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Assessment of zooplankton abundance, distribution, community composition, and temporal variability is critical to understanding the effects of climate variability and change on lower trophic level production and availability for consumption by larger consumers. Zooplankton sampling is performed across the Canadian continental shelf system by Fisheries and Oceans Canada's Atlantic Zone Monitoring Programme (AZMP). Sampling includes semi-monthly to monthly collection of zooplankton using vertical net tows (VNTs) deployed from near-bottom to surface at stations on the central Scotian Shelf (Stn 2, 150 m depth) and Newfoundland Shelf (Stn 27, 175 m depth), and by Continuous Plankton Recorders (CPRs) in the near-surface layers along routes over the Scotian and Newfoundland shelves (0–10 m depth). Here, we compare abundance metrics for 11 copepod taxa collected using both gear types in both regions between 1999 and 2015. Seasonal cycles of VNT and CPR abundance were similar for near-surface residents. VNT: CPR abundance ratios varied year-round for vertical migrants, as ontogenetic migrants shifted their vertical distribution, and as diel migrants changed their migratory behaviour. For some taxa, differences in annual average VNT: CPR abundance ratios between regions suggest differences in vertical distribution, while for others differences in inter-annual variability for VNT and CPR abundances suggest differences in the dynamics of the near- and sub-surface components of the populations.

Copepods of the genus Calanus play a critical trophic role in the North Atlantic ecosystems, where they serve as an important source of energy-rich food for fish and marine mammals, including the endangered North Atlantic right whale. As a strategy for coping with unfavorable near-surface conditions, Calanus enter diapause and migrate to deep water in late summer and fall after feeding and accumulating lipid stores in spring and summer. In order to assess the most important physical drivers of vertical distribution of diapausing Calanus, we synthesized existing depth-stratified abundance data of Calanus finmarchicus and Calanus hyperboreus from the Northwest Atlantic continental shelf and slope regions, spanning Newfoundland in the northeast to the Gulf of Maine in the southwest. Bottom depth strongly constrained the depth and shape of vertical distributions, with distributions becoming deeper and less compact as bottom depth increased. Diapausing Calanus, observed across a broad range of temperature (T) and in-situ density (σ) conditions (T = −1.0 to 14.4 °C, σ = 25.3–28.1 kg m−3), tended to distribute at depths with the coldest temperatures locally available. Over the shelf, diapausing Calanus in the GOM and SS generally did not have access to temperatures considered optimal for diapause (

The Labrador Sea is an ideal region to study the biogeographical, physiological, and biogeochemical implications of phytoplankton community composition due to sharp transitions between distinct water masses across its shelves and central basin. We have investigated the multi-year (2005–2014) distributions of late spring and early summer (May to June) phytoplankton communities in the various hydrographic settings of the Labrador Sea. Our analysis is based on pigment markers (using CHEMTAX analysis), and photophysiological and biogeochemical characteristics associated with each phytoplankton community. Diatoms were the most abundant group, blooming first in shallow mixed layers of haline-stratified Arctic shelf waters. Along with diatoms, chlorophytes co-dominated at the western end of the section (particularly in the polar waters of the Labrador Current (LC)), whilst Phaeocystis co-dominated in the east (modified polar waters of the West Greenland Current (WGC)). Pre-bloom conditions occurred in deeper mixed layers of the central Labrador Sea in May, where a mixed assemblage of flagellates (dinoflagellates, prasinophytes, prymnesiophytes, particularly coccolithophores, and chrysophytes/pelagophytes) occurred in low-chlorophyll areas, succeeding to blooms of diatoms and dinoflagellates in thermally stratified Atlantic waters in June. Light-saturated photosynthetic rates and saturation irradiance levels were highest at stations where diatoms were the dominant phytoplankton group ( > 70 % of total chlorophyll a), as opposed to stations where flagellates were more abundant (from 40 up to 70 % of total chlorophyll a). Phytoplankton communities from the WGC (Phaeocystis and diatoms) had lower light-limited photosynthetic rates, with little evidence of photoinhibition, indicating greater tolerance to a high light environment. By contrast, communities from the central Labrador Sea (dinoflagellates and diatoms), which bloomed later in the season (June), appeared to be more sensitive to high light levels. Ratios of accessory pigments (AP) to total chlorophyll a (TChl a) varied according to phytoplankton community composition, with polar phytoplankton (cold-water related) having lower AP : TChl a. Polar waters (LC and WGC) also had higher and more variable particulate organic carbon (POC) to particulate organic nitrogen (PON) ratios, suggesting the influence of detritus from freshwater input, derived from riverine, glacial, and/or sea ice meltwater. Long-term observational shifts in phytoplankton communities were not assessed in this study due to the short temporal frame (May to June) of the data. Nevertheless, these results add to our current understanding of phytoplankton group distribution, as well as an evaluation of the biogeochemical role (in terms of C : N ratios) of spring phytoplankton communities in the Labrador Sea, which will assist our understanding of potential long-term responses of phytoplankton communities in high-latitude oceans to a changing climate.

The Labrador Sea is an important oceanic sink for atmospheric CO2 because of intensive convective mixing during winter and extensive phytoplankton blooms that occur during spring and summer. Therefore, a broad-scale investigation of the responses of phytoplankton community composition to environmental forcing is essential for understanding planktonic food-web organisation and biogeochemical functioning in the Labrador Sea. Here, we investigated the phytoplankton community structure (>4 ?m) from near surface blooms (1.2 mg chla m?3) occurred on and near the shelves in May and in offshore waters of the central Labrador Sea in June due to haline- and thermal-stratification, respectively. Sea ice-related (Fragilariopsis cylindrus and F. oceanica) and Arctic diatoms (Fossula arctica, Bacterosira bathyomphala and Thalassiosira hyalina) dominated the relatively cold (

Two hypotheses have been proposed to account for trophic dynamic control of the eastern Scotian Shelf ecosystem off Atlantic Canada: (1) top-down: fishery induced trophic cascade and (2) bottom-up: climate variability. We evaluate the evidence in support of these hypotheses: including observations on top-down drivers (fishing effort and predation by grey seals), bottom-up drivers (nutrient supply and water column stratification), and the several trophic levels (groundfish, macro-invertebrates, small pelagic fish, and plankton). There is limited support for the fishery-induced trophic cascade hypothesis. The predictions of the climate variability hypothesis are generally met for the lower and middle trophic levels, but the ongoing high levels of natural mortality of groundfish are not accounted for. We propose an alternative hypothesis encompassing concurrent top-down and bottom-up processes, and conclude that many species of groundfish (including cod) and small pelagic fish stocks (including herring) will not recover with the ongoing high levels of natural mortality generated by grey seal predation. Predictions on future trends in abundance of the commercially important macro-invertebrate species (lobster, snow crab, and shrimp) are not possible based on the available evidence.

Here we present a new, pan-North-Atlantic compilation of data on key mesozooplankton species, including the most important copepod, Calanus finmarchicus. Distributional data of eight representative zooplankton taxa, from recent (2000–2009) Continuous Plankton Recorder data, are presented, along with basin-scale data of the phytoplankton colour index. Then we present a compilation of data on C. finmarchicus, including observations of abundance, demography, egg production and female size, with accompanying data on temperature and chlorophyll. This is a contribution by Canadian, European and US scientists and their institutions: http://doi.pangaea.de/10.1594/PANGAEA.820732, http://doi.pangaea.de/10.1594/PANGAEA.824423, http://doi.pangaea.de/10.1594/PANGAEA.828393 (please also see Melle et al., 2013; Castellani and Licandro, 2013; Jónasdóttir et al., 2014).

