Brian P. V. Hunt

Current state and trends in Canadian Arctic marine ecosystems: II. Heterotrophic food web, pelagic-benthic coupling, and biodiversity

As part of the Canadian contribution to the International Polar Year (IPY), several major international research programs have focused on offshore arctic marine ecosystems. The general goal of these projects was to improve our understanding of how the response of arctic marine ecosystems to climate warming will alter food web structure and ecosystem services provided to Northerners. At least four key findings from these projects relating to arctic heterotrophic food web, pelagic-benthic coupling and biodiversity have emerged: (1) Contrary to a long-standing paradigm of dormant ecosystems during the long arctic winter, major food web components showed relatively high level of winter activity, well before the spring release of ice algae and subsequent phytoplankton bloom. Such phenological plasticity among key secondary producers like zooplankton may thus narrow the risks of extreme mismatch between primary production and secondary production in an increasingly variable arctic environment. (2) Tight pelagic-benthic coupling and consequent recycling of nutrients at the seafloor characterize specific regions of the Canadian Arctic, such as the North Water polynya and Lancaster Sound. The latter constitute hot spots of benthic ecosystem functioning compared to regions where zooplankton-mediated processes weaken the pelagic-benthic coupling. (3) In contrast with another widely shared assumption of lower biodiversity, arctic marine biodiversity is comparable to that reported off Atlantic and Pacific coasts of Canada, albeit threatened by the potential colonization of subarctic species. (4) The rapid decrease of summer sea-ice cover allows increasing numbers of killer whales to use the Canadian High Arctic as a hunting ground. The stronger presence of this species, bound to become a new apex predator of arctic seas, will likely affect populations of endemic arctic marine mammals such as the narwhal, bowhead, and beluga whales.

Poles Apart: The “Bipolar” Pteropod Species Limacina helicina Is Genetically Distinct Between the Arctic and Antarctic Oceans

The shelled pteropod (sea butterfly) Limacina helicina is currently recognised as a species complex comprising two sub-species and at least five “forma”. However, at the species level it is considered to be bipolar, occurring in both the Arctic and Antarctic oceans. Due to its aragonite shell and polar distribution L. helicina is particularly vulnerable to ocean acidification. As a key indicator of the acidification process, and a major component of polar ecosystems, L. helicina has become a focus for acidification research. New observations that taxonomic groups may respond quite differently to acidification prompted us to reassess the taxonomic status of this important species. We found a 33.56% (±0.09) difference in cytochrome c oxidase subunit I (COI) gene sequences between L. helicina collected from the Arctic and Antarctic oceans. This degree of separation is sufficient for ordinal level taxonomic separation in other organisms and provides strong evidence for the Arctic and Antarctic populations of L. helicina differing at least at the species level. Recent research has highlighted substantial physiological differences between the poles for another supposedly bipolar pteropod species, Clione limacina. Given the large genetic divergence between Arctic and Antarctic L. helicina populations shown here, similarly large physiological differences may exist between the poles for the L. helicina species group. Therefore, in addition to indicating that L. helicina is in fact not bipolar, our study demonstrates the need for acidification research to take into account the possibility that the L. helicina species group may not respond in the same way to ocean acidification in Arctic and Antarctic ecosystems.

Biodiversity and Biogeography of the Lower Trophic Taxa of the Pacific Arctic Region: Sensitivities to Climate Change

The lower trophic level taxa underpin the marine ecosystems of the Pacific Arctic Region (PAR). Recent field observations indicate that range shifts, and changes in the relative abundance of particular taxa have occurred within the last decade. Here we provide a region wide survey of the diversity and distribution of viruses, bacteria, archaea, auto- and heterotrophic protists, as well as metazoan zooplankton and benthic organisms in the PAR. Our aim is to provide a foundation for the assessment of the changes within the lower trophic level taxa of the PAR and to document such change when possible. Sensitivities to the effects of climate change are also discussed. Our vision is to enable data-based predictions regarding ecological succession in the PAR under current climate scenarios, and to deepen our understanding regarding what the future holds for higher trophic level organisms and the carbon cycle.

