Tropical Southwest Pacific Research Articles

Comparative analysis of day and night micronekton abundance estimates in west Pacific between acoustic and trawl surveys

Micronekton organisms are a central component of the trophic organization in the pelagic ecosystem, being prey to top predators and participating in the export of carbon from the surface to the deep layers. Despite their importance, the abundance estimates and species distribution of micronekton remain largely uncertain. This study aimed to compare and assess two sampling methods classically used for the estimation of micronekton abundance in mesopelagic acoustic scattering layers: scientific echosounder and trawl sampling. Measurements of 38 and 70 kHz hull-mounted echosounders were examined with biological trawl samples from 8 surveys in the South-West tropical Pacific. A model of acoustic observation was built from the trawl sampling species composition and forward scattering models. Predicted and observed acoustic responses are compared to assess the variability and the difference between the acoustic and trawl sampling methods in various scattering layers, for day and night periods. The difference between methods decreased with depth and with increasing abundance of fish with swimbladders caught in trawls. Notably, this difference was found to be minimal in the nocturnal deep scattering layer (mesopelagic zone, depth>200 m). This study emphasizes potential lower estimates of organisms’ abundance by trawling and a bias towards mesopelagic fish. Understanding the differences between methods and their variability within different scattering layers is essential for studying micronekton and improving the precision of biomass estimates.

A Review of Ocean Color Algorithms to Detect Trichodesmium Oceanic Blooms and Quantify Chlorophyll Concentration in Shallow Coral Lagoons of South Pacific Archipelagos

The oceanic waters of the Southwest Tropical Pacific occupy a vast region including multiple Pacific Island Countries. The state of these waters is determinant for fisheries and the blue economy. Ocean color remote sensing is the main tool to survey the variability and long-term evolution of these large areas that are important for economic development but are affected by climate change. Unlike vast oligotrophic gyres, tropical waters are characterized by numerous archipelagos and islands, with deep and shallow lagoons subjected to the large impacts of the land. Strikingly large dendritic phytoplankton (Trichodesmium) blooms with high levels of chlorophyll, developing within archipelagos, as well as coastal enrichments from various origins may be observed. Algorithms to detect the presence of Trichodesmium have been developed or adapted, as well as algorithms to estimate the chlorophyll concentration ([Chl-a)]. Adapting existing [Chl-a] algorithms does not always yield high, i.e., sufficient, accuracy. A review of published regional bio-optical algorithms developed taking into account the specific phytoplankton composition and minimizing the adverse impacts of particles and the seabed bottom on [Chl-a] determination is presented, as well the bio-optical database that allowed their development. The interest of such algorithms for a variety of applications and scientific accomplishments is highlighted, with a view to further addressing the main biology and biogeochemistry questions, e.g., to determine the true impact of diazotrophs and assess lagoon [Chl-a] variability with the highest confidence. This work anticipates the use of future coarse and high-spatial-resolution and multi- and hyper-spectral satellite imagery in the Pacific.

Towards improved seasonal rainfall prediction in the tropical Pacific Islands

The El Niño Southern Oscillation (ENSO) is a major influence on interannual variability of rainfall in stations in the tropical southwest Pacific. Predictions of seasonal rainfall, especially a season or two ahead, are of great value to these countries. This paper therefore examines the correlations over ~ 60 years between seasonal rainfall and 8 ENSO indicators at 16 island stations, allowing for lead times. The results show the influence on rainfall of the position and movement of the South Pacific Convergence Zone (SPCZ) during ENSO events, and that the Southern Oscillation Index (SOI), the sea surface temperature anomaly in the central Pacific (Niño 3.4), and the warm water volume in the eastern Pacific (WWV1) have longer lead times compared to most other ENSO indicators. These indicators can therefore be used with confidence in SCOPIC, a widely used statistical tool for prediction of seasonal rainfall. (As global climate models generally have systematic errors in their depiction of the SPCZ, they cannot yet be used directly to reliably predict seasonal rainfall in this region.) For several sites near the SPCZ, we find that a moderately good forecast of rainfall in both spring and summer can be made from indicators measured in June–July (i.e., 3–6 months in advance.)