Diatoms are a keystone algal group, with diverse cell morphology and a global distribution. The biogeography of morphological, functional, and life-history traits of marine diatoms were investigated in Arctic and Atlantic waters of the Labrador Sea during the spring bloom (2013-2014). In this study, trait-based analysis using community-weighted means showed that low temperatures (< 0°C) in Arctic waters correlated positively with diatom species that have traits such as low temperature optimum growth and the ability to produced ice-binding proteins, highlighting their sea ice origin. High silicate concentrations in Arctic waters, as well as sea ice cover and shallow bathymetry, favoured diatom species that were heavily silicified, colonial and capable of producing resting spores, suggesting that these are important traits for this community. In Atlantic waters, diatom species with large surface area to volume ratios were dominant in deep mixed layers, whilst low silicate to nitrate ratios correlated positively with weakly silicified species. Sharp cell projections, such as processes or spines, were positively correlated with water-column stratification, indicating that these traits promote positive buoyancy for diatom cells. Our trait-based analysis directly links cell morphology and physiology with diatom species distribution, allowing new insights on how this method can potentially be applied to explain ecophysiology and shifting biogeographical distributions in a warming climate. Copyright © 2018 Fragoso, Poulton, Yashayaev, Head, Johnsen and Purdie. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

This paper addresses relationships between the distribution and abundance of zooplankton and its habitat in the northern North Atlantic Ocean. Distributions of ten representative zooplankton taxa, from recent (2000-2009) Continuous Plankton Recorder data, are presented, along with basin-scale patterns of annual sea surface temperature and phytoplankton color. The distribution patterns represent the manifestation of very different physiological, life history and ecological interactions of each taxon with the North Atlantic habitat characteristics. The paper then focuses on a pan-Atlantic compilation of demographic and life history information for the planktonic copepod, Calanus finmarchicus, perhaps one of the most ecologically important and certainly the most studied zooplankton species in the North Atlantic. Abundance, dormancy, egg production and mortality in relation to temperature and phytoplankton biomass, using chlorophyll a as a proxy, are analyzed in the context of understanding factors involved in determining the distribution and abundance of C. finmarchicus across its range. Several themes emerge: (1) transport of C. finmarchicus is from the south to the north in the northeast Atlantic, but from the north to the south in the western North Atlantic, which has implications for understanding population responses to climate forcing on coastal shelves, , (2) recruitment to the youngest copepodite stages occurs during or just after the phytoplankton bloom in the east while it occurs after the bloom in many western sites, (3) while the deep basins in the Labrador Sea and Norwegian Sea are primary sources of C. finmarchicus production, the western North Atlantic marginal seas have an important role in sustaining high C. finmarchicus abundance on the western North Atlantic shelves, (4) differences in mean temperature and chlorophyll concentration between the western and eastern North Atlantic are reflected in regional differences in female body size and egg production responses, (5) differences in functional responses in egg production rate may reflect genetic differences between western and eastern populations, (6) dormancy duration is generally shorter in the deep waters adjacent to the lower latitude western North Atlantic shelves than in the east, and (7) differences in stage-specific mortality rates are related to bathymetry, temperature and potential predators, notably the abundance of congeners Calanus hyperboreus and C. glacialis, which likely feed on early life stages of C. finmarchicus. Two modeling approaches have previously been used to interpret the abundance and distribution of C. finmarchicus in relation to the North Atlantic habitat. A statistical approach based on ecological niche theory and a dynamical modeling approach, based on knowledge of spatial population dynamics and life history and implemented by recent developments in coupled physical-life cycle modeling. The strengths and weaknesses of each approach are discussed. A synthesis of the two modeling approaches to predict North Atlantic zooplankton species shifts, not only for C. finmarchicus, but also for other major taxa, is advocated. While the computational resource requirements and lack of species-specific life history information for physical-biological modeling hinder full application for many zooplankton taxa, use of the approach, where possible, to understand advective influences will provide insight for interpretation of statistical predictions from species distribution models.

The Norwegian Sea is generally warmer than the Labrador Sea because it is influenced more by Atlantic Water inflows from the south, whereas the latter receives relatively larger inputs of Arctic Water from the north. Despite its more northerly location, the spring bloom generally starts earlier in the Norwegian Sea. Within each of the two seas, however, there are regional and interannual differences in temperature and the timing of the spring bloom. The responses of Calanus finmarchicus populations to these differences in environmental conditions include differences in physical characteristics (e.g. female size), physiological rates (egg production rates) and seasonal cycles of abundance. Females are generally larger in the Labrador Sea and have higher egg production rates for a given chlorophyll concentration than do those in the Norwegian Sea. Within and among areas in both seas, as temperatures increase and spring blooms tend to occur earlier, C. finmarchicus start to reproduce earlier, the new generation develops faster, and in some areas a second generation ensues. In areas where near surface temperatures are relatively high in summer and/or where phytoplankton growth rates are relatively low in summer or autumn, reproduction and development cease, and C. finmarchicus desert the surface layers for their overwintering depths. This occurs in the Norwegian Sea in summer and in the central Labrador Sea in autumn. By contrast, in areas where near surface temperatures remain cool in summer and where phytoplankton growth persists through the autumn, reproduction and development can continue through summer and autumn, probably until winter vertical mixing prevents phytoplankton growth. This occurs on the southern Newfoundland Shelf. Even in areas where the growth season is prolonged, however, a proportion of the first generation, and probably subsequent generations, descends to overwinter. If the size of the overwintering population is used as an index of net productivity, then for equivalent regions in the Norwegian Sea and Labrador Sea (the areas of each most affected by Atlantic inflow), the differences in ambient temperatures and bloom dynamics apparently have little impact. With global warming, as temperatures in the Norwegian and Labrador Seas increase up to a certain threshold, the timing of life history events for C. finmarchicus will likely be advanced and the number of generations produced per year could increase. The time spent in the near surface layers will probably decrease, however, while the overall effect on population size may not be large. Once the temperature threshold for unfavourable survival of C. finmarchicus has been exceeded, the distribution range for C. finmarchicus will likely contract northwards, with important consequences for dependent species in the affected regions.

We investigated the drift of passive particles on the Newfoundland Shelf and western Labrador Sea using numerical simulations to assess the possible sources of plankton collected at a high frequency sampling site (S27; 47.55°N, 52.59°W) located near the coast of Newfoundland, Canada. We also summarized data detailing the seasonal stage succession of Calanus finmarchicus at that site, as well as along three oceanographic sections sampled in the spring, summer and autumn across the adjacent continental shelf. Simulations indicated that the Labrador and Newfoundland Shelves represent the major sources of particles transiting through the S27 site, with relatively minor contributions from the western Labrador Sea which are significant during a few months each year. The latter point may be affected by uncertainty in the representation of cross-shelf transport associated with seasonal or short-term variations in atmospheric and oceanic forcing, which may also affect the strength and location of bifurcation of the inner branch of the Labrador Current around the Grand Banks. Nevertheless, our results indicated that drift along the inner shelf is likely to be the primary source of copepods collected at S27 throughout most of the year. This in turn suggested that there may be a higher degree of connectivity between conditions in coastal areas of Newfoundland and those in Baffin Bay and west Greenland than with the southern half of the Labrador Sea.

