Hawaii Ocean Time-series Research Articles

Temporal dynamics of Prochlorococcus ecotypes in the Atlantic and Pacific oceans

To better understand the temporal and spatial dynamics of Prochlorococcus populations, and how these populations co-vary with the physical environment, we followed monthly changes in the abundance of five ecotypes-two high-light adapted and three low-light adapted-over a 5-year period in coordination with the Bermuda Atlantic Time Series (BATS) and Hawaii Ocean Time-series (HOT) programs. Ecotype abundance displayed weak seasonal fluctuations at HOT and strong seasonal fluctuations at BATS. Furthermore, stable 'layered' depth distributions, where different Prochlorococcus ecotypes reached maximum abundance at different depths, were maintained consistently for 5 years at HOT. Layered distributions were also observed at BATS, although winter deep mixing events disrupted these patterns each year and produced large variations in ecotype abundance. Interestingly, the layered ecotype distributions were regularly reestablished each year after deep mixing subsided at BATS. In addition, Prochlorococcus ecotypes each responded differently to the strong seasonal changes in light, temperature and mixing at BATS, resulting in a reproducible annual succession of ecotype blooms. Patterns of ecotype abundance, in combination with physiological assays of cultured isolates, confirmed that the low-light adapted eNATL could be distinguished from other low-light adapted ecotypes based on its ability to withstand temporary exposure to high-intensity light, a characteristic stress of the surface mixed layer. Finally, total Prochlorococcus and Synechococcus dynamics were compared with similar time series data collected a decade earlier at each location. The two data sets were remarkably similar-testimony to the resilience of these complex dynamic systems on decadal time scales.

Microbial community structure in the North Pacific ocean

We report a ribosomal tag pyrosequencing study of the phylogenetic diversity of Archaea, Bacteria and Eucarya over a depth profile at the Hawaii Ocean Time-Series Station, ALOHA. The V9 region of the SSU rRNA gene was amplified from samples representing the epi- (10 m), meso- (800 m) and bathy- (4400 m) pelagia. The primers used are expected to amplify representatives of approximately 80% of known phylogenetic diversity across all three domains. Comparisons of unique sequences revealed a remarkably low degree of overlap between communities at each depth. The 444 147 sequence tags analyzed represented 62 975 unique sequences. Of these, 3707 (5.9%) occurred at two depths, and only 298 (0.5%) were observed at all three depths. At this level of phylogenetic resolution, Bacteria diversity decreased with depth but was still equivalent to that reported elsewhere for different soil types. Archaea diversity was highest in the two deeper samples. Eucarya observations and richness estimates are almost one order of magnitude higher than any previous marine microbial Eucarya richness estimates. The associations of many Eucarya sequences with putative parasitic organisms may have significant impacts on our understanding of the mechanisms controlling host population density and diversity, and point to a more significant role for microbial Eucarya in carbon flux through the microbial loop. We posit that the majority of sequences detected from the deep sea that have closest matches to sequences from non-pelagic sources are indeed native to the marine environment, and are possibly responsible for key metabolic processes in global biogeochemical cycles.

Genomics of deep‐sea and sub‐seafloor microbes

Over two‐thirds of the surface of the earth is covered by oceans, which have an average depth of about 3800 m. As each drop of ocean water contains > 105 cells, the > 1030 microbial cells in the ocean represent the largest reservoir of microbes on earth (Whitman et al., 1998). Communities of bacteria, archaea, protists and unicellular fungi account for most of the oceanic biomass and metabolism. Marine microbes are known to play an essