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Pan-Arctic patterns of planktonic heterotrophic microbial abundance and processes: Controlling factors and potential impacts of warming
... Sarmento et al. (2010) compiled data of prokaryotic production and grazing rates from temperate systems to Antarctic waters, and have shown a higher sensitivity to temperature for prokaryotic production than for mortality rates. The same was found by Maranger et al. (2015), who included also the rate of lysed prokaryotes for the Arctic Ocean into the dataset. It is then important to elucidate the sensitivity of these processes in the Antarctic Ocean, in which this information is lacking, since they may have consequences in the fate of the microbial carbon in a warmer ocean. ...
... Temperature played an important role in the studied microbial processes (PHP, GZ and RLC) and PHP and RLC responded quicker to small changes than grazing rates (Figure 4). In contrast, Maranger et al. (2015) showed that PHP and GZ were more sensitive to temperature than viral activity in Arctic regions. This could be due to the fact that in the Arctic, during the summer season, the range of temperature is wider (−1.87 to 9.0, Maranger et al., 2015) than in Antarctic waters (−1.87 to 4.0, Danovaro et al., 2011). ...
... In contrast, Maranger et al. (2015) showed that PHP and GZ were more sensitive to temperature than viral activity in Arctic regions. This could be due to the fact that in the Arctic, during the summer season, the range of temperature is wider (−1.87 to 9.0, Maranger et al., 2015) than in Antarctic waters (−1.87 to 4.0, Danovaro et al., 2011). Thus, Arctic HFs were probably well adapted to this wide range, and responded quickly to small increases of temperature. ...
During the Austral summer 2009 we studied three areas surrounding the Antarctic
Peninsula: the Bellingshausen Sea, the Bransfield Strait and the Weddell Sea. We aimed
to investigate, whether viruses or protists were the main agents inducing prokaryotic
mortality rates, and the sensitivity to temperature of prokaryotic heterotrophic production
and mortality based on the activation energy (Ea) for each process. Seawater samples
were taken at seven depths (0.1–100 m) to quantify viruses, prokaryotes and protists
abundances, and heterotrophic prokaryotic production (PHP). Viral lytic production,
lysogeny, and mortality rates of prokaryotes due to viruses and protists were estimated
at surface (0.1–1 m) and at the Deep Fluorescence Maximum (DFM, 12–55 m) at eight
representative stations of the three areas. The average viral lytic production ranged from
1.0 ± 0.3 × 107 viruses ml−1 d−1 in the Bellingshausen Sea to1.3 ± 0.7 × 107 viruses
ml−1 d−1 in the Bransfield Strait, while lysogeny, when detectable, recorded the lowest
value in the Bellingshausen Sea (0.05 ± 0.05 × 107 viruses ml−1 d−1) and the highest
in the Weddell Sea (4.3 ± 3.5 × 107 viruses ml−1 d−1). Average mortality rates due to
viruses ranged from 9.7 ± 6.1 × 104 cells ml−1 d−1 in the Weddell Sea to 14.3 ± 4.0
× 104 cells ml−1 d−1 in the Bellingshausen Sea, and were higher than averaged grazing
rates in the Weddell Sea (5.9 ± 1.1 × 104 cells ml−1 d−1) and in the Bellingshausen
Sea (6.8 ± 0.9 × 104 cells ml−1 d−1). The highest impact on prokaryotes by viruses and
main differences between viral and protists activities were observed in surface samples:
17.8 ± 6.8 × 104 cells ml−1 d−1 and 6.5 ± 3.9 × 104 cells ml−1 d−1 in the Weddell
Sea; 22.1 ± 9.6 × 104 cells ml−1 d−1 and 11.6 ± 1.4 × 104 cells ml−1 d−1 in the
Bransfield Strait; and 16.1 ± 5.7 × 104 cells ml−1 d−1 and 7.9 ± 2.6 × 104 cells ml−1
d−1 in the Bellingshausen Sea, respectively. Furthermore, the rate of lysed cells and PHP
showed higher sensitivity to temperature than grazing rates by protists.We conclude that
viruses were more important mortality agents than protists mainly in surface waters and
that viral activity has a higher sensitivity to temperature than grazing rates. This suggests
a reduction of the carbon transferred through the microbial food-web that could have
implications in the biogeochemical cycles in a future warmer ocean scenario.
... Furthermore, it has also been assessed that the increases of bacterial production (BP) due to warming, triggers greater bacterial carbon transfer to higher trophic levels rather than the flux of dissolved organic carbon from bacteria lysed by viruses (Lara et al., 2013;Maranger et al., 2015). This could be due to the observed success of lysogeny respect to lysis in warming conditions (Lara et al., 2013). ...
... Furthermore, we also observed that bacterial mortality due to grazing was almost always higher than that caused by viral lysis, mainly at 6 • C (Figures 2B,C). These results agree with other studies both, in situ where Maranger et al. (2015) reported a stronger temperature-control on grazing rates compared to bacterial lysis rates in a pan-arctic study, as well as in warming controlled experiments (Lara et al., 2013). In contrast, Vaqué et al. (2017) showed that BP and mortality rates due to viruses were more sensitive to temperature than grazing activity in Antarctic regions. ...
Ocean acidification and warming are two main consequences of climate change that can directly affect biological and ecosystem processes in marine habitats. The Arctic Ocean is the region of the world experiencing climate change at the steepest rate compared with other latitudes. Since marine planktonic microorganisms play a key role in the biogeochemical cycles in the ocean it is crucial to simultaneously evaluate the effect of warming and increasing CO 2 on marine microbial communities. In 20 L experimental microcosms filled with water from a high-Arctic fjord (Svalbard), we examined changes in phototrophic and heterotrophic microbial abundances and processes [bacterial production (BP) and mortality], and viral activity (lytic and lysogenic) in relation to warming and elevated CO 2. The summer microbial plankton community living at 1.4 • C in situ temperature, was exposed to increased CO 2 concentrations (135-2,318 µatm) in three controlled temperature treatments (1, 6, and 10 • C) at the UNIS installations in Longyearbyen (Svalbard), in summer 2010. Results showed that chlorophyll a concentration decreased at increasing temperatures, while BP significantly increased with pCO 2 at 6 and 10 • C. Lytic viral production was not affected by changes in pCO 2 and temperature, while lysogeny increased significantly at increasing levels of pCO 2 , especially at 10 • C (R 2 = 0.858, p = 0.02). Moreover, protistan grazing rates showed a positive interaction between pCO 2 and temperature. The averaged percentage of bacteria grazed per day was higher (19.56 ± 2.77% d −1) than the averaged percentage of lysed bacteria by virus (7.18 ± 1.50% d −1) for all treatments. Furthermore, the relationship among microbial abundances and processes showed that BP was significantly related to phototrophic pico/nanoflagellate abundance in the 1 • C and the 6 • C treatments, and BP triggered viral activity, mainly lysogeny at 6 and 10 • C, while bacterial mortality rates was significantly related to bacterial abundances at 6 • C. Consequently, our experimental results suggested that future increases in water temperature and pCO 2 in Arctic waters will produce a decrease of phytoplankton biomass, enhancement of BP and changes in the carbon fluxes within the microbial food web. All these heterotrophic processes will contribute to weakening the CO 2 sink capacity of the Arctic plankton community.
... According to Maranger et al. (2015), next to temperature, bacterioplankton abundance and chlorophyll a concentration are also parameters affecting bacterial production in the Arctic. Among 720 samples collected throughout the Arctic, these parameters accounted for the variability of bacterial production in 57%. ...
Bacterial production and the accompanying environmental factors were measured in the water columns of two Arctic fjords during the cruise in July and August 2013. Water samples were collected at six stations located in the central part of Hornsund and Kongsfjorden. In Hornsund, where average water temperatures were 1.25-fold lower than in Kongsfjorden, the bacterial production was twice as high (0.116±0.102 vs 0.05±0.03mgCm⁻³h⁻¹). Statistical analysis indicated that chlorophyll a concentration itself was not a significant factor that affected bacterial production, in contrast to its decomposition product, pheophytin, originating from senescent algal cells or herbivorous activity of zooplankton. Single and multiple regression analysis revealed that water temperature, dissolved organic carbon (DOC), and pheophytin concentration were the main factors affecting bacterial production in both fjords.
... (1) differentiation of the protistan plankton structure into two separated domains at the 6 • C threshold (2) domination of Phaeocystis (colonial and flagellated cells) in the colder domain in two evaluated summers (3) higher abundance of heterotrophic protists in the warmer domain in both investigated years (4) the possible earlier shift of the communities into the post-bloom stage in the warmer 2016, indicated by the domination of flagellates in the early summer Given the recently demonstrated beneficial effects of warming on picoeukaryotes including Micromonas pusilla (Hoppe et al., 2018) and bacteria (Maranger et al., 2015), as well as our findings, we hypothesize that the further climate change of the European Arctic seas (especially the Atlantic domain) will favor the dominance of small, mobile protists. Restructuring of communities toward pico-and nanoplankton will likely lengthen the carbon pathway through the food web by increasing the significance of heterotrophic protists and, thus, will result in increased microbial loop activity. ...
The European Arctic is rapidly changing where increasing water temperatures and rapid loss of sea ice will likely influence the structure and functioning of the entire ecosystem. This study aimed to describe the taxonomic composition and spatial distribution of early summer (2015–2016) nano- and microplanktonic protists in the Nordic (Norwegian, Greenland) Seas and the Fram Strait (70.99°N to 78.84°N; 1.52°E to 19.90°E) and to determine the distribution patterns of the communities from the aspect of hydrography, as deduced from in situ measurements. Here we identify some generalized regularity in the protistan distribution, indicating the two separated domains at the 6°C threshold. While Phaeocystis seemed to be a fairly conservative representative of the colder area (<6°C), the taxonomic structure of the warmer waters (>6°C) may vary significantly between successive summers: from mostly Bacillariophyceae-dominated communities in 2015 to flagellate-dominated in 2016. Based on our results, we hypothesized that the more intense phototroph development in the area, as deduced from higher remotely sensed chlorophyll a concentrations in 2016, i.e., record warm year in the observational period, could lead to faster depletion of nutrients and, thus, an earlier shift into the post-bloom community stage. Taking into account the possible phenological shift toward early summer domination of flagellates in a warmer year, as well as a higher number of heterotrophic protists associated with the warmer domain in two evaluated summers, it is highly likely that climatic warming of this region will have an impact on energy transfer to higher trophic levels. Although generalized patterns could be elucidated, more information is needed to predict and understand how the changing Arctic will alter protistan communities and, thus, higher-order consumers.