Egg production rates (EPRs) were measured for Calanus finmarchicus from the Labrador Sea during annual cruises between May and July from 1997 to 2010. EPRs ranged between 0 and 91 eggs f day at 95 stations. Clutch size (CS) was related to female size (prosome length, PL) and both were smallest on the Greenland (2.76 mm, 55.2 eggs) and Labrador (2.86 mm, 60.1 eggs) shelves and largest in the central basin (2.94 mm, 73.0 eggs). CS and spawning frequency (SF, the proportion of females spawning during a 24 h experiment) both increased with increasing female nitrogen content and decreased with increasing C:N ratio. CS and SF were also positively correlated and both increased with an increasing in situ chlorophyll concentration. The relationships between SF (% day) and EPR (as eggs f day or % body C or N day) and in situ chlorophyll concentration were well described by Ivlev functions (r 1⁄4 0.30–0.34). The residuals from the Ivlev correlations for EPR (but not SF) were significantly negatively correlated with temperature for the central basin (r 1⁄4 0.14), but unrelated over all subregions. Over the course of the bloom, as in situ nitrate concentrations decreased, the female N content and the CS decreased and the female C content and C:N increased. Overall, the EPR was most influenced by (i) local feeding conditions (in situ chlorophyll concentration), (ii) the state of the bloom (via effects on female age/condition) and (iii) long-term feeding and temperature history (via effects on female size), although a significant amount of variability remains unexplained.

article i nfo The seasonal variability of inorganic carbon in the surface waters of the Scotian Shelf region of the Canadian northwestern Atlantic Ocean was investigated. Seasonal variability was assessed using hourly measurements, covering a full annual cycle, of the partial pressure of CO2, (pCO2), and hydrographic variables obtained by an autonomous moored instrument (44.3°N and 63.3°W). These measurements were complemented by seasonal shipboard sampling of dissolved inorganic carbon (DIC), total alkalinity (TA), and pCO2, at the mooring site, and over the larger spatial scale. Biological processes were found to be the dominant control on mixed-layer DIC, with the delivery of carbon-rich subsurface waters also playing an important role. Annual mixed-layer net community production was 2.4 mol C m �2 yr �1 . The air-sea fluxes of CO2 were computed using observed hourly wind speeds from the Sable Island Meteorological Station (43.9°N and 60.3°W). The region

We report size, lipid sac volume and fatty acid composition of C5 copepodites of Calanus finmarchicus collected in late autumn 2007 on the Newfoundland Shelf and in slope waters in the Labrador Sea, to assess differences in feeding histories between these populations. Copepodites from the slope waters were generally relatively large and were mainly at depth and in diapause, whereas those from shelf waters were relatively small and were either in the surface layers or had only recently descended into the deeper layer. Multivariate analyses revealed a strong pattern of separation among the levels of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA), bacterial fatty acid markers and ω3 PUFAs in copepods from the different areas. There was clear separation of diatom (16:1ω7, 16:4ω1 and 20:5ω3) from prymnesiophyte–dinophyte (18:1ω9, 18:4ω3 and 22:6ω3) fatty acid markers. Larger body size and greater energy reserves were associated with C5 C. finmarchicus from the slope waters and with increases in the proportion of fatty acid biomarkers for diatoms and omnivory. Smaller body sizes were associated with C5s on the shelf and with a greater proportion of fatty acid biomarkers for dinoflagellates and prymnesiophytes. C. finmarchicus collected near the coast had significantly higher levels of biomarkers indicative of terrestrial input in their diet.

Characterizing environmental effects on copepod production and their ecological roles is complicated by multiple physical (e.g. temperature) and biological (e.g. food, predation) factors controlling multiple aspects of copepod physiology and demography. For example, data for two regions in eastern Canada (St. John’s, Newfoundland and Halifax, Nova Scotia) indicate that subtle differences in environmental conditions lead to significant differences in seasonal copepod (Calanus finmarchicus) recruitment timing and magnitude. Here, we quantify how environmental variability influences C. finmarchicus physiology and demography leading to observed regional and seasonal variations in abundance off St. John’s and Halifax. We apply a stochastic individual-based model (IBM) for copepod population dynamics to simulate the seasonal variation in C. finmarchicus abundance of egg through copepodite 1 (C1) stages at the two sites using year-specific local forcing from multi-year data. The model includes individual variability in development, egg production and mortality rates with resulting seasonal C1 abundance averaged among years and compared to analogous observations. We find temperature has a dominant effect on both development and egg production rates while egg recruitment is affected by temperature and female abundance at both sites. We show that mortality rate characterization has a strong influence on modeled abundances, and site-specific environmentally dependent mortality rates are necessary to produce results consistent with observations (temperature vs. food vs. cannibalism via females). Results indicate that prediction of climate change effects on copepod abundance and their ecological roles requires consideration of biological (e.g. chlorophyll a, female abundance) as well as physical (e.g. temperature) factors. In particular, estimates of abundances during the onset of C1 recruitment (i.e. their arrival on the larval fish prey field) are improved by 67–94% when the influence of biological factors on mortality rates are considered.

Phytoplankton abundance in the NW Atlantic was measured by continuous plankton recorder (CPR) sampling along tracks between Iceland and the western Scotian Shelf from 1998 to 2006, when sea-surface chlorophyll (SSChl) measurements were also being made by ocean colour satellite imagery using the SeaWiFS sensor. Seasonal and inter-annual changes in phytoplankton abundance were examined using data collected by both techniques, averaged over each of four shelf regions and four deep ocean regions. CPR sampling had gaps (missing months) in all regions and in the four deep ocean regions satellite observations were too sparse between November and February to be of use. Average seasonal cycles of SSChl were similar to those of total diatom abundance in seven regions, to those of the phytoplankton colour index in six regions, but were not similar to those of total dinoflagellate abundance anywhere. Large inter-annual changes in spring bloom dynamics were captured by both samplers in shelf regions. Changes in annual (or 8 months) averages of SSChl did not generally follow those of the CPR indices within regions and multi-year averages of SSChl, and the three CPR indices were generally higher in shelf than in deep ocean regions. Remote sensing and CPR sampling provide complementary ways of monitoring phytoplankton in the ocean: the former has superior temporal and spatial coverage and temporal resolution, and the latter provides better taxonomic information.

The results of Continuous Plankton Recorder sampling in the NW Atlantic between 1958 and 2006 are presented for 11 plankton taxa in eight shelf and deep ocean regions. For shelf regions, phytoplankton abundances increased in the early 1990s, mainly in winter, as the contribution of Arctic-derived freshwater to the Newfoundland (NLS) and Scotian shelves (SS) increased. Farther east, in the sub-polar gyre, phytoplankton levels increased with rising temperatures during the 1990s and 2000s. In both areas, the changes can be explained by increased stratification. The increased influx of arctic water to the NLS in the 1990s was also probably directly responsible for the increased abundances of two arctic Calanus species (C. glacialis and C. hyperboreus) and indirectly responsible for the decreased abundance of Calanus I–IV (mainly C. finmarchicus), perhaps via changes in food composition. On the SS the arctic Calanus species increased in abundance in the 2000s, likely as the result of increased transport from the Arctic via the Gulf of St Lawrence. In the deep ocean, plankton seasonal cycles changed little over the decades and increasing phytoplankton levels in the 2000s were accompanied by increases in zooplankton abundance, suggesting bottom-up control. In shelf regions, phytoplankton increases in the 1990s were in winter and Calanus I–IV appeared earlier in spring than in previous decades. Zooplankton levels generally did not change overall however, perhaps because the species examined were mainly inactive during winter.

