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We studied the input of ammonium coming from the dissolution of food pellets commonly used in salmon farming as well as its potential for mineralization via ammonification and ammonium oxidation by natural bacterioplankton communities. Through experiments in central and southern Chile, we evaluated the potential for photo‐ and bio‐ammonification of food pellets in surface and subsurface waters (30 m depth) using irradiated (PAR + UV; IPS) and non‐irradiated dissolved‐pellet solutions (NIPS). Ammonium release was observed in both IPS and NIPS treatments and occurred at short time scales (

Chile is the second largest global producer of farmed salmon. The growth of salmon production has not been free of environmental challenges, such as the increasing use of pesticides to control the parasitic load of the sea lice Caligus rogercresseyi. The lack of the specificity of pesticides can potentially affect non-target organisms, as well as the structure and functioning of aquatic ecosystems. The aim of this study, was to understand the effect of pesticides on natural microbial communities to the addition of the anti-lice pesticide azamethiphos, deltamethrin and emamectin benzoate, and their potential impact in ammonium uptake rates in the coast off central-southern Chile and Northern Patagonia. The addition of pesticides on natural microbial communities resulted in a rapid response in ammonium uptake, which was significant for the single use of pesticide, azamethiphos and emamectin benzoate, as well as the combination, azamethiphos, deltamethrin and emamectin benzoate. In northern Patagonia, azamethiphos addition produced a 53% decrease in photoautotrophic uptake. However, an increase, although variable, was observed in chemoautotrophic uptake. Emamectin benzoate produced a 36 to 77% decrease in chemo and photoautotrophic ammonium uptake, respectively. The combined use of pesticides, also produced up to 42% decrease in both photo and chemoautotrophic assimilation. We conclude that the use of pesticides in salmon farming produces diverse responses at the microbial level, stimulating and/or inhibiting microbial communities with subsequent impact on nitrogen budgets. Further studies are necessary to understand the impact of pesticides in the ecology of central-southern and northern Patagonia, Chile.

The landscapes of high-altitude wetland ecosystems are characterized by different kinds of aquatic sites, including ponds holding conspicuous microbial life. Here, we examined a representative pond of the wetland landscape for dynamics of greenhouse gases, and their association with other relevant biogeochemical conditions including diel shifts of microbial communities' structure and activity over two consecutive days. Satellite image analysis indicates that the area of ponds cover 238 of 381.3 Ha (i.e., 62.4%), representing a significant landscape in this wetland. Solar radiation, wind velocity and temperature varied daily and between the days sampled, influencing the biogeochemical dynamics in the pond, shifting the pond reservoir of inorganic versus dissolved organic nitrogen/phosphorus bioavailability, between day 1 and day 2. Day 2 was characterized by high dissolved organic nitrogen/phosphorus and N2O accumulation. CH4 presented a positive excess showing maxima at hours of high radiation during both days. The microbial community in the sediment was diverse and enriched in keystone active groups potentially related with GHG recycling including bacteria and archaea, such as Cyanobacteria, Verrucomicrobia, Rhodobacterales and Nanoarchaeaota (Woesearchaeia). Archaea account for the microbial community composition changes between both days and for the secondary productivity in the water measured during day 2. The results indicate that an intense recycling of organic matter occurs in the pond systems and that the activity of the microbial community is correlated with the availability of nutrients. Together, the above results indicate a net sink of CO2 and N2O, which has also been reported for other natural and artificial ponds. Overall, our two-day fluctuation study in a representative pond of a high-altitude wetland aquatic landscape indicates the need to explore in more detail the short-term besides the long-term biogeochemical variability in arid ecosystems of the Andes plateau, where wetlands are hotspots of life currently under high anthropogenic pressure.

