Northern Humboldt Current System Research Articles

Species Delimitation of Argonauta Paralarvae Reveals an Extensive Morphological Variability in the Northern Humboldt Current System

Argonauts or paper nautiluses are pelagic octopod cephalopods with a cosmopolitan distribution in tropical and subtropical waters around the world. Unlike other species of octopus, these are characterized by the fact that the female has a shell that serves as the breeding chamber for the eggs. Over time, this structure has been used as a taxonomic diagnostic character, causing problems in the systematics of this genus, with around 50 synonymies reported. Only two species, Argonauta argo and A. nouryi, have been reported in the Northern Humboldt Current System; however, there is taxonomic uncertainty regarding these species, which is reflected in the paralarvae (the first stage of life after hatching). In the paralarvae, the chromatophore patterns are considered to be conservative and reliable taxonomic characteristics. The objective of this study is to demonstrate the extensive variability in the chromatophore arrangement of Argonauta paralarvae in the Northern Humboldt Current using DNA barcoding and five species delimitation models. Our results include up to 11 different paralarvae morphotypes according to the pattern of chromatophores (number and arrangement) and 2 shell morphotypes. Species delimitation methods divided the 13 Argonauta morphotypes into two consensus molecular taxonomic units (MOTUs), A. argo and A. nouryi. Additionally, the results revealed an extensive morphological variability in the paralarvae and female shells of A. nouryi, demonstrating the importance of molecular data in studies involving species with different life stages, especially when this extensive morphological variability obscures conventional analyses.

Spatiotemporal variability and drivers of modeled primary production rates in the Northern Humboldt Current System

A coupled physical-biogeochemical model was employed to explore the spatiotemporal dynamics of primary production (PP) rates within the Northern Humboldt Current System (NHCS). The coastal zone spanning 250 km from the shore, from 3°to 18°S, stands out as a highly productive upwelling region, exhibiting an average surface PP value of 2.5 mol C m−3 yr−1. Correspondingly, the average vertically integrated PP within the euphotic layer amounts to 13 mol C m−2 yr−1. In this context, summer emerges as the peak of productivity, yielding 18 mol C m−2 yr−1, while winter signifies the period of least productivity, with 9 mol C m−2 yr−1. Our study revealed that surface PP variability is primarily driven by changes in surface chlorophyll and phytoplanktonic biomass (mainly diatoms), followed by changes in photosynthetically active radiation (PAR) levels. During summertime, these three drivers contribute to substantial positive anomalies in surface PP. However, the reduction in nutrient availability resulting from weakened upwelling-favorable winds has a slight negative impact on surface PP rates. Yet, this decline is offset by a positive thermal effect during the warmer season. In contrast, during the winter season, a significant decrease in surface chlorophyll concentrations due to a vertical redistribution into a deeper mixed layer significantly diminishes surface PP. Furthermore, the reduction in both PAR levels and biomass concentrations has a comparable effect, further contributing to the decrease in surface PP rates during wintertime. At a depth of 20 m, changes in PP are primarily driven by variations between the opposing influences of PAR and chlorophyll concentrations. While PAR adheres to the seasonal cycle of warming and cooling throughout the year, chlorophyll-driven anomalies exhibit an inverse pattern to those at the surface, influenced by the vertical dilution effect within the mixed layer. Overall, this study provides valuable insights into the complex interplay of drivers that govern PP dynamics across various depths within one of the world’s most productive marine regions.

Comprehensive characterization of Marine Heatwaves in a coastal Northern Humboldt Current System regional model over recent decades