Zooplankton community structure and dynamics in the Arctic Canada Basin during a period of intense environmental change (2004–2009)

Mesozooplankton were sampled in the Canada Basin in the summers of 2004, 2006, 2007, 2008, and fall 2009. Six taxa (Calanus hyperboreus, Calanus glacialis, Oithona similis, Limacina helicina, Microcalanus pygmaeus, and Pseudocalanus spp.) accounted for 77–91% of the abundance in all years, and 70–80% of biomass in 2004–2008. The biomass of C. hyperboreus and C. glacialis was reduced in 2009, likely due to seasonal migration below the sampling depth. Mean abundance was consistent across surveys while biomass increased from 18.92 to 32.56 mg dry weight m−3 between 2004 and 2008. Multivariate analysis identified a clear separation between shelf and deep basin (>1000 m) assemblages. Within the deep basin abundance and biomass were higher in the west, associated with a higher chlorophyll maximum. In 2007 and 2008 considerable heterogeneity developed in the assemblage structure, associated with variability in the contribution of the short-lived (<1 year) copepod species O. similis and M. pygmaeus. Conversely, the long lived (≥2 years) C. hyperboreus and C. glacialis showed an increasingly consistent spatial distribution of high biomass from 2004 to 2008. We propose that a greater dependence on autochthonous basin production by the short-lived species resulted in their decreased secondary production in the freshening Beaufort Gyre in 2007 and 2008. Conversely, long-lived species were supported by high allochthonous production on the Beaufort and Chukchi shelves and lipid stores accumulated from this source enabled them to persist in the low chlorophyll a biomass conditions of the Canada Basin.

Predation impact of carnivorous macrozooplankton in the vicinity of the Prince Edward Island archipelago (Southern Ocean) in austral autumn 1998

The composition, biomass, feeding and predation impact of carnivorous macrozooplankton (>2cm) on mesozooplankton (0.2-2cm) in the sub-Antarctic waters of the southwest Indian Ocean were investigated at 19 stations in austral autumn (April/May) 1998. Zooplankton abundances and biomass ranged from 6.9 to 95.2 ind m -3 and between 1.8 and 18.1 mg Dwt m -3 , respectively. Throughout the investigation, mesozooplankton comprising mainly copepods numerically and by biomass dominated net samples. Among the copepods, Calanus simillimus, Clausocalanus brevipes, Ctenocalanus vanus, and Oithona spp. dominated. The carnivore component of the macrozooplankton consisted mainly of five groups: decapods, amphipods, chaetognaths, euphausiids and gelatinous zooplankton. Among these, chaetognaths (Eukrohnia hamata and Sagitta gazellae) and euphausiids (Nematoscelis megalops and Euphausia longirostris) were the most prominent. Collectively, the carnivorous macrozooplankton comprised between 11% and 72% of total zooplankton biomass. Total predation impact of the carnivorous macrozooplankton varied considerably but generally accounted for <5% (range 0.7-44%) of the total mesozooplankton standing stock. Tentative calculations suggest that carnivorous macrozooplankton may contribute, via vertical migrations and production of fast sinking faecal pellets, to a downward flux of carbon equivalent to up to 9% of the total mesozooplankton stock per day within the Antarctic Polar Frontal Zone. As a consequence, carnivorous macrozooplankton may increase the localised efficiency of the biological pump.

Determination of particulate organic carbon sources to the surface mixed layer of the Canada Basin, Arctic Ocean

Stable isotope ratios of particulate organic carbon (POC), together with other tracers, were analyzed in samples from the Canada Basin surface mixed layer in 2008 and 2009. Sampling was conducted during the end of the 2008 melt season and at the beginning of the 2009 freezeup under a variety of surface conditions, including open water, newly formed seasonal ice, and multiyear ice. In both years, POC exhibited a wide isotopic range (δ13C-POC −24.5 to −31.1‰), with the most isotopically depleted material generally found in the central basin. Isotopically enriched material was found on the shelves, consistent with higher biological production and strongly correlated with in situ carbon-uptake rates. In contrast, offshore in the central basin, there was no significant relationship between δ13C-POC distributions and either chlorophyll a or aqueous CO2 concentrations, suggesting that in situ biological production was not the dominant control. Analysis of freshwater sources suggested that the sea ice melt contribution of POC to surface waters in the central Canada Basin exerted a negligible influence on δ13C-POC distributions, and instead isotopically depleted POC in the surface waters of the central Canada Basin were sourced externally through advective transport of riverine organic matter. We show that alkalinity and meteoric water content can be used to distinguish POC inputs from North American and Russian rivers and our analysis suggests that Russian river inputs are the predominant source of 13C-depleted organic matter to the mixed layer of the central Canada Basin.

