undertaken around the sub-Antarctic island of South Georgia during austral spring, 1997, to investigate the capacity of the OPC to accurately describe fine-scale plankton distributions. Initially, formalin-preserved developmental stages of representative zooplankton were passed through a benchtop OPC and a relationship established between maximum cross sectional area (CSA) of the plankton and the digital output value (DOV) of the OPC. The LHPR with an OPC attached to the net frame was then deployed at two stations representing contrasting plankton abundances. Outputs from two oblique hauls taken at a shelf station, and one at an oceanic station, were compared. Following corrections for volume swept by both instruments, and standardization of OPC particle and net plankton counts on the basis of sampler mouth area, the OPC was found to ‘overestimate' relative to the net by up to several orders of magnitude. However, following the correction for rate of flow though the OPC, and the application of a size threshold to the OPC data excluding the proportion of particles <0.25 mm2 CSA, based on the expected retention of plankton by the net there was a greatly improved and significant correlation. Overall there was no consistent over- or undersample by the OPC relative to the net, with mean OPC:net ratios being 1–1.8 for net plankton densities of up to 6000 ind. m–3. There was also a generally good agreement between the abundances of the rarer large biomass dominant calanoid copepods estimated by both samplers. An OPC overestimate of 2–3× relative to the net was seen on the ascent portion of the haul at the oceanic station, attributable to the presence of high abundances (estimated density ≥1600–4000 ind. l–1) of large centric diatoms which appeared to cause coincident counts. We conclude that with careful interpretation, OPC data can accurately describe small-scale plankton distributions in the Southern Ocean as well as order of magnitude changes.

The spring mesozooplankton community at South Georgia: a comparison of shelf and oceanic sites

Mesozooplankton (predominantly 200–2000 μm) were sampled at a shelf and an oceanic station close to South Georgia, South Atlantic, during austral spring (October/November) 1997. Onshelf zooplankton biomass was extremely high at 10–16 g dry mass m−2 (0–150 m), 70% comprising the small neritic clausocalaniid copepod Drepanopus forcipatus. Large calanoid species, principally Calanoides acutus and Rhincalanus gigas, contributed only 8–10%. At the oceanic station, biomass in the sampled water column (0–1000 m) was ∼6.5 g dry mass m−2 and 4–6 g dry mass m−2 in the top 200 m. Here, large calanoids composed 40–50% of the standing stock. Antarctic krill (Euphausia superba) occurred in low abundances at both stations. Vertical profiles obtained with a Longhurst Hardy Plankton Recorder indicated that populations of C. acutus and R. gigas, which overwinter at depth, had completed their spring ascent and were resident in surface waters. Dry mass, carbon and lipid values were lower than found in summer but were consistent with overwintered populations. Phytoplankton concentrations were considerably higher at the oceanic station (2–3 mg chlorophyll a m−3) and increased over the time on station. In response to this, egg production of both large calanoid species and growth rates of R. gigas approached those measured in summer. Onshelf phytoplankton concentrations were lower (<1 mg m−3), and low egg production rates suggested food limitation. Here phytoplankton rations equivalent to 6% zooplankton body C would have been sufficient to clear primary production whereas at the oceanic station daily carbon fixation was broadly equivalent to zooplankton carbon biomass.

Mesozooplankton biomass in the Celtic Sea: a first approach to comparing and combining CPR and LHPR data

Zooplankton biomass varies on temporal, horizontal and vertical scales. However, data sets which incorporate all these dimensions at high resolution are very rare. Two devices which measure all these aspects have recently been simultaneously deployed in the Celtic Sea, the continuous plankton recorder (CPR) and the Longhurst–Hardy plankton recorder (LHPR). This demonstrates that integrated biomass derived from the LHPR are not significantly different from those derived using the CPR. Values have, therefore, been combined for the first time to describe the vertical distribution of mesozooplankton biomass at the Celtic Sea shelf edge through an annual cycle. This suggests that the surface biomass peak is broader at the shelf break than in the open ocean and in the autumn the main biomass peak may be below the depth sampled by the CPR.