Traditional plankton models do not simulate the life history, physiology or phenology of mesozooplankton with sufficient realism to represent their ecological roles and associated environmental dependencies. In contrast, individual-based models (IBMs) have proven utility in characterizing fish population dynamics as they bridge the gap between the level at which environmental impacts occur (individuals) and the level at which observations are made (populations). IBMs are under-utilized in zooplankton ecology, possibly because of their apparent complexity, reliance on uncertain parameters, and/or computational expense. Here, we show that such limitations are not inherent, and in fact, IBMs can offer considerable power for quantitative hypothesis testing. We present a conceptually and programmatically simple approach for building a stochastic stage-based IBM for copepods (abundant mesozooplankters). Our model incorporates physiological rates that vary with environmental conditions, and includes stochasticity via a “Fitness” parameter, which characterizes individual variability, and may be prescribed a priori (e.g. genetic), or solved dynamically based on individual history, condition, environment, etc. We demonstrate how this Fitness allows for easy evaluation of the relative importance of forcing factors (e.g. temperature vs. food) on modelled rates and abundances as well as statistical comparison to observations. The latter is a powerful feature of our modelling approach, which we exploit to test hypotheses regarding the seasonal dynamics of the copepod Calanus finmarchicus in the northwest Atlantic as a case study. The result is a model description (including flow chart and code) that can be adapted for a wide range of species and novel applications in a quantitative manner.

Plourde, S., Pepin, P., and Head, E. J. H. 2009. Long-term seasonal and spatial patterns in mortality and survival of Calanus finmarchicus across the Atlantic Zone Monitoring Programme region, Northwest Atlantic. – ICES Journal of Marine Science, 66: 1942–1958. The vertical life table method was used to estimate stage-specific daily mortality rates and survival from 1999 to 2006 for Calanus finmarchicus sampled in the Canadian Atlantic Zone Monitoring Programme, which covers the Newfoundland–Labrador Shelf (NLS), Gulf of St Lawrence (GSL), and Scotian Shelf (SS). Stage-specific mortality rates and survival showed significant regional and seasonal differences, with the largest signal associated with variations in temperature. Density-dependent mortality, associated with the abundance of C6 females, was the main factor influencing mortality in the egg–C1 transition during the period of population growth in spring on the SS, and in summer in the GSL and on the NLS. In autumn, mortality in egg–C1 was positively related to temperature and negatively related to phytoplankton biomass, with particularly high mortality rates on the SS. The integration of our results into stage-specific recruitment rates from egg to C5 revealed that C. finmarchicus populations experience their greatest loss (mortality) during the egg–C1 transition. Loss during development to C1 was greater in the GSL than in the other regions during the period of population growth, resulting in lower recruitment success in the GSL. In autumn, C. finmarchicus showed low stage-specific daily recruitment rates on the SS at high temperatures, and low phytoplankton biomass compared with those in the GSL and on the NLS. Our findings reinforce the necessity of describing regional and seasonal patterns in mortality and survival to understand factors controlling the population dynamics of C. finmarchicus.

We report on the variation in energy reserves of individual C5 copepodites of Calanus finmarchicus from the Newfoundland continental shelf and the Labrador Sea collected from surface and overwintering (or bottom) depths prior to, during and after the expected timing of the onset of diapause. Overall, the trend was for a decreasing average prosome length as the year progressed for all locations although the decline was smallest in the Labrador Sea and greatest in the deep waters of the continental shelf. The size of the oil sac was closely linked to the weight of the copepodite but the form of this relationship showed substantial variations with depth and season. We show a clear increase in the relative oil sac volume for C. finmarchicus between late spring and late summer, by which time some animals had descended to diapause depths. The progressive decrease in oil sac volume of animals sampled at depth in the Labrador Sea between September and December suggests a significant loss of energy reserves during diapause. From the distribution of volumes and body sizes in December we estimate that 23–53% of individuals would not be able to meet the energetic cost of moulting and early gonad development. Overall, some of our observations appear to invalidate earlier hypotheses concerning the governing role of lipids in the life history of C. finmarchicus . However, assessment of the factors that influence entry into dormancy should be based on the relative probabilities of alternative strategies for successful reproduction (e.g. entering dormancy vs. continuing into a second generation).

Johnson, C. L., Leising, A. W., Runge, J. A., Head, E. J. H., Pepin, P., Plourde, S., and Durbin, E. G. 2008. Characteristics of Calanus finmarchicus dormancy patterns in the Northwest Atlantic. – ICES Journal of Marine Science, 65: 339–350. Demographic time-series from four fixed stations in the Northwest Atlantic Ocean demonstrate variable timing of entry into and emergence from dormancy in subpopulations of the planktonic copepod Calanus finmarchicus. A proxy for timing of entry was established as the date each year when the proportion of the fifth copepodid stage (CV) in the subpopulation rose to half its overall climatological maximum CV proportion at that station. The proxy for timing of emergence at each station was set as the first date when adults were more than 10% of the total abundance of copepodid stages. An alternate emergence proxy date was determined by back-calculating the spawning dates of the first early copepodid stages appearing in spring, using a stage-structured, individual-based model. No single environmental cue (photoperiod, surface temperature, or average surface-layer chlorophyll a concentration) consistently explained entry or emergence dates across all stations. Among hypotheses put forward to explain dormancy in Calanus species, we cannot eliminate the lipid accumulation window hypothesis for onset of dormancy or a lipid-modulated endogenous timer controlling dormancy duration. The fundamental premise of these hypotheses is that individuals can only enter dormancy if their food and temperature history allows them to accumulate sufficient lipid to endure overwintering, moult, and undergo early stages of gonad maturation.

Continuous Plankton Recorder (CPR) sampling on the Newfoundland and Scotian shelves covers three multi-year periods characterised by negative (1962–1971), positive (1992–2000) and negative/neutral (2001–2003) values of the NAO index. Water temperatures respond differently to changes in the NAO in different regions: a positive NAO index tends to lead to reduced temperatures on the Newfoundland shelf and to increased temperatures on the central/western Scotian shelf, and a negative NAO index to the reverse. Since the 1960s, the hydrographic changes due to the NAO have been superimposed on a freshening of the water column throughout the region, which is attributed to increased contribution of Arctic water outflow. Changes in plankton abundance measured by the CPR for the three time periods were generally, but not always, similar on the Newfoundland and eastern and western regions of the Scotian shelf, although Arctic species (e.g. Calanus glacialis , Calanus hyperboreus ) were notably more abundant and warm water species (e.g. Metridia lucens , euphausiids) less abundant on the Newfoundland shelf than on the Scotian shelf. Three categories of phytoplankton (colour, diatoms, dinoflagellates) increased in abundance in the 1990s, and these increases generally persisted into 2001–2003. This is believed to be a response to the persistent freshening of the water column, probably due to increased stratification. The Arctic species C. glacialis and C. hyperboreus also showed persistent increases in abundance after 1992, perhaps due to increased transport from the Arctic, although the abundance of the Arctic slope water species Metridia longa decreased. Two groups, Calanus 1–4 and euphausiids, both thought to play important roles in the food chain, showed persistent decreases in abundance after 1992, especially on the Newfoundland shelf. In all regions, Calanus finmarchicus 5–6, Oithona spp. and Centropages hamatus abundance changed in association with variations in the NAO, although no common mechanism could be identified. C. finmarchicus 5–6 abundance decreased in the 1990s and increased after 2001, while the other two species showed the opposite pattern. Centropages typicus and M. lucens abundance on the Scotian shelf increased with rising temperature. This is attributed to increased production rates for the former and an increased influx of warm, M. lucens- rich, slope water on to the shelf for the latter. A comparison between ring net and CPR sampling on the Newfoundland shelf suggests that the Calanus 1–4 category is dominated by C. finmarchicus and that late stage C. glacialis and C. hyperboreus are grossly under-sampled compared to late stage C. finmarchicus .