Poriferans rift apart: molecular demosponge biodiversity in Central and French Polynesia and comparison with adjacent marine provinces of the Central Indo-Pacific

The distribution of marine sponges in the tropical Southwest Pacific Ocean is largely unexplored despite the vital ecological role of sponges in coral reefs and their value as sources of metabolites for drug design. Several collection campaigns to the French Polynesian archipelagos (Society, Marquesas, Tuamotu, Gambier, and Austral) were conducted to assess the bio- and chemodiversity of the island groups. In the course of these scientific expeditions, more than 200 identified sponge specimens were acquired, for which we were able to assign 102 Molecular Operational Taxonomic Units (MOTUs). Based on these MOTUs, we assessed, in the largest analysis of its kind for this area to date, the sponge composition and faunistic overlaps of the marine province Southeast Polynesia with Marquesas and Central Polynesia. We also compared the sponge fauna of these Eastern Indo-Pacific provinces with marine provinces of the adjacent Central Indo-Pacific realm. Our findings corroborate that sponge faunal similarity within marine realms is higher than among realms, and follows the marine barriers to gene flow observed for other taxa. We detected high levels of provincial endemism for marine sponges, consistent with findings from other Indo-Pacific regions. At the level of province, geographical distance and ocean surface currents influence faunal similarity, and constitute the primary factors for the connectivity of sponge faunas between the disjunct and remote island groups in the tropical Southwest Pacific Ocean.

An analysis of the 10 MW Butoni wind farm in the Tropical Southwest Pacific Island of Fiji

This study carries out an analysis of the 10 MW Butoni wind farm in the tropical southwest Pacific island of Fiji using 6 years of uninterrupted near-surface wind observations (2013–2018). The standard wind-industry software, WAsP is used to analyse and evaluate the wind characteristics of the wind farm and the surrounding areas. The modelled and operational AEP are discussed with the related economic analysis together with the main causes for the under-performance of the wind farm. The results revealed that the mean wind speed, power density and the AEP at the Butoni wind farm are below the utility-scale standard of 6.4 m/s, 300 W/m2 and 500 MWh/year/turbine respectively, at 55 m above ground level (AGL). The main reason for the under-performance of the wind farm is that it was commissioned for a low mean wind speed regime of Wind Power Class 1. The wind farm has a lower-than-expected capacity factor of 5.4% and a higher wind shear coefficient of 0.35. An economic analysis revealed that the payback time is 24.5 years, and the cost of energy generation is FJD $ 0.55/kWh.

Linking Seasonal‐to‐Interannual Variability of Intermediate Currents in the Southwest Tropical Pacific to Wind Forcing and ENSO

AbstractSeasonal‐to‐interannual variability of intermediate circulations in the southwest tropical Pacific is key to interhemisphere and interbasin water mass exchange and is shown to be interconnected with wind forcing and the El Niño–Southern Oscillation (ENSO) based on numerical model results validated by mooring observations. The Lower Equatorial Intermediate Current and the intermediate part of the New Guinea Coastal Undercurrent show clear seasonal variability and a significant interannual response to ENSO at a 7–12‐month lag, both of which are induced by Rossby waves. Baroclinic modes 1 and 2 of the direct wind‐forced Rossby waves contribute mainly to the seasonal variability. In contrast, the Rossby waves from baroclinic modes 2 and 4 and the reflection of Kelvin waves are the main contributors to the variability on ENSO time scales. The presence of the southwest tropical Pacific coastline enhances the intermediate seasonal variance and the interannual relationship between intermediate currents and ENSO.

Multimodel Forecasting of Precipitation at Subseasonal Timescales Over the Southwest Tropical Pacific

AbstractMultimodel ensemble (MME) reforecasts of rainfall at subseasonal time scales in the southwest tropical Pacific are constructed using six models (BoM, CMA, ECCC, ECMWF, Météo‐France, and UKMO) from the Subseasonal‐to‐Seasonal (S2S) database by member pooling. These reforecasts are verified at each grid point of the 110°E to 200°E; 30°S to 0° domain for the 1996–2013 DJF period. The evaluation is based on correlation and on the ROC skill score of the upper quintile of precipitation for both weekly targets and Weeks 3–4 outlook. Confirming previous results at the seasonal time scales, the MME reaches the highest skill and also improves the reliability of probabilistic forecasts. However, an equivalent ensemble size comparison between the MME and the individual models shows that the better performance of the MME compared to the best individual models is significantly related to the larger ensemble size of the MME. Forecast skill is then explained in light of potential sources of predictability by evaluating the performance of the models depending on the initial ENSO and MJO state. While the role of ENSO on predictability is quite consistent with its related rainfall anomalies, the role of the MJO is more ambiguous and strongly depends on the location: An initialization in active MJO conditions does not necessarily imply better forecasts. This influence of ENSO and the MJO on predictability does not change when switching from individual models to the MME.