We investigated the distribution of microplastics in the water column along a large remote estuarine system located between the Northern and Southern Patagonian Ice Fields in Chilean Patagonia, and connected with the Pacific Ocean through the Gulf of Penas. Microplastic particles were found in all samples, with abundances ranging from 0.1 to 7 particles/m3. Polymers identified were principally acrylics, PET, and cellophane. The average abundance of microplastics in surface waters was similar along the whole estuary (0.4 ± 0.3 particles/m3) with acrylics and epoxy resins being more abundant near Caleta Tortel, the only small village in the area. The observed higher abundance of microplastics in the deeper waters towards the Gulf of Penas points to intrusions of subsurface waters transporting plastic particles from the ocean into the channel system. This underlines the potential of ocean currents in transporting plastic pollution into pristine fjords and channels in Chilean Patagonia.

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Northern Chile harbors different bioclimatic zones including hyper-arid and arid ecosystems and hotspots of microbial life, such as high altitude wetlands, which may contribute differentially to greenhouse gases (GHG) such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). In this study, we explored ground level GHG distribution and the potential role of a wetland situated at 3800 m.a.s.l, and characterized by high solar radiation 40 phyla), including archaea and bacteria potentially active in the different matrices studied (water, sediments and mats). Functional microbial groups associated with GHG recycling were detected at low frequency, i.e.

Aquaculture has become an important component of Chilean economy, especially in the southern region, where salmon farming is an active industry. However, high density in salmon cages can increase stress and susceptibility to parasitic outbreaks of the copepod Caligus rogercresseyi. The pesticides used against sea lice can have potential effects on non-target microbiota and on the structure and functioning of aquatic ecosystems. The objective of this study was to investigate the response of natural microbial communities to the addition of the anti-lice pesticides azamethiphos, deltamethrin and emamectin benzoate and their potential impact on photoautotrophic and chemoautotrophic carbon fixation in central-southern Chile (37°S to 42°S). The addition of pesticides on primary production samples was related to changes in carbon uptake, which were significant if a single pesticide was applied, mainly emamectin benzoate and azametiphos. In surface waters of central Chile, emamectin benzoate produced a 60–90% decrease for both photo and chemoautotrophic carbon fixation. Enhanced rates were also observed for in situ primary production as a result of azamethiphos addition in northern Patagonia. Such stimulation, although limited, was possibly related to the supply of nitrogen and phosphate for phytoplankton requirements by this organophosphate compound.

Chromophoric dissolved organic matter (CDOM) is a biologically active component of dissolved organic matter that influences the optical properties of aquatic environments, thereby playing an important role in the photochemical and photobiology processes occurring in the euphotic layer. Three cruises were carried out at three contrasting oceanographic areas off central and southern Chile (36.8°S, 44.6°S, 54.9°S) between November 2013 and July 2017 in order to assess the variability of the CDOM pool's optical properties (a325, SCDOM, SR). Marked differences, including vertical variability, were found for a325, SCDOM, and SR within and among each of the three zones, although surface DOC concentrations remained constant at all sites (87.59 ± 26.04 µmol L−1). At the coastal area close to the Biobio River (36.8°S), a325 values varied between 0.26–1.93 m−1 with maximum measured in surface waters. For the Puyuhuapi fjord in northern Patagonia (44.6°S), a325 ranged between 0.69 and 3.86 m−1, while in the Beagle Channel (54.9°S) it oscillated between 0.59 and 1.73 m−1. We identified the terrestrial organic matter input to surface waters as the main factor influencing surface CDOM concentrations in central Chile. In the Puyuhuapi fjord, terrestrial influence was heterogeneous because of its seasonal variability of the Channel but it was overall less significant than in situ produced CDOM. We also observed the lowest contribution of terrestrial CDOM inputs compared to the in situ production in southern Patagonia (Beagle Channel). We observed that the distribution of CDOM is potentially dominated by biological in situ production below the surface freshwater-influenced layer in the southern area while highly influenced by terrestrial input in the coastal upwelling area at 36°S. Overall, this study shows that the optical properties of dissolved organic matter (mainly SR) can be suitable for identifying different CDOM pools in coastal areas of contrasting oceanographic conditions. In stratified systems, freshwater inputs determine the distribution of surface CDOM while in well-mixed conditions, the in situ production determine the general CDOM concentration.