... However, a decreasing pattern of viral abundance with increasing temperature can be identified when global data are grouped together (Danovaro et al., 2011). Maranger et al. (2015) reported that BP and grazing on bacteria increase similarly and more rapidly in the warming Arctic Ocean than rates of viral lyses, therefore it results in more efficient transfer of bacterial carbon within 'microbial loop'. ...
Mediterranean Sea (including Adriatic Sea) has been identified as a ‘hotspot’ for climate change, with prediction of the increase in water temperature of 2-4 °C over next few decades. Being mainly oligotrophic, and strongly phosphorus limited, Adriatic Sea is characterised by important role of microbial food web in production and transfer of biomass and energy towards higher trophic levels. We hypothesized that predicted rise in temperature of 3 °C in the near future may be resulted with increase of bacterial production and bacterial losses to grazers, which could significantly enlarged trophic base for metazoans. This empirical study, based on combined ‘space-for-time-substitution’ analysis (performed on 3583 data sets) and experimental approach (36 in situ grazing experiments performed at different temperatures), showed that predicted temperature increase of 3 °C in the near future, as a result of global warming, could cause a significant increase in bacterial growth at temperatures lower than 16o C (during the colder winter-spring period, as well as in the deeper layers). The effect of temperature on bacterial growth could be additionally doubled in conditions without phosphorus limitation. Furthermore, temperature increase of 3 °C, could double the grazing on bacteria by HNF and ciliate predators, and could increase the proportion of bacterial production transferred to metazoan food web by 42%. Therefore, it is expected that global warming may further strengthen the role of microbial food web in a carbon cycle in the Adriatic Sea.
... Additionally, the growth of PICO in aquatic systems is affected by environmental factors (e.g., light, temperature, etc.), nutrient availability, grazing, and viral lysis [11,12,13]. It is well-established that temperature significantly influences microbiological processes such as production [14], growth rate [15,16], and growth efficiency [17], as well as grazing on bacteria [9,18,19] and viral lysis [20][21][22][23]. However, these processes have shown different sensitivity to temperature increases, which ultimately determines how the microbial community will respond to warming [24,25]. ...
A recent analysis of the Mediterranean Sea surface temperature showed significant annual warming. Since small picoplankton microorganisms play an important role in all major biogeochemical cycles, fluxes and processes occurring in marine systems (the changes at the base of the food web) as a response to human-induced temperature increase, could be amplified through the trophic chains and could also significantly affect different aspects of the structure and functioning of marine ecosystems. In this study, manipulative laboratory growth/grazing experiments were performed under in situ simulated conditions to study the structural and functional changes within the microbial food web after a 3 °C increase in temperature. The results show that a rise in temperature affects the changes in: (1) the growth and grazing rates of picoplankton, (2) their growth efficiency, (3) carrying capacities, (4) sensitivity of their production and grazing mortality to temperature, (5) satisfying protistan grazer carbon demands, (6) their preference in the selection of prey, (7) predator niche breadth and their overlap, (8) apparent uptake rates of nutrients, and (9) carbon biomass flow through the microbial food web. Furthermore, temperature affects the autotrophic and heterotrophic components of picoplankton in different ways.
... Most of the studies addressing both controls simultaneously were conducted in temperate and polar regions (e.g. Maranger et al. 2015;Bowman et al. 2017;Morán et al. 2018). In the Mediterranean,Šolić et al. (2009) concluded that bacterioplankton stocks were strongly regulated by bottom-up control during colder months and top-down regulated during the warmer season. ...
Bacterioplankton play a pivotal role in marine ecosystems. However, their temporal dynamics and underlying control mechanisms are poorly understood in tropical regions such as the Red Sea. Here we assessed the impact of bottom-up (resource availability) and top-down (viruses and heterotrophic nanoflagellates) controls on bacterial abundances by weekly sampling a coastal central Red Sea in 2017. We monitored microbial abundances by flow cytometry together with a set of environmental variables including temperature, salinity, dissolved organic and inorganic nutrients and chlorophyll a. We distinguished five groups of heterotrophic bacteria depending on their physiological properties, such as relative nucleic acid content, membrane integrity and cell-specific respiratory activity, two groups of Synechococcus cyanobacteria and three groups of viruses. Viruses controlled heterotrophic bacteria for most of the year, as supported by a negative correlation between their respective abundances and a positive one between bacterial mortality rates and mean viral abundances. On the contrary, heterotrophic nanoflagellates abundance covaried with that of heterotrophic bacteria. Heterotrophic nanoflagellates showed preference for larger bacteria from both the high and low nucleic acid content groups. Our results demonstrate that top-down control is fundamental in keeping heterotrophic bacterioplankton abundances low (< 3 × 105 cells mL-1) in Red Sea coastal water.
... Studies on diversity and functioning of microbial communities in the Arctic Ocean have been focused mostly on epipelagic waters (0-200 m) [17][18][19][20][21][22][23][24][25][26]. To date, only few studies have investigated microbes in the deeper mesopelagic layers (down to 1000 m) [27,28], some of them specifically focusing on deep water Archaea [29,30] rather than on the whole prokaryotic community. ...
The deep Arctic Ocean is increasingly vulnerable to climate change effects, yet our understanding of its microbial processes is limited. We collected samples from shelf waters, mesopelagic Atlantic Waters (AW) and bathypelagic Norwegian Sea Deep Waters (NSDW) in the eastern Fram Strait, along coast-to-offshore transects off Svalbard during boreal summer. We measured community respiration, heterotrophic carbon production (HCP), and dissolved inorganic carbon utilization (DICu) together with prokaryotic abundance, diversity, and metagenomic predictions. In deep samples, HCP was significantly faster in AW than in NSDW, while we observed no differences in DICu rates. Organic carbon uptake was higher than its inorganic counterpart, suggesting a major reliance of deep microbial Arctic communities on heterotrophic metabolism. Community structure and spatial distribution followed the hydrography of water masses. Distinct from other oceans, the most abundant OTU in our deep samples was represented by the archaeal MG-II. To address the potential biogeochemical role of each water mass-specific microbial community, as well as their link with the measured rates, PICRUSt-based predicted metagenomes were built. The results showed that pathways of auto- and heterotrophic carbon utilization differed between the deep water masses, although this was not reflected in measured DICu rates. Our findings provide new insights to understand microbial processes and diversity in the dark Arctic Ocean and to progress toward a better comprehension of the biogeochemical cycles and their trends in light of climate changes.
... Such warming beyond a threshold of 5°C has been shown to enhance a decline in the phytoplankton biomass [3,5]. Interestingly this type of escalating warming may elevate the marine bacterial population which may trigger bacterial carbon transfer to the upper trophic levels [6]. Indeed the recent study based on the observation of the responses of phytoplankton biomass, the heterotrophic microorganisms and viruses to the contemporaneous changes in warming along the elevation of CO 2 unraveled an attention-grabbing dynamics in microbial abundance [3]. ...
The consequences of global warming as well as climate changes have recently been appearing as a huge threat round the globe because of the associated decline in the environmental sustainability which in turn is noticed to evoke the mass public health deterioration. Promulgation of re-emerging diseases has been connected by several groups of scientists to one of the ghastly effects of global warming. The significance of understanding the changes in the biodiversity; i.e., the variations in disease causing bacteria and viruses, vectors and hosts is noteworthy upon the environmental perturbations caused by the global warming and by the climate change as well. Such a study would benefit the overall public health globally. Present review concisely described environmental issues raised by the global warming and about the associated vector-borne re-emerging diseases.
... Edwards and Richardson, 2004;Wiltshire et al., 2008;Montoya and Raffaelli, 2010). It is well established that temperature significantly influences microbiological processes such as production (Rivkin et al., 1996;Hoppe et al., 2008;Šoli c et al., 2018a), growth rate (White et al., 1991;Shiah and Ducklow, 1994) and growth efficiency (Rivkin and Legendre, 2001), as well as grazing on bacteria (Vaqué et al., 1994;Krstulovi c, 1994, 1995;Vázquez-Dominguez et al., 2012) and viral lysis (Danovaro et al., 2011;Lara et al., 2013;Maranger et al., 2015;Tsai et al., 2015;Ordulj et al., 2017). Furthermore, temperature influences the complex microbial trophic interactions, altering food web structure and ecosystem functioning (Petchey et al., 1999). ...
Temperature and phosphorus positively interacted in controlling picoplankton biomass production and its transfer towards higher trophic levels. Two complementary approaches (experimental and field study) indicated several coherent patterns: (1) the impact of temperature on heterotrophic bacteria was high at temperatures lower than 16oC and levelled off at higher temperatures, whereas this impact on autotrophic picoplankton was linear along the entire range of the investigated temperatures; (2) the addition of phosphorus increased the values of picoplankton production and grazing, but did not change the nature of their relationships with temperature substantially; (3) the picoplankton carbon flux towards higher trophic levels was larger during the warmer months (grazing by HNF dominated during the warmer period and by ciliates during the colder period) and also strengthened in conditions without phosphorus limitation; (4) the hypothesis that the available phosphorus can be better utilized at higher temperatures was confirmed for both autotrophic and heterotrophic picoplankton; (5) the hypothesis that the rise in temperature stimulates growth only in conditions of sufficient phosphorus was confirmed only for heterotrophic bacteria. Therefore, in the global warming scenario, an increase of the picoplankton carbon flux towards higher trophic levels can be expected in the Adriatic Sea, particularly under unlimited phosphorus conditions.
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... This seasonal data bias is owing to the logistical difficulties of sampling in the Arctic during the dark and colder seasons. The predictions are further implicated by a regional bias since the majority of data for heterotrophic microbial processes are collected in the Beaufort Sea and Chukchi Sea [29]. Our study aimed at filling seasonal gaps by investigating DOM dynamics and bacterial activity in the Fram Strait during summer and autumn. ...
