Western North Pacific Integrated Physical-Biogeochemical Ocean Observation Experiment (INBOX): Part 2. Biogeochemical responses to eddies and typhoons revealed from the S1 mooring and shipboard measurements

An interdisciplinary project called S1-INBOX (Western North Pacific Integrated PhysicalBiogeochemical Ocean Observation Experiment conducted around the S1 biogeochemical mooring site) was carried out during the summer of 2011 in the oligotrophic, subtropical North Pacific Ocean near biogeochemical mooring S1 (30° N, 145° E). Results from the S1 mooring during S1-INBOX revealed a large export flux at a depth of 200 m, a high chlorophyll a concentration in the deep chlorophyll maximum layer, and a high potential photochemical efficiency of photosystem II. These phenomena were associated with vertical uplift of isopycnal surfaces at the edge of a cyclonic eddy and a transition from the cyclonic eddy to an anticyclonic eddy. Shipboard biogeochemical surveys conducted during oligotrophic conditions in July 2011 revealed that the phytoplankton community in these waters was dominated by small species that are responsive to intermittent supplies of nutrients. Surface wind forcing because of Typhoons MA-ON and SONCA may have generated near-inertial oscillations. Diapycnal mixing associated with near-inertial waves was also related to high export fluxes, the indication being that propagation of near-inertial internal waves and subsequent mixing may have been important to biogeochemical phenomena during S1-INBOX

Reappraisal of meridional differences of factors controlling phytoplankton biomass and initial increase preceding seasonal bloom in the northwestern Pacific Ocean

Multiplatform observations of ocean biogeochemical data were used to elucidate meridional differences in the factors that limit phytoplankton biomass (Chl-a) and the mechanisms that trigger the seasonal winter or spring phytoplankton bloomin the northwestern Pacific Ocean (NWPO). During the winter, Chl-a north (south) of 30°N is limited by light (nutrients). During the spring and fall, Chl-a in much of the area east of the Japan/Kuril Islands and/or north of 40°N(south of 35°N) is limited by light (nutrients). During the summer, nutrients limit Chl-a over much of the NWPO, except in the areas east of the Japan/Kuril Islands and north of 45°N. In the area south of around 31°N, phytoplankton biomass is nutrient limited throughout the year, and the seasonal bloom emerges in the winter, begins in the fall which is associated with mixed layer deepening. Between 31°N and 40°N, the spring bloom onset is mainly associated with a cessation of mixed layer deepening. In much of the area north of 40°N, including the Oyashio area, the onset of the spring bloom is consistent with Sverdrup’s critical depth hypothesis. The spatial extents of the light- and nutrient-limited areas and the areas associated with a single bloom onset mechanism are by no means constant. They are expected to undergo meridional shifts as a result of large-scale climatic changes and global warming

Sixteen-year phytoplankton biomass trends in the northwestern Pacific Ocean observed by the SeaWiFS and MODIS ocean color sensors

Using multisensor/platform biophysical data collected from 1997 to 2013, we investigated trends of the concentrations of phytoplankton biomass (Chl) in the northwestern Pacific Ocean (NWPO) and the probable responsible factors. The trend of rising sea surface temperature (SST) was the main factor maintaining phytoplankton positive net growth and resulted in a trend of increasing Chl at high latitudes in all seasons. At latitudes of 36-46°N, east of 160°E, the trend of rising SST was accompanied by a trend of declining Chl, markedly in spring and fall, which could be ascribed to strengthened stratification. The trends of environmental variables in the Oyashio area have modified conditions in a way detrimental to phytoplankton growth, the result being a trend of declining Chl from spring to fall. Chl south of roughly 36°N exhibited different trends in different seasons because of the different trends of vertical stratification. Whereas the observed 16-year Chl trends were not primarily influenced by interannual climate variability, to some degree they were likely modified by decadal variability associated with a weakened Aleutian Low pressure. This work prompts further comprehensive studies to investigate the probable ecological consequences of the observed Chl trend for high-trophic-level marine organisms in the NWPO

Seasonal variations in the nitrogen isotopic composition of settling particles at station K2 in the western subarctic North Pacific

Intensive observations using hydrographical cruises and moored sediment trap deployments during 2010 and 2012 at station K2 in the North Pacific western subarctic gyre (WSG) revealed seasonal changes in δ15N of both suspended and settling particles. Suspended particles (SUS) were collected from depths between the surface and 200 m; settling particles by drifting traps (DST; 100-200 m) and moored traps (MST; 200 and 500 m). All particles showed higher δ15N values in winter and lower in summer, contrary to the expected by isotopic fractionation during phytoplankton nitrate consumption. We suggest that these observed isotopic patterns are due to ammonium consumption via light-controlled nitrification, which could induce variations in δ15N(SUS) of 0.4-3.1 ‰ in the euphotic zone (EZ). The δ15N(SUS) signature was reflected by δ15 N(DST) despite modifications during biogenic transformation from suspended particles in the EZ. δ15 N enrichment (average: 3.6 ‰) and the increase in C:N ratio (by 1.6) in settling particles suggests year-round contributions of metabolites from herbivorous zooplankton as well as TEPs produced by diatoms. Accordingly, seasonal δ15 N(DST) variations of 2.4-7.0 ‰ showed a significant correlation with primary productivity (PP) at K2. By applying the observed δ15 N(DST) vs. PP regression to δ15 N(MST) of 1.9-8.0 ‰, we constructed the first annual time-series of PP changes in the WSG. Moreover, the monthly export ratio at 500 m was calculated using both estimated PP and measured organic carbon fluxes. Results suggest a 1.6 to 1.8 times more efficient transport of photosynthetically-fixed carbon to the intermediate layers occurs in summer/autumn rather than winter/spring

Single-Turnover Variable Chlorophyll Fluorescence as a Tool for Assessing Phytoplankton Photosynthesis and Primary Productivity: Opportunities, Caveats and Recommendations

Phytoplankton photosynthetic physiology can be investigated through single-turnover variable chlorophyll fluorescence (ST-ChlF) approaches, which carry unique potential to autonomously collect data at high spatial and temporal resolution. Over the past decades, significant progress has been made in the development and application of ST-ChlF methods in aquatic ecosystems, and in the interpretation of the resulting observations. At the same time, however, an increasing number of sensor types, sampling protocols, and data processing algorithms have created confusion and uncertainty among potential users, with a growing divergence of practice among different research groups. In this review, we assist the existing and upcoming user community by providing an overview of current approaches and consensus recommendations for the use of ST-ChlF measurements to examine in-situ phytoplankton productivity and photo-physiology. We argue that a consistency of practice and adherence to basic operational and quality control standards is critical to ensuring data inter-comparability. Large datasets of inter-comparable and globally coherent ST-ChlF observations hold the potential to reveal large-scale patterns and trends in phytoplankton photo-physiology, photosynthetic rates and bottom-up controls on primary productivity. As such, they hold great potential to provide invaluable physiological observations on the scales relevant for the development and validation of ecosystem models and remote sensing algorithms

MR18-04 Leg.1 Preliminary Cruise Report

調査海域: 西部北太平洋 / Area: The western North Pacific ; 期間: 2018年7月19日～2018年8月9日 / Operation Period: July 19, 2018～August 9, 2018http://www.godac.jamstec.go.jp/darwin/cruise/mirai/mr18-04_leg1/

MR19-02 Preliminary Cruise Report

調査海域: 北西太平洋 / Area: The western North Pacific ; 期間: 2019年5月24日～2019年6月14日 / Operation Period: May 24, 2019～June 14, 2019http://www.godac.jamstec.go.jp/darwin/cruise/mirai/mr19-02/