In this study, a high-resolution hydrodynamic model simulation was used to analyze the three-dimensional characteristics of marine heatwaves (MHWs) in the coastal region of the Northern Humboldt Current System (NHCS) over the period 2000–2019. Three distinct vertical layers were identified. The near-surface layer, extending down to 75 m depth, is identified as the region where both the maximum MHW intensity and cumulative intensity are found, predominantly concentrated along the thermocline depth. The intermediate layer, spanning from 75 to 125 m depth, exhibited the longest MHW durations and the lowest MHW frequencies. MHWs in this layer tend to be of moderate intensity and are often associated with ENSO variability. In contrast, the deep layer, from 125 to 250 m depth, was characterized by short-lived MHWs of low intensity. These MHWs exert a lesser thermal impact on the region. To understand the drivers of MHWs, a composite analysis was performed. The results highlighted the significant role exerted by wind-driven anomalies in the life cycle of MHWs. It was found that the combined effect of a sluggish coastal upwelling circulation and suppressed oceanic heat loss, as a result of weakened winds, preceded the MHW formation. Conversely, the strengthening of winds, which enhanced net heat loss, primarily driven by latent heat flux anomalies, and an enhancement of the cooling effect due to the recovery of the coastal upwelling circulation, are key processes in ending a typical MHW event in the coastal region. These findings expand our understanding of the MHWs and their associated potential drivers and help to reveal which regions are most susceptible to extreme climate events in one of the most productive marine ecosystems worldwide.

Total Mass Flux in the Northern Humboldt Current System: Rates and Contribution Sources from Central Peru (12° S)

The total mass flux (TMF) of particulate organic matter (POM) is key for understanding the energetic transfer within the “biological pump” (i.e., involving the carbon cycle), reflecting a critical connection between the surface and the bottom. A fixed multi-sediment trap was installed at 30 m depth in Callao Bay, central Peru from March to December 2020. After recovery, samples were dried and weighed to calculate the TMF and pellet flux. The average TMF was 601.9 mg·m−2·day−1, with 70.2 and 860 mg·m−2·day−1 as the lowest and highest values during “normal conditions”. Zooplankton fecal pellets (ZFP) were found in ovoid (e.g., larvae) and cylindrical (e.g., adult copepods) shapes and their flux contribution to TMF was low, ranging from 0.17 to 85.59 mg·m−2·day−1. In contrast with ZFP, fish fecal pellets (FFP) were found in fragments with a cylindrical shape, and their contribution to the TMF was higher than ZFP, ranging from 1 to 92.56 mg·m−2·day−1. Mean sinking velocities were 4.63 ± 3.47 m·day−1 (ZFP) and 432.27 ± 294.26 m·day−1 (FFP). There is a considerable difference between the ZFP and FFP contributions to TMFs. We discuss the implications of these results regarding a still poorly understood process controlling the POM flux off the Peruvian coast.

Influence of combined temperature and food availability on Peruvian anchovy (Engraulis ringens) early life stages in the northern Humboldt Current system: A modelling approach

In the northern Humboldt Current system (NHCS), the Peruvian anchovy (Engraulis ringens) constitutes the bulk of landings and has a significant socioeconomic contribution. Understanding the impact of environment on the early-life stages of anchovy and further population dynamics remains challenging. Climate variability at a variety of scales modulates currents velocity, temperature and food availability, impacting early-life stages drift, growth and survival. In order to investigate these impacts, we developed Ichthyop-DEB, an individual-based model including larval retention processes and a Dynamic Energy Budget (DEB) bioenergetic module for larval growth. First, we assessed the effect of hydrodynamic simulations horizontal resolution on simulated larval retention patterns using a recruitment age-criterion of 30 days. Then, we evaluated the impact of the following biological processes on simulated larval recruitment patterns: (i) a minimum size-criterion (2 cm), as opposed to a minimum age-criterion (30 days), to be considered as recruited, (ii) the upper larval thermal limit tolerance of the species, for which lab experiments are lacking, and (iii) a constant larval mortality rate. We found that using different resolutions of the hydrodynamic model (10 and 2 km) led to similar simulated larval retention patterns. Retention was highest when spawning occurred in the superficial layer (0–15 m) in austral winter and in the deepest considered layer (30–45 m) in summer. Coupling with the DEB model produced contrasted growth patterns on the continental shelf with a strong month-latitude interaction. Larval recruitment was strongest from 6° to 10° S in austral summer, largely contributing to the average seasonal pattern. Depending on the temperature correction function tested with the bioenergetic module, simulated larval recruitment could also be strong in the northernmost zone (2°–4°S), an area not known for abundant anchovy populations, which suggests a possible thermal growth limitation. Finally, sensitivity tests performed on larval growth limitation by food suggested a deficiency in food supply in the southernmost zone (18°–20°S).