Seasonal patterns of vertical flux over two years (2001–2003) at two stations in the Bras d'Or Lakes, a large estuary in Cape Breton Island, Nova Scotia, were determined using aluminum and organic carbon in settled particles collected in moored traps to calculate lithogenic (terrigenous) and marine biogenic fractions. On an annual basis, lithogenic material comprised 53% and 81% of dry weight and 48% and 66% of organic carbon in settled particles at the deep- (128 m) and shallow-water (41 m) locations, respectively. Peaks in runoff, inferred from rain and snowfall precipitation, ice cover and possible horizontal transport of resuspended sediment coincided with maximum sedimentation rates of lithogenic material during unstratified periods between October and March. Biological factors correlated with phytoplankton and ice algal primary production and seasonal changes in zooplankton grazing inferred from phaeopigments controlled biogenic particle fluxes under stratified conditions between June and September and during winter. Year-to-year variations in deposition of lithogenic and marine biogenic material depended on seasonal differences in stratification, precipitation, freeze/melt conditions and the extent of ice cover.

The annual cycle of phytoplankton cell abundance is coherent across diverse ecosystems in the temperate North Atlantic Ocean. In Bedford Basin, on the Scotian Shelf and in the Labrador Sea, the numerical abundance of phytoplankton is low in spring and high in autumn, thus in phase with the temperature cycle. Temperature aligns abundance on a common basis, effectively adjusting apparent cell discrepancies in waters that are colder or warmer than the regional norm. As an example of holistic simplicity arising from underlying complexity, the variance in a community variable (total abundance) is explained by a single predictor (temperature) to the extent of 75% in the marginal seas. In the estuarine basin, weekly averages of phytoplankton and temperature computed from a 13 year time-series yield a predictive relationship with 91% explained variance. Temperature-directed assembly of individual phytoplankton cells to form communities is statistically robust, consistent with observed biomass changes, amenable to theoretical analysis, and a sentinel for long-term change. Since cell abundance is a community property in the same units for all marine microbes at any trophic level and at any phylogenetic position, it promises to integrate biological oceanography into general ecology and evolution.

Since the 1990s, phytoplankton biomass on the continental shelf of Nova Scotia and in the Labrador Sea has undergone sustained changes in the spring and fall, which are accompanied by changes in bacterioplankton that are dampened in amplitude but coherent in the direction of change. A reversal of trend in biomass change, so-called sign switching, occurs both in time and in space. Thus, whenever (spring or fall) and wherever (Scotian Shelf or Labrador Sea) phytoplankton increase or decrease, so also does bacterioplankton. This tandem sign switch indicates coupling of the trophic levels at a multiyear time scale and contributes to an ecological fingerprint of systemwide forcing.

The number of settled year-0 haddock on the eastern Scotian Shelf (ESS) in July 1999 was the highest seen since record keeping began in 1970. The average length indicated that most had hatched in late February, well before the historical peak spawning period. In the data record above average year-0 abundance leads to good recruitment, suggesting that early spawning incurs survivability advantages. Conditions may have been particularly favourable for haddock recruitment in 1999: (i) because an unusually large number of adults spawned early and (it) because the earliest larval stages encountered an unusually plentiful supply of food, since the spring bloom and Calanus finmarchicus reproduction peaked in late February, 2-4 weeks earlier than usual.

The global association between heterotrophic bacteria and phytoplankton in the open ocean was examined to identify the macroecological limits of bacterial abundance. A consolidated dataset was constructed using 13,973 paired measurements of bacterial abundance and chlorophyll concentration from 15 years of observation in various biogeochemical provinces covering the Atlantic, Pacific, Arctic and Antarctic Oceans as well as the Arabian Sea. The bivariate relationship between bacteria and chlorophyll is a filled polygon whose upper boundary undergoes a change in slope from positive to negative at about 1 mgChl m −3 , suggesting a transition from bottom-up to top-down control of bacteria along a resource supply gradient. The upper limit of bacterial abundance in the ocean is everywhere set by phytoplankton, but the limit is not realized in productive waters because of mortality losses. A carrying capacity of about 7 trillion bacteria for 1 mg of chlorophyll is indicated by empirical observations, consistent with theoretical considerations and predictions from food web modelling.

We collected mesozooplankton samples in the upper 100 m in spring or early summer each year between 1995 and 2000 along a section from Hamilton Bank (Labrador) to Cape Desolation (Greenland), and along additional sections in spring 1997 and early summer 1995. The North Atlantic waters of the central basin were characterised by the presence of the copepods Calanus finmarchicus , Euchaeta norvegica and Scolecithrocella minor and euphausiids. Calanus glacialis , Calanus hyperboreus and Pseudocalanus spp. were associated with the Arctic waters over the shelves. Amongst the other enumerated groups larvaceans were concentrated over the shelves and around the margins. Amphipods, pteropods and the copepods Oithona spp. and Oncaea spp. showed no definable relationships with water masses or bathymetry, while the diel migrant ostracods and chaetognaths were confined to deep water. Metrida longa , also a strong diel migrant, and Microcalanus spp., a mainly deep water species and possible diel migrant, were both sometimes quite abundant on the shelves as well as in the central basin, consistent with their likely Arctic origins. Analysis of community structure along the section across the Labrador Sea indicated that stations could be grouped into five different zones corresponding to: the Labrador Shelf; the Labrador Slope; the western and central Labrador Sea; the eastern Labrador Sea and Greenland Slope; and, the Greenland Shelf. The boundaries between zones varied spatially between years, but community composition was relatively consistent within a given zone and a given season (spring versus early summer). The relationship between community composition and water masses was not entirely straightforward. For example, Labrador Shelf water was generally confined to the shelf, but in spring 2000 when it also dominated the adjacent slope zone, the community in the Labrador Slope zone was similar to those found in other years. Conversely, in spring 1997, when Arctic organisms were unusually abundant in the Labrador Slope zone, there was no increased contribution of shelf water. In addition, North Atlantic organisms were often found on the shelves when no slope or central basin water was present. Although other organisms were sometimes very abundant, the mesozooplankton preserved dry weight biomass was dominated everywhere by the three species of Calanus , which together always accounted for ≥70%. One species, C. finmarchicus , comprised >60% of the total mesozooplankton biomass and >80% of the abundance of large copepods in spring and summer throughout the central Labrador Sea. In western and central regions of the central basin average C. finmarchicus biomass was ca 4 g dry weight m −2 and average abundance, ca 17 000 m −2 over both seasons. Highest levels ( ca 7 g dry weight m −2 , >100 000 m −2 ) occurred in the northern Labrador Sea in spring and in eastern and southwest regions in early summer. C. hyperboreus contributed ca 20% of the total mesozooplankton biomass in the central basin in spring and C. glacialis accounted for C. hyperboreus contributed a maximum of 54% and 3.6 g dry weight m −2 , and C. glacialis , a maximum of 29% and 1 g dry weight m −2 , to the total mesozooplankton biomass.

Populations of the copepod species Calanus finmarchicus often dominate the springtime biomass and secondary production of shelf ecosystems throughout the North Atlantic Ocean. Recently, it has been hypothesised that interannual to interdecadal fluctuations observed in such populations are driven primarily by climate-associated changes in ocean circulation. Here, we compare evidence from the North Sea and Gulf of Maine/Western Scotian Shelf (GoM/WSS) linking fluctuations in C. finmarchicus abundance to changes in ocean circulation associated with the North Atlantic Oscillation (NAO). A particularly striking contrast emerges from this Trans-Atlantic comparison: whereas the North Sea C. finmarchicus population exhibits a negative correlation with the NAO index, the GoM/WSS population exhibits a more complex, positive association with the index. The physical processes underlying these contrasting population responses are discussed in the context of regional- to basin-scale circulation changes associated with the NAO.