Krill-copepod interactions at South Georgia, Antarctica, II. Euphausia superba as a major control on copepod abundance

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 176:63-79 (1999) - doi:10.3354/meps176063 Krill-copepod interactions at South Georgia, Antarctica, II. Euphausia superba as a major control on copepod abundance A. Atkinson*, P. Ward, A. Hill, A. S. Brierley, G. C. Cripps British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, United Kingdom *E-mail: a.atkinson@bas.ac.uk ABSTRACT: Euphausia superba (hereafter 'krill') and copepods are major zooplankton taxa in the Southern Ocean, but there is little information on how they interact. This paper investigates their coincidence across a wide range of temporal and spatial scales to examine whether copepod distribution is related to that of krill. During 2 summers of high krill abundance near South Georgia (1996 and 1997) copepod abundance was <40% of that during an abnormally low krill year (1994). No such depletion was found north of the Polar Front, where krill were rare. Analysis of 2 mesoscale data sets showed that krill, rather than food or environmental factors, were most strongly implicated in copepod distribution. An area of persistently high krill abundance just north of South Georgia was characterised by exceptionally few copepods. Fine-scale relationships between patches of krill and copepods were studied with a Longhurst Hardy Plankton Recorder. Within krill swarms copepod abundance was low, but more dispersed krill associated with high concentrations of copepods. Copepods also appeared to live deeper and to make more extensive vertical migrations when krill were present. The inverse relationship between krill and copepod abundances thus occurred repeatedly and across a wide range of scales. The facts that krill swarms are mobile and were unrelated to hydrography further suggest that the inverse relationship was caused by krill. This could arise from competitive exclusion, direct predation or both. Evidence for competition is that South Georgia copepods rely on high phytoplankton biomass for recruitment and krill can remove this. Predation is suggested by the fact that crustaceans were found in krill guts in this region during both summer and winter. During the 1996 summer, experimentally derived predation rates on copepods, combined with krill biomass values, suggested a significant impact on small copepods. Therefore we suggest that copepod numbers can be controlled by a combination of competition and predation by krill. KEY WORDS: Antarctic krill · Euphausia superba · Copepods · South Georgia · Southern Ocean · Distribution · Population control · Feeding interactions Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 176. Publication date: January 18, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Acoustic discrimination of Southern Ocean zooplankton

Acoustic surveys in the vicinity of the sub-Antarctic island of South Georgia during a period of exceptionally calm weather revealed the existence of a number of horizontally extensive yet vertically discrete scattering layers in the upper 250 m of the water column. These layers were fished with a Longhurst-Hardy plankton recorder (LHPR) and a multiple-opening 8 m 2 rectangular mid-water trawl (RMT8). Analysis of catches suggested that each scattering layer was composed predominantly of a single species (biovolume>95%) of either the euphausiids Euphausia frigida or Thysanöessa macrura, the hyperiid amphipod Themisto gaudichaudii, or the eucalaniid copepod Rhincalanus gigas. Instrumentation on the nets allowed their trajectories to be reconstructed precisely, and thus catch data to be related directly to the corresponding acoustic signals. Discriminant function analysis of differences between mean volume backscattering strength at 38, 120 and 200 kHz separated echoes originating from each of the dominant scattering layers, and other signals identified as originating from Antarctic krill ( Euphausia superba), with an overall correct classification rate of 77%. Using echo intensity data alone, gathered using hardware commonly employed for fishery acoustics, it is therefore possible to discriminate in situ between several zooplanktonic taxa, taxa which in some instances exhibit similar gross morphological characteristics and have overlapping length– frequency distributions. Acoustic signals from the mysid Antarctomysis maxima could also be discriminated once information on target distribution was considered, highlighting the value of incorporating multiple descriptors of echo characteristics into signal identification procedures. The ability to discriminate acoustically between zooplankton taxa could be applied to provide improved acoustic estimates of species abundance, and to enhance field studies of zooplankton ecology, distribution and species interactions.

