Kı̄lauea volcano is one of the most active volcanoes in the world with nearly continuous seismic activity from 1983 to 2018. In May 2018, the Kı̄lauea volcano erupted and released volcanic ash into the atmosphere. Carried by easterly winds, the volcanic ash plume dispersed westward and by June 2018, the plume was observed over the central part (150E to 160W) of the nutrient‐poor North Pacific Subtropical Gyre (NPSG). Coincident with precipitation during the same period, anomalously high dust deposition comprised mostly of wet dust was observed over the same region. Consequently, patches of high chlorophyll (chl ) waters were observed approximately 5 north of the high dust deposition area from the middle of June to early August 2018 via satellite images. The phytoplankton bloom peaked in July encompassing 1.5 million , about 5 or 50 times the size of Malaysia or Taiwan, respectively. In addition to the large dust deposition, shoaling of the mixed layer in the range of 25–50 m is believed to have concentrated the bloom within the optical depth detected by satellite. Net primary production and export production estimated from satellite observations show that the July 2018 bloom generated an additional 1.91 Tg C of net carbon production, and 0.34 Tg C was exported from the euphotic zone.

Photosynthesis acts as a fundamental control in the cycling of biologically reactive elements in the ocean. Modeling photosynthesis requires an understanding of its response to light, specifically the maximum rate of photosynthesis per photon absorbed and the irradiance level at which it becomes light-saturated (E-k), though field measurements of these parameters are both time and labor intensive. As absorbed light either drives photosynthesis, is re-emitted as fluorescence, or is converted to heat, fluorescence can be related to the photosynthetic response to light in that, as light increases, there exists an inflection point where the probability that excess absorbed energy is dissipated as heat increases and fluorescence yield is decreased. Accordingly, we use a combination of in vivo chlorophyll fluorescence, particulate matter absorption spectra, and photosynthetically active radiation measurements to approximate this inflection irradiance (termed E-FT) and relate it to modeled E-k along a transect from the oligotrophic North Pacific Subtropical Gyre to the edge of the more eutrophic subpolar gyre (similar to 45 degrees N). We find that E-FT declines by a factor of 4x from values of 200-300 mu mol photons m(-2) s(-1) in the oligotrophic gyre to 50-100 mu mol photons m(-2) s(-1) north of the transition zone and correlates well with E-k from traditional data and models. This latitudinal pattern is associated with changes in biomass concentrations and the phytoplankton carbon to chlorophyll ratio, as well as with changes in particulate carbon to nitrogen ratios. Collectively, these results demonstrate a promising framework to capture high-resolution variability in a key photosynthetic parameter.

Particulate adenosine-5′-triphosphate (P-ATP) and particulate carbon (PC) concentrations were measured on approximately monthly intervals throughout the upper water column (0–1000 m) over a 30-yr (1989–2018) period at Station ALOHA to track the seasonal-to-decadal variability in total microbial biomass and the dynamics of living-to-nonliving particulate organic matter pools. On selected cruises, samples were also collected to a depth of ∼4800 m. P-ATP concentrations were relatively uniform (27–34 ng l−1) throughout the upper euphotic zone (0–100 m) with a distinct peak at 45 m. P-ATP concentrations were significantly higher (p < 0.001) in summer (Jun-Aug) than in winter (Dec-Feb), especially between 45 and 100 m where the seasonal differences averaged 28%. Below 100 m, P-ATP concentrations decreased rapidly with depth to a 30-yr mean value of 3.5 ng ATP l−1 at 250 m, and then decreased more gradually to a 30-yr mean value of 0.9 ng ATP l−1 at 1000 m. Between 125 and 175 m, the seasonal peak in P-ATP shifted to spring (Mar-May), with minima in fall (Sep-Nov) and winter (Dec-Feb). No consistent seasonal variations in P-ATP were detected at depths >175 m, suggesting a temporally stable habitat. Assuming a PC:P-ATP ratio of 250:1 (g g−1), the 0–100 m, 100–250 m, and 250–1000 m depth-integrated microbial biomass estimates were 775, 425, and 350 mg C m−2, respectively. Bathypelagic zone (>1000 m) P-ATP concentrations, based on a more limited data set than the upper portions of the water column, were low (0.4–0.7 ng ATP l−1). However, when integrated over the entire deep water habitat (1000–4800 m), bathypelagic zone microbial biomass was substantial (425 mg C m−2). Expressed as a percentage of total PC, microbial biomass ranged from ∼30% in the upper euphotic zone to ∼3% at depths >3000 m, emphasizing the preponderance of detrital PC throughout the entire water column. The total water column inventory of microbial biomass at Station ALOHA was 2 g C m−2 compared to ∼18 g C m−2 for total suspended PC; approximately 50% of the total microbial biomass is resident in the aphotic zone (>175 m). Although daily gross primary production at Station ALOHA is on par with the total euphotic zone (0–175 m)-integrated microbial biomass (∼1 g C m−2 d−1 and ∼1 g C m−2, respectively), the sources of C and energy fueling the substantial aphotic zone microbial biomass (∼1 g m−2) are not well understood at the present time.