Carbon budgets of copepod communities in the northern Humboldt Current System off Peru

Abundance, biomass and respiration rates of dominant medium- to larger-sized copepod species (ML class) from the upwelling system off Peru (8.5-16°S) were determined along with their carbon ingestion and egestion rates. Small copepods (S class) were included for comparisons of community rates. Overall, abundance/biomass was highest in the upper 50 m and decreased with depth and thus also community ingestion and egestion. Ingestion of the ML class (0-50 m) in shelf regions (14-515 mg C m-2 d-1) was lower in the south compared to the north and central study areas, while their offshore ingestion (11-502 mg C m-2 d-1) was comparable across regions (8.5-16°S). Ingestion rates (0-50 m) of the S class were in a range similar to those of the ML class in shelf regions (100-417 mg C m-2 d-1) but were higher offshore (177-932 mg C m-2 d-1). Calanus chilensis and the S class contributed most to total ingestion in the north, while in the south, Centropages brachiatus had the highest community ingestion aside from the S class. Egestion varied from 3-155 mg C m-2 d-1 for the ML class and 30-280 mg C m-2 d-1 for the S class. The high community rates highlight the crucial role of both size classes for carbon budgets in the northern Humboldt Current System off Peru and indicate that the ML class may enhance passive vertical carbon flux, whereas the S class may support carbon remineralization rates in surface waters.

The oxygen minimum zone relative depth is a key driver of dolphin habitats in the northern Humboldt Current System

Highly mobile odontocetes need habitats with environmental conditions with the potential of aggregating enough and high-quality prey, to maximize foraging success. Until now, the characterization of those habitats was in terms of physical and biological indicators of high production, capable of attracting and sustaining prey. Nevertheless, there has been no approach to quantifying the effects of a biophysical characteristic of the ocean with proven effects on the vertical distribution of prey for cetaceans: The oxygen minimum zone (OMZ) depth. In the northern branch of the Humboldt Current System off Peru (~6-18° S), a shallow OMZ (30-50 m) affects the distribution of the Peruvian anchovy (Engraulis ringens), main prey for several marine predators, including dolphins. We hypothesized these predators would aggregate in productive areas, but with preference for places where the relative OMZ depth can constrain prey vertically, making it more accessible and maximizing foraging success. We fitted Bayesian habitat models for three dominant odontocete species in this region, with multiple combinations of environmental covariates, smoothing techniques, and temporal and spatial random effects. Cetacean data came from 23 dedicated surveys spanning 2001-2019. Habitat predictors included the spatial anomalies of sea surface temperature, surface chlorophyl-a, pycnocline depth and OMZ depth. Dusky (Lagenorhynchus obscurus) and common dolphins (Delphinus delphis) preferred productive, cold areas with a very shallow OMZ, regardless of the season, while bottlenose dolphins (Tursiops truncatus) aggregated in both cold and warm waters, also with shallow OMZ. The former two species of higher metabolic demands would maximize energy intake by selecting areas with highly aggregated prey, while the latter, of more moderate metabolic needs and more diverse prey, would exploit less restricted habitats.

Understanding the drivers of fish variability in an end-to-end model of the Northern Humboldt Current System

