Upwelling System Off Research Articles

Improving the Remote Sensing Retrieval of Phytoplankton Functional Types (PFT) Using Empirical Orthogonal Functions: A Case Study in a Coastal Upwelling Region

An approach that improves the spectral-based PHYSAT method for identifying phytoplankton functional types (PFT) in satellite ocean-color imagery is developed and applied to one study case. This new approach, called PHYSTWO, relies on the assumption that the dominant effect of chlorophyll-a (Chl-a) in the normalized water-leaving radiance (nLw) spectrum can be effectively isolated from the signal of accessory pigment biomarkers of different PFT by using Empirical Orthogonal Function (EOF) decomposition. PHYSTWO operates in the dimensionless plane composed by the first two EOF modes generated through the decomposition of a space–nLw matrix at seven wavelengths (412, 443, 469, 488, 531, 547, and 555 nm). PFT determination is performed using orthogonal models derived from the acceptable ranges of anomalies proposed by PHYSAT but adjusted with the available regional and global data. In applying PHYSTWO to study phytoplankton community structures in the coastal upwelling system off central Chile, we find that this method increases the accuracy of PFT identification, extends the application of this tool to waters with high Chl-a concentration, and significantly decreases (~60%) the undetermined retrievals when compared with PHYSAT. The improved accuracy of PHYSTWO and its applicability for the identification of new PFT are discussed.

Seasonal and annual variability of coastal sulphur plumes in the northern Benguela upwelling system.

We investigated the seasonal and annual variability of surface sulphur plumes in the northern Benguela upwelling system off Namibia because of their significant impacts on the marine ecosystem, fishing industry, aquaculture farming and tourism due to their toxic properties. We identified the sulphur plumes in ocean colour satellite data of the medium resolution imaging spectrometer (MERIS) for the 2002–2012 time period using the differences in the spectral properties of Namibian Benguela optical water types. The sulphur events have a strong seasonal cycle with pronounced main and off-seasons forced by local and remote-driven processes. The main peak season is in late austral summer and early austral autumn at the beginning of the annual upwelling cycle caused by increasing equatorwards alongshore winds. The sulphur plume activity is high between February and April during the seasonal oxygen minimum associated with the seasonal reduction of cross-shore ventilation of the bottom waters, the seasonal southernmost position of the Angola Benguela Frontal Zone, the seasonal maximum of water mass fractions of South Atlantic and Angola Gyre Central Waters as well as the seasonal arrival of the downwelling coastal trapped waves. The off-season is in austral spring and early austral summer during increased upwelling intensity and enhanced oxygen supply. The annual variability of sulphur events is characterized by very high activities in years 2004, 2005 and 2010 interrupted by periods of lower activity in years 2002 to 2003, 2006 to 2009 and 2011 to 2012. This result can be explained by the relative contributions or adding effects of local and remote-driven forces (from the equatorial area). The probability for the occurrence of sulphur plumes is enhanced in years with a lower annual mean of upwelling intensity, decreased oxygen supply associated with decreased lateral ventilation of bottom waters, more southern position of the Angola Benguela Frontal Zone, increased mass fraction of South Atlantic Central Water and stronger downwelling coastal trapped waves. Understanding of the variability and forcing processes of the toxic sulphur events will help in the future to monitor and forecast them as well as to manage their social and economic consequences in the northern Benguela upwelling system off Namibia.

Vertical segregation among pathways mediating nitrogen loss (N<sub>2</sub> and N<sub>2</sub>O production) across the oxygen gradient in a coastal upwelling ecosystem