Drinkwater, K. F., Hunt, G. L. Jr, Astthorsson, O. S., and Head, E. J. H. 2012. Comparative studies of climate effects on polar and subpolar ocean ecosystems, progress in observation and prediction: an introduction. – ICES Journal of Marine Science, 69: . Background to and content of this part-product of the second ESSAS (Ecosystem Studies of Sub-Arctic Seas) symposium is provided, along with a call for future work of such nature to be continued, expanded, and enhanced, specifically with a view to determining global variations in resources and dynamics attributable to climate change.

Here we present a new, pan-Atlantic compilation of data on key mesozooplankton species, including the possibly most important copepod, Calanus finmarchicus. Distributional data of ten representative zooplankton taxa, from recent (2000–2009) Continuous Plankton Recorder data, are presented, along with basin-scale data of the phytoplankton colour index. Then we present a compilation of data on C. finmarchicus including observations of abundance, demography, egg production and female size with accompanying data on temperature and chlorophyll. This is a contribution by Canadian, European and US scientists and their institutions. http://doi.pangaea.de/10.1594/PANGAEA.820732, http://doi.pangaea.de/10.1594/PANGAEA.824423, http://doi.pangaea.de/10.1594/PANGAEA.828393.

The fluorescence excitation spectrum is sometimes used as a proxy for the action spectrum of photosynthesis in phytoplankton. The main assumption behind this approximation is that the shapes of absorption and fluorescence excitation spectra are similar except for the absorption by photoprotective pigments, which do not contribute to the fluorescence spectrum. In this study, we compare the shapes of the absorption and fluorescence spectra in three species of phytoplankton grown at different irradiances: two diatoms (Thalassiosira weissflogii and Chaetoceros sp.) and a cyanophyte (Synechococcus sp.). The contribution to absorption by photoprotective pigments was estimated for each experiment. Results showed that the differences between the shapes of absorption and fluorescence spectra were similar to the estimated absorption by photoprotective pigments only in the case of T. weissflogii. In Synechococcus sp., and to a lesser degree in Chaetoceros sp., the differences between the two types of spectra were larger than the absorption by photoprotective pigments. In the case of Synechococcus sp., the difference between these spectra was apparently due mainly to the extreme imbalance of chlorophyll a distribution between the two photosystems. Chaetoceros sp. seemed to be an intermediate case: a small part of the chlorophyll a of the cell appeared to be exclusively associated with photosystem I and therefore did not contribute to fluorescence. Fluorescence and absorption values were normalized to their values at 545 nm, and the ratio of normalized absorption to normalized fluorescence was computed for the blue (439 nm) and red (676 nm) peaks in the spectra. The results showed that these peak ratios can be used to distinguish between the effects of photoprotective pigments and the arrangement of the photosynthetic apparatus on differences between fluorescence and absorption spectra.

The life cycle of Calanus finmarchicus on and around Western Bank, 1991–1992, was essentially annual; the overwintered generation (G0) produced G1 that developed at temperature-dependent rates and then largely disappeared after June to winter as late copepodids at depth. However, a small fraction of G1 matured to spawn a less rapidly developing G2 that outnumbered G1 in the depths of Emerald Basin in autumn 1990. Estimated mortality on the central Scotian Shelf for G1 from egg to copepodid stage 5 in June was ~4%·day–1 but subsequently for G1 and G2 was nearly constant at ~1%·day–1. Populations on Western Bank mostly derived from reproduction by overwintered G2 on the Scotian Shelf. Larger populations in Emerald Basin in 1987–1988 were augmented from advected animals enabled to diapause there at depth and on Emerald Bank by inputs from beyond the shelf break, ultimately from farther north. Our observations and analyses match the physical circulation in the region.

Three trans-Atlantic oceanographic surveys (Nova Scotia to Canary Islands) were carried out during fall 1992 and spring 1993 to describe the large-scale variability in hydrographic, chemical and biological properties of the upper water column of the subtropical gyre and adjacent waters. Significant spatial and temporal variability characterized a number of the biological pools and rate processes whereas others were relatively invariant. Systematic patterns were observed in the zonal distribution of some properties. Most notable were increases (eastward) in mixed-layer temperature and salinity, depths of the nitracline and chlorophyll- a maximum, regenerated production (NH 4 uptake) and bacterial production. Dissolved inorganic carbon (DIC) concentrations, phytoplankton biomass, mesozooplankton biomass and new production (NO 3 uptake) decreased (eastward). Bacterial biomass, primary production, and community respiration exhibited no discernible zonal distribution patterns. Seasonal variability was most evident in hydrography (cooler/fresher mixed-layer in spring), and chemistry (mixed-layer DIC concentration higher and nitracline shallower in spring) although primary production and bacterial production were significantly higher in spring than in fall. In general, seasonal variability was greater in the west than in the east; seasonality in most properties was absent west of Canary Islands (∼20°W). The distribution of autotrophs could be reasonably well explained by hydrography and nutrient structure, independent of location or season. Processes underlying the distribution of the microheterophs, however, were less clear. Heterotrophic biomass and metabolism was less variable than autotrophs and appeared to dominate the upper ocean carbon balance of the subtropical North Atlantic in both fall and spring. Geographical patterns in distribution are considered in the light of recent efforts to partition the ocean into distinct “biogeochemical provinces”.

Campbell, R. W. and Head, E. J. H. 2000. Viability of Calanus finmarchicus eggs in situ: does the presence of intact phytoplankton reduce hatching success? – ICES Journal of Marine Science, 57: 1780–1785. In order to test whether or not the presence of intact diatoms (and other phytoplankton) inhibits hatching of copepod eggs, hatching success was measured for Calanus finmarchicus eggs collected from freshly captured females incubated in filtered seawater (FSW), in natural phytoplankton mixtures (collected at the depth of the chlorophyll maximum), and in a culture of Thalassiosira weissflogii. When compared with eggs incubated in FSW, hatching success was generally the same in the natural phytoplankton mixtures, but lower for eggs incubated in the T. weissflogii culture. Hatching success was also uncorrelated with the abundance of phytoplankton available in situ. The results suggest that hatching success is generally high for eggs produced by female C. finmarchicus feeding during spring bloom conditions in the Northwest Atlantic, and also that it is not generally reduced by toxins exuded by natural phytoplankton assemblages in situ. 2000 International Council for the Exploration of the Sea

Natural 15 N / 14 N ( δ 15 N) was determined in suspended particulate matter (SPM) and plankton tow samples (20- and 233-μm mesh nets) during extensive surveys along E–W transects from Halifax, Nova Scotia, to the Canary Islands in the fall of 1992 and the spring of 1993. This is the first study reporting δ 15 N measurements along with NO − 3 and NH + 4 uptake rates in the North Atlantic. During both cruises, the δ 15 N of SPM and net-plankton was lower in oligotrophic waters compared with values in eutrophic areas. The lack of correlation between the euphotic zone integrated δ 15 N SPM and NH + 4 concentration and NH + 4 uptake rate at both times suggested that the contrast between regions could not be explained by regenerated production, even though NH + 4 was the preferred N substrate in most regions. In fact, based on a correlation analysis, this study suggested that isotope fractionation during nitrate uptake and the residence time of N in SPM ( τ ) were the two major factors controlling the δ 15 N SPM in 1992. During the fall of 1992, the δ 15 N SPM of the oligotrophic region ( τ of 100–170 d) was consistent with the accumulated product equation in closed systems and a δ 15 N of new nitrate of 4–5‰. In contrast, in eutrophic regions ( τ of 7–40 d), the δ 15 N SPM was consistent with an open system equation. Isotope fractionation ( e ) for growth on nitrate was estimated to be 2.4‰ in the upwelling off Morocco. In 1993, no correlations were observed between δ 15 N SPM and nitrate dynamics, probably because of horizontal advection. Based on the low τ of 5–30 d, an open system equation was used with nitrate as the reactant, and it suggested variations of 2–3‰ in either the δ 15 N NO3 or e .