The Northern Humboldt Current System is the most productive eastern boundary upwelling system, generating about 10 % of the global fish production, mainly coming from small pelagic fish. It is bottom-up and top-down affected by environmental and anthropogenic variability, such as El-Niño Southern Oscillation and fishing pressure, respectively. The high variability of small pelagic fish in this system, as well as their economic importance, call for a careful management aided by the use of end-to-end models. This type of models represent the ecosystem as a whole, from the physics, through plankton up to fish dynamics. In this study, we utilised an end-to-end model consisting of a physical–biogeochemical model (CROCO-BioEBUS) coupled one-way with an individual-based fish model (OSMOSE). We investigated how time-variability in plankton food production affects fish populations in OSMOSE and contrasted it against the sensitivity of the model to two parameters with high uncertainty: the plankton accessibility to fish and fish larval mortality. Relative interannual variability in the modelled fish is similar to plankton variability. It is, however, small compared with the high variability seen in fish observations in this productive ecosystem. In contrast, changes in larval mortality have a strong effect on anchovies. In OSMOSE, it is a common practice to scale plankton food for fish, accounting for processes that may make part of the total plankton in the water column unavailable. We suggest that this scaling should be done constant across all plankton groups when previous knowledge on the different availabilities is lacking. In addition, end-to-end modelling systems should consider environmental impacts on other biological processes such as larval mortality in order to better capture the interactions between environmental processes, plankton and fish.

Distribution and biomass of anchovy (Engraulis ringens) and pelagic red squat lobster (Pleuroncodes monodon) in marine ecosystem of Paracas National Reserve, southern Peru

The present study determined the distribution and biomass of the Peruvian anchovy and pelagic red squat lobster related to the oceanographic characteristics in the marine protected area of the Paracas National Reserve in the Northern Humboldt Current System. Acoustic, biological, and oceanographic information was collected on the pelagic habitat in surveys carried out by the Peruvian Marine Research Institute during the summer and spring of 2018, 2019, and 2020. The results obtained indicated that the spatial occupation of the pelagic habitat of Peruvian anchovy and pelagic red squat lobster were influenced by the dynamics of upwelling, water masses, and a shallow and intense Oxygen Minimum Zone. Spatial overlap of high biomass of Peruvian anchovy and pelagic red squat lobster were recorded in the surface layer during the night. Although, with the presence of dense swarms of múnida in anoxic intermediate waters of the Oxygen Minimum Zone during the day. The Marine Protected Area off the Paracas National Reserve functions as a protection zone for the structure and functioning of the pelagic-neritic ecosystem and strengthens the fishery production of Peruvian anchovy stock in the Northern Humboldt Current System

Climate vulnerability assessment of key fishery resources in the Northern Humboldt Current System

The Northern Humboldt Current System sustains one of the most productive fisheries in the world. However, climate change is anticipated to negatively affect fish production in this region over the next few decades, and detailed analyses for many fishery resources are unavailable. We implemented a trait-based Climate Vulnerability Assessment based on expert elicitation to estimate the relative vulnerability of 28 fishery resources (benthic, demersal, and pelagic) to the impacts of climate change by 2055; ten exposure factors (e.g., temperature, salinity, pH, chlorophyll) and 13 sensitivity attributes (biological and population-level traits) were used. Nearly 36% of the species assessed had “high” or “very high” vulnerability. Benthic species were ranked the most vulnerable (gastropod and bivalve species). The pelagic group was the second most vulnerable; the Pacific chub mackerel and the yellowfin tuna were amongst the most vulnerable pelagic species. The demersal group had the relatively lowest vulnerability. This study allowed identification of vulnerable fishery resources, research and monitoring priorities, and identification of the key exposure factors and sensitivity attributes which are driving that vulnerability. Our findings can help fishery managers incorporate climate change into harvest level and allocation decisions, and assist stakeholders plan for and adapt to a changing future.

Toward a Solution of the “Peruvian Puzzle”: Pelagic Food-Web Structure and Trophic Interactions in the Northern Humboldt Current Upwelling System Off Peru