Composition, carbon and nitrogen uptake, and gene transcription of microbial mat communities in Porcelana neutral hot spring (Northern Chilean Patagonia) were analyzed using metagenomics, metatranscriptomics and isotopically labeled carbon (H13CO3) and nitrogen (15NH4Cl and K15NO3) assimilation rates. The microbial mat community included 31 phyla, of which only Cyanobacteria and Chloroflexi were dominant. At 58°C both phyla co-occurred, with similar contributions in relative abundances in metagenomes and total transcriptional activity. At 66°C, filamentous anoxygenic phototrophic Chloroflexi were >90% responsible for the total transcriptional activity recovered, while Cyanobacteria contributed most metagenomics and metatranscriptomics reads at 48°C. According to such reads, phototrophy was carried out both through oxygenic photosynthesis by Cyanobacteria (mostly Mastigocladus) and anoxygenic phototrophy due mainly to Chloroflexi. Inorganic carbon assimilation through the Calvin–Benson cycle was almost exclusively due to Mastigocladus, which was the main primary producer at lower temperatures. Two other CO2 fixation pathways were active at certain times and temperatures as indicated by transcripts: 3-hydroxypropionate (3-HP) bi-cycle due to Chloroflexi and 3-hydroxypropionate-4-hydroxybutyrate (HH) cycle carried out by Thaumarchaeota. The active transcription of the genes involved in these C-fixation pathways correlated with high in situ determined carbon fixation rates. In situ measurements of ammonia assimilation and nitrogen fixation (exclusively attributed to Cyanobacteria and mostly to Mastigocladus sp.) showed these were the most important nitrogen acquisition pathways at 58 and 48°C. At 66°C ammonia oxidation genes were actively transcribed (mostly due to Thaumarchaeota). Reads indicated that denitrification was present as a nitrogen sink at all temperatures and that dissimilatory nitrate reduction to ammonia (DNRA) contributed very little. The combination of metagenomic and metatranscriptomic analysis with in situ assimilation rates, allowed the reconstruction of day and night carbon and nitrogen assimilation pathways together with the contribution of keystone microorganisms in this natural hot spring microbial mat.

High-altitude wetland holds freshwater springs, evaporitic ponds and lagoon with variable salinity and nutrients, potentially influencing the ecology of nitrifying communities. In this study, nitrifying microorganisms in Salar de Huasco (Chile) were surveyed to determine bacterial and archaeal contribution to ammonium (AO), nitrite oxidation (NO), ammonium uptake (AU) during wet and dry seasons. The activity signals from these groups were assessed by specific amoA-qPCR transcription, 15N tracer studies and addition of group specific inhibitor experiments for nitrifying microorganisms (N1-guanyl-1, 7-diaminoheptane [GC7]-archaeal specific and allylthiourea [ATU]-bacterial specific). Nitrifying communities, i.e. Nitrosopumilus, Nitrosospira, Nitrosomonas, Kuenenia and Nitrospira, were more frequent (∼0.25% of 16S rRNA sequences) at low salinity sites. Bacterial amoA-qPCR transcripts also increased at low salinity and along in situ ammonium increase observed between wet/dry seasons. Nutrient changes through time and 15N tracer experiments results showed that AO and NO were detected and peaked mainly at low salinity-high ammonium sites (0.3 μM), whereas AU was predominant at evaporitic sites. Our results indicate that salinity and ammonium affect the nitrifying communities that are potentially more active at low-salinity sites but persistent at saltier evaporitic areas of the wetland when ammonium is available.