Improving subseasonal precipitation forecasts through a statistical–dynamical approach : application to the southwest tropical Pacific

Subseasonal forecasts are based on coupled general circulation models that often have a good representation of large-scale climate drivers affecting rainfall. Yet, they have more difficulty in providing accurate precipitation forecasts. This study proposes a statistical-dynamical post-processing scheme based on a bayesian framework to improve the quality of subseasonal forecasts of weekly precipitation. The method takes advantage of dynamically-forecast precipitation (calibration) and large-scale climate features (bridging) to enhance forecast skill through a statistical model. It is applied to the austral summer precipitation reforecasts in the southwest tropical Pacific, using the Meteo-France and ECMWF reforecasts in the Subseasonal-to-seasonal (S2S) database. The large-scale predictors used for bridging are climate indices related to El Nino Southern Oscillation and the Madden–Julian Oscillation, that are the major sources of predictability in the area. Skill is assessed with a Mean Square Skill Score for deterministic forecasts, while probabilistic forecasts of heavy rainfall spells are evaluated in terms of discrimination (ROC skill score) and reliability. This bayesian method leads to a significant improvement of all metrics used to assess probabilistic forecasts at all lead times (from week 1 to week 4). In the case of the Meteo-France S2S system, it also leads to strong error reduction. Further investigation shows that the calibration part of the method, using forecast precipitation as a predictor, is necessary to achieve any improvement. The bridging part, and particularly the ENSO-related information, also provides additional discrimination skill, while the MJO-related information is not really useful beyond week 2 over the region of interest.

Tropical Cyclone Induced Wave Setup around New Caledonia during Cyclone COOK (2017)

Jullien, S.; Aucan, J.; Lefevre, J.; Peltier, A., and Menkes, C.E., 2020. Tropical cyclone induced wave setup around New Caledonia during cyclone COOK (2017). In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1454-1459. Coconut Creek (Florida), ISSN 0749-0208.Tropical cyclone (TC)-induced coastal flooding arises from a combination of atmospheric surge, and energetic wave-induced setup. TC-generated waves in deep water considerably evolve in shallow water before reaching the coast. Islands surrounded by a barrier reef and lagoon are a particular case of steep and irregular bathymetry over which waves undergo various transformations, and induce complex wave-current interactions. The present study investigates the TC-induced coastal impact on the reef environment of New Caledonia in the tropical southwest Pacific. An unstructured wave-current model simulation encompassing New Caledonia and Vanuatu is used to describe the complete TC-induced waves impact, from wave generation to propagation and transformation in the coastal area. Comparison with in-situ measurements are performed during cyclone COOK in 2017. The results show that coastlines with a double protection (barrier and fringing reefs) are relatively sheltered from the waves induced by TCs (significant wave height is 3 m at the coast vs. 13 m offshore) while shores fronting the reef passes are more severely exposed with waves reaching 9 m close to the coast. Setups and currents that are associated to wave breaking on the barrier reef induce strong lagoon currents and localized sea level anomalies that does not necessarily reach the coast. In more sheltered bays, wave breaking at the entrance of the bay acts against outgoing currents. This phenomenon leads to an additional elevation of the mean water level in the bay.

First record of the seagrass-boring shipworm Zachsia sp. (Bivalve: Teredinidae) in natural and transplanted Enhalus acoroides (Hydrocharitaceae) rhizomes in tropical Southwest Pacific

Seagrass ecosystems are under threat worldwide. Seagrass transplantation is one restoration approach which is has been tried with uneven success. While transplantation methods and oceanographic parameters have been investigated in attempts to understand the factors affecting transplant survival rate and seagrass transplantation success, little attention has been paid to the possible impact of pests. We observed borer molluscs in a seagrass Enhalus acoroides both from natural bed and transplant area. This is thought to be the first record from Indonesian waters of these bivalve molluscs of the genus Zachsia, family Teredinidae, the only mollusc genus known to bore into seagrass rhizomes. In a further eleven E. acoroides shoots (average length 10.5cm) we found between one and four calcareous burrows 2mm to 5mm in diameter inside the rhizome. Further research is required to determine the species present and its distribution in Indonesian waters as well as the true ecological impact of Zachsia sp. on seagrasses.