The Arctic Ocean is considerably affected by the consequences of global warming, including more extreme seasonal fluctuations in the physical environment. So far, little is known about seasonality in Arctic marine ecosystems in particular microbial dynamics and cycling of organic matter. The limited characterization can be partially attributed to logistic difficulties of sampling in the Arctic Ocean beyond the summer season. Here, we investigated the distribution and composition of dissolved organic matter (DOM), gel particles and heterotrophic bacterial activity in the Fram Strait during summer and autumn. Our results revealed that phytoplankton biomass influenced the concentration and composition of semi-labile dissolved organic carbon (DOC), which strongly decreased from summer to autumn. The seasonal decrease in bioavailability of DOM appeared to be the dominant control on bacterial abundance and activity, while no temperature effect was determined. Additionally, there were clear differences in transparent exopolymer particles (TEP) and Coomassie Blue stainable particles (CSP) dynamics. The amount of TEP and CSP decreased from summer to autumn, but CSP was relatively enriched in both seasons. Our study therewith indicates clear seasonal differences in the microbial cycling of organic matter in the Fram Strait. Our data may help to establish baseline knowledge about seasonal changes in microbial ecosystem dynamics to better assess the impact of environmental change in the warming Arctic Ocean.
This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning’.
... In cold polar waters the bacteriophage-induced bacterial mortality varies considerably: from less than 1% up to 100% of the daily bacterial production (Guixa-Boixereu et al., 2002;Wells andDeming, 2006, Steward et al., 2007). The majority of research projects on virioplankton conducted in the Arctic region took place from late spring till early autumn and only a few studies were performed in early spring (Steward et al., 1996(Steward et al., , 2007Wells and Deming, 2006;Maranger et al., 2015;Venger et al., 2016). ...
... Temperature is an extremely influencing factor on microbiological processes such as production (Šolić and Krstulović 1994;Rivkin, Anderson and Lajzerowicz 1996;Hoppe et al. 2008), growth rate (White et al. 1991;Shiah and Ducklow 1994;Šolić et al. 2017) and growth efficiency (Rivkin and Legendre 2001), as well as grazing on bacteria (Vaqué, Gasol and Marrasé 1994;Sarmento et al. 2010;Vásquez-Domínguez et al. 2012) and viral lysis (Danovaro et al. 2011;Lara et al. 2013;Maranger et al. 2015;Tsai, Gong and Shiau 2015;Ordulj et al. 2017). Furthermore, temperature influences the complex microbial trophic interactions, altering food web structure and ecosystem functioning (Petchey et al. 1999). ...
An assessment of the temperature increase effect on processes within the microbial food web provides a better insight into the carbon transfer and energy flow processes in marine environments in the global warming perspective. Modified laboratory dilution experiments that allow simultaneous estimates of protozoan grazing and viral lysis on picoplankton groups (bacteria, Prochlorococcus, Synechococcus and pico-eukaryotic algae) under in situ and 3°C above in situ temperatures were performed at seasonal scale. Picoplankton mortality due to grazing was generally higher than that caused by viral lysis, especially in the cold months. The largest part of HNF carbon demand was satisfied by grazing on bacteria throughout the year. Although ciliates satisfied their carbon demand predominantly through grazing on HNF and bacteria, the role of autotrophic picoplankton (APP) as their prey increased significantly in the cold months. Bacteria constituted the most important host for viruses throughout the year. However, during the warm months, APP groups were also significant hosts for viral infection. Under the warming condition the amount of picoplankton biomass transferred to protozoan grazers exceeded the lysed biomass, suggesting that global warming could further increase picoplankton carbon flow toward higher trophic levels in the Adriatic Sea.
... Temperature is an extremely influencing factor on microbiological processes such as production (Šolić and Krstulović, 1994;Rivkin et al., 1996;Hoppe et al., 2008), growth rate (White et al., 1991;Shiah and Ducklow, 1994;Šolić et al., 2017) and growth efficiency (Rivkin and Legendre, 2001), as well as grazing on bacteria (Vaqué et al., 1994;Sarmento et al., 2010;Vásquez-Domínguez et al., 2012) and viral lysis (Danovaro et al., 2011;Lara et al., 2013;Maranger et al., 2015;Tsai et al., 2015). However, these processes have shown different sensitivity to temperature increase that ultimately determines how a microbial community will respond to warming (López-Urrutia et al., 2006;Yvon-Durocher et al., 2010). ...
Global and atmospheric climate change is altering the thermal conditions in the Adriatic Sea and, consequently, the marine ecosystem. Along the eastern Adriatic coast sea surface temperature (SST) increased by an average of 1.03 °C during the period from 1979 to 2015, while in the recent period, starting from 2008, a strong upward almost linear trend of 0.013 °C/month was noted. Being mainly oligotrophic, the middle Adriatic Sea is characterized by the important role played by the microbial food web in the production and transfer of biomass and energy towards higher trophic levels. It is very important to understand the effect of warming on microbial communities, since small temperature increases in surface seawater can greatly modify the microbial role in the global carbon cycle. In this study, the Self-Organizing Map (SOM) procedure was used to analyse the time series of a number of microbial parameters at two stations with different trophic status in the central Adriatic Sea. The results show that responses of the microbial food web (MFW) structure to temperature changes are reproducible in time. Furthermore, qualitatively similar changes in the structure of the MFW occurred regardless of the trophic status. The rise in temperature was associated with: (1) the increasing importance of microbial heterotrophic activities (increase bacterial growth and bacterial predator abundance, particularly heterotrophic nanoflagellates) and (2) the increasing importance of autotrophic picoplankton (APP) in the MFW.
... Bacteria follow similar distributions to algae over time and space in the sea ice because the majority of DOC required for bacterial production is sourced from the ice algae (e.g., Rysgaard and Glud 2004;Comeau et al. 2013). Other factors have also been suggested to affect bacterial abundance and production in sea ice including the number of bacterivores like choanoflagellates (Sime-Ngando et al. 1997;Riedel et al. 2007;Deming 2010), temperature, and cell lysis following viral infection (Maranger et al. 2015). Quantifying the impact of these controls on bacteria is required to better understand variability in bacterial production that is observed across the Arctic (Bunch and Harland 1990;Maranger et al. 1994;Kaartokallio et al. 2013) and the role of sea ice in the microbial food web (Sarmiento and Gruber 2006). ...
The abundance of diatoms and heterotrophic bacteria in sea ice rapidly increases during the spring. However, the number and activity of these microorganisms vary with changing environmental conditions and potentially the taxonomic composition of the algal community during this time. In this study, we assessed the spring bottom-ice community composition in Dease Strait, Nunavut, and investigated potential controls of chlorophyll a (chl a), particulate organic carbon (POC), cell abundance, and production from early March until early June. We found that using flow cytometry to estimate photosynthetic nanoeukaryote remove the plural (2–20 μm) abundance gave results very similar to light microscopy counts, except when pennate diatoms with lengths close to 20 μm, the maximum size detected by flow cytometry, were abundant. Using the average abundance of nanoeukaryotes from the two methods, we documented a change in the size of cells comprising the ice algal community over the spring, from largely pico- (<2 μm), to nano- and microeukaryotes (20–200 μm). This shift in ice algal size corresponded to a bloom in diatoms that drove increases in chl a, POC, and primary productivity. Low-salinity surface waters, limited nutrient availability, as well as seasonally intensifying light in the bottom ice appeared to support dominance of the centric diatom Attheya spp. Increases in the number and productivity of heterotrophic bacteria in this study were correlated with the number of photosynthetic picoeukaryote cells, potentially due to their supply of dissolved organic carbon substrate. Our results suggest that future conditions predicted for the Arctic that include low nutrients and greater light transmission to the bottom of sea ice may favor an ice algal community dominated by centric diatoms versus the more characteristic pennate diatom-dominated community.
In September 2017, studies were conducted in the East Siberian Sea along the transect from a Indigirka delta to the ice edge at the outer shelf margin. The abundances of planktonic prokaryotes (NPR) and free viruses (NV), frequency of visibly infected prokaryotic cells (FVIC), and virus-mediated mortality of prokaryotes (VMPR) varied within (0.5–3.4) × 10⁶ (on average (1.67 ± 0.69) × 10⁶) cells/mL; (2.3–10.3) × 10⁶ (on average (5.28 ± 1.91) × 10⁶) viruses/mL; 0.6–2.5 (on average 1.1 ± 0.4) % of NPR; and 4.5–22.3 (on average 9.0 ± 3.8) % of the total prokaryotic production, respectively. The proportions of viruses attached to prokaryotic cells (NVPR) and suspended particles (NVP) were 1.9–26.3 (on average 8.0 ± 5.0) % of NV and 0.1–65.9 (on average 8.0 ± 13.3) % of NV, respectively. High concentrations of detrital and mineral particles, to which a significant number of viruses were attached and, as a result, loss of their activity were recorded in the river–sea water mixing zone. In such a situation, the number of virus attacks on prokaryotes and cases of their infection decreased. There was a negative relationship between the concentration of suspended particles 0.5–5.0 μm in size and the abundance of infected prokaryotic cells. Thus, we conclude that viruses played a substantial role in controlling the abundance and production of heterotrophic prokaryotic plankton in the low-productive East Siberian Sea at the beginning of autumn.
Regional index terms: Russian Federation; East Siberian Sea.
Geographic bounding coordinates: 70–78° N; 150–165° E.
Planktonic heterotrophic prokaryotes make up the largest living biomass and process most organic matter in the ocean. Determining when and where the biomass and activity of heterotrophic prokaryotes are controlled by resource availability (bottom-up), predation and viral lysis (top-down) or temperature will help in future carbon cycling predictions. We conducted an extensive survey across subtropical and tropical waters of the Atlantic, Indian and Pacific Oceans during the Malaspina 2010 Global Circumnavigation Expedition and assessed indices for these three types of controls at 109 stations (mostly from the surface to 4000 m depth). Temperature control was approached by the apparent activation energy in eV (ranging from 0.46 to 3.41), bottom-up control by the slope of the log-log relationship between biomass and production rate (ranging from -0.12 to 1.09) and top-down control by an index that considers the relative abundances of heterotrophic nanoflagellates and viruses (ranging from 0.82 to 4.83). We conclude that temperature becomes dominant (i.e. activation energy >1.5 eV) within a narrow window of intermediate values of bottom-up (0.3-0.6) and top-down 0.8-1.2) controls. A pervasive latitudinal pattern of decreasing temperature regulation towards the Equator, regardless of the oceanic basin, suggests that the impact of global warming on marine microbes and their biogeochemical function will be more intense at higher latitudes. Our analysis predicts that 1°C ocean warming will result in increased biomass of heterotrophic prokaryoplankton only in waters with <26°C of mean annual surface temperature. This article is protected by copyright. All rights reserved.
The distribution of viruses and their impact on prokaryotes were studied using the epifluorescence and transmission electron microscopy along a transect, located at 130°E from the area adjacent to the Lena River delta next to the shelf area, the continental slope area and the deep-sea regions of the Laptev Sea in September 2015. The abundance of planktonic prokaryotes (NP) and free viral particles (NV), the frequency of visibly virus-infected prokaryotic cells (FVIC), and the viral-mediated prokaryotic mortality (VMP) varied within the following ranges: (0.1–3.1) × 10⁶ cells mL⁻¹, (0.1–4.4) × 10⁶ viruses mL⁻¹, 0.3–1.7% of NP, and 2.1–12.2% of the total mortality of prokaryotes, respectively, and reached maximum values at the inner shelf area. The percentage of viruses attached to prokaryotic cells varied from 2 to 6% of NV in the surface waters of the outer shelf and the deep-sea areas to 43–46% at the depth of 100–140 m in the continental slope area. The percentage of viruses that were attached to detrital or mineral microparticles varied from less than 1% in the waters of the deep-sea area to 27–50% in the waters of the inner shelf area adjacent to the Lena River delta. A negative correlation was found between the fraction of lysogenic prokaryotes and the FVIC values (r = – 0.60, p < 0.05).
In the 20-cm bottom water layer above the sediment the NP, NV, FVIC and VMP values were (0.8–3.0) × 10⁶ cells mL⁻¹, (1.6–3.4) × 10⁶ viruses mL⁻¹, 0.5–1.2%, and 3.7–9.5%, respectively. In the surface 2-cm layer of the sediment these parameters were (1.8–4.4) × 10⁹ cells cm⁻³, (0.8–2.3) × 10⁹ viruses cm⁻³, 0.2–1.0%, and 1.4–7.8%, respectively. In the bottom water layer the percentage of viral particles attached to prokaryotes and to detrital and mineral particles was 10–29% and 20–79% of NV, respectively. In the surface sediments the percentage of viral particles attached to prokaryotes was considerably higher compared to the bottom water layer and ranged from 44 to 57% of NV. The capsid size of the viral particles from the pelagic zone, bottom water layer and surface sediments was on average 71 ± 14, 74 ± 7 and 63 ± 7 nm, respectively. In all these habitats, viruses from the size class of 60–100 nm prevailed.
The relatively high abundance of prokaryotes and viruses, as well as the high FVIC values in the freshwater-seawater mixing zone (the highest values were recorded in the waters with a salinity of 9.6 psμ) were apparently caused by the influx of considerable amounts of nutrients, and dissolved and particulate organic matter that come with the river runoff. At the same time, the high concentration of small detrital and mineral particles in the freshwaters lead to considerable adsorption of viruses by these particles and caused a decrease in the FVIC in the coastal areas adjacent to the Lena River delta.
We explored how changes of viral abundance and community composition among four contrasting regions in the Southern Ocean relied on physicochemical and microbiological traits. During January–February 2015, we visited areas north and south of the South Orkney Islands (NSO and SSO) characterized by low temperature and salinity and high inorganic nutrient concentration, north of South Georgia Island (NSG) and west of Anvers Island (WA), which have relatively higher temperatures and lower inorganic nutrient concentrations. Surface viral abundance (VA) was highest in NSG (21.50 ± 10.70 × 106 viruses mL−1) and lowest in SSO (2.96 ± 1.48 × 106 viruses mL−1). VA was positively correlated with temperature, prokaryote abundance and prokaryotic heterotrophic production, chlorophyll a, diatoms, haptophytes, fluorescent organic matter, and isoprene concentration, and was negatively correlated with inorganic nutrients (NO3−, SiO42−, PO43−), and dimethyl sulfide (DMS) concentrations. Viral communities determined by randomly amplified polymorphic DNA–polymerase chain reaction (RAPD-PCR) were grouped according to the sampling location, being more similar within them than among regions. The first two axes of a canonical correspondence analysis, including physicochemical (temperature, salinity, inorganic nutrients—NO3−, SiO42−, and dimethyl sulfoniopropionate -DMSP- and isoprene concentrations) and microbiological (chlorophyll a, haptophytes and diatom, and prokaryote abundance and prokaryotic heterotrophic production) factors accounted for 62.9% of the variance. The first axis, temperature-related, accounted for 33.8%; the second one, salinity-related, accounted for 29.1%. Thus, different environmental situations likely select different hosts for viruses, leading to distinct viral communities.
During August 2009, measurements of bacterial abundance and nucleic acid content were made along with production and respiration in coastal waters of the Beaufort Sea (Arctic Ocean), an area influenced by the Mackenzie River inflow. The main purpose was to evaluate bacterial organic carbon processing with respect to local sources, mainly primary production and river inputs. Bacterial production and abundance generally decreased from river to offshore waters and from surface to deep waters. In contrast, the percentage of high nucleic acid bacteria was higher in deep waters rather than in surface or river waters. Statistical analyses indicated that bacterial production was primarily controlled by temperature and the availability of labile organic matter, as indicated by total dissolved amino acid concentrations. Direct comparisons of bacterial carbon demand and primary production indicated net heterotrophy was common in shelf waters. Net autotrophy was observed at stations in the Mackenzie River plume, suggesting that the carbon fixed in plume waters helped fuel net heterotrophy in the Beaufort Sea margin.
Some protists from both marine and freshwater environments function at more than one trophic level by combining photosynthesis and particle ingestion. Photosynthetic algae from several taxa (most commonly chrysomonads and dinoflagellates) have been reported to ingest living prey or nonliving particles, presumably obtaining part of their carbon and/or nutrients from phagocytosis. Conversely, some ciliates and sarcodines sequester chloroplasts after ingestion of algal prey. Plastid retention or “chloroplast symbiosis” by protists was first demonstrated <20 years ago in a benthic foraminiferan. Although chloroplasts do not divide within these mixotrophic protists, they continue to function photosynthetically and may contribute to nutrition. Sarcodines and ciliates that harbor endosymbiotic algae could be considered mixotrophic but are not covered in detail here. The role of mixotrophy in the growth of protists and the impact of their grazing on prey populations have received increasing attention. Mixotrophic protists vary in their photosynthetic and ingestion capabilities, and thus, in the relative contribution of photosynthesis and phagotrophy to their nutrition. Abundant in both marine and freshwaters, they are potentially important predators of algae and bacteria in some systems. Mixotrophy may make a stronger link between the microbial and classic planktonic food webs by increasing trophic efficiency.
Key words. Amoebae, chloroplast symbiosis, ciliates, flagellates, mixotrophy, phagotrophic phytoflagellates
The annual migration of bowhead whales (Balaena mysticetus) past Barrow, Alaska, has provided subsistence hunting to Iñupiat for centuries. Bowheads recurrently feed on aggregations of zooplankton prey near Barrow in autumn. The mechanisms that form these aggregations, and the associations between whales and oceanography, were investigated using field sampling, retrospective analysis, and traditional knowledge interviews. Oceanographic and aerial surveys were conducted near Barrow during August and September in 2005 and 2006. Multiple water masses were observed, and close coupling between water mass type and biological characteristics was noted. Short-term variability in hydrography was associated with changes in wind speed and direction that profoundly affected plankton taxonomic composition. Aggregations of ca. 50-100 bowhead whales were observed in early September of both years at locations consistent with traditional knowledge. Retrospective analyses of records for 1984-2004 also showed that annual aggregations of whales near Barrow were associated with wind speed and direction. Euphausiids and copepods appear to be upwelled onto the Beaufort Sea shelf during E or SE winds. A favorable feeding environment is produced when these plankton are retained and concentrated on the shelf by the prevailing westward Beaufort Sea shelf currents that converge with the Alaska Coastal Current flowing to the northeast along the eastern edge of Barrow Canyon.
The Arctic Ocean ecosystem is particularly vulnerable to ocean acidification (OA) related alterations due to the relatively high CO2 solubility and low carbonate saturation states of its cold surface waters. Thus far, however, there is only little known about the consequences of OA on the base of the food web. In a mesocosm CO2-enrichment experiment (overall CO2 levels ranged from ∼ 180 to 1100 μatm) in Kongsfjorden off Svalbard, we studied the consequences of OA on a natural pelagic microbial community. OA distinctly affected the composition and growth of the Arctic phytoplankton community, i.e. the picoeukaryotic photoautotrophs and to a lesser extent the nanophytoplankton thrived. A shift towards the smallest phytoplankton as a result of OA will have direct consequences for the structure and functioning of the pelagic food web and thus for the biogeochemical cycles. Besides being grazed, the dominant pico-and nanophytoplankton groups were found prone to viral lysis, thereby shunting the carbon accumulation in living organisms into the dissolved pools of organic carbon and subsequently affecting the efficiency of the biological pump in these Arctic waters.
Mixotrophy is a valuable functional trait used by microbes when environmental conditions vary broadly or resources are limited. In the sunlit waters of the ocean, photoheterotrophy, a form of mixotrophy, is often mediated by proteorhodopsin (PR), a seven helices transmembrane protein binding the retinal chromophore. Altogether, they allow bacteria to capture photic energy for sensory and proton gradient formation cell functions. The seasonal occurrence and diversity of the gene coding for PR in cold oligotrophic polar oceans is not known and PR expression has not yet been reported. Here we show that PR is widely distributed among bacterial taxa, and that PR expression decreased markedly during the winter months in the Arctic Ocean. Gammaproteobacteria-like PR sequences were always dominant. However, within the second most common affiliation, there was a transition from Flavobacteria-like PR in early winter to Alphaproteobacteria-like PR in late winter. The phylogenetic shifts followed carbon dynamics, where patterns in expression were consistent with community succession, as identified by DNA community fingerprinting. Although genes for PR were always present, the trend in decreasing transcripts from January to February suggested reduced functional utility of PR during winter. Under winter darkness, sustained expression suggests that PR may continue to be useful for non-ATP forming functions, such as environmental sensing or small solute transport. The persistence of PR expression in winter among some bacterial groups may offer a competitive advantage, where its multifunctionality enhances microbial survival under harsh polar conditions.The ISME Journal advance online publication, 20 February 2015; doi:10.1038/ismej.2015.1.