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International audience; Cyanobacteria from Subsection V (Stigonematales) are important components of microbial mats in non-acidic terrestrial hot springs. Despite their diazotrophic nature (N2 fixers), their impact on the nitrogen cycle in such extreme ecosystems remains unknown. Here, we surveyed the identity and activity of diazotrophic cyanobacteria in the neutral hot spring of Porcelana (Northern Patagonia, Chile) during 2009 and 2011–2013. We used 16S rRNA and the nifH gene to analyze the distribution and diversity of diazotrophic cyanobacteria. Our results demonstrate the dominance of the heterocystous genus Mastigocladus (Stigonematales) along the entire temperature gradient of the hot spring (69–38 °C). In situ nitrogenase activity (acetylene reduction), nitrogen fixation rates (cellular uptake of 15N2) and nifH transcription levels in the microbial mats showed that nitrogen fixation and nifH mRNA expression were light-dependent. Nitrogen fixation activities were detected at temperatures ranging from 58 °C to 46 °C, with maximum daily rates of 600 nmol C2H4 cm−2 per day and 94.1 nmol N cm−2 per day. These activity patterns strongly suggest a heterocystous cyanobacterial origin and reveal a correlation between nitrogenase activity and nifH gene expression during diurnal cycles in thermal microbial mats. N and C fixation in the mats contributed ~3 g N m−2 per year and 27 g C m−2 per year, suggesting that these vital demands are fully met by the diazotrophic and photoautotrophic capacities of the cyanobacteria in the Porcelana hot spring.

The high availability of electron donors occurring in coastal upwelling ecosystems with marked oxyclines favours chemoautotrophy, in turn leading to high N2O and CH4 cycling associated with aerobic NH4+ (AAO) and CH4 oxidation (AMO). This is the case of the highly productive coastal upwelling area off central Chile (36° S), where we evaluated the importance of total chemolithoautotrophic vs. photoautotrophic production, the specific contributions of AAO and AMO to chemosynthesis and their role in gas cycling. Chemolithoautotrophy was studied at a time-series station during monthly (2007–2009) and seasonal cruises (January 2008, September 2008, January 2009) and was assessed in terms of the natural C isotopic ratio of particulate organic carbon (δ13POC), total and specific (associated with AAO and AMO) dark carbon assimilation (CA), and N2O and CH4 cycling experiments. At the oxycline, δ13POC averaged −22.2‰; this was significantly lighter compared to the surface (−19.7‰) and bottom layers (−20.7‰). Total integrated dark CA in the whole water column fluctuated between 19.4 and 2.924 mg C m−2 d−1, was higher during active upwelling, and contributed 0.7 to 49.7% of the total integrated autotrophic CA (photo plus chemoautotrophy), which ranged from 135 to 7.626 mg C m−2 d−1, and averaged 20.3% for the whole sampling period. Dark CA was reduced by 27 to 48% after adding a specific AAO inhibitor (ATU) and by 24 to 76% with GC7, a specific archaea inhibitor. This indicates that AAO and AMO microbes (most of them archaea) were performing dark CA through the oxidation of NH4+ and CH4. Net N2O cycling rates varied between 8.88 and 43 nM d−1, whereas net CH4 cycling rates ranged from −0.41 to −26.8 nM d−1. The addition of both ATU and GC7 reduced N2O accumulation and increased CH4 consumption, suggesting that AAO and AMO were responsible, in part, for the cycling of these gases. These findings show that chemically driven chemolithoautotrophy (with NH4+ and CH4 acting as electron donors) could be more important than previously thought in upwelling ecosystems, raising new questions concerning its relevance in the future ocean.