Abstract. The upwelling system off central Chile (36.5° S) is seasonally subjected to oxygen (O2)-deficient waters, with a strong vertical gradient in O2 (from oxic to anoxic conditions) that spans a few metres (30–50 m interval) over the shelf. This condition inhibits and/or stimulates processes involved in nitrogen (N) removal (e.g. anammox, denitrification, and nitrification). During austral spring (September 2013) and summer (January 2014), the main pathways involved in N loss and its speciation, in the form of N2 and/or N2O, were studied using 15N-tracer incubations, inhibitor assays, and the natural abundance of nitrate isotopes along with hydrographic information. Incubations were developed using water retrieved from the oxycline (25 m depth) and bottom waters (85 m depth) over the continental shelf off Concepción, Chile. Results of 15N-labelled incubations revealed higher N removal activity during the austral summer, with denitrification as the dominant N2-producing pathway, which occurred together with anammox at all times. Interestingly, in both spring and summer maximum potential N removal rates were observed in the oxycline, where a greater availability of oxygen was observed (maximum O2 fluctuation between 270 and 40 µmol L−1) relative to the hypoxic bottom waters ( < 20 µmol O2 L−1). Different pathways were responsible for N2O produced in the oxycline and bottom waters, with ammonium oxidation and dissimilatory nitrite reduction, respectively, as the main source processes. Ammonium produced by dissimilatory nitrite reduction to ammonium (DNiRA) could sustain both anammox and nitrification rates, including the ammonium utilized for N2O production. The temporal and vertical variability of δ15N-NO3− confirms that multiple N-cycling processes are modulating the isotopic nitrate composition over the shelf off central Chile during spring and summer. N removal processes in this coastal system appear to be related to the availability and distribution of oxygen and particles, which are a source of organic matter and the fuel for the production of other electron donors (i.e. ammonium) and acceptors (i.e. nitrate and nitrite) after its remineralization. These results highlight the links between several pathways involved in N loss. They also establish that different mechanisms supported by alternative N substrates are responsible for substantial accumulation of N2O, which are frequently observed as hotspots in the oxycline and bottom waters. Considering the extreme variation in oxygen observed in several coastal upwelling systems, these findings could help to understand the ecological and biogeochemical implications due to global warming where intensification and/or expansion of the oceanic OMZs is projected.

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

The classical paradigm about marine N2 fixation establishes that this process is mainly constrained to nitrogen-poor tropical and subtropical regions, and sustained by the colonial cyanobacterium Trichodesmium spp. and diatom-diazotroph symbiosis. However, the application of molecular techniques allowed determining a high phylogenic diversity and a wide distribution of marine diazotrophs, which extends the range of ocean environments where biological N2 fixation may be relevant. Between February 2014 and December 2015, we carried out 10 one-day samplings in the upwelling system off NW Iberia in order to: 1) investigate the seasonal variability in the magnitude of N2 fixation, 2) determine its biogeochemical role as a mechanism of new nitrogen supply, and 3) quantify the main diazotrophs in the region under contrasting hydrographic regimes. Our results indicate that the magnitude of N2 fixation in this region was relatively low (0.001±0.002 – 0.095±0.024 µmol N m-3 d-1), comparable to the lower-end of rates described for the subtropical NE Atlantic. Maximum rates were observed at surface during both upwelling and relaxation conditions. The comparison with nitrate diffusive fluxes revealed the minor role of N2 fixation (2 fixation activity detected in the region. Quantitative PCR targeting the nifH gene revealed the highest abundances of two sublineages of Candidatus Atelocyanobacterium thalassa or UCYN-A (UCYN-A1 and UCYN-A2) mainly at surface waters during upwelling and relaxation conditions, and of Gammaproteobacteria γ-24774A11 at deep waters during downwelling. Maximum abundance for the three groups were up to 6.7 × 102, 1.5 × 103 and 2.4 × 104 nifH copies L-1, respectively. Our findings demonstrate measurable N2 fixation activity and presence of diazotrophs throughout the year in a nitrogen-rich temperate region.

Sedimentary trace element sinks in a tropical upwelling system

The accumulation of trace elements in sediments from highly productive continental margins may depend on the affinity of these elements for organic matter and their degrees of further incorporation into pyrite (FeS2). We tested the hypothesis that the relative contributions of these geochemical phases play a substantial role as trace element (As, Cd, Cr, Cu, Mn, Ni, and Zn) sinks in the highly bioturbated sediments from the tropical upwelling system off Cabo Frio, southeastern Brazil. Four sediment cores sampled across the Cabo Frio continental shelf were submitted to a sequential extraction procedure performed to separate three different operationally defined fractions, i.e., the geochemical phases soluble in 1 M HCl (considered as the “reactive” fraction), concentrated H2SO4 (considered as the organic matter-bound phase), and concentrated HNO3 (considered as the pyrite-bound phase). The trace metal incorporation into pyrite was assessed by estimating the degree of trace metal pyritization (DTMP), while the pyrite sulfur stable isotope signatures (δ 34SPyr) were used as proxies for sulfur redox cycling intensity. Relative contributions of trace element fixation by organic matter and pyrite were positively correlated for Mn, Cr, and Ni on one hand, and negative correlated for Cu, on the other hand. The positive correlations imply in synergistic roles of these geochemical phases in determining the trace elements sedimentary sinks, while the negative relationship found for Cu reflects differences in the predominant retention mechanisms along with sediment burial. The δ34SPyr signatures were negatively correlated with DTMP values of As, Cd, and Mn, suggesting a diminishing effect of the sulfur redox cycling on trace elements pyritization. These δ 34SPyr signatures were not correlated with DTMP values of Cr, Cu, and Ni, which were dominantly associated with the high organic matter contents found in this upwelling system. The role of pelagic organic matter scavenging of metals and later fueling of benthic microbial sulfate reduction and pyrite accumulation were evidenced as highly variable across the Cabo Frio shelf sediments. Differences in the organic matter accumulation in response to upwelling-enhanced primary productivity and in the intensity of bioturbation-driven sulfur redox cycling help to explain the spatial variability in the biogeochemical processes affecting the sedimentary trace metal sinks.

Relationship between SST gradients and upwelling off Peru and Chile: model/satellite data analysis

ABSTRACTThe upwelling system off Peru/Chile is characterized by significant mesoscale to submesoscale surface variability that results from the instability of the coastal currents (due to the strong vertical and horizontal shears) and to the marked density cross-shore gradients (associated with the mean upwelling). Here we investigate to what extent upwelling intensity can be inferred from sea surface temperature (SST) derived from remote sensing. As a first step in validation, a comparison between SST observations is performed, which indicates that the 1 km gridded multi-scale ultra-high-resolution (MUR) SST data set is defining a zone of maximum SST gradients closer to shore than the low-resolution National Centers for Environmental Information 0.25° resolution data set. Two model versions, at nominal resolutions of 2 km and 4 km, of the Massachusetts Institute of Technology general circulation model are analysed. A high-resolution version at 2 km is examined for the period 13 September 2011–23 January 2013, while a 4 km version is examined for 6 March 2011–22 April 2013. MUR shows maxima SST gradients in the range of 0.03 ± 0.02 K km−1 while the model showed higher gradients around 0.05 ± 0.02 K km−1. Based on coherence spectra, the relationship between upwelling rate (as inferred from the vertical velocity) and SST gradient is documented in the model from intraseasonal to annual timescales. It suggests that changes in SST gradient magnitudes are related to changes in the intensity of coastal upwelling off Peru and Chile. Such a relationship between SST gradients and vertical velocity would allow for the use of satellite-derived SSTs to monitor the intensity of coastal upwelling from the intraseasonal to annual timescales.

Intraseasonal patterns in coastal plankton biomass off central Chile derived from satellite observations and a biochemical model

Abstract Subseasonal (5–130 days) environmental variability can strongly affect plankton dynamics, but is often overlooked in marine ecology studies. We documented the main subseasonal patterns of plankton biomass in the coastal upwelling system off central Chile, the southern part of the Humboldt System. Subseasonal variability was extracted from temporal patterns in satellite data of wind stress, sea surface temperature, and chlorophyll from the period 2003–2011, and from a realistically forced eddy-resolving physical-biochemical model from 2003 to 2008. Although most of the wind variability occurs at submonthly frequencies (

Linking diverse nutrient patterns to different water masses within anticyclonic eddies in the upwelling system off Peru

Abstract. Ocean eddies can both trigger mixing (during their formation and decay) and effectively shield water encompassed from being exchanged with ambient water (throughout their lifetimes). These antagonistic effects of eddies complicate the interpretation of synoptic snapshots typically obtained by ship-based oceanographic measurement campaigns. Here we use a coupled physical–biogeochemical model to explore biogeochemical dynamics within anticyclonic eddies in the eastern tropical South Pacific Ocean. The goal is to understand the diverse biogeochemical patterns that have been observed at the subsurface layers of the anticyclonic eddies in this region. Our model results suggest that the diverse subsurface nutrient patterns within eddies are associated with the presence of water masses of different origins at different depths.

Coastal-oceanic distribution gradient of coccolithophores and their role in the carbonate flux of the upwelling system off Concepción, Chile (36°S)

Coastal-oceanic distribution gradient of coccolithophores and their role in the carbonate flux of the upwelling system off Concepción, Chile (36°S)

Scales and properties of cold filaments in the Benguela upwelling system off Lüderitz

AbstractThe Benguela upwelling system is one of the four strongest upwelling systems in the world. Meso‐scale and submeso‐scale structures like eddies and filaments build up at the front between the cold upwelled water and the warm surface water offshore. From three and a half years of satellite data (MODIS) of sea surface temperature (SST) more than 450 filaments were identified and used for extensive statistics of the scales, the occurrence frequency and the location of filaments in the Lüderitz upwelling cell and its vicinity. In situ data of SST, sea surface salinity and profiles of temperature, salinity and velocity collected during six cruises between September 2011 and February 2014 provide information on the properties of filaments and on their vertical structure. The occurrence of the filaments shows a maximum in austral summer and a minimum in September similar to the seasonal intensity of the upwelling. Within the Lüderitz cell, filaments have a meridional extent between 5 km and 80 km, an offshore extent between 20 km and more than 500 km and a vertical extent between 70 m and 180 m. The temperature within the filaments is 0.25 – 2.5°C lower than the ambient temperature and their salinity is up to 0.6 lower than the ambient salinity. Within the filaments westerly currents with velocities up to 30 cm/s were observed. Mean filament properties and occurrence result in an offshore volume transport of 3 Sv and a heat transport anomaly of −3 × 1013W.

Benthic remineralisation rates in shelf and slope sediments of the northern Benguela upwelling margin

The Benguela Upwelling System off Namibia is a region of intensive plankton production. Remineralisation of this biomass frequently causes the formation of an oxygen minimum zone. A part of the organic matter is further deposited on the broad shelf in form of an extensive mudbelt with high TOC concentrations. During February 2011 we retrieved sediment samples from shelf and slope sediment along the Namibian coast to establish fluxes of nutrients, oxygen, and N2 on the basis of pore water concentrations. In mudbelt sediment, fluxes were estimated as high as 8mmol NH4+ m−2d−1 and 0.9mmol PO43− m−2d−1, which is probably attributable to the activity of large sulphur bacteria. Especially phosphate is mobilised from sediment overlain by oxygen deficient bottom water when and where bottom water oxygen concentrations fall below 50µmoll−1. In comparison to nutrient transport by Southern Atlantic Central Water flowing onto the Namibian shelf, benthic nutrient fluxes of the mudbelt contribute less than 5% to the nutrient budget of the shelf.

Aragonite saturation state in a monsoonal upwelling system off Java, Indonesia

Aragonite saturation state (Ωarag) and its influence from upwelling along the southern coast of Java, Indonesia were examined using carbonate and hydrographic data collected from 22 September to 2 October 2013. Results showed that sea surface Ωarag was lower in the upwelling area (2.97–3.44) than in the nonupwelling area (3.45–3.57), with the lowest value in the eastern part of the study area. We used a two end-member mixing model to separate contributions on Ωarag from two processes associated with upwelling: physical transport vs. biological production. Results indicated that physical transport induced at least a Ωarag decrease of 0.8, whereas biological production caused Ωarag to increase by up to 0.6. Additionally, the influence on Ωarag of interannual upwelling variability modulated by the Indian Ocean Dipole (IOD), a unique climate phenomenon in the Indian Ocean, was roughly estimated. We argue that the effect of interannual upwelling variability modulated by IOD events was possibly larger than what was imposed on Ωarag by increasing atmospheric CO2 levels over the past decade.

Water–air carbon fluxes in the coastal upwelling zone off northern Baja California

An important challenge in marine science is to quantify the air–sea flux of carbon because of its possible relation to climate change and the decreasing pH in seawater. In coastal areas of Mexico, the magnitude and variability of carbon fluxes between the ocean and the atmosphere is almost completely unknown. Therefore, the objective of this research was to determine the variability of air–sea carbon fluxes in the upwelling system off northern Baja California. A one-year time series of carbon fluxes was generated from data on partial pressure of carbon dioxide (pCO2) from an oceanographic buoy installed immediately south of Punta Banda (Ensenada, Baja California), which measured salinity, temperature, and pCO2 in the water (pCO2water) and air (pCO2air) every 3 h during 2009. The study area presented several upwelling events with low sea surface temperatures (≤14 ºC), salinity <34.0, and high values of pCO2water, which resulted in an export of excess CO2 in the water to the atmosphere, identifying the area as a temporary source of CO2. In contrast, during the relaxation of upwelling, low values of pCO2water and intermediate surface temperatures resulted in the sea acting as a sink for CO2. The average daily flux was 0.6 ± 0.11 mmol C m–2 in winter, 2.8 ± 0.27 mmol C m–2 in spring, 0.3 ± 0.11 mmol C m–2 in summer, and –0.3 ± 0.03 mmol C m–2 in autumn, the positive sign indicating fluxes from the water to the air and vice versa. The annual balance of CO2 fluxes was 0.3 ± 0.06 mol C m–2 yr–1; thus, the system turned out to be a net source of CO2 from the ocean to the atmosphere.

Local differences in phytoplankton-bacterioplankton coupling in the coastal upwelling off Galicia (NW Spain)

We simultaneously studied microbial-mediated carbon fluxes at 2 contrasting sites within the coastal upwelling system off Galicia, near A Coruna and Vigo, Spain, over an annual cycle in order to compare the fraction of primary production released as dissolved organic carbon (DOC) and the degree of coupling between bacteria and phytoplankton. A significant fraction of primary production was released as DOC at both sites, averaging ~30%. DOC release ac counted for, on average, 30% of the total bacterial carbon demand, which is indicative of a moderate trophic dependence of bacteria on phytoplankton in these coastal ecosystems. Nevertheless, dif- ferences in hydrographic conditions associated with stronger upwelling pulses off Vigo, and deeper upper mixed layers during the downwelling period off A Coruna, led to significant differ- ences in phytoplankton dynamics and the subsequent direct coupling with heterotrophic bacteria. Strong direct coupling between phytoplankton extracellular release and bacterial production (BP) was found off Vigo, which could be related to the quality of the DOC produced by actively grow- ing phytoplankton. By contrast, DOC release and BP rates were decoupled off A Coruna, likely due to unaccounted DOC associated with indirect trophic processes or to the low availability of freshly produced exudates associated with overflow or photoinhibition mechanisms.

Low nutrient and high chlorophyll a coastal upwelling system – A case study in the southern Taiwan Strait

Using the field data during four summer cruises of 2005, 2006, 2008 and 2013, an interesting phenomenon of low nutrient and high Chl.a (LNHC) was observed in the coastal upwelling system off Dongshan in the southern Taiwan Strait. The results indicated that the upwelling region was dominated by cold (<25 °C) and saline (>33.9 psu) upwelled water, and the concentration of nitrate (<1 μM) and phosphate (<0.1 μM) was very low, while with high Chl.a content (mean 1.98–3.56 μg L−1, maximum 8.3 μg L−1) during the stronger summer upwelling cruise of 2005, 2006 and 2008. The upwelled water originated from the 50–100 m layer off Shanwei with a low concentration of nitrate (<7.5 μmol·L−1) and phosphate (<0.5 μmol·L−1), which was transported to Shantou-Dongshan by the northeastward bottom current. During the time that the upwelled water moved the long-distance (150–300 km) from Shanwei to Shantou-Dongshan and was transported through the euphotic zone alongshore, the phytoplankton grew rapidly due to the favorable temperature, N/P ratio and illumination, and consumed most of the nutrients in the upwelled water. These unique physical, chemical and biological processes are the main reasons for the formation of the low nutrients/high chlorophyll a in this coastal upwelling system in the southern Taiwan Strait.

Description of a new epibiotic relationship (Suctorian–Copepoda) in NE Atlantic waters: from morphological to phylogenetic analyses

AbstractParaeuchaeta hebes is one of the most important carnivorous copepods in the coastal upwelling system off Galician waters (Ría de Vigo, NE Atlantic). A suctorian epibiont of the genus Pelagacineta was found attached to the surface of these copepods. The abundance and distribution on the copepod surface were analysed, taking into account the sex of the crustacean, revealing some preference for females and also a different attachment point in both sexes. The morphological study allowed us to identify a new species of this Suctoria epibiont as Pelagacineta hebensis. Moreover, the 18S rRNA gene was partially sequenced to inspect the phylogenetic position of Pelagacineta hebensis within the subclass Phyllopharyngea. The maximum‐likelihood (ML) tree obtained was consistent with the morphological and with previous molecular studies and showed that P. hebensis belongs to the order Endogenina, as a sister clade of the few taxa sequenced within this order. Including new genetic data to the Endogenina will allow building new hypothesis about the evolution of the most derived clade of suctorians.

Time series of in-situ particle properties and sediment trap fluxes in the coastal upwelling filament off Cape Blanc, Mauritania

We compared particle data from a moored video camera system with sediment trap derived fluxes at ∼1100m depth in the highly dynamic coastal upwelling system off Cape Blanc, Mauritania. Between spring 2008 and winter 2010 the trap collected settling particles in 9-day intervals, while the camera recorded in-situ particle abundance and size-distribution every third day. Particle fluxes were highly variable (40–1200mgm−2d−1) and followed distinct seasonal patterns with peaks during spring, summer and fall. The particle flux patterns from the sediment traps correlated to the total particle volume captured by the video camera, which ranged from1 to 22mm3l−1. The measured increase in total particle volume during periods of high mass flux appeared to be better related to increases in the particle concentrations, rather than to increased average particle size. We observed events that had similar particle fluxes, but showed clear differences in particle abundance and size-distribution, and vice versa. Such observations can only be explained by shifts in the composition of the settling material, with changes both in particle density and chemical composition. For example, the input of wind-blown dust from the Sahara during September 2009 led to the formation of high numbers of comparably small particles in the water column. This suggests that, besides seasonal changes, the composition of marine particles in one region underlies episodical changes. The time between the appearance of high dust concentrations in the atmosphere and the increase lithogenic flux in the 1100m deep trap suggested an average settling rate of 200md−1, indicating a close and fast coupling between dust input and sedimentation of the material.