The metabolism of the Arctic Ocean is marked by extremely pronounced seasonality and spatial heterogeneity associated with light conditions, ice cover, water masses and nutrient availability. Here we report the marine planktonic metabolic rates (net community production, gross primary production and community respiration) along three differ-ent seasons of the year, for a total of eight cruises along the western sector of the European Arctic (Fram Strait – Sval-bard region) in the Arctic Ocean margin: one at the end of 2006 (fall/winter), two in 2007 (early spring and summer), two in 2008 (early spring and summer), one in 2009 (late spring–early summer), one in 2010 (spring) and one in 2011 (spring). The results show that the metabolism of the west-ern sector of the European Arctic varies throughout the year, depending mostly on the stage of bloom and water tempera-ture. Here we report metabolic rates for the different periods, including the spring bloom, summer and the dark period, in-creasing considerably the empirical basis of metabolic rates in the Arctic Ocean, and especially in the European Arctic corridor. Additionally, a rough annual metabolic estimate for this area of the Arctic Ocean was calculated, resulting in a net community production of 108 g C m −2 yr −1 .
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.
Photosynthetic picoeukaryotes (PPE) are recognized as major primary producers and contributors to phytoplankton biomass in oceanic and coastal environments. Molecular surveys indicate a large phylogenetic diversity in the picoeukaryotes, with members of the Prymnesiophyceae and Chrysophyseae tending to be more common in open ocean waters and Prasinophyceae dominating coastal and Arctic waters. In addition to their role as primary producers, PPE have been identified in several studies as mixotrophic and major predators of prokaryotes. Mixotrophy, the combination of photosynthesis and phagotrophy in a single organism, is well established for most photosynthetic lineages. However, green algae, including prasinophytes, were widely considered as a purely photosynthetic group. The prasinophyte Micromonas is perhaps the most common picoeukaryote in coastal and Arctic waters and is one of the relatively few cultured representatives of the picoeukaryotes available for physiological investigations. In this study, we demonstrate phagotrophy by a strain of Micromonas (CCMP2099) isolated from Arctic waters and show that environmental factors (light and nutrient concentration) affect ingestion rates in this mixotroph. In addition, we show size-selective feeding with a preference for smaller particles, and determine P vs I (photosynthesis vs irradiance) responses in different nutrient conditions. If other strains have mixotrophic abilities similar to Micromonas CCMP2099, the widespread distribution and frequently high abundances of Micromonas suggest that these green algae may have significant impact on prokaryote populations in several oceanic regimes.The ISME Journal advance online publication, 20 February 2014; doi:10.1038/ismej.2014.16.
Standard metabolic theory predicts that both respiration and
photosynthesis should increase with increasing temperature, albeit at
different rates. However, test of this prediction for ocean planktonic
communities is limited, despite the broad consequences of this
prediction in the present context of global ocean warming. We compiled a
large data set on volumetric planktonic metabolism in the open ocean and
tested the relationship between specific metabolic rates and water
temperature. The relationships derived are consistent with predictions
derived from metabolic theory of ecology, yielding activation energy for
planktonic metabolism consistent with predictions from the metabolic
theory. These relationships can be used to predict the effect of warming
on ocean metabolism and, thus, the role of planktonic communities in the
flow of carbon in the global ocean.
We analyzed heterotrophic, pelagic bacterial production and specific growth rate data from 57 studies conducted in fresh, marine and estuarine/coastal waters. Strong positive relationships were identified between 1) bacterial production and bacterial abundance and 2) bacterial production and algal biomass. The relationship between bacterial production and bacterial abundance was improved by also considering water temperature. The analysis of covariance model revealed consistent differences between fresh, marine and estuarine/coastal waters, with production consistently high in estuarine/coastal environments. The log-linear regression coefficient of abundance was not significantly different from 1.00, and this linear relationship permitted the use of specific growth rate (SGR in day(-1)) as a dependent variable. A strong relationship was identified between specific growth rate and temperature. This relationship differed slightly across the three habitats. A substantial portion of the residual variation from this relationship was accounted for by algal biomass, including the difference between marine and estuarine/coastal habitats. A small but significant difference between the fresh- and saltwater habitats remained. No significant difference between the chlorophyll effect in different habitats was identified. The model of SGR against temperature and chlorophyll was much weaker for freshwater than for marine environments. For a small subset of the data set, mean cell volume accounted for some of the residual variation in SGR. Pronounced seasonality, fluctuations in nutrient quality, and variation of the grazing environment may contribute to the unexplained variation in specific growth.
In August–September 2009, the concentration of dissolved organic matter and quantitative distribution of virioplankton, bacterioplankton, and heterotrophic nanoflagellates were studied in the coastal waters of the Kara Sea, the fresh waters of the islands and the coasts of the sea, and the estuaries of the Ob’ and Yenisei rivers. A high positive correlation was observed between the abundances of viruses and bacteria. The frequency of visibly infected bacteria in marine waters ranged from 0.6 to 4.3% (an average of 1.6%); in the fresh waters of islands and coastline and in estuaries, it ranged from 0.3 to 3.9% (an average of 1.5%) and from 0.5 to 1.6% (an average of 1.1%) respectively. In most surveyed water bodies, the role of viruses in bacterioplankton mortality was considerably higher than that of heterotrophic flagellates.
The effect of Arctic warming, which is 3 times faster than the global average, on microbial communities was evaluated experimentally to determine how increasing temperatures affect bacterial and viral abundance and production, protist community composition, and bacterial loss rates (bacterivory and lysis) in 2 contrasting Arctic marine systems. In July 2009, we collected samples from open Arctic waters in the Barents Sea and Atlantic-influenced waters in Isfjorden, Svalbard Islands (Fjord waters). The samples were used in 2 microcosm experiments at 7 temperatures, ranging from 1.0 to 10.0 degrees C. In the open Arctic microbial community, collected at <1.0 degrees C, bacterial and viral abundances, bacterial production and grazing rates due to protists increased significantly above 5.5 degrees C, and remained at high values at even higher experimental temperatures. The abundance of protists, such as some heterotrophic pico/nanoflagellates, as well as some ciliates, also increased with warming. In contrast, the biomass of phototrophs decreased above 5.5 degrees C. The water temperature in Fjord waters was 6.2 degrees C at the time of sampling, and the microbial community showed smaller variations than the Arctic community. These results indicate that increases in temperature stimulate heterotrophic microbial biomass and activity compared to that of phototrophs, which has important implications for carbon and nutrient cycling in the system. In addition, open Arctic communities were more vulnerable to warming than those already adapted to the warmer Fjord waters influenced by Atlantic seawater.
Narrow annual ranges of temperature characterize polar waters. Consequently, small increases in temperature could significantly affect the metabolic processes of marine microorganisms. We investigated the response of bacterial heterotrophic production (BHP) and grazing rates to small temperature changes in 3 zones near the western Antarctic Peninsula-Bransfield and Gerlache Straits, and Bellingshausen Sea-during December 2002. We performed 8 grazing experiments with water samples collected from depths where chlorophyll a (chl a) concentration was maximum, and incubated the samples at ambient temperature and at -1, 1, 2 and 5 degrees C. We expected that grazing would increase in parallel with BHP at increasing temperatures; however, temperature differentially affected these 2 microbial activities. Thus, grazing rates increased maximally at temperatures <= 2 degrees C, except in 1 station in the Gerlache Strait, while BHP increased maximally at temperatures <= 2 degrees C, except in 1 station in the Bellingshausen Sea. The percentage of grazed bacteria to BHP at the highest experimental temperatures was low (56 +/- 19%) in the Gerlache Strait, high (395 +/- 137%) in the Bransfield Strait and approximately balanced (97 +/- 24%) in the Bellingshausen Sea. This suggests that differential microbial processes in each zone at increasing temperatures will also depend on the autochthonous community. The present study contributes to the understanding of the variability of polar biogeochemical fluxes, and may aid in predicting the response of microorganisms in future scenarios with local and seasonal changes in temperature.
Few of us may ever live on the sea or under it, but all of us are mak-ing increasing use of it either as a source of food and other materi-als, or as a dump. As our demands upon the ocean increase, so does our need to understand the ocean as an ecosystem. Basic to the un-derstanding of any ecosystem is knowledge of its food web, through which energy and materials flow. (Pomeroy 1974, p. 499)
The distribution, abundance, and production of viruses and bacteria were investigated during an August to September 1992 cruise aboard the RV 'Alpha Helix' in the Bering and Chukchi Seas. Viruses were abundant in seawater samples at all stations (10^9 to 10^10 per L) and exceeded the bacteria concentration by an order of magnitude on average. Virus-like particles and bacteria were also observed in the pore water of a sediment sample at 27 and 2.1 X 10^9 per L, respectively. The concentrations of viruses and bacteria in pelagic samples were correlated (r= 0.83, n = 43). In a detailed depth profile from the deepest and northernmost station (72° N), bacteria and viruses displayed subsurface maxima in the upper 100 m. Below 100 m, the concentrations declined, but were detectable even in the deepest-collected samples (402 m). Integrated bacterial biomass estimates were similar to results from a previous study in this area, but bacterial production measurements ranging from 0.3 to 0.45 g C per m^2 per d were an order of magnitude higher. Production rates of bacterial viruses (also known as bacteriophages or simply phages) measured by radiolabeling ranged from 0.5 to 4.2 X 10^9 viruses per L per d, which are similar to previous estimates for temperate coastal waters. The production measurements indicated turnover times ranging from 0.4 to 17 d for bacteria and maximum estimates of 1.2 to 15 d for bacterial viruses. Viral mortality of bacteria was estimated from the frequency of visibly infected cells (FVIC) and flagellate grazing was calculated from flagellate and bacterial abundances together with an assumed flagellate clearance rate. Overall, estimated viral lysis was roughly comparable to estimated grazing by flagellates as a source of bacterial mortality. Averaged over the water column, viral mortality of bacteria in the Chukchi Sea was estimated to be 23% of the bacterial production at 2 southern stations and approximately 10% at 2 northern stations. FVIC was correlated with bacterial production (r = 0.75, n = 18) and specific growth rate (r = 0.74, n = 18),but not with bacterial abundance (r = 0.22, n = 27). These data suggest viruses to be a ubiquitous and dynamic feature and a significant source of bacterial mortality in Arctic marine microbial communities. The implications of bacterial and viral production for C and N cycling in the Chukchi Sea are discussed.