Tracer experiments ( 15 N and 13 C) were carried out off the Peruvian upwelling system in order to evaluate the potential role of regenerated nitrogen in sustaining biological productivity in surface waters (0–15 m depth range). Rates of daily carbon uptake in surface waters showed values ranging between 6 and 200 mg C m −3 d −1 . Maximum integrated rates were obtained in northern Peru (up to 5.5 g m −2 d −1 ). Parallel measurements of nitrogen uptake showed a stronger utilization of ammonium compared to nitrate at all stations, with surface ammonium uptake values reaching up to 0.15 μmol N L −1 d −1 . Ammonium was also actively regenerated in surface waters, at rates as high as 0.5 μmol N L −1 d −1 , whereas ammonium oxidation to nitrate (nitrification) occurred at lower rates ( −1 d −1 ). Results obtained in this study showed active regeneration of N within the euphotic layer. Regenerated nitrate and ammonium are, therefore, available for primary producers within the euphotic zone and sustain a fraction of the observed photoautotrophic carbon fixation. The contribution of nitrogen regenerating processes to primary production in terms of DIN could represent as much as 50% of NH 4 assimilated in surface waters (through ammonium regeneration) as well as a variable fraction (2–16%) of nitrate through nitrification.

International audience; Although N 2 fixation could represent a supplementary source of bioavailable nitrogen in coastal upwelling areas and underlying oxygen minimum zones (OMZs), the limited data available prevent assessing its variability and biogeochemical significance. Here we report the most extensive N 2 fixation data set gathered to date in the upwelling area off central Chile (368S). It covers interannual to high frequency time scales in an area of about 82,500 km 2 in the eastern South Pacific (ESP). Because heterotrophic N 2 fixation may be regulated by DOM availability in the ESP, we conducted experiments at different oxygen conditions and included DOM amendments in order to test diazotrophic activity. Rates in the euphotic zone showed strong temporal variability which resulted in values reaching 0.5 nmol L 21 d 21 in 2006 (average 0.32 6 0.17 nmol L 21 d 21) and up to 126.8 nmol L 21 d 21 (average 24.75 6 37.9 nmol L 21 d 21) in 2011. N 2 fixation in subsurface suboxic conditions (1.5 6 1.16 nmol L 21 d 21) also occurred mainly during late summer and autumn while virtually absent in winter. The diversity of diazotrophs was dominated by heterotrophs, with higher richness in surface compared to OMZ waters. Rates in oxygen depleted conditions could exceed values obtained in the euphotic layer, but rates were not dependent on the availability of dissolved organic matter. N 2 fixation also showed a positive correlation with total chlorophyll and the C:N ratio of phytoplankton, but not to the P excess compared to N. We conclude that the diazotrophic community responds to the composition of phytoplankton rather than the extent of N deficiency and the availability of bulk DOM in this system.

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We analyzed the coupling between bacterioplankton and phytoplankton in Laguna de Rocha through an experimental approach. A freshwater zone of high turbidity and macrophytes growth and a brackish zone of higher light penetration and lower macrophytes biomass characterize this coastal lagoon. It has been shown that dissolved inorganic nitrogen, especially NH4+, has decreased to undetectable levels during the last decade. One hypothesis for this trend is the rapid removal by phytoplankton and bacterioplankton uptake. In an attempt to test this, we performed incubations using lagoon water from both zones split in two treatments (pre-filtered by 1.2 µm and unfiltered water) and amended with 15N-NH4+. After 4 h incubation we found that in both zones bacterioplankton showed significantly higher NH4+ uptake rates when incubated together with phytoplankton and that uptake rates of both microbial communities were higher in freshwater incubations. These results suggest that bacterial NH4+ uptake would be coupled to phytoplankton-derived exudates and hence that depletion of dissolved NH4+ in this system could be linked to rapid microbial uptake. The degree of this coupling would vary according to hydrological dynamics in this ecosystem.