Station ALOHA (A Long-term Oligotrophic Habitat Assessment) was established in the North Pacific Subtropical Gyre (22 degrees 45' N, 158 degrees W) as an oligotrophic ocean benchmark to improve our understanding of processes that govern the fluxes of carbon (C) into and from the surface ocean. At approximately monthly intervals, measurements of the primary production of particulate C (PC) using the C-14 method, and the export of PC and particulate nitrogen (PN) using surface-tethered sediment traps deployed at 150 m have been made along with a host of complementary physical, biological, and biogeochemical measurements. Euphotic zone depth-integrated (0-200 m) primary production ranged from 220.2 (standard deviation, SD, 10.8) mg C m (-2) d (-1) in Feb 2018 to 1136.5 (SD = 17.1) mg C m (-2) d (-1) in Jun 2000, with a 30-yr (1989-2018) mean of 536.8 (SD = 135.0) mg C m(-2) d (-1) (n = 271). Although the monthly primary production climatology was fairly well constrained, we observed substantial sub-decadal variability and a significant 0-125 m depth-integrated increasing trend of 4.0 (p < 0.01; 95% confidence interval, CI, 2.1-5.9) (mg C m(-2) d (-1)) yr 1 since 1989, displaying a large relative increase of 37% (CI = 18-55%) in the lower portion (75-125 m) of the euphotic zone. Chlorophyll (Chl) a and suspended PC and PN concentrations also displayed significant (p < 0.01) increases in the 75-125 m region of the euphotic zone. PC export at 150 m exhibited both short-term (monthly) and longer-scale variability with a 30-yr mean of 27.9 (SD = 9.7, n = 265) mg C m(-2) d (-1). PC and PN export exhibited extended, multi-year periods of significantly lower or higher values compared to the 30-yr mean. These multi-year periods of anomalously low and high particle export, in the absence of contemporaneous variations in primary production, probably reflect periodic changes in remineralization efficiencies. The PC export ratio (e-ratio; PC export at 150 m divided by 0-150 m depth-integrated C-14-based primary production) was low, with a 30-yr mean of 0.054 (SD = 0.021, n = 248), and exhibited a significant (p < 0.01) long-term decreasing trend over the 30-yr observation period. The 30-yr long-term increases in primary production (similar to 37%), Chl a, and suspended PC and PN concentrations (similar to 17%, 8%, and 8%, respectively) in the 75-125 m portion of the water column are hypothesized to result from an enhanced supply of nutrients to the lower portion of the water column over the past three decades.

A large-volume mesocosm-based nutrient perturbation experiment was conducted off the island of Hawai‘i, USA, to investigate the response of surface ocean phytoplankton communities to the addition of macronutrients, trace metals, and vitamins and to assess the feasibility of using mesocosms in the open ocean. Three free-drifting mesocosms (~60 m3) were deployed: one mesocosm served as a control (no nutrient amendments); a second (termed +P) was amended with nitrate (N), silicate (Si), phosphate (P), and a trace metal + vitamin mixture; and a third (termed -P) was amended with N, Si, and a trace metal + vitamin mixture but no P. These mesocosms were unreplicated due to logistical constraints and hence differences between treatments are qualitative. After 6 d, the largest response of the phytoplankton community was observed in the +P mesocosm, where chlorophyll a and 14C-based primary production were 2-3× greater than in the -P mesocosm and 4-6× greater than in the control. Comparison between mesocosm and ‘microcosm’ incubations (20 l) revealed differences in the magnitude and timing of production and marked differences in community structure with a reduced response of diatoms in microcosm treatments. Notably, we also observed pronounced declines in Prochlorococcus populations in all treatments, although these were greater in microcosms (up to 99%). Overall, this study confirmed the feasibility of deploying free-drifting mesocosms in the open ocean as a potentially powerful tool to investigate ecological impacts of nutrient perturbations and constitutes a valuable first step towards scaling plankton manipulation experiments.