A new species of the genus Rapturella (Gastropoda: Acteonidae) from southeast Brazil.

Acteonidae d'Orbigny, 1843 is the largest family within the superfamily Acteonoidea (Bouchet et al. 2017) and includes small gastropods typical of infralittoral environments. Most acteonids living on the continental shelf or in deep water have been described on the basis of shell morphology alone (Simone 2006; Valdés 2008; Cunha 2011; Salvador Cunha 2016), because little material with soft parts has been collected and examined. Consequently, little is known about the ecology of the species. Recently, many new species of the genus Acteon Montfort, 1810 have been described from tropical Southwest Pacific waters (Valdés 2008), suggesting that a high diversity of acteonid species may await discovery in other deep water environments, including those of Brazil.

Observed Characteristics and Vertical Structure of Mesoscale Eddies in the Southwest Tropical Pacific

AbstractIn the Southwest Pacific Ocean, waters transit from the subtropical gyre before being redistributed equatorward and poleward. While the mean pathways are known, the contribution to the mixing and transport of the water from mesoscale eddies has not been comprehensively investigated. In this research, satellite altimetry data, combined with an eddy detection and tracking algorithm is used to investigate the distribution and surface characteristics of mesoscale eddies in this region of complex bathymetry (10°S–30°S, 140°E–190°E). Detected eddies are then colocalized with in situ data from Argo floats to determine their vertical structure and the effect of eddies on the water masses. The numerous islands affect the eddy behavior as most eddies are formed in the lee of islands, propagate westward and decay when encountering shallow bathymetry. Eddies are sparse and short‐lived in the tropical area north of Fiji, impacting only the top 200 meters of water. They do not appear to be able to trap and transport waters in this region. In the Coral Sea, a region of lateral shear between currents transporting waters of different origins, eddies are more numerous and energetic. They affect the water properties down to at least 500 m depth, and anticyclonic eddies trap water to ∼200 m, contributing to the upper thermocline waters mixing and transport. South of New Caledonia, mesoscale eddies are ubiquitous, with typical lifetimes longer than 5 months. They affect the temperature, salinity, and velocities down to ∼1,000 m depth, and weakly contribute to the mixing of lower thermocline waters.

Geochemical characteristics of sinking particles in the Tonga arc hydrothermal vent field, southwestern Pacific

Studies of sinking particles associated with hydrothermal vent fluids may help us to quantify mass transformation processes between hydrothermal vent plumes and deposits. Such studies may also help us understand how various types of hydrothermal systems influence particle flux and composition. However, the nature of particle precipitation out of hydrothermal vent plumes in the volcanic arcs of convergent plate boundaries has not been well studied, nor have the characteristics of such particles been compared with the characteristics of sinking particles at divergent boundaries. We examined sinking particles collected by sediment traps for about 10 days at two sites, each within 200m of identified hydrothermal vents in the south Tonga arc of the southwestern Pacific. The total mass flux was several-fold higher than in the non-hydrothermal southwest tropical Pacific. The contribution of non-biogenic materials was dominant (over 72%) and the contribution of metals such as Fe, Mn, Cu, and Zn was very high compared to their average levels in the upper continental crust. The particle flux and composition indicate that hydrothermal authigenic particles are the dominant source of the collected sinking particles. Overall, our elemental ratios are similar to observations of particles at the divergent plate boundary in the East Pacific Rise (EPR). Thus, the nature of the hydrothermal particles collected in the south Tonga arc is probably not drastically different from particles in the EPR region. However, we observed consistent differences between the two sites within the Tonga arc, in terms of the contribution of non-biogenic material, the radiocarbon content of sinking particulate organic carbon, the ratios of iron to other metals (e.g. Cu/Fe and Zn/Fe), and plume maturity indices (e.g. S/Fe). This heterogeneity within the Tonga arc is likely caused by differences in physical environment such as water depth, phase separation due to subcritical boiling and associated sub-seafloor precipitation.