It has been proposed that current levels of ultraviolet B (UVB) radiation could cause lethal or sublethal effects on fish embryos located in the upper layers of the water column. Observed levels of UVB off central Chile (36°S, 73°W) indicate that planktonic fish embryos could be exposed to harmful UVB radiation. From July 2011 to January 2012 embryos from anchoveta (Engraulis ringens) and common sardine (Strangomera bentincki) were used to test experimentally whether the UVB levels in central Chile produce lethal or sublethal effects in epipelagic fish embryos. Simultaneously, whether the embryos might be exposed to harmful UVB levels in the field was investigated. Our experimental results show that UVB may cause a decrease in hatching success, changes in buoyancy and embryonic malformations. These results, along with the observed vertical distribution of embryos and UVB radiation levels in the field during late spring suggest that lethal and sublethal effects may be occurring in the embryos of both species.

The main processes involved in nitrogen cycling (as NO3 � and NH4 + assimilation and their re- generation) were studied using N-tracer experiments in the coastal upwelling system off central Chile (36°S). The study area shows seasonal surface fertil- ization and the development of subsurface hypoxia in austral spring and summer. The rates of NO3 � uptake during active upwelling were 5 times higher than in non-upwelling seasons. Uptake of NH4 + was almost half that of NO3 � uptake rates during upwelling peri- ods, and similar to NO3 � uptake rates in the absence of upwelling. Nitrification experiments showed higher rates during active, compared to non-active, upwelling seasons. NH4 + oxidation was coupled with NO2 � oxi- dation in near bottom (suboxic) waters, while in the oxycline (hypoxic water), total fluxes of NO3 � regener- ation via nitrification resulted from higher activity of NO2 � oxidation compared to NH4 + oxidation. On the other hand, archaeal NH4 + oxidation had the potential for processing a large fraction of NH4 + and could therefore co-occur with bacterial NO2 � oxidation. NH4 + utilization in this coastal up welling is thus in the same range as NO3 � assimilation. NH4 + oxidation is af- fected by oxygen concentration in the water column, leading to occasional decoupling of NH4 + and NO2 � oxidation, particularly at the oxycline. This study gives preliminary evidence of the importance of archaeal- bacterial interactions in the nitrification process and highlights the role of ammonium in fueling annual primary production in coastal upwelling systems.

Mesoscale features are recurrently found in the Coastal Transition Zone (CTZ) off central-southern Chile (∼35–40°S), a region characterized by seasonal wind-driven coastal upwelling. In this study, oceanographic data from two consecutive cruises carried out during the upwelling season (January 2009) were combined with satellite time series data in order to characterize the structure and evolution of mesoscale eddies and to explore their influence on phytoplankton structure and nutrient distribution, in the context of organic carbon exchanges between the coastal upwelling zone and the CTZ. Two coastally-generated, contiguous mesoscale eddies (∼2 months old) were characterized: a surface cyclonic eddy (CC1) and a subsurface anticyclonic eddy (AA1). The distributions of salinity and dissolved oxygen, together with nutrients, suggested that both eddies were sites of vertical injection of high nutrient concentrations from the Equatorial Subsurface Water mass (ESSW) into the upper layer in the CTZ. In addition, eddy AA1, in combination with an upwelling plume, was involved in the offshore advection of nutrients from the core of the ESSW located in the coastal zone. At shallow depths (

Nitrogen excretion by zooplankton was studied in the coastal upwelling area off Concepcion (36° S) in central/southern Chile. Experiments were performed using numerically dominant copepods of the mesozooplankton community during two contrasting periods (winter and spring 2009). The experimental setup was aimed at characterizing the organic and inorganic nitrogen excretion compounds, such as dissolved organic nitrogen (including urea), and ammonium. Moreover, the seasonal variability of excretion products was studied and analyzed as a function of microphytoplankton and nanophytoplankton abundance, and nutrient concentrations (NH 4 + , NO 3 − , NO 2 − , PO 4 −3 , Si(OH) 4 ). We tested the hypothesis that ammonium supplied by mesozooplankton (represented by copepods) can support a significant fraction of the nitrogen demand of phytoplankton. To do so, excretion rates were determined and the budget of each nitrogen compound was assessed. Results showed that among the organic and inorganic nitrogen compounds excreted by copepods off central Chile, total DON (i.e., ureic and non-ureic components) may be the most important excretion component, contrasting with the view of ammonium as the main excretion compound of this group. We conclude that total (organic and inorganic) nitrogen excretion per group of dominant copepods in the continental shelf could have a significant impact on the nitrogen requirements of phytoplankton communities, potentially sustaining up to 26.7% of the phytoplankton ammonium demand.

The flux of organic material sinking to depth is a major control on the inventory of carbon in the ocean. To first order, the oceanic system is at equilibrium such that what goes down must come up. Because the export flux is difficult to measure directly, it is routinely estimated indirectly by quantifying the amount of phytoplankton growth, or primary production, fuelled by the upward flux of nitrate. To do so it is necessary to take into account other sources of biologically available nitrogen. However, the generation of nitrate by nitrification in surface waters has only recently received attention. Here we perform the first synthesis of open-ocean measurements of the specific rate of surface nitrification and use these to configure a global biogeochemical model to quantify the global role of nitrification. We show that for much of the world ocean a substantial fraction of the nitrate taken up is generated through recent nitrification near the surface. At the global scale, nitrification accounts for about half of the nitrate consumed by growing phytoplankton. A consequence is that many previous attempts to quantify marine carbon export, particularly those based on inappropriate use of the f-ratio (a measure of the efficiency of the 'biological pump'), are significant overestimates.

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Three cruises were carried out as part of the Programme Ocean Multidisciplinaire Meso Echelle (POMME) program between February and October 2001, in the northeast Atlantic basin (16°W–22°W, 38°N–45°N). The hydrographic survey showed a permanent frontal zone at 41°N–42°N characterized by an intense isopycnal displacement. North of 41°N, average mixed layer depths evolved from 100 m in winter to 75 m in spring. In that area, vertical mixing was modulating the available nutrients in the surface layer, which resulted in surface nitrate concentrations close to 7 μM at 44.5°N. The area located south of the front was characterized by low surface nutrient values (∼1 μM NO 3 ) caused mainly by biological activity. During these surveys, several mesoscale features were studied. Cyclonic structures at 42°N had a continuous effect on the vertical nitrate distribution in the euphotic layer from winter to spring. The presence of an anticyclonic structure north of the frontal zone resulted in lifting the seasonal thermocline in summer along with a depression of the permanent thermocline, which induced anomalously high nitrate concentrations (5–9 μM) between 50 and 300 m depth. South of the frontal zone, the presence of an anticyclonic eddy at 40.5°N also enhanced nutrient enrichment in the surface layer. As the presence of mesoscale eddies influenced the nutrient distribution, the processes responsible for nutrient injection to the euphotic zone are discussed.

[1] A dual 13C/15N isotopic technique was used to estimate carbon and nitrogen uptake rates during the 1-year-long Programme Ocean Multidisciplinaire Meso Echelle (POMME) experiment in the northeast basin of the Atlantic Ocean (16°–22°W, 38°–45°N). Rates of primary production ranged from 2.26 μmoles L−1 d−1 in winter to maximum surface values of 7.8 μmoles L−1 d−1 during the spring season. Nitrate assimilation was intense since the beginning of winter and accounted for 3–7 mmoles m−2 d−1 in the first 80 m depth of the water column. Rates increased to 5–9 mmoles m−2 d−1 during the spring season. NO3− uptake rates during the late summer season were close to winter values. Ammonium assimilation activity balanced the uptake of nitrate for all seasons. The seasonal f ratio was close to 0.28 in winter, increased to 0.34 in spring, and, finally, decreased to 0.25 in late summer. Annual carbon uptake rates for the entire POMME area accounted for 16.9 moles C m−2 yr−1. From this result, annual new production could be estimated to 5.7 moles C m−2 yr−1.