Diagnosing CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes in the upwelling system off the Oregon–California coast

Abstract. It is generally known that the interplay between the carbon and nutrients supplied from subsurface waters via biological metabolism determines the CO2 fluxes in upwelling systems. However, quantificational assessment of such interplay is difficult because of the dynamic nature of both upwelling circulation and the associated biogeochemistry. We recently proposed a new framework, the Ocean-dominated Margin (OceMar), for semi-quantitatively diagnosing the CO2 source/sink nature of an ocean margin over a given period of time, highlighting that the relative consumption between carbon and nutrients determines if carbon is in excess (i.e., CO2 source) or in deficit (i.e., CO2 sink) in the upper waters of ocean margins relative to their off-site inputs from the adjacent open ocean. In the present study, such a diagnostic approach based upon both couplings of physics–biogeochemistry and carbon–nutrients was applied to resolve the CO2 fluxes in the well-known upwelling system off Oregon and northern California of the US west coast, using data collected along three cross-shelf transects from the inner shelf to the open basin in spring/early summer 2007. Through examining the biological consumption on top of the water mass mixing revealed by the total alkalinity–salinity relationship, we successfully predicted and semi-analytically resolved the CO2 fluxes showing strong uptake from the atmosphere beyond the nearshore regions. This CO2 sink nature primarily resulted from the higher utilization of nutrients relative to dissolved inorganic carbon (DIC) based on their concurrent inputs from the depth. On the other hand, the biological responses to intensified upwelling were minor in nearshore waters off the Oregon–California coast, where significant CO2 outgassing was observed during the sampling period and resolving CO2 fluxes could be simplified without considering DIC/nutrient consumption, i.e., decoupling between upwelling and biological consumption. We reasoned that coupling physics and biogeochemistry in the OceMar model would assume a steady state with balanced DIC and nutrients via both physical transport and biological alterations in comparable timescales.

Episodic events imposed on the seasonal nutrient dynamics of an upwelling system off northeastern Taiwan

An upwelling center has been frequently observed on the shelf break of the East China Sea (ECS) off northeastern Taiwan, where the Kuroshio encounters an abrupt shoaling topography. The region is also located under the track of Asian dust (AD) storms and frequently disturbed by typhoons. To examine the seasonal nutrient dynamics of the upwelling and the potential impacts of these episodic events, transect stations across the upwelling center were visited at intervals of 1–3months from November 2003 to January 2005. The results of the water temperature profiles indicated that the upwelling persisted throughout the year with the most intensified events occurring in winter. Seasonally, a higher nitrate concentration was observed during the stronger upwelling period. The highest nitrate value was however observed during an AD storm, with an average value of 8.3μM for the top 30m. Interestingly, the N/P ratio was larger than 16 during these episodic events, and the N/P ratios were equal to or smaller than 16 during other periods. On average, the estimated values from wet deposition constituted 17.3% of the total estimated nitrogen input. This suggests that atmospheric deposition, particularly during the episodic events, might be an important external nutrient source.

Ammonium production off central Chile (36°S) by photodegradation of phytoplankton-derived and marine dissolved organic matter.

We investigated the production of ammonium by the photodegradation of dissolved organic matter (DOM) in the coastal upwelling system off central Chile (36°S). The mean penetration of solar radiation (Z1%) between April 2011 and February 2012 was 9.4 m, 4.4 m and 3.2 m for Photosynthetically Active Radiation (PAR; 400–700 nm), UV-A (320–400 nm) and UV-B (280–320 nm), respectively. Ammonium photoproduction experiments were carried out using exudates of DOM obtained from cultured diatom species (Chaetoceros muelleri and Thalassiosira minuscule) as well as natural marine DOM. Diatom exudates showed net photoproduction of ammonium under exposure to UVR with a mean rate of 0.56±0.4 µmol L−1 h−1 and a maximum rate of 1.49 µmol L−1 h−1. Results from natural marine DOM showed net photoproduction of ammonium under exposure to PAR+UVR ranging between 0.06 and 0.2 µmol L−1 h−1. We estimated the potential contribution of photochemical ammonium production for phytoplankton ammonium demand. Photoammonification of diatom exudates could support between 117 and 453% of spring-summer NH4 + assimilation, while rates obtained from natural samples could contribute to 50–178% of spring-summer phytoplankton NH4 + requirements. These results have implications for local N budgets, as photochemical ammonium production can occur year-round in the first meters of the euphotic zone that are impacted by full sunlight.