Narrowband shortwave minima in spectra of backscattered light from the sea obtained from ocean color scanners as a remote indication of algal blooms

Summary We propose a new approach to indication of algal blooms. It stems from analysis of the multispectral satellite reflectance R rs of areas where blooms were documented during recent decades. We found that spectra of algal blooms exhibit minima at wavelengths of channels of Moderate Resolution Imaging Spectroradiometer (MODIS) λ = 443 and λ = 488 nm (Baltic, Black, and Caspian seas), λ = 443 nm (Southwest Tropical Pacific (SWTP)), and λ = 443 nm and λ = 469 nm (Patagonian Continental Shelf (PCS)), attributable to absorption bands of chlorophyll a and accessory pigments. We quantified the minima using indices D1 = R rs (443) − R rs (412) and D2 = R rs (488) − R rs (469) and proved their diagnostic potential by comparing their distributions to that of R rs (555). Linear dependence of D1 upon chlorophyll a was found from MODIS data for the bloom of Nodularia spumigena . Time dependences of D1 and D2 point to the latter as a probable remote forerunner of cyanobacterial blooms. In the PCS, D1 and D2 proved to be too simplistic owing to diversity of spectral shapes at λ

Oceanic transports through the Solomon Sea: The bend of the New Guinea Coastal Undercurrent

Thermocline waters of the tropical southwest Pacific can be traced back to the center of the South Pacific basin and have a potential influence on equatorial surface conditions and on the characteristics of the El Niño Southern Oscillation on decadal timescales. The Solomon Sea is traversed by this influential flow, and therefore is an optimal place for exploring this oceanic connection to the equator. From a high‐resolution hydrographic survey at which we applied an inverse box model, we describe the main pathways at the entrance of the Solomon Sea, and more particularly the extremely sharp bend of the western boundary current around the south‐east tip of Papua New Guinea. Of the 30 Sv subtropical waters transported into the Coral Sea from the east, above 1300 m, 29 ± 5 Sv makes its way through the Solomon Sea with a large part transported in a boundary current, at the entrance of the Solomon Sea. Around the south‐east tip of Papua New Guinea, the Gulf of Papua Current turns abruptly to the north, in a very sharp bend as it merges into the New Guinea Coastal Undercurrent, on its way, toward the equator. The warm currents transport large amounts of internal energy, with a total of 1.0 ± 0.3 1015W entering the Solomon Sea from the south.

Mid- to late Holocene environmental and climatic changes in New Caledonia, southwest tropical Pacific, inferred from the littoral plain Gouaro-Déva

Multiproxy analysis of three littoral cores from western New Caledonia supports the hypothesis that the main controlling factors of environmental changes are sea-level change, ENSO variability and extra-tropical phenomena, such as the Medieval Warm Period (MWP) marked by a tendency for La Niña-like conditions in the tropical Pacific. The record starts during the late Holocene sea-level rise when the terrestrial vegetation indicated wet and cool conditions. The site was a coastal bay definitely transformed into a freshwater swamp at around 3400 cal yr BP, after the rapid drawdown of sea level to its current level. Sediments and foraminiferal assemblages indicated subsequent episodes of freshwater infillings, emersion or very high-energy conditions, likely related to climatic changes and mostly controlled by ENSO variability. Between 2750 and 2000 cal yr BP, relatively dry and cool climate prevailed, while wetter conditions predominated between ca. 1800 and 900 cal yr BP. The Rhizophoraceae peak between ca. 1080 and 750 cal yr BP, coeval with the MWP, may indicate a global phenomenon. Microcharcoal particles present throughout the record increased after 1500 cal yr BP, suggesting an anthropogenic source. From ca. 750 cal yr BP the appearance of current type of vegetation marks the human impact.

A new species of mud shrimp of the genus Espeleonaushonia Juarrero & Martínez-Iglesias, 1997 (Decapoda: Gebiidea: Laomediidae) from Palau

A new species of Espeleonaushonia is described from a single specimen collected in an underwater cave in Babeldaob Island, Republic of Palau. The new species is placed in Espeleonaushonia based on the following characters: exopod of third maxilliped well developed, propodus of first pereopod with spines on dorsal surface and along mesial and lateral margins; second pereopod not chelate with dactylus densely setose, fifth pereopod with simple dactylus, fourth to sixth abdominal pleura with serrate ventrodistal angle, and both rami of uropods completely divided. The description of this new species represents the first report of the genus from the tropical southwest Pacific.