The seasonal development of the zooplankton community in a British Columbia (Canada) fjord during two years with different spring bloom timing

Abstract Variability in phytoplankton and zooplankton dynamics were studied during spring and early summer of 2006 and 2007 in Rivers Inlet, a fjord on the central coast of British Columbia, Canada. The onset of the spring bloom was shifted by 3 weeks in 2007 (26 April) compared to 2006 (7 April). The later 2007 bloom was accompanied by a later and lower zooplankton biomass peak and a different zooplankton composition. Mean spring–summer total zooplankton biomass was 57 and 27 mg m−3 in 2006 and 2007, respectively. This was explained by reduced juvenile densities and changes in dominance of most abundant zooplankton taxa in the fjord. Between 2006 and 2007 mean densities of copepod nauplii and calanoid copepodites decreased from 224.3 to 95.8 and from 197.7 to 35.9 ind. m−3, respectively. Adult Calanoidae and Metridinidae decreased in abundance while densities of Acartiidae and Ectinosomatidae increased. We postulate that the later bloom delayed the seasonal increase in zooplankton biomass by inhibiting the survival of early developing stages of certain zooplankton species through a mismatch between their appearance and the spring bloom. This study provides the first baseline observation on the temporal scale of zooplankton phenological changes in the region.

The winter pack-ice zone provides a sheltered but food-poor habitat for larval Antarctic krill

A dominant Antarctic ecological paradigm suggests that winter sea ice is generally the main feeding ground for krill larvae. Observations from our winter cruise to the southwest Atlantic sector of the Southern Ocean contradict this view and present the first evidence that the pack-ice zone is a food-poor habitat for larval development. In contrast, the more open marginal ice zone provides a more favourable food environment for high larval krill growth rates. We found that complex under-ice habitats are, however, vital for larval krill when water column productivity is limited by light, by providing structures that offer protection from predators and to collect organic material released from the ice. The larvae feed on this sparse ice-associated food during the day. After sunset, they migrate into the water below the ice (upper 20 m) and drift away from the ice areas where they have previously fed. Model analyses indicate that this behaviour increases both food uptake in a patchy food environment and the likelihood of overwinter transport to areas where feeding conditions are more favourable in spring.Winter sea ice is thought to provide critical grazing habitat for overwintering Antarctic krill. In contrast, here the authors show that the pack-ice zone is a food-poor habitat, but does serve as an important sheltering ground for developing larvae.

Nutrients and Other Environmental Factors Influence Virus Abundances across Oxic and Hypoxic Marine Environments

Virus particles are highly abundant in seawater and, on average, outnumber microbial cells approximately 10-fold at the surface and 16-fold in deeper waters; yet, this relationship varies across environments. Here, we examine the influence of a suite of environmental variables, including nutrient concentrations, salinity and temperature, on the relationship between the abundances of viruses and prokaryotes over a broad range of spatial and temporal scales, including along a track from the Northwest Atlantic to the Northeast Pacific via the Arctic Ocean, and in the coastal waters of British Columbia, Canada. Models of varying complexity were tested and compared for best fit with the Akaike Information Criterion, and revealed that nitrogen and phosphorus concentrations, as well as prokaryote abundances, either individually or combined, had significant effects on viral abundances in all but hypoxic environments, which were only explained by a combination of physical and chemical factors. Nonetheless, multivariate models of environmental variables showed high explanatory power, matching or surpassing that of prokaryote abundance alone. Incorporating both environmental variables and prokaryote abundances into multivariate models significantly improved the explanatory power of the models, except in hypoxic environments. These findings demonstrate that environmental factors could be as important as, or even more important than, prokaryote abundance in describing viral abundance across wide-ranging marine environments.

Spatial and temporal variability of zooplankton off New Caledonia (Southwestern Pacific) from acoustics and net measurements

Spatial and temporal distribution of zooplankton off New Caledonia in the eastern Coral Sea was studied during two multidisciplinary cruises in 2011, during the cool and the hot seasons. Acoustic measurements of zooplankton were made using a shipborne acoustic Doppler current profiler (S-ADCP), a scientific echosounder and a Tracor acoustic profiling system (TAPS). Relative backscatter from ADCP was converted to biomass estimates using zooplankton weights from net-samples collected during the cruises. Zooplankton biomass was estimated using four methods: weighing, digital imaging (ZooScan), ADCP and TAPS. Significant correlations were found between the different biomass estimators and between the back-scatters of the ADCP and the echosounder. There was a consistent diel pattern in ADCP derived biomass and echosounder backscatter resulting from the diel vertical migration (DVM) of zooplankton. Higher DVM amplitudes were associated with higher abundance of small zooplankton and cold waters to the south of the study area, while lower DVM amplitudes in the north were associated with warmer waters and higher abundance of large organisms. Zooplankton was largely dominated by copepods (71–73%) among which calanoids prevailed (40–42%), with Paracalanus spp. as the dominant species (16–17%). Overall, zooplank-ton exhibited low abundance and biomass (mean night dry biomass of 4.7 6 2.2 mg m 3 during the cool season and 2.4 6 0.4 mg m 3 during the hot season) but high richness and diversity (Shannon index ∼4). Substantially enhanced biomass and abundance appeared to be episodically associated with mesoscale features contributing to shape a rather patchy zooplankton distribution.