Marine microbial communities have been little studied in Arctic waters, especially
during the winter–spring transition before the development of extensive phytoplankton blooms. This
study investigated microbial plankton in the ice-covered polar surface waters of the northwestern
Fram Strait (75 to 80°N) at the onset of the 24 h light period in spring (April to May). The system we
encountered was characterised by low concentrations of chlorophyll a (< 0.2 μg l −1) and a low abundance of both bacteria (1.4 to 2.5 × 10^8 cells l −1) and protists (1 to 1.7 × 10^5 cells l −1). Bacterial production was very low (≤0.63 μg C l−1 d−1), despite the dominance of nucleic-acid-rich bacteria (58 ± 6%
of total bacterial abundance). Small (2 to 5 μm) phototrophs dominated the eukaryotic assemblage in
the surface and most probably had profound effects on the composition and metabolic balance of the
microbial community as a whole. Most stations appeared to have been net-autotrophic, and calcula-
tions of phagotrophy indicated a balanced carbon budget for the microbial community. Mixotrophy
was seen in a large part of the ciliate assemblage and may have contributed to the productivity and
stability of the pre-bloom system that we encountered.
ABSTRACT: Various parameters of bacterial activity were measured in the Kara Sea system (Arctic)
including the freshwater endmembers of the rivers Ob and Yenisei during August/September 2001.
Average bacterial production in surface water was highest in the rivers (13.5 μg C l–1 d–1) and decreased
towards the estuaries (5.8 μg C l–1 d–1) and the open Kara Sea (2.4 μg C l–1 d–1). Bacterial production
in the salinity gradient was significantly correlated to chlorophyll a concentrations (r = 0.78, p
< 0.001), indicating a tight coupling between primary production and bacterial growth. Similar to
bacterial production mean turnover-rate constants of dissolved free amino acids (DFAA) and glucose
decreased from the rivers (1.5 to 1.9 d–1) towards the Kara Sea (0.4 d–1). In combination with low in
situ concentrations of DFAA (<50 nM) and glucose (<10 nM) and the immediate stimulation of bacterial
growth upon addition of glucose in incubation experiments this suggests that bacteria were limited
by the availability of organic carbon and that the supply of substrates governed the distribution
pattern of bacterial growth. Mean uptake of DFAA contributed more to bacterial production
(24 to >100%) than glucose uptake (<10%). Addition of filtered water from the Yenisei to untreated
water from the Kara Sea caused no increase in bacterial production relative to the controls, indicating
that riverine dissolved organic material is not the primary source of labile compounds for the bacterial
community of the Kara Sea. Based on a bacterial growth efficiency of 27% that was derived from
bacterial production and respiration measurements, our estimates for the bacterial carbon demand
confine the export of surplus primary production from the Kara Sea to the central Arctic.
Climate warming is especially severe in the Arc-tic, where the average temperature is increasing 0.4 • C per decade, two to three times higher than the global average rate. Furthermore, the Arctic has lost more than half of its summer ice extent since 1980 and predictions suggest that the Arctic will be ice free in the summer as early as 2050, which could increase the rate of warming. Predictions based on the metabolic theory of ecology assume that temperature increase will enhance metabolic rates and thus both the rate of primary production and respiration will increase. How-ever, these predictions do not consider the specific metabolic balance of the communities. We tested, experimentally, the response of Arctic plankton communities to seawater tem-perature spanning from 1 • C to 10 • C. Two types of com-munities were tested, open-ocean Arctic communities from water collected in the Barents Sea and Atlantic influenced fjord communities from water collected in the Svalbard fjord system. Metabolic rates did indeed increase as suggested by metabolic theory, however these results suggest an ex-perimental temperature threshold of 5 • C, beyond which the metabolism of plankton communities shifts from autotrophic to heterotrophic. This threshold is also validated by field measurements across a range of temperatures which sug-gested a temperature 5.4 • C beyond which Arctic plankton communities switch to heterotrophy. Barents Sea communi-ties showed a much clearer threshold response to temperature manipulations than fjord communities.
Respiration rates are fundamental to understanding ecosystem C flux; however, respiration remains poorly characterized in polar oceans. The Circumpolar Flaw Lead (CFL) study provided a unique opportunity to sample the Amundsen Gulf, from November 2007 to July 2008 and follow microbial C dynamics. This study shows that bacterial production (BP) was highly variable, ranging from 0.01 to 2.14 μg C L -1 d -1 (CV = 192%), whereas the range in community respiration (CR) was more conservative from 3.8 to 44.2 μg C L -1 d -1 (CV = 55%), with measurable rates throughout the year. The spring-summer peak in BP preceded the peak in CR suggesting differential predominant control. From May until July, BP was more related to chlorophyll a concentration (r = 0.68) whereas CR was not. Given the observed high variability, BP was the main driver of bacterial growth efficiency (BGE) (r 2 = 0.86). The overall average BGE was low at 4.6%, ranging from 0.20 in winter to a peak of 18.6% during the spring bloom. This study confirms that respiration is an important fate for C in the Amundsen Gulf, and our rate-based estimates of ecosystem scale CR suggests that considerably more C is respired than could be accounted for by gross primary production (GPP). One of the most plausible explanations for this observed discrepancy is that regenerated primary production is currently underestimated. © 2012. American Geophysical Union. All Rights Reserved.
High virus counts were found in Arctic sea ice samples taken during the spring ice algal bloom near Resolute, Northwest Territories, Canada. Viral abundances in the sea ice ranged from 3.7 x 10(11) viruses m(-2) (or 9.0 x 10(6) ml(-1)) to 4.9 x 10(12) m(-2) (1.3 x 10(8) ml(-1)) which is 10- to 100-fold greater than the concentration of viruses in the underlying water column (1.1 x 10(6) ml(-1)). This increase in viral abundance corresponds with the 10- to 100-fold increase in bacterial abundance in sea ice as compared with the water column. Bacterial abundances ranged from 6.1 x 10(9) bacteria m(-2) (1.5 x 10(5) ml(-1)) to 4.2 x 10(11) m(-2) (1.0 x 10(7) ml(-1)) from early to late spring respectively. The virus-to-bacteria ratios (VBR) were among the highest reported in natural samples. The greatest viral abundances occurred in the 0.5 to 1.5 cm layer of the ice profile, where the bacteria were most active. The VBR generally decreased during the spring although viruses were increasing in abundance. The disequilibrium between phage and bacterial growth and abundance maxima during the spring bloom is suggested to be due to (1) a change in the makeup of the bacterial community, such that phage-resistant bacteria proliferated later in the spring, or (2) an increase in viral lytic activity with higher bacterial cell-specific growth rates; both viral lytic activity and bacterial growth rates declined later in the spring as the bacterial population reached its peak.
In order to investigate the factors controlling viral abundance, 22 lakes in Quebec were surveyed. We measured viral and bacterial abundance, bacterial production, chlorophyll a, total phosphorus and DOC (dissolved organic carbon) concentrations. Regression models built with these data were compared to models based on literature data, which to date have been collected largely from marine sites. Positive empirical relationships were found between viral abundance and (1) chlorophyll a concentrations, (2) bacterial abundances, (3) bacterial production, and (4) total phosphorus concentration. There was little to no trend in the virus-to-bacteria ratio with increasing trophy. Analysis of covariance revealed significant differences between relations in marine and freshwater systems. The virus-to-bacteria ratio was significantly higher in freshwater (mode = 22.5) than marine environments (mode = 2.5), and there were significantly more bacteria per unit chlorophyll in our freshwater samples. We suggest that this difference is related to the increased dependence of freshwater bacteria on allochthonous material relative to marine systems, as well as the increased relative importance of photosynthetic cyanobacteria in lakes.
During August 2009, measurements of bacterial
abundance and nucleic acid content were made along with
production and respiration in coastal waters of the Beaufort
Sea (Arctic Ocean), an area influenced by the Mackenzie
River inflow. The main purpose was to evaluate bacterial
organic carbon processing with respect to local sources,
mainly primary production and river inputs. Bacterial production
and abundance generally decreased from river to offshore
waters and from surface to deep waters. In contrast, the
percentage of high nucleic acid bacteria was higher in deep
waters rather than in surface or river waters. Statistical analyses
indicated that bacterial production was primarily controlled
by temperature and the availability of labile organic
matter, as indicated by total dissolved amino acid concentrations.
Direct comparisons of bacterial carbon demand and
primary production indicated net heterotrophy was common
in shelf waters. Net autotrophy was observed at stations in
the Mackenzie River plume, suggesting that the carbon fixed
in plume waters helped fuel net heterotrophy in the Beaufort
Sea margin.
This study examined the abundance, cell size, and activity of Bacteria and Archaea in the Chukchi Sea and the Canada Basin of the western Arctic Ocean in the spring (May-June) and summer (July-August) of 2002 and 2004. Data from fluorescence in situ hybridization (FISH) analyses indicate that bacterial abundance as a percent of total prokaryotes decreased with depth, whereas in contrast, Crenarchaeota increased from about 10% of prokaryotes in surface waters to as much as 40% in samples from 100 to 200 m. Euryarchaeota were detectable in only a few samples. Relative abundance of Crenarchaeota, expressed as a percent of total prokaryotes, correlated with ammonium concentrations, but relative bacterial abundance did not. Crenarchaeota cells were significantly larger than Bacteria by 1.5- to 2-fold in the upper 200 m. Data collected from a combination of FISH and microautoradiography indicate that often the fraction of both Bacteria and Crenarchaeota assimilating organic compounds was high (up to 55%), and both microbial groups were more active in assimilating amino acids than other compounds. However, Crenarchaeota were usually less active than Bacteria in assimilating amino acids and glucose, but were nearly as active as Bacteria in assimilating protein and diatom extracellular polymers. The fraction of Bacteria and Crenarchaeota assimilating CO2 in surface waters was higher than expected by anaplerotic fixation alone, suggesting that many of these microbes are chemoautotrophic. These data add to a growing body of evidence indicating how the roles of Archaea and Bacteria differ in biogeochemical cycles of the oceans. © 2007, by the American Society of Limnology and Oceanography, Inc.