The northern Humboldt Current upwelling system (HCS) belongs to the most productive marine ecosystems, providing five to eight times higher fisheries landings per unit area than other coastal upwelling systems. To solve this “Peruvian puzzle”, to elucidate the pelagic food-web structure and to better understand trophic interactions in the HCS, a combined stable isotope and fatty acid trophic biomarker approach was adopted for key zooplankton taxa and higher trophic positions with an extensive spatial coverage from 8.5 to 16°S and a vertical range down to 1,000 m depth. A pronounced regional shift by up to ∼5‰ in the δ15N baseline of the food web occurred from North to South. Besides regional shifts, δ15N ratios of particulate organic matter (POM) also tended to increase with depth, with differences of up to 3.8‰ between surface waters and the oxygen minimum zone. In consequence, suspension-feeding zooplankton permanently residing at depth had up to ∼6‰ higher δ15N signals than surface-living species or diel vertical migrants. The comprehensive data set covered over 20 zooplankton taxa and indicated that three crustacean species usually are key in the zooplankton community, i.e., the copepods Calanus chilensis at the surface and Eucalanus inermis in the pronounced OMZ and the krill Euphausia mucronata, resulting in an overall low number of major trophic pathways toward anchovies. In addition, the semi-pelagic squat lobster Pleuroncodes monodon appears to play a key role in the benthic-pelagic coupling, as indicated by highest δ13C’ ratios of −14.7‰. If feeding on benthic resources and by diel vertical migration, they provide a unique pathway for returning carbon and energy from the seafloor to the epipelagic layer, increasing the food supply for pelagic fish. Overall, these mechanisms result in a very efficient food chain, channeling energy toward higher trophic positions and partially explaining the “Peruvian puzzle” of enormous fish production in the HCS.

Migration behavior of anchoveta (Engraulis ringens) in the Northern Humboldt Current System between September 2019 and September 2020

Migrational behavior of anchoveta ( Engraulis ringens ) in the Northern Humboldt Current System between September 2019 and September 2020 is analyzed. Acoustic, biological, and oceanographic data and information from the several research cruises carried out by the Instituto del Mar del Peru (IMARPE) during this period were considered. The results showed the presence of transitory warm oceanographic events that induced the anchoveta population to make horizontal and vertical migrations in their search for cold conditions, the adult specimens (bigger than 12 cm of total length) retracted near the bottom of the platform being inaccessible to the fishing fleet; after the normalization of environmental conditions, their abundance was accessible to the fishing fleet. Therefore, the fishing activity in the second season of anchoveta fishing in 2019 had a negative impact due to the joint action of environmental events and events of a biological nature (entry of strong recruitment and migration). The anchoveta was preferably found in cold coastal waters that varied in the seasons. In the spring of 2019, it was found in temperatures lower than 22.1°C, in the summer of 2020 at temperatures lower than 23.1°C, and in the winter of 2020 at temperatures lower than 17.2°C; as for, salinity was always found in values lower than 35.2. The strategy of the migratory behavior of the anchoveta concerning its habitat in this period has allowed it to remain in its abundance in the Humboldt Current System, which added to adequate fishing management will allow it to maintain its sustainability.

Zooplankton mortality effects on the plankton community of the northern Humboldt Current System: sensitivity of a regional biogeochemical model

Abstract. Small pelagic fish off the coast of Peru in the eastern tropical South Pacific (ETSP) support around 10 % of global fish catches. Their stocks fluctuate interannually due to environmental variability which can be exacerbated by fishing pressure. Because these fish are planktivorous, any change in fish abundance may directly affect the plankton and the biogeochemical system. To investigate the potential effects of variability in small pelagic fish populations on lower trophic levels, we used a coupled physical–biogeochemical model to build scenarios for the ETSP and compare these against an already-published reference simulation. The scenarios mimic changes in fish predation by either increasing or decreasing mortality of the model's large and small zooplankton compartments. The results revealed that large zooplankton was the main driver of the response of the community. Its concentration increased under low mortality conditions, and its prey, small zooplankton and large phytoplankton, decreased. The response was opposite, but weaker, in the high mortality scenarios. This asymmetric behaviour can be explained by the different ecological roles of large, omnivorous zooplankton and small zooplankton, which in the model is strictly herbivorous. The response of small zooplankton depended on the antagonistic effects of mortality changes as well as on the grazing pressure by large zooplankton. The results of this study provide a first insight into how the plankton ecosystem might respond if variations in fish populations were modelled explicitly.