Egg production rates of Calanus finmarchicus were measured in the Labrador Sea (spring) and on the Scotian Shelf (spring, summer, and autumn), in conjunction with indices of gonad maturity and food availability, female body size, surface layer temperature, and surface nitrate concentrations. Egg production rates were often two to four times higher than any previously reported at similar temperatures. Estimates of egg production rates of populations were most closely related to the proportion of reproductively mature females (as measured by a gonad stage index) and female size. Specifically, the gonad stage index (proportion of females capable of spawning) was correlated with the proportion of females that spawned during 24-h incubations, and female size determined the clutch size (i.e., number of eggs produced). Although there were detectable effects of food and temperature on egg production rates, they were generally weaker than the effect of female gonad maturity. Thus, over the broad spatial and temporal scales of this study, the reproductive (i.e., life history) status of a population was more important in determining its egg production rate than direct external factors (i.e., temperature and food conditions).

During mid-May-early June 1997 observations of hydrography, phytoplankton and nitrate concentrations, and abundance and stage distribution of Calanus finmarchicus populations were made in the Labrador Sea and south of Greenland. Egg production rates were also measured for isolated C. finmarchicus females. Surface nitrate and integrated phytoplankton concentrations indicated that, in the deep water, the phytoplankton bloom had ended in the north and east, was in progress in the north central Labrador Sea and near the basin margins, and had not yet become established in an area stretching from the central Labrador Sea to the south of Greenland. C. finmarchicus egg production rates and stage distributions at stations in the 3 areas designated as early, mid- and late/post-bloom zones, suggested that development rates of the overwintered GO generation into mature adults (females and males) were probably low before the bloom, but accelerated during its development. Individual and areal rates of egg production were highest in the early bloom zone, whereas nauplii were more abundant in the bloom and late/post-bloom zones. Differences in naupliar abundance may have been related to food limitation, or predation. Following development through to the young copepodite stages (CI-III), which were most abundant in the late/post-bloom zone, morality rates were apparently lower and growth rates less dependent on high phytoplankton concentrations and perhaps more dependent on temperature. In the Labrador Sea, where the annual growth season is relatively short and C. finmarchicus produces only 1 generation per year, the timing of the spring bloom may have a significant impact on recruitment of the new year's generation. In areas where the bloom is early and intense, maturation of the overwintered adults will be rapid and egg-laying will occur when phytoplankton concentrations are high. Subsequent survival success of eggs through to later stages will also probably be relatively high and individuals from the new year's generation will have ample time to reach stages capable of overwintering. By contrast, if the bloom is late or of low intensity, adult maturation will be delayed and egg-laying may occur when phytoplankton concentrations are low. Under these conditions relatively few eggs may survive and individuals that do survive will have a shorter period in which to attain stages which can overwinter.

During the spring of 1996, phytoplankton samples were collected along a transect from South Wolf Island (Labrador) to Cape Desolation (Greenland). Dense blooms of diatoms were found over the shelf near the coast of Labrador, whereas high concentrations of the colony-forming prymnesiophyte Phaeocystis pouchetii were found close to Greenland. Phytoplankton samples were separated into 2 major groups (diatoms or prymnesiophytes) on the basis of chlorophyll (chl) chl c 3 /chl a ratios (determined by HPLC analysis), and the effects of species composition on the absorption and photosynthetic characteristics of these 2 high-latitude phytoplankton populations were studied. At all pigment concentrations and all wavelengths examined (apart from 623 nm), the diatom population had a much lower absorption coefficient than the prymnesiophyte population; this was attributed to an increased pigment-packaging effect in the larger diatom cells. Varying proportions of photo-protective pigments also influenced the absorption characteristics of these populations. The low specific-absorption coefficient of the diatom population resulted in a higher maximum photosynthetic quantum yield relative to that of the prymnesiophyte population. The initial slope of the photosynthesis-irradiance (P-E) curve (α B ) also appeared to be taxon-specific, with higher α B values being recorded for the smaller prymnesiophytes than for the larger diatom cells. The implications of species-dependent variations in phytoplankton absorption coefficients for the retrieval of remotely-sensed chl a are discussed.

Observations of hydrography and Calanus spp. abundance were made at stations on lines across the central and eastern Scotian Shelf (SS) in April 1995 and 1997 and on additional lines on the western SS and across Cabot Strait (CS) and the Laurentian Channel (LC) in 1997. Calanus finmarchicus was more abundant on the central and western SS than in the east, and Calanus hyperboreus was concentrated in the east and in areas influenced by the Nova Scotia Current. Calanus spp. on the eastern SS probably originated mainly from populations overwintering in the Gulf of St. Lawrence. High abundances of C. finmarchicus occurred at or beyond the shelf break when warm slope water was present, suggesting that they derived from a population overwintering in the deep water south of the SS. In both years, slope water was present in the near-surface layers at shelf stations of the Halifax Line (HL), but in 1997 the degree of intrusion was greater. Calanus finmarchicus abundances were also substantially higher at shelf stations of the HL in 1997, implying contribution from the offshore population to the populations in central and western SS regions. Consistent with the suggested difference in their origins, C. finmarchicus on the western SS belonged primarily to the new year's generation, whereas those on the eastern SS belonged mainly to that of the previous year.

The biomass, species and chemical composition of the mesozooplankton and their impact on lower food levels were estimated along a transect across the Arctic Ocean. Mesozooplankton biomass in the upper 200 m of the water column was signi"cantly higher (19}42 mg DW m~3) than has previously been reported for the Arctic Ocean, and it reached a maximum at ca. 873N in the Amundsen Basin. The lowest values were recorded in the Chukchi Sea and Nansen Basin, where ice cover was lower (50}80%) than in the central Arctic Ocean. In the deeper strata (200}500 m) of the Canadian and Eurasian Basins, the biomass was always much lower (4.35}16.44 mg DW m~3). The C/N (g/g) ratio for the mesozooplankton population was high (6.5}8.5) but within the documented range. These high values (when compared to 4.5 at lower latitudes) may be explained by the high lipid content. Mesozooplankton accounted for approximately 40% of the total particulate organic carbon in the upper 100 m of the water column. Mesozooplankton species composition was homogeneous along the transect, consisting mainly of copepods (70}90% of the total number). It was dominated by four large copepod species (Calanus hyperboreus, C. glacialis, C. ,nmarchicus and Metridia longa), which together accounted for more than 80% of the total biomass. According to measurements of gut pigment and gut turnover rates, the mesozooplankton on average ingested between 6 and 30% of their body carbon per day as phytoplankton. Microzooplankton may have provided an additional source of energy for the mesozooplankton community. These data emphasize the importance of mesozooplankton in the arctic food web and reinforce the idea that the Arctic Ocean should no longer be considered to be a &&biological desert’’. ( 1999 Elsevier Science Ltd. All rights reserved.