More than a decade has passed since the realization that bacteria are quantitatively important consumers of organic carbon in marine food webs. The basic information on the significance of the microbial food web was put forth eloquently by Pomeroy (1974), who pieced together data from a variety of sources that all indicated a major role of small heterotrophs consuming dissolved and particulate material. However, these ideas did not gain wide recognition until the high abundance of marine bacteria was shown by epifluorescence microscopy (Ferguson and Rublee, 1976; Hobbie et al., 1977), and the bacterial heterotrophic production was shown to be large (i.e., 10–30%) compared to primary production (Hagstrőm et al., 1979; Fuhrman and Azam, 1980; 1982). With reasonable estimates of bacterial growth efficiency (i.e., near 50%), it became clear that heterotrophic bacteria consume an amount of carbon equivalent to approximately 20–60% of total primary production. Williams (1981) reached this conclusion when he synthesized the extant results on bacterial biomass and production. He also showed that “normal” well-known processes and mechanisms could lead to as much as 60% of the primary production becoming dissolved organic carbon (DOC), and subsequently, being taken up by bacteria. Azam et al. (1983) formalized the concept of the microbial loop by which significant quantities of organic matter are produced or processed through prokaryotic and very small eukaryotic organisms, eventually feeding into the larger macrozooplankton.
Microzooplankton grazing impact on phytoplankton in the Bering Sea
during spring was assessed in 2008, 2009 and 2010 using two-point
dilution assays. Forty-nine experiments were completed in a region
encompassing shelf to slope waters, including the 70 m line along the
edge of the shelf. A variety of conditions were encountered, with a
concomitant range of trophic states, from pre-bloom low chlorophyll-a
(Chl-a)<3 μg l-1 during heavy ice cover to late spring
open water diatom blooms with Chl-a up to 40 μg l-1.
Microzooplankton biomass was dominated by large heterotrophic
dinoflagellates and ciliates. Both athecate and thecate dinoflagellates,
as well as some species of ciliates, fed on diatom cells and chains.
Other types of protists, notably thecate amoebae and parasitoid
flagellates, were also observed preying on diatoms. Total
microzooplankton biomass ranged from 0.1 to 109 μg C l-1
and was positively related to Chl-a concentration. Significant rates of
microzooplankton herbivory were found in 55% of dilution experiments.
Maximum grazing rate was 0.49 d-1, and average grazing rate,
including experiments with no significant grazing, was 0.09±0.10
d-1. Phytoplankton intrinsic growth rates varied from
slightly negative growth to >0.4 d-1. Microzooplankton
grazing was significant in both non-bloom and bloom conditions,
averaging 46±75% of phytoplankton daily growth. Based on the
amount of phytoplankton carbon consumed, we estimated potential
microzooplankton community growth rates of up to 1.3 d-1. Our
results confirm the importance of protist grazers in planktonic food
webs of high latitude ecosystems. We also conclude that our finding of
significant grazing by microzooplankton on spring blooms in the Bering
Sea does not support theories about phytoplankton bloom formation based
on escape from grazing, due either to predation resistance or to slow
growth of herbivorous protists at cold temperature.
Mooring data indicate the Bering Strait throughflow increases ˜50%
from 2001 (˜0.7 Sv) to 2011 (˜1.1 Sv), driving heat and
freshwater flux increases. Increase in the Pacific-Arctic pressure-head
explains two-thirds of the change, the rest being attributable to weaker
local winds. The 2011 heat flux (˜5 × 1020J)
approaches the previous record high (2007) due to transport increases
and warmer lower layer (LL) temperatures, despite surface temperature
(SST) cooling. In the last decade, warmer LL waters arrive earlier
(1.6 ± 1.1 days/yr), though winds and SST are
typical for recent decades. Maximum summer salinities, likely set in the
Bering Sea, remain remarkably constant (˜33.1 psu) over the decade,
elucidating the stable salinity of the western Arctic cold halocline.
Despite this, freshwater flux variability (strongly driven by transport)
exceeds variability in other Arctic freshwater sources. Remote data
(winds, SST) prove insufficient for quantifying variability, indicating
interannual change can still only be assessed by in situ year-round
measurements.
Microzooplankton grazing impact on phytoplankton was assessed using the Landry–Hassett dilution technique in the Western Arctic Ocean during spring and summer 2002 and 2004. Forty experiments were completed in a region encompassing productive shelf regions of the Chukchi Sea, mesotrophic slope regions of the Beaufort Sea off the North Slope of Alaska, and oligotrophic deep-water sites in the Canada Basin. A variety of conditions were encountered, from heavy sea-ice cover during both spring cruises, moderate sea-ice cover during summer of 2002, and light to no sea ice during summer of 2004, with a concomitant range of trophic conditions, from low chlorophyll-a (Chl-a; <0.5 μg L−1) during heavy ice cover in spring and in the open basin, to late spring and summer shelf and slope open-water diatom blooms with Chl-a >5 μg L−1. The microzooplankton community was dominated by large naked ciliates and heterotrophic gymnodinoid dinoflagellates. Significant, but low, rates of microzooplankton herbivory were found in half of the experiments. The maximum grazing rate was 0.16 d−1 and average grazing rate, including experiments with no significant grazing, was 0.04±0.06 d−1. Phytoplankton intrinsic growth rates varied from the highest values of about 0.4 d−1 to the lowest values of zero to slightly negative growth, on average 0.16±0.15 d−1. Light limitation in spring and post-bloom senescence during summer were likely explanations of observed low phytoplankton growth rates. Microzooplankton grazing consumed 0–120% (average 22±26%) of phytoplankton daily growth. Grazing and growth rates found in this study were low compared to rates reported in another Arctic system, the Barents Sea, and in major geographic regions of the world ocean.
Variability in abundance of virus-like particles (VLP), VLP decay rates and prokaryotic mortality due to viral infection were determined in three Antarctic areas: Bellingshausen Sea, Bransfield Strait and Gerlache Strait, during December 1995 and February 1996. VLP abundance showed very small spatial variability in the three areas (7×106–2×107 VLP ml−1). VLP abundance, on the other hand, decreased one order of magnitude from the surface to the bottom, in two stations where deep vertical profiles were sampled. Low seasonal variability in VLP abundance was found when comparing each area separately. Diel VLP variability was also very low. VLP abundance showed the lowest values when solar irradiation was maximal, in two of the three stations where diel cycles were examined. Viral decay rates (VDR) were determined using KCN in two kinds of experiments. Type 1 experiments were performed in 6 stations to determine viral decay. Type 2 experiments were carried out in 2 stations to examine the influence of temperature and organic matter concentration on viral decay. VDR was not influenced by these parameters. Prokaryotic mortality due to viral infection was always higher than that due to bacterivores in the stations where both factors of prokaryotic mortality were measured. Viral infection accounted for all the prokaryotic heterotrophic production in Bellingshausen Sea and Gerlache Strait and for half of the prokaryotic heterotrophic production in Bransfield Strait. These high values of prokaryotic mortality due to viral infection are difficult to reconcile in nature, and more work is necessary to determine the mechanisms involved in the disappearance of viruses.
A satellite-based study was conducted to document daily changes in net primary production (NPP) by phytoplankton in the Arctic Ocean from 1998 to 2009 using fields of sea ice extent, sea surface temperature, and chlorophyll a concentrations. Total annual NPP over the Arctic Ocean exhibited a statistically significant 20% increase between 1998 and 2009 (range = 441-585 Tg C yr-1), due mainly to secular increases in both the extent of open water (+27%) and the duration of the open water season (+45 days). Increases in NPP over the 12 year study period were largest in the eastern Arctic Ocean, most notably in the Kara (+70%) and Siberian (+135%) sectors. NPP per unit area for the Arctic Ocean averaged 101 g C m-2 yr-1 with no significant change over the study period. In the western sectors, NPP ranged from 71.3 ± 11.0 g C m-2 yr-1 in the Beaufort to 96.9 ± 7.4 g C m-2 yr-1 in the Chukchi, while in the more productive eastern Arctic, annual NPP between 1998 and 2009 ranged from 101 ± 15.8 in the Siberian sector to 121 ± 20.2 in the Laptev. Results of a statistical analysis suggest that between 1979 and 1998 (prior to the launch of SeaWiFS and MODIS), total Arctic NPP likely averaged 438 ± 21.5 Tg C yr-1. Moreover, when summer minimum ice cover drops to zero sometime during the first half of this century, annual NPP in the Arctic Ocean could reach ˜730 Tg C yr-1. Nutrient fluxes into Arctic surface waters need to be better understood to determine if these projected increases are sustainable.
Active heterotrophic bacterial communities exist in all marine environments, and although their growth rates or respiratory rates may be limited by the interaction of low substrate concentrations with temperatures near their lower limit for growth, temperature and substrate concentrations are rarely considered together as limiting factors. Moreover, attempts to evaluate metabolic limits by both temperature and substrate concentration have sometimes led to confusing conclusions, because, while we can measure dissolved organic carbon (DOC) concentrations in natural waters, much of it is not readily available to heterotrophic bacteria. In spite of this procedural limitation, it can be helpful to regard temperature and substrate concentration as potential limiting factors that interact. In temperate ocean surface waters and estuarine waters, where bacterial growth is often reduced in winter, growth and respiration may be increased experimentally either by raising the temperature or by increasing organic substrate concentrations, providing indirect evidence that the limitation is an effect of temperature on substrate uptake or assimilation. Experimental work with bacterial isolates also has shown a temperature-substrate interaction. In permanently cold polar waters, most heterotrophic bacteria appear to be living at temperatures well below their optima for growth. Nevertheless, bacteria in permanently cold surface waters can achieve activity rates in summer that are as high as those in temperate waters. In sea ice, rates of bacterial production are most often low, even though concentrations of substrates, including free amino acids, are sometimes much higher than they are in seawater. This suggests that at sea ice temperatures heterotrophic bacteria have lowered ability to take up or utilize organic substrates.