Stable isotope and fatty acid analyses reveal significant differences in trophic niches of smooth hammerhead Sphyrna zygaena (Carcharhiniformes) among three nursery areas in northern Humboldt Current System.

Fishery pressure on nursery areas of smooth hammerhead in northern Peruvian coast have become a serious threat to sustainability of this resource. Even though, some management actions focused on conservation of the smooth hammerhead populations were proposed in recent years, their scientific foundations are often limited, and biomass of smooth hammerhead in Peruvian waters continues to decrease. To inform management and conservation, this study aims to evaluate the trophic niche of smooth hammerhead juveniles from three nursery areas in the northern Peruvian coast using stable isotope and fatty acid analyses. First, we compared the environmental characteristics of each nursery area (i.e., sea surface temperature and chlorophyll-a concentration) and concluded that nursery areas differed significantly and consistently in sea surface temperature. Subsequently, we evaluated isotopic composition of carbon and nitrogen and fatty acid profiles of muscle and liver tissues collected from juvenile smooth hammerhead from each nursery area. We found that juvenile smooth hammerhead captured in San José were enriched in heavier 13C and 15N isotopes compared to those captured in Máncora and Salaverry. Furthermore, the broadest isotopic niches were observed in juveniles from Máncora, whereas isotopic niches of juveniles from Salaverry and San José were narrower. This difference is primarily driven by the Humboldt Current System and associated upwelling of cold and nutrient rich water that drives increased primary production in San José and, to a less extent, in Salaverry. Compared to smooth hammerhead juveniles from Máncora, those from San José and Salaverry were characterised by higher essential fatty acid concentrations related to pelagic and migratory prey. We conclude that smooth hammerhead juveniles from three nursery areas in the northern Peruvian coast differ significantly in their trophic niches. Thus, management and conservation efforts should consider each nursery area as a unique juvenile stock associated with a unique ecosystem and recognize the dependence of smooth hammerhead recruitment in San José and Salaverry on the productivity driven by the Humboldt Current System.

Modelling seasonal patterns of larval fish parasitism in two northern nearshore areas in the Humboldt Current System

Macro- and micro-environmental factors modulate parasite loads in fish, determining parasitic abundances, diversity, and interaction dynamics. In this study, seasonal variations in larval ectoparasites on fish larvae in the northern Humboldt Current System (HCS) were evaluated using a delta-gamma generalized linear model to predict their occurrence frequencies. Fish larvae were collected from two nearshore areas during austral spring–summer and autumn–winter. Only five (of 38) larval fish species were parasitized by copepods: Gobiesox marmoratus, Ophiogobius jenynsi, Helcogrammoides cunninghami, Myxodes sp., and Auchenionchus crinitus. A binomial model showed that the presence/absence of parasitized fish larvae varied among the fish species and their larval abundances, while a positive delta-gamma model showed that ectoparasite frequency varied among the seasons and fish species. Seasonal variations in parasitized fish larvae frequency could be associated with host and parasite reproductive processes, which are related to oceanographic features responsible for larval retention and subsequent higher infestation probabilities. Host length was positively correlated with ectoparasite length, suggesting early infection and combined growth until the detachment of the ectoparasite. Our results suggest that infestation patterns in larval fish species can be identified using delta-gamma models and that they respond to local (retention) and high-scale (HCS) processes.

Physical and biogeochemical impacts of RCP8.5 scenario in the Peru upwelling system

Abstract. The northern Humboldt Current system (NHCS or Peru upwelling system) sustains the world's largest small pelagic fishery. While a nearshore surface cooling has been observed off southern Peru in recent decades, there is still considerable debate on the impact of climate change on the regional ecosystem. This calls for more accurate regional climate projections of the 21st century, using adapted tools such as regional eddy-resolving coupled biophysical models. In this study three coarse-grid Earth system models (ESMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5) are selected based on their biogeochemical biases upstream of the NHCS, and simulations for the RCP8.5 climate scenario are dynamically downscaled at ∼12 km resolution in the NHCS. The impact of regional climate change on temperature, coastal upwelling, nutrient content, deoxygenation, and the planktonic ecosystem is documented. We find that the downscaling approach allows us to correct major physical and biogeochemical biases of the ESMs. All regional simulations display a surface warming regardless of the coastal upwelling trends. Contrasted evolutions of the NHCS oxygen minimum zone and enhanced stratification of phytoplankton are found in the coastal region. Whereas trends of downscaled physical parameters are consistent with ESM trends, downscaled biogeochemical trends differ markedly. These results suggest that more realism of the ESM circulation, nutrient, and dissolved oxygen fields is needed in the eastern equatorial Pacific to gain robustness in the projection of regional trends in the NHCS.