There has been a long history of interest on how (if) the absorption coefficient of “pure” fresh water (afwλ) and “pure” seawater (aswλ) changes with temperature (T), yet the impact of T reported in the literature differs significantly in the blue domain. Unlike the previous studies based on laboratory measurements, we took an approach based on ~18 years (2002–2020) of MODIS ocean color and temperature measurements in the oligotrophic oceans, along with field measured chlorophyll concentration and phytoplankton absorption coefficient, to examine the relationship between T and the total absorption coefficient (aλ) at 412 and 443 nm. We found that the values of a412 and a443 in the summer are nearly flat (slightly decreasing) for the observed T range of ~19–27 °C. Since there are no detectable changes of chlorophyll during this period, the results suggest that T has a negligible impact on asw412 and asw443 in this T range. As a complement, the impact of salinity on afwλ was also evaluated using three independent determinations of aswλ and afwλ, where good agreements were found from these observations.

Coastal upwelling regions represent less than 2% of the marine biosphere's volume. Nonetheless, local biogeochemical processes have a considerable impact on the structure and functioning of the global oceans. In this study we characterize the evolution of biogeochemical properties during the upwelling seasons of 2009 and 2010 in the coastal ecosystem off central Oregon (west coast of USA). Both years displayed similar cumulative upwelling favorable winds and total input of preformed nutrients into the midshelf despite of their marked differences in the timing of the onset of the upwelling season, its duration and the number of upwelling relaxation events. However, these differences in upwelling forcing induced inter-annual variability in oxygen and nitrogen deficiencies, particulate matter accumulation and nutrient regeneration. Our results suggest that the summer evolution of nitrogen loss and oxygen depletion over the shelf does not follow a simple relationship with the cumulative upwelling index. We hypothesize that other factors, such as the intensity and persistence of the upwelling, which affect the residence time of water masses over the continental shelf, play a key role in the cumulative effect of biological processes leading to hypoxia in these coastal environments.

The major biogeochemical cycles of marine ecosystems are driven by solar energy. Energy that is initially captured through photosynthesis is transformed and transported to great ocean depths via complex, yet poorly understood, energy flow networks. Herein we show that the chemical composition and specific energy (Joules per unit mass or organic carbon) of sinking particulate matter collected in the North Pacific Subtropical Gyre reveal dramatic changes in the upper 500 m of the water column as particles sink and age. In contrast to these upper water column processes, particles reaching the deep sea (4000 m) are energy-replete with organic carbon-specific energy values similar to surface phytoplankton. These enigmatic results suggest that the particles collected in the abyssal zone must be transported by rapid sinking processes. These fast-sinking particles control the pace of deep-sea benthic communities that live a feast-or-famine existence in an otherwise energy-depleted habitat.

From June to August 2018, the eruption of Kīlauea volcano on the island of Hawai'i injected millions of cubic meters of molten lava into the nutrient-poor waters of the North Pacific Subtropical Gyre. The lava-impacted seawater was characterized by high concentrations of metals and nutrients that stimulated phytoplankton growth, resulting in an extensive plume of chlorophyll a that was detectable by satellite. Chemical and molecular evidence revealed that this biological response hinged on unexpectedly high concentrations of nitrate, despite the negligible quantities of nitrogen in basaltic lava. We hypothesize that the high nitrate was caused by buoyant plumes of nutrient-rich deep waters created by the substantial input of lava into the ocean. This large-scale ocean fertilization was therefore a unique perturbation event that revealed how marine ecosystems respond to exogenous inputs of nutrients.

The supply of nutrients is a fundamental regulator of ocean productivity and carbon sequestration. Nutrient sources, sinks, residence times, and elemental ratios vary over broad scales, including those resulting from climate-driven changes in upper water column stratification, advection, and the deposition of atmospheric dust. These changes can alter the proximate elemental control of ecosystem productivity with cascading ecological effects and impacts on carbon sequestration. Here, we report multidecadal observations revealing that the ecosystem in the eastern region of the North Pacific Subtropical Gyre (NPSG) oscillates on subdecadal scales between inorganic phosphorus (P i ) sufficiency and limitation, when P i concentration in surface waters decreases below 50-60 nmol⋅kg-1 In situ observations and model simulations suggest that sea-level pressure changes over the northwest Pacific may induce basin-scale variations in the atmospheric transport and deposition of Asian dust-associated iron (Fe), causing the eastern portion of the NPSG ecosystem to shift between states of Fe and P i limitation. Our results highlight the critical need to include both atmospheric and ocean circulation variability when modeling the response of open ocean pelagic ecosystems under future climate change scenarios.