Despite the relevance of high latitude oceans to models and budgets of biogenic carbon, and the central role of heterotrophic microbes in global biogeochemical cycles, the patterns of energy flow through the lower food web in polar regions are poorly understood. To assess bacteria-based food webs in polar regions, the distribution, growth, and respiration and grazing losses of bacteria must be characterized. We report on the results of a seasonal (late winter through late summer) study of protist grazing in both Resolute Bay, Northwest Territories, Canadian Arctic and McMurdo Sound, Antarctica, and summarize the literature on the relations between the growth and grazing mortality of polar bacterioplankton. Bacterial abundance varied 5-fold in the Arctic and 25-fold in the Antarctic. Average bacterial growth rates ranged from 0.1 to 1.1 d -1. During comparable seasons, bacterial abundance was 2- to 3-fold higher and growth rates were 2- to 3-fold lower in the Antarctic than the Arctic. When grazing occurred, microzooplankton consumed nearly all of the local bacterial production. Grazing losses of bacteria were negligible immediately before and after the phytoplankton bloom. We propose that at these times, bacterioplankton were nutrient limited and protists were predominantly herbivorous. Protozoan grazers appear to alternate between bacterivorous and herbivorous nutritional modes as prey fields change in response to the seasonal progression in submarine irradiance and concentration of dissolved nutrients. The timing and magnitude of the phytoplankton bloom and the duration of the post-bloom period exert a significant influence on the flux of bacterioplankton carbon through microzooplankton and ultimately the coupling of the microbial and metazoan food webs.
The deep, central basins of the Arctic Ocean have been thought to support little biological production. However, summer dissolved oxygen data from the upper mixed layer of the ice-covered central Arctic Ocean yield estimates of primary production which are high enough to account for oxygen utilization in the halocline. Thus, it may not be necessary to postulate either that all significant primary production is on or near the continental shelves, or that organic matter is transported along isopycnals over decades of time to support respiration in the halocline over the deep basins. Because dissolved oxygen data are available for many parts of the Arctic Basin, it may be possible to begin to look for regional differences in productivity. It remains true that more primary production is occurring on the extensive continental shelves than in the basins, and some dissolved organic matter produced on continental shelves must be entering the basins via the halocline. Some of that dissolved organic matter may also contribute to secondary production and to the observed oxygen utilization. However, the evidence from dissolved oxygen measurements, as well as from the observations on consumers, from bacteria to bears, suggests the presence of a complete, locally supported food web in the permanently ice-covered regions of the Arctic Ocean.
On a transect across the Lomonosov Ridge stratified zooplankton tows were made to the bottom at seven stations. A species inventory was established and compared with earlier observations in the Arctic Ocean. Differences between the Amundsen and Makarov basins are relatively small and correspond well with the general circulation patterns for Atlantic, Pacific, and neritic waters, suggesting slow mixing rates for the different basins. There were no remarkable differences in the species composition or their vertical distribution between the two sides of the Lomonosov Ridge. This indicates effective faunistic exchange across the ridge, although several bathy-pelagic species were almost or completely absent on top of the Ridge. Biomass showed a strong gradient along the transect, with a pronounced peak (9.5g dry weight m−2) in the core of Atlantic water over the ridge, and minima over the deep basins. These differences were related to the effect of bottom topography for deep-living species, and the dynamics of the Atlantic layer for the meso- and epipelagic species. The maximum was formed mainly by the copepods Calanus hyperboreus and Metridia longa together with chaetognaths and ostracods. The presence of young developmental stages in some of the abundant species (C. hyperboreus, M. longa) suggests successful reproduction at all stations but C. finmarchicus was almost exclusively represented as old stages and adults. Comparison with earlier data on abundance and biomass from the Canada Basin (Russian Drift station “North Pole-22”) shows a pronounced difference with respect to both absolute quantities and relative composition. The copepod C. finmarchicus is completely absent in the central Canada Basin, and the portion of non-copepod zooplankton is dramatically decreased. This points to a reduced advection of Atlantic water or more severe food conditions in this basin.
Satellite thermal infrared data on surface temperatures provide pan-Arctic coverage from 1981 to 2001 during cloud-free conditions and reveal large warming anomalies in the 1990s compared to the 1980s and regional variability in the trend. The rms error of the derived surface temperatures when compared with in situ data ranges from 1.5 to 3 K over the 20-yr period. Average temperature trends are generally positive at 0.33 6 0.168C decade21 over sea ice, 0.50 6 0.228C decade21 over Eurasia, and 1.06 6 0.228C decade21 over North America. The trend is slightly negative and insignificant at 20.09 6 0.258C decade21 in Greenland with the negatives mainly at high elevations. The trends are also predominantly positive in spring, summer, and autumn causing the lengthening of the melt season by 10-17 days per decade while they are generally negative in winter. The longer-term in situ surface temperature data shows that the 20-yr trend is 8 times larger than the 100-yr trend suggesting a rapid acceleration in the warming that may be associated with the recent change in phase of the Arctic Oscillation that has been linked to increasing greenhouse gases in the atmosphere.
Standing stocks and production rates for phytoplankton and heterotrophic bacteria were examined during four expeditions in the western Arctic Ocean (Chukchi Sea and Canada Basin) in the spring and summer of 2002 and 2004. Rates of primary production (PP) and bacterial production (BP) were higher in the summer than in spring and in shelf waters than in the basin. Most surprisingly, PP was 3-fold higher in 2004 than in 2002; ice-corrected rates were 1581 and 458mgCm−2d−1, respectively, for the entire region. The difference between years was mainly due to low ice coverage in the summer of 2004. The spatial and temporal variation in PP led to comparable variation in BP. Although temperature explained as much variability in BP as did PP or phytoplankton biomass, there was no relationship between temperature and bacterial growth rates above about 0°C. The average ratio of BP to PP was 0.06 and 0.79 when ice-corrected PP rates were greater than and less than 100mgCm−2d−1, respectively; the overall average was 0.34. Bacteria accounted for a highly variable fraction of total respiration, from 3% to over 60% with a mean of 25%. Likewise, the fraction of PP consumed by bacterial respiration, when calculated from growth efficiency (average of 6.9%) and BP estimates, varied greatly over time and space (7% to >500%). The apparent uncoupling between respiration and PP has several implications for carbon export and storage in the western Arctic Ocean.
In this study we evaluate the partitioning of organic carbon between the particulate and dissolved pools during spring phytoplankton blooms in the Ross Sea, Antarctica, and the Sargasso Sea. As part of a multidisciplinary project in the Ross Sea polynya we investigated the dynamics of the dissolved organic carbon (DOC) pool and the role it played in the carbon cycle during the 1994 spring phytoplankton bloom. Phytoplankton biomass during the bloom was dominated by an Antarctic Phaeocystis sp. We determined primary productivity (PP; via H'CO, incubations), particulate organic carbon (POC), bacterial productivity (BP; via ('Hlthymidine incorporation), and DOC during two occupations of 76"3O'S from 175"W to 168"E. Results from this bloom are compared to blooms observed in the Sargasso Sea in the vicinity of the Bermuda Atlantic Time-Series Study station (BATS). We present data that demonstrate clear differences in the production, biolability, and accumulation of DOC between the two ocean regions. Despite four- to fivefold greater PP in the Ross Sea, almost an order of magnitude less DOC (mmol m ?) accumulated during the Ross Sea bloom compared to the Sargasso Sea blooms. In the Ross Sea 89% (- 1 mol C m ') of the total organic carbon (TOC) that accumulated during the bloom was partitioned as POC, with the remaining 11% (- 0.1 mol C rn') partitioned as DOC. In contrast, a mean of 86% (0.7.5-1.0 mol m ') of TOC accumulated as DOC during the 1992, 1993, and 1995 blooms in the Sargasso Sea, with as little as 14% (0.08-0.29 mol C m-?) accumulating as POC. Although a relatively small portion of the fixed carbon was produced as DOC in the Ross Sea, the bacterial carbon demand indicated that a qualitatively more labile carbon was produced in the Ross Sea compared to the Sargasso Sea. There are fundamental differences in organic carbon partitioning between the two systems that may be controlled by plankton community structure and food-web dynamics.
To illuminate the role of Pacific Waters in the 2007 Arctic sea-ice retreat, we use observational data to estimate Bering Strait volume and heat transports from 1991 to 2007. In 2007, both annual mean transport and temperatures are at record-length highs. Heat fluxes increase from 2001 to a 2007 maximum, 5-6 × 1020 J/yr. This is twice the 2001 heat flux, comparable to the annual shortwave radiative flux into the Chukchi Sea, and enough to melt 1/3rd of the 2007 seasonal Arctic sea-ice loss. We suggest the Bering Strait inflow influences sea-ice by providing a trigger for the onset of solar-driven melt, a conduit for oceanic heat into the Arctic, and (due to long transit times) a subsurface heat source within the Arctic in winter. The substantial interannual variability reflects temperature and transport changes, the latter (especially recently) being significantly affected by variability (> 0.2 Sv equivalent) in the Pacific-Arctic pressure-head driving the flow.
The response of the bacterial (Bacteria and Archaea) community to vernal phytoplankton blooms was examined over a grid of stations in Gerlache Strait, Antarctic Peninsula, during the RACER II program (29 October to 26 November 1989). Total bacterial production (0.13 to 10.6 mg C m(-3) d(-1)). based on the incorporation of H-3-leucine into protein, increased with increasing chlorophyll a (chl a) concentration. Bacterial cell-specific growth rate also increased with increasing primary production among stations. Nevertheless, bacterial cell abundance was greatest at the sites that had the lowest chi a concentrations, and declined wherever phytoplankton bloomed. Early bloom communities had few nanoprotist grazers; grazing was undetectable by the Landry-Hassett dilution method during this period. Fully developed bloom communities (chl a > 10 mg m(-3)) had a profusion of nanoprotist grazers (median 3000 cells ml(-1)). Despite relatively low ingestion rates per individual (0.9 bacteria cell(-1) h(-1)), the abundant grazing community kept bacterial biomass very low in Gerlache Strait, to the point that the metabolism of the pelagic bacterial surface community was only a minor fraction of total ecosystem metabolism. Grazing was the apparent cause, although biomass limitation of the bacteria due to lack of resources (e.g. bioavailable dissolved organic matter) may be the ultimate cause of the uncoupling of bacterial and phytoplanktonic communities in these habitats.
Cited By (since 1996): 21, Export Date: 12 April 2011, Source: Scopus