ENSO-driven CO2 efflux variability and the role of the upwelling region on the carbon exchange in the Northern Humboldt Current System

Abstract The role played by El Nino-Southern Oscillation (ENSO) on modulating the oceanic carbon outflux in the Northern Humboldt Current System (NHCS) is assessed with the use of outputs from a coupled physical-biogeochemical hindcast which permits to evaluate the interannual variability of the surface seawater partial pressure of carbon dioxide, hereinafter pCO 2 s w , and reconstruct associated air-sea CO2 fluxes from 1998 through 2015 at monthly timescales. In addition, surface dissolved inorganic carbon (DIC) concentrations were inferred from a recent neural network methodology prescribed with the same outputs of the biogeochemistry hindcast. Results show a large spatiotemporal variability of the CO2 exchange to nine El Nino and La Nina episodes throughout the period of analysis. Within the coastal and equatorial upwelling region, that variability results from combined ENSO-driven ΔpCO2 (pCO 2 s w - pCO 2 a i r ) and wind speed variations, although the latter in a minor proportion during ENSO peaks. Specifically, it is found that the weak CO2 source behavior during an average El Nino episode is mainly caused by a weakening of ΔpCO2 which is partially compensated by more efficient CO2 exchange at the air-sea interface due to the strengthening of the upwelling-favorable winds. Conversely, the relatively strong CO2 source behavior during an average La Nina episode results from more efficient upwelling which brings colder but CO2-rich waters to the surface, therefore, increasing pCO 2 s w and the associated CO2 efflux. Moreover, it is estimated that during an average La Nina episode, 3.3 million metric tons of carbon is additionally emitted contributing to the atmospheric CO2 accumulation. In contrast, during an average El Nino episode, the total amount of carbon retained is about 24.7 million metric tons of carbon that normally would have been lost to the atmosphere as CO2, rendering the NHCS a key region for the global atmospheric carbon budget.

Spatio-temporal trends in zooplankton biomass in the northern Humboldt current system off Peru from 1961-2012

Anchovy (Engraulis ringens) is the most important exploited fish species in the Northern Humboldt Current System (NHCS) off Peru. This species, as well as most other pelagic resources, mainly forage on zooplankton. The NHCS is bottom-up controlled at a variety of scales. Therefore, fish biomass is driven by the abundance of their prey. In this context, we studied the spatiotemporal patterns of zooplankton biomass in the NHCS from 1961-2012. Data were collected with Hensen net all along the Peruvian coast. To transform zooplankton biovolume into biomass we used a regression that was calibrated from 145 zooplankton samples collected during four surveys and, for which, precise information was available on both biovolume and wet weight. The regression model was then applied on a time-series encompassing 158 cruises performed by the Peruvian Institute of the Sea (IMARPE) between 1961 and 2012. We observed a clear multidecadal pattern and two regime shifts, in 1973 and 1992. Maximum biomass occurred between 1961 and 1973 (61.5 g m−2). The lowest biomass (17.8 g m−2) occurred between 1974 and 1992. Finally, the biomass increased after 1993 (26.6 g m−2) but without reaching the levels observed before 1973. A seasonal pattern was observed with significantly more biomass in spring than in other seasons. Spatially, zooplankton biomass was higher offshore and in northern and southern Peru. Interestingly, the zooplankton sampling was performed using classic zooplankton net that are well fitted to mesozooplankton and are known to underestimate the macrozooplankton; however, the spatiotemporal patterns we observed are consistent with those of macrozooplankton, in particular euphausiids. This suggests that in the NHCS, when and where macrozooplankton dominates it also dominates the biomass obtained using classic zooplankton net samples. Finally, until now, in the NHCS only time-series on zooplankton biovolume were available. The biomass data we provide are more directly usable in trophic or end-to-end models.