Sustained ocean time series are critical for characterizing marine ecosystem shifts in a time of accelerating, and at times unpredictable, changes. They represent the only means to distinguish between natural and anthropogenic forcings, and are the best tools to explore causal links and implications for human communities that depend on ocean resources. Since the inception of sustained ocean observations, ocean time series have withstood many challenges, most prominently availability of uninterrupted funding and retention of trained personnel. This OceanObs'19 review article provides an overarching vision for sustained ocean time series observations for the next decade, focusing on the growing challenges of maintaining sustained ocean time series, including ship-based and autonomous coastal and open-ocean platforms, as well as remote sensing. In addition to increased diversification of funding sources to include the private sector, NGOs, and other groups, more effective engagement of stakeholders and other endusers will be critical to ensure the sustainability of ocean time series programs. Building a cohesive international time series network will require dedicated capacity to coordinate across observing programs and leverage existing infrastructure and platforms of opportunity. This review article outlines near-term observing priorities and technology needs; explores potential mechanisms to broaden ocean time series data applications and end-user communities; and describes current tools and future requirements for managing increasingly complex multi-platform data streams and developing synthesis products that support science and society. The actionable recommendations outlined herein ultimately form the basis for a robust, sustainable, fit-for-purpose time series network that will foster a predictive understanding of changing ocean systems for the benefit of society.

The biodiversity and high productivity of coastal terrestrial and aquatic habitats are the foundation for important benefits to human societies around the world. These globally distributed habitats need frequent and broad systematic assessments, but field surveys only cover a small fraction of these areas. Satellite-based sensors can repeatedly record the visible and near-infrared reflectance spectra that contain the absorption, scattering, and fluorescence signatures of functional phytoplankton groups, colored dissolved matter, and particulate matter near the surface ocean, and of biologically structured habitats (floating and emergent vegetation, benthic habitats like coral, seagrass, and algae). These measures can be incorporated into Essential Biodiversity Variables (EBVs), including the distribution, abundance, and traits of groups of species populations, and used to evaluate habitat fragmentation. However, current and planned satellites are not designed to observe the EBVs that change rapidly with extreme tides, salinity, temperatures, storms, pollution, or physical habitat destruction over scales relevant to human activity. Making these observations requires a new generation of satellite sensors able to sample with these combined characteristics: (1) spatial resolution on the order of 30 to 100-m pixels or smaller; (2) spectral resolution on the order of 5 nm in the visible and 10 nm in the short-wave infrared spectrum (or at least two or more bands at 1,030, 1,240, 1,630, 2,125, and/or 2,260 nm) for atmospheric correction and aquatic and vegetation assessments; (3) radiometric quality with signal to noise ratios (SNR) above 800 (relative to signal levels typical of the open ocean), 14-bit digitization, absolute radiometric calibration <2%, relative calibration of 0.2%, polarization sensitivity <1%, high radiometric stability and linearity, and operations designed to minimize sunglint; and (4) temporal resolution of hours to days. We refer to these combined specifications as H4 imaging. Enabling H4 imaging is vital for the conservation and management of global biodiversity and ecosystem services, including food provisioning and water security. An agile satellite in a 3-d repeat low-Earth orbit could sample 30-km swath images of several hundred coastal habitats daily. Nine H4 satellites would provide weekly coverage of global coastal zones. Such satellite constellations are now feasible and are used in various applications.

