Dissolved Organic Matter Cycling in the Coastal Upwelling System Off Central Peru During an “El Niño” Year

The Peruvian upwelling system (PUS) is among the most productive regions in the ocean, with high rates of primary production and an intense oxygen minimum zone (OMZ). The main perturbation of this system is associated to “El Niño” (EN), which affects water mass distribution and reduces primary production. Previous studies in the PUS provided first insights into the dynamics of dissolved organic matter (DOM), but high-resolution studies involving the molecular characterization of the DOM pool to reveal the processes that affect the carbon cycle in this highly productive system are lacking. We characterized the molecular composition of solid-phase extractable DOM (SPE-DOM) in the coastal upwelling system off Central Peru and linked it to specific processes that affect DOM cycling. Seasonal sampling (April, August, and December) was carried out off Central Peru (12°S) during 2015, a low productivity year marked by EN conditions. The DOM molecular composition was obtained via Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Solid-phase extractable dissolved organic carbon (SPE-DOC) concentrations showed significant differences (p < 0.05) between the water masses present off central Peru. In order to explore if changes in SPE-DOC concentrations were the result of water mass mixing, we applied a conservative mixing (CM) model. The model revealed a non-conservative behavior of SPE-DOC and allowed us to identify two distinct groups of samples with increased and decreased SPE-DOC concentrations, respectively, and one group of samples inside the CM range. Differences in environmental parameters characterizing these groups were in accordance with respective processes associated to production and degradation of SPE-DOC. The trends observed for molecular parameters revealed the imprint of processes related to DOM production and DOM degradation, both biotic (microbial degradation) and abiotic (photodegradation). Our study suggests that even under low productivity conditions like EN, there is an active cycling of the DOM pool off central Peru

Factors controlling plankton community production, export flux, and particulate matter stoichiometry in the coastal upwelling system off Peru

Eastern boundary upwelling systems (EBUS) are among the most productive marine ecosystems on Earth. The production of organic material is fueled by upwelling of nutrient-rich deep waters and high incident light at the sea surface. However, biotic and abiotic factors can modify surface production and related biogeochemical processes. Determining these factors is important because EBUS are considered hotspots of climate change, and reliable predictions of their future functioning requires understanding of the mechanisms driving the biogeochemical cycles therein. In this field experiment, we used in situ mesocosms as tools to improve our mechanistic understanding of processes controlling organic matter cycling in the coastal Peruvian upwelling system. Eight mesocosms, each with a volume of ∼55 m3, were deployed for 50 d ∼6 km off Callao (12∘ S) during austral summer 2017, coinciding with a coastal El Niño phase. After mesocosm deployment, we collected subsurface waters at two different locations in the regional oxygen minimum zone (OMZ) and injected these into four mesocosms (mixing ratio ≈1.5 : 1 mesocosm: OMZ water). The focus of this paper is on temporal developments of organic matter production, export, and stoichiometry in the individual mesocosms. The mesocosm phytoplankton communities were initially dominated by diatoms but shifted towards a pronounced dominance of the mixotrophic dinoflagellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in surface layers. The community shift coincided with a short-term increase in production during the A. sanguinea bloom, which left a pronounced imprint on organic matter C : N : P stoichiometry. However, C, N, and P export fluxes did not increase because A. sanguinea persisted in the water column and did not sink out during the experiment. Accordingly, export fluxes during the study were decoupled from surface production and sustained by the remaining plankton community. Overall, biogeochemical pools and fluxes were surprisingly constant for most of the experiment. We explain this constancy by light limitation through self-shading by phytoplankton and by inorganic nitrogen limitation which constrained phytoplankton growth. Thus, gain and loss processes remained balanced and there were few opportunities for blooms, which represents an event where the system becomes unbalanced. Overall, our mesocosm study revealed some key links between ecological and biogeochemical processes for one of the most economically important regions in the oceans

Molecular characterization of the sedimentary organic matter deposited off central Peru (12 - 14ºS): first insights into preservation processes

International audiencePhytoplankton production represents the ultimate source of organic matter in the ocean; thus, the study of organic compounds can give us information related to organic matter (OM) origin and transformations. Usually less than 1% of the OM produced in the ocean surface reaches the seafloor, although in highly productive regions nearly 10% can be buried and subjected to further degradation. The Peruvian upwelling system is among the most productive marine ecosystems in the world ocean, with high primary production sustained mainly in a year-round upwelling. Along the Peruvian continental margin, variations in primary production, bottom dissolved oxygen, and depth influence OM accumulation and preservation, and thus determine the existence of different depositional environments. Previous geochemical and palaeoceanographic studies have shown that the best records of well-preserved OM are found towards the central area of the Peruvian continental margin, between 12°S and 14°S. Therefore, the study of organic compounds, particularly lipids, deposited in surface sediments could give us information regarding early diagenetic processes related to OM degradation/preservation. The objective of this study was to characterize both the solvent extractable OM fraction (i.e. free lipids) and the insoluble OM fraction (i.e. protokerogen) in order to elucidate possible preservation mechanisms involved in OM accumulation. A total of 14 surface sediment samples from different locations between 12°S and 14°S were analyzed by means of gas chromatography mass spectrometry. Organic compounds such as short-chain and long-chain alkanes and fatty acids were quantified in the solvent-extractable OM fraction, which allowed the calculation of a pristane/phytane index and a carbon preference index. In the insoluble OM fraction, alkanes and fatty acids were also quantified together with dithiophene and benzothiophene compounds and organic sulfur heterocompounds. Overall, our results allowed a detailed geochemical molecular characterization of the OM deposited in surface sediments beneath one of the most productive areas of the Peruvian coast. The differences observed in both the n-alkane and fatty acids distribution between the solvent-extractable OM fraction and the insoluble OM fraction, together with the quantification of sulfur compounds in the insoluble fraction, suggests that complex diagenetic processes occur in surface sediments. An important part of the freshly-produced OM in the highly productive surface waters off central Peru reaches the seafloor and undergoes preservation mechanisms mainly related to natural sulfurization and selective preservation, tightly coupled to the reduced conditions that characterize surface sediments in the area

Latitudinal variability of preserved sedimentary organic matter along the Peruvian continental margin as inferred from petrographic and geochemical properties

International audienceProcesses controlling preservation and accumulation of organic matter (OM) in marine sediments are highly sensitive towards both natural and anthropogenic impacts. Thus, the sedimentary OM is considered a key proxy for the characterization of recent and past sedimentary environments. Along the Peruvian continental margin (PCM), high primary production and an intense oxygen minimum zone (OMZ) favor the accumulation and preservation of OM in sediments. Spatial variations in oceanographic conditions impact on both productivity and the redox conditions, in particular the OMZ intensity. These factors in turn lead to spatial differences in depositional conditions and OM accumulation and preservation. Therefore, this study aims at characterizing nature and types of recent sedimentary OM along the PCM covering a wide latitudinal and bathymetric range. With this purpose, a total of 29 surface sediment samples were collected from Tumbes (3°S) to San Juan (15°S) and analyzed by means of organic petrography (palynofacies analysis) and Rock-Eval pyrolysis. Results allowed characterizing the OM from recent sediments. Along the PCM, the sedimentary OM is predominantly amorphous, revealing its overwhelming marine origin. The highest abundances of gelified amorphous OM, total organic carbon, S1 and S2 Rock-Eval fractions occurred towards the southern part of the study area, especially off Callao (12°S) and Pisco (14°S), considered important upwelling and primary productivity centers. A principal component analysis performed allowed the description of main OM depositional environments. In the outer continental shelf of Callao (12°S) and OMZ of Pisco (14°S), accumulation of well-preserved OM occurs whereas in the inner shelf of Callao redox oscillations lead to less OM accumulation and pyrite formation in sediments. North of Callao, from Tumbes (3°S) to Huacho (11°S), a mixed signal of OM accumulation was observed. Finally, along the slope, transportation processes seem to play a key role in OM accumulation. This study also allowed the evaluation of certain proxies (e.g. Rock-Eval hydrogen index and oxygen index) routinely used in palaeoceanographic studies

Characterization of hypoxic events in Paracas bay (Peru, 13.8°S) through intensity and biological effect indexes

International audienceOver the past decade, there has been a significant increase in low oxygen conditions within marine coastal areas, profoundly impacting ecosystem processes and living coastal resources. Coastal bays in highly productive upwelling regions, where hypoxia occurs naturally, are special areas affected by both local and adjacent shelf-related processes. Paracas Bay (13.8°S) is a traditional shellfish fishing and intense farming area highly influenced by one of the most active upwelling centers of the Peruvian coast. Despite the small dimensions of the bay (35 km2), a key feature is its complex physical dynamics and high environmental variability. Recently, important efforts have been made in the study of both the spatial and temporal oxygen concentration variability, nevertheless, information regarding the ecological and biological impact of hypoxic events is still lacking. In this study, the spatial and temporal distribution of hypoxic events was analyzed across Paracas bay at different depths by means of high-frequency hourly dissolved oxygen records collected by data-loggers deployed across the bay during the periods September 2012 – February 2013 and March 2015 – February 2017. To study the ecological impact of hypoxic events, we developed a hypoxia intensity index, while the biological impact was studied through the development of a hypoxia biological effect index using as model species the Peruvian scallop (Argopecten purpuratus). Our results showed that hypoxic events have an intrinsic variability across the bay. The deeper areas of the bay, towards the northwest and center, were characterized by long, intense, and lethal events, while the southeast and southwest, shallower areas, were characterized by shorter events of low intensity and either sublethal or innocuous. We propose that the observed variability is not only related to the large-scale environmental context in which the events occurred, but also to small-scale variability linked to local circulation, biological activity, and sediment biogeochemistry. We expect that our research will be useful not only for scientific purposes, but also for coastal resource management and aquaculture, underlining the importance of developing high-resolution oxygen monitoring systems in coastal bays

KOSMOS 2017 Peru mesocosm study: overview data

Eastern boundary upwelling systems (EBUS) are among the most productive marine ecosystems on Earth. The production of organic material is fueled by upwelling of nutrient-rich deep waters and high incident light at the sea surface. However, biotic and abiotic factors can mod- ify surface production and related biogeochemical processes. Determining these factors is important because EBUS are considered hotspots of climate change, and reliable predic- tions of their future functioning requires understanding of the mechanisms driving the biogeochemical cycles therein. In this field experiment, we used in situ mesocosms as tools to improve our mechanistic understanding of processes con- trolling organic matter cycling in the coastal Peruvian up- welling system. Eight mesocosms, each with a volume of ∼ 55 m3, were deployed for 50 d ∼ 6 km off Callao (12◦ S) during austral summer 2017, coinciding with a coastal El Niño phase. After mesocosm deployment, we collected sub- surface waters at two different locations in the regional oxy- gen minimum zone (OMZ) and injected these into four meso- cosms (mixing ratio ≈ 1.5 : 1 mesocosm: OMZ water). The focus of this paper is on temporal developments of organic matter production, export, and stoichiometry in the indi- vidual mesocosms. The mesocosm phytoplankton commu- nities were initially dominated by diatoms but shifted to- wards a pronounced dominance of the mixotrophic dinoflag- ellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in surface layers. The community shift coincided with a short-term increase in production during the A. san- guinea bloom, which left a pronounced imprint on organic matter C : N : P stoichiometry. However, C, N, and P export fluxes did not increase because A. sanguinea persisted in the water column and did not sink out during the experiment. Accordingly, export fluxes during the study were decou- pled from surface production and sustained by the remain- ing plankton community. Overall, biogeochemical pools and fluxes were surprisingly constant for most of the experiment. We explain this constancy by light limitation through self- shading by phytoplankton and by inorganic nitrogen limita- tion which constrained phytoplankton growth. Thus, gain and loss processes remained balanced and there were few oppor- tunities for blooms, which represents an event where the sys- tem becomes unbalanced. Overall, our mesocosm study re- vealed some key links between ecological and biogeochem- ical processes for one of the most economically important regions in the oceans