Upwelling modulation of functional traits of a dominant planktonic grazer during "warm-acid" El Niño 2015 in a year-round upwelling area of Humboldt Current.

Climate change is expected to exacerbate upwelling intensity and natural acidification in Eastern Boundaries Upwelling Systems (EBUS). Conducted between January-September 2015 in a nearshore site of the northern Humboldt Current System directly exposed to year-round upwelling episodes, this study was aimed at assessing the relationship between upwelling mediated pH-changes and functional traits of the numerically dominant planktonic copepod-grazer Acartia tonsa (Copepoda). Environmental temperature, salinity, oxygen, pH, alkalinity, chlorophyll-a (Chl), copepod adult size, egg production (EP), and egg size and growth were assessed through 28 random oceanographic surveys. Agglomerative clustering and multidimensional scaling identified three main di-similitude nodes within temporal variability of abiotic and biotic variables: A) “upwelling”, B) “non-upwelling”, and C) “warm-acid” conditions. Nodes A and B represented typical features within the upwelling phenology, characterized by the transition from low temperature, oxygen, pH and Chl during upwelling to higher levels during non-upwelling conditions. However, well-oxygenated, saline and “warm-acid” node C seemed to be atypical for local climatology, suggesting the occurrence of a low frequency oceanographic perturbation. Multivariate (LDA and ANCOVA) analyses revealed upwelling through temperature, oxygen and pH were the main factors affecting variations in adult size and EP, and highlighted growth rates were significantly lower under node C. Likely buffering upwelling pH-reductions, phytoplankton biomass maintained copepod reproduction despite prevailing low temperature, oxygen and pH levels in the upwelling setting. Helping to better explain why this species is among the most recurrent ones in these variable yet productive upwelling areas, current findings also provide opportune cues on plankton responses under warm-acid conditions, which are expected to occur in productive EBUS as a consequence of climate perturbations.

Temporal changes in mesoscale aggregations and spatial distribution scenarios of the Peruvian anchovy (Engraulis ringens)

The Peruvian anchovy (Engraulis ringens) is the most important small pelagic of the Humboldt Current System (HCS), supporting the largest mono-specific fishery in the world. The spatial behavior of this species tends to be very dynamic at different spatial scales, influenced mostly by its biomass level and environmental factors. The aim of this study was to analyze temporal and spatial fluctuations in anchovy spatial distribution off Peru, based and modeled on acoustic data, focusing on large- and meso-scale spatial structures. We employed data from 41 scientific surveys (1994–2016) and Bayesian hierarchical spatial models to obtain the anchovy's spatial distribution, allowing us to identify spatial structures at specific scales. Our results showed similar temporal trends in the number of mesoscale structures, their areas and anchovy density, exhibiting altogether two breakpoints in the time series: ~1999 and ~2013. The last period (2013–2016) was similar to the earlier one (1994–1999), in terms of low values of mesoscale structure indicators. On the other hand, we identified four spatial scenarios differentiated by the aggregative behavior, which were highly influenced by seasons and El Niño events. Each scenario had recurrent, or fidelity, areas placed in different locations. For instance, for the ‘El Niño scenario’ a particularly coastal recurrent area was identified, which might be a refuge zone for this species during these unfavorable events. Finally, we assessed differences in biomass estimates of each scenario. The highest biomass values were estimated for the ‘Summer favorable scenario’ and the lowest ones for the ‘El Niño scenario’, which supports the MacCall's basin hypothesis for this species. This study expands the current knowledge of the Peruvian anchovy and it is a first step to understand the effects on this species of the last El Niño events (2014–2016) that occurred in the northern Humboldt Current System.