Layers of Antarctic krill, Euphausia superba : are they just long krill swarms?

Antarctic krill, Euphausia superba Dana, a major component in the southern ocean food web, typically occur in aggregations that range from small, discrete swarms and schools through to layers and superswarms that extend horizontally for several kilometres. A large Longhurst–Hardy plankton recorder has been used to obtain high-resolution serial samples from within two layers (up to 4 km in length) that were found near Elephant Island, north of the Antarctic Peninsula. Krill length, sex and maturity stage, net and acoustic estimates of number density are shown to vary significantly within these large layers. The variation occurring within a single layer is comparable with that occurring in a set of 38 swarms sampled contemporaneously with the layers. Thus, unlike a krill swarm, a whole krill layer may account for a substantial amount of the variation in the local krill population, although individual parts of the layer cannot be considered in this way. The layers play an important role in the ecology of the krill within the area. Firstly, these large layers may contain a significant proportion of the biomass within an area. Secondly, the structure of the layers gives some insight into the ways in which krill swarm formation and dispersal may be occurring.

Diel variations in the vertical distribution of copepods off the north coast of Spain

Diel variations in the vertical distribution of copepods were studied in the north coast of Spain and related to the physical and biological structure of the water column. Zooplankton samples were taken at three different stations during a 24 h period, in the proximity of a drogue buoy using a Longhurst Hardy Plankton Recorder (LHPR) sampling net, so that it is assumed that the same water mass was sampled continuously. Physical and other biological parameters were monitored by Conductivity Temperature Depth Recorder (CTD) profiles and vertical Undulating Oceanographic Recorder (UNDULATOR) dips for temperature, salinity, and chlorophyll a. Copepods were counted and classified into seven categories according to their size, and their distribution in the vertical plane was analysed using univariate and multivariate statistical techniques.Vertical distribution of copepods and their developmental stages were mainly determined by chlorophyll distribution and, when a subsurface chlorophyll maximum was present, small copepods and copepodite stages appeared aggregated in this layer. Nocturnal upward movements of the population and an ontogenetic component in the position and behaviour of the community were also important features that characterised the distribution of the community. Those patterns may explain a part of the observed variability of the abundances and distribution of the copepods in the water column. However, the water mass displacement in the 24 h period of study must also be considered as a main variability factor in the spatial and temporal distribution of the zooplankton populations.

Diel periodicity of Subantarctic copepods: relationships between vertical migration, gut fullness and gut evacuation rate

A zooplankton community in the Polar Frontal Zone north of South Georgia was sampled for 5 days in February 1994. Feeding of various copepodite stages of six copepod species was assessed by a series of gut fluorescence/gut evacuation experiments. Feeding periodicity was compared to ver- tical distribution and migration patterns, as revealed by Longhurst Hardy Plankton Recorder (LHPR) and ring net catches. Despite chlorophyll a levels reaching only 0.8 mg m~3 and daily carbon rations based on phytoplankton intake being low, feeding of all the copepods was restricted mainly to the 8 h night-time period. During the daytime, the epipelagic community was vertically dispersed within the top 100 m. At night, upward migration by most species led to a convergence of almost all zooplankters in the upper half of the surface mixed layer. However, large-amplitude vertical migrations which crossed the thermocline were performed only by Metridia lucens and Pleuromamma robusta. Although feeding by both migrants and non-migrants was mainly at night, there was no diel signal in gut evacuation rate. The gut evacuation rates of the nine copepod species and stages differed significantly (5-fold) and were negatively related to the extent of their diel vertical migration. The long gut-passage times of the migratory species, M.lucens and P.robusta, would have allowed them time to defaecate some of the food eaten near the surface at depth, contributing to an active carbon transfer out of the mixed layer. However, their scarcity at this site meant that their grazing comprised only ~<1.4% of total copepod mgestion, so their combined gut flux is likely to have been negligible (-0.4 mg carbon m-2day-').