The Effect of Typhoons on the Diversity and Distribution Pattern of Aquatic Plants on Hainan Island, South China

ABSTRACTTyphoons are an important meteorological feature of the tropical southwest Pacific. However, little is known about their effects on aquatic plants. Here, we describe the potential influence of typhoons on the diversity and distribution of aquatic plants on Hainan Island, South China. Using a combination of field survey and published data, 266 species or sub‐species belonging to 115 genera in 57 families were found across typhoon impacted regions of the island. Multiple regression analysis indicated that species richness of aquatic plants was correlated with typhoon incidence rather than the area of aquatic habitat within each region. Heavy rainfall, flooding, and strong winds are likely to create suitable habitats and increase the diversity of aquatic habitats in regions frequently affected by typhoons, which may explain the higher species richness found in those regions. Beta diversity and cluster analyses showed that distinct aquatic floras occurred in regions that were impacted by different typhoon pathways. By comparing the aquatic floras of the Philippines and the typhoon‐affected regions of Hainan, we suggest that typhoons have the potential to transport aquatic plants long distances, although there are a number of other potential explanations for the patterns observed in the aquatic flora of Hainan Island.

Marine mammal strandings in the New Caledonia region, Southwest Pacific

Four hundred twenty three marine mammals, in 72 stranding events, were recorded between 1877 and 2005 in New Caledonia, the Loyalty Islands, and Vanuatu in the southwest Pacific. Sixteen species were represented in this count, including: minke whale, Balaenoptera acutorostrata (1 single stranding), sei whale, B. borealis (1 single stranding), blue whale, B. musculus (1 single stranding), humpback whale, Megaptera novaeangliae (2 single strandings), giant sperm whale, Physeter macrocephalus (18 single strandings, 2 pair strandings), pygmy sperm whale, Kogia breviceps (5 single strandings), dwarf sperm whale, K. sima (2 single strandings, 1 triple stranding), Blainville's beaked whale, Mesoplodon densirostris (2 single strandings), short-finned pilot whale, Globicephala macrorhynchus (4 strandings, 56 individuals), melon-headed whale, Peponocephala electra (1 single stranding and 2 mass strandings totalling 231 individuals), common dolphin, Delphinus delphis (1 single stranding), spinner dolphin, Stenella longirostris (1 pair stranding and 2 mass strandings of groups of ∼30 individuals each), Indian Ocean bottlenose dolphin, Tursiops aduncus (2 single strandings), dugong, Dugong dugon (14 single strandings), and New Zealand fur seal, Arctocephalus forsteri (3 single strandings). A stranded rorqual identified as an Antarctic minke whale ( B. bonaerensis), with coloration patterns that did not match known descriptions, was also reported. Sei whale was recorded for the first time in the tropical Southwest Pacific region and Antarctic minke whale, melon-headed whale, and Indian Ocean bottlenose dolphin were recorded for the first time in New Caledonia. Strandings of sperm whales were most frequent in the spring, but also occurred in autumn months, suggesting a seasonal pattern of occurrence possibly related to seasonal migration. One stranded humpback whale bore the scars of a killer whale's attack and one dugong was injured by a shark. Scars left by propellers were noted on several stranded animals including one Antarctic minke whale, one pygmy sperm whale, one dwarf sperm whale, and four dugongs. Collisions with vessels were suspected to be a frequent cause of death for dugong. To cite this article: P. Borsa, C. R. Biologies 329 (2006).

SPATIAL AND TEMPORAL ANALYSIS OF SEAWIFS CHLOROPHYLL IN THE SOUTH TROPICAL PACIFIC OCEAN

The South West Tropical Pacific (SWTP) is generally considered as an N-limiting oligotrophic area with parts characterized by permanent deep thermocline and nutricline as the Melanesian Archipelago. However, ocean color imagery shows in this region a strong seasonal chlorophyll cycle, with large blooms which have for long been attributed to nitrogen-fixing cyanobacteria as Trichodesmium. SeaWiFS surface chlorophyll analysis of the area between 5°S-25°S, 150°E-170°W has been conducted from 1997 to 2004. A strong seasonal and interannual variability is described, which relates well with the sea-truth measured chlorophyll concentrations. An empirical orthogonal function analysis helped in determining principal explicative factors of this variability, and in identifying different patterns, i.e., seasonality (austral winter and summer blooms), which may be driven by different fertilization processes, and a unexplained interannual variability. This work represents part of the French PROOF-IRD DIAPAZON (DIAzotrophy in a PAcific ZONe) program aiming at estimating nitrogen and carbon fixation by cyanobacteria at the SWTP scale