A carbon flux study was carried out off the coast of Morocco, at 31°N, in a region characterized by the presence of a persistent cyclonic eddy. Two short-term (4 and 3 day) deployments of free-floating sediment traps were combined with water column sampling and rate process measurements as the ship followed the traps. For a period of 36 h between trap deployments, a hydrographic section was run along 31°30'N as part of a larger scale survey being carried out simultaneously on the R.V. A. von Humboldt. The first trap deployment was near the eastern margin of the eddy and the traps moved to the north and west in a frontal jet associated with its northern boundary. After the second deployment, which was at the recovery point of the first, the traps moved to the west and then to the southwest. Throughout the study, chlorophyll concentrations varied between 27 and 125 mg m−2 (0–100 m), with highest concentrations in the upwelled water nearest the coast and in upwelled water generated within the cyclonic eddy. Particulate organic carbon (POC) and particulate organic nitrogen (PON) concentrations were relatively uniform (13.6±1.8 and 1.63±28 g m−2 with phytoplankton carbon accounting for 16–85% of total POC. Bacterial carbon was ∼ 5% of total POC and mesozooplankton carbon concentrations were equivalent to ∼9% of total POC. Microzooplankton biomass was not assessed but POC:PON ratios in the water column were often high, suggesting there was sometimes a large detrital component in the POC. Primary production rates varied between 1.0 and 2.5 g C m−2 day−1. Bacterial consumption accounted for ∼50% of primary production. Metabolic rates suggested that copepods were ingesting more than 0.4 g C m−2 day−1. while filtration rates suggested that ingestion of phytoplankton carbon was only ∼0.2 g C m−2day−1, even when phytoplankton constituted ∼85% of the POC. f-ratios (based on uptake rates for 15N-nitrate and ammonia) were between 0.1 and 0.4, and excretion by mesozooplankton could account for ∼ 40% of the daily ammonium uptake by phytoplankton. HPLC pigment analysis showed that when chlorophyll biomass was high, diatoms were dominant, whereas when it was low, small prymnesiophytes, chlorophytes and diatoms were all important. The composition of the fluoresecent pigments in material in the sediment traps indicated that intact phytoplankton and copepod faecal pellets were the main sources but the relative rates of sedimentation of pigment, POC and PON for the two trapping periods did not reflect differences that were observed in the overlying water column. This was likely to be the result of spatial heterogeneity and strong horizontal currents heterogeneity and strong horizontal currents within the euphotic zone. Thus, material collected at 100 m probably did not originate in the water column immediately overlying the traps and trapping efficiencies might also have been variable.

During recent decades, historically unprecedented changes have been observed in the Arctic as climate warming has increased precipitation, river discharge, and glacial as well as sea-ice melting. Additionally, shifts in the Arctic's atmospheric pressure field have altered surface winds, ocean circulation, and freshwater storage in the Beaufort Gyre. These processes have resulted in variable patterns of freshwater export from the Arctic Ocean, including the emergence of great salinity anomalies propagating throughout the North Atlantic. Here, we link these variable patterns of freshwater export from the Arctic Ocean to the regime shifts observed in Northwest Atlantic shelf ecosystems. Specifically, we hypothesize that the corresponding salinity anomalies, both negative and positive, alter the timing and extent of water-column stratification, thereby impacting the production and seasonal cycles of phytoplankton, zooplankton, and higher-trophic-level consumers. Should this hypothesis hold up to critical evaluation, it has the potential to fundamentally alter our current understanding of the processes forcing the dynamics of Northwest Atlantic shelf ecosystems.

Arising from D. G. Boyce, M. R. Lewis & B. Worm , 591–596 (2010)10.1038/nature09268 ; Boyce et al. reply Phytoplankton account for approximately 50% of global primary production, form the trophic base of nearly all marine ecosystems, are fundamental in trophic energy transfer and have key roles in climate regulation, carbon sequestration and oxygen production. Boyce et al.1 compiled a chlorophyll index by combining in situ chlorophyll and Secchi disk depth measurements that spanned a more than 100-year time period and showed a decrease in marine phytoplankton biomass of approximately 1% of the global median per year over the past century. Eight decades of data on phytoplankton biomass collected in the North Atlantic by the Continuous Plankton Recorder (CPR) survey2, however, show an increase in an index of chlorophyll (Phytoplankton Colour Index) in both the Northeast and Northwest Atlantic basins3,4,5,6,7 (Fig. 1), and other long-term time series, including the Hawaii Ocean Time-series (HOT)8, the Bermuda Atlantic Time Series (BATS)8 and the California Cooperative Oceanic Fisheries Investigations (CalCOFI)9 also indicate increased phytoplankton biomass over the last 20–50 years. These findings, which were not discussed by Boyce et al.1, are not in accordance with their conclusions and illustrate the importance of using consistent observations when estimating long-term trends.

Particulate samples from the water column, to depths of 80–120 m, were collected over a 10-day period during a bloom in the North Atlantic, near 37°N, 40°W, between 12 and 22 April 1990. These samples were analysed by HPLC for their chlorin and carotenoid content. The same analysis was carried out for samples of faecal pellets collected from copepods from depths of 0–100 m, which had fed in situ, and for material from sediment traps in the 100–500 m depth range. Water column pigment compositions were consistent with observations that diatoms and Phaeocystis were major components of the bloom. They also sometimes showed high concentrations of chlorophyllide a and a phaephorbide a-like pigment, which may be due to algal die-off and cell autolysis. Faecal pellets contained high concentrations of pyrophaeophorbide a lower amounts of unidentified phaeophorbides and some chlorophyll a, but none of the phaeophorbide a-like pigment found in the water column. Sediment traps contained high concentrations of the phaeophorbides characteristics of copepod faecal pellets and lower, approximately equal amounts of the water column phaeophorbide a-like pigment and chlorophyll a. The sampling area appeared to be in a jet of current, which was moving southwesterly along a front between Eastern Basin Water (to the north) and North Atlantic Central Water (to the south) and which seemed to be drawing in water from both these water masses. Areas of downwelling, thought to be associated with these convergent currents, may have accelerated the sedimentation of dead and dying phytoplankton cells.

An investigation of the spring bloom was carried out in the western North Atlantic (40–50°W) as one component of the multi-nation Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment (NABE). The cruise track included an extended hydrographic section from 32 to 47°N and process studies at two week-long time-series stations at 40 and 45°N. Biological and chemical data collected along the transect indicated that the time-series stations were located in regions where the spring bloom was well developed; algal biomass was high and surface nutrient concentrations were reduced from maximum wintertime levels. Despite similarities in the vertical structure and magnitude of phytoplankton biomass and productivity, the two stations clearly differed in physical, chemical and other biological characteristics. Detailed depth profiles of the major autotrophic and heterotrophic microplankton groups (bacteria, phytoplankton, microzooplankton) revealed a strong vertical coherence in distribution at both sites, with maximum concentrations in the upper 50 m being typical of the spring bloom. Ultraplankton (< 10 μm) were an important component of the primary producers at 40°N, whereas larger netplankton (diatoms, dinoflagellates) were more important at 45°N. Silicate depletion was clearly evident in surface waters at 45°N, where diatoms were most abundant. Despite the relative importance of diatoms at 45°N, dinoflagellates dominated the biomass of the netplankton at both sites; however, much of this community may have been heterotrophic. Bacterial biomass and production were high at both stations relative to phytoplankton levels, particularly at 45°N, and may have contributed to the unexpectedly high residual ammonium concentrations observed below the chlorophyll maximum layer at both stations. Microzooplankton grazing dominated phytoplankton losses at both stations, with consumption as high as 88% of the daily primary production. Grazing losses to the mesozooplankton, on the other hand, were small (