Global analyses of satellite-and modeled data suggest decreased phytoplankton abundance and primary productivity in oligotrophic gyres as they expand in response to increased surface temperatures, shoaling of surface mixed layers, and decreased supply of subsurface macronutrients. However, concomitant changes have not been evident in situ at Hawaii Ocean Time-series (HOT) Station ALOHA, suggesting physiological or structural reorganization not observed from space, uncharacterized spatiotemporal variability, or uncorrected sensor drift. To address the spatiotemporal variability hypothesis, we compared interannual patterns of in situ data to gyre geography based on multiple metrics including dynamic topography, satellite surface chlorophyll (chl a), and multivariate seascapes using modelled or satellite data. There was only weak evidence of secular increases in the extent of the subtropical gyre; rather, interannual oscillations were evident in physical, biological, and multivariate biophysical definitions of the gyre. Modelled and satellite-based multivariate seascapes agreed well in terms of expansion (surface area of seascapes) and isolation of Station ALOHA (distance to seascape boundary) resulting in combined data record of nearly three decades. Isolation was associated positively with the North Pacific Gyre Oscillation (NPGO), and negatively with Multivariate ENSO Index (MEI) and Pacific Decadal Oscillation (PDO); the converse was true for the gyre's expansion. This expansion followed a shoaling and freshening of the surface mixed layer and declines in in situ 14C assimilation rates measured over 12 hours in ambient light suggesting that Station ALOHA may serve as an leading indicator of gyre biogeographic patterns. Lags between geographic indicators and in situ conditions appear to partially explain past observed discrepancies between patterns from satellite remote sensing and those from in situ conditions at Station ALOHA appear to be partially explained by lags between geographic indicators and in situ conditions as well as satellite sensor drift and data record length.

We conducted a series of experiments to examine short-term (2–5 days) effects of abrupt increases in the partial pressure of carbon dioxide (pCO2) in seawater on rates of primary and bacterial production at Station ALOHA (22°45’ N, 158° W) in the North Pacific Subtropical Gyre (NPSG). The majority of experiments (8 of 10 total) displayed no response in rates of primary production (measured by 14C-bicarbonate assimilation; 14C-PP) under elevated pCO2 (~1100 μatm) compared to ambient pCO2 (~387 μatm). In 2 of 10 experiments, rates of 14C-PP decreased significantly (~43%) under elevated pCO2 treatments relative to controls. Similarly, no significant differences between treatments were observed in 6 of 7 experiments where bacterial production was measured via incorporation of 3H-leucine (3H-Leu), while in 1 experiment, rates of 3H-Leu incorporation measured in the dark (3H-LeuDark) increased more than 2-fold under high pCO2 conditions. We also examined photoperiod-length, depth-dependent (0–125 m) responses in rates of 14C-PP and 3H-Leu incorporation to abrupt pCO2 increases (to ~750 μatm). In the majority of these depth-resolved experiments (4 of 5 total), rates of 14C-PP demonstrated no consistent response to elevated pCO2. In 2 of 5 depth-resolved experiments, rates of 3H-LeuDark incorporation were lower (10% to 15%) under elevated pCO2 compared to controls. Our results revealed that rates of 14C-PP and bacterial production in this persistently oligotrophic habitat generally demonstrated no or weak responses to abrupt changes in pCO2. We postulate that any effects caused by changes in pCO2 may be masked or outweighed by the role that nutrient availability and temperature play in controlling metabolism in this ecosystem.

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.

To constrain the energy fueling photosynthesis in the North Pacific Subtropical Gyre (NPSG) we characterize the variability of phytoplankton absorption spectra in conjunction with that of the light field at Station ALOHA (22 degrees 45'N, 158 degrees 00'W). Furthermore, we decompose the phytoplankton absorption into photosynthetic and photoprotective components based on high-performance liquid chromatography pigment analysis. Between January 2006 and December 2012 the variability in chlorophyll-specific absorption (a(psi)(not asymptotic to)) above the deep chlorophyll maximum (DCM) layer was driven by changes in photoprotective carotenoid concentrations while the chlorophyll-specific absorption of photosynthetic pigments (a(phi(PSP))(not asymptotic to))remained nearly constant with a mean (+/- SD) value of 0.008 +/- 0.001 m(2) (mg chl a)(-1). In contrast, below the DCM layer changes in a(phi)(not asymptotic to) resulted from increases in the relative contribution of photosynthetic pigments with depth, suggesting that the constancy in a(phi(PSP))(not asymptotic to) above the DCM layer is controlled by nutrient limitation. While the daily photon fluxes absorbed by photosynthetic pigments in the upper 45 m did not vary at a seasonal scale, averaging 0.45 +/- 0.12 mol quanta m(-2) d(-1) in winter and 0.46 +/- 0.10 mol quanta m(-2) d(-1) in summer, when integrated over the upper 200 m these fluxes ranged from 0.64 +/- 0.16 to 0.79 +/- 0.19 mol quanta m(-2) d(-1) in winter and summer, respectively. Based on the rate of photons trapped by the photosynthetic pigments and on the seasonal euphotic zone depth integrated gross O-2 evolution rates derived from (H2O)-O-18 in situ incubations we estimate a mean photosynthetic yield of similar to 0.1 mol O-2 evolved per mol quanta absorbed by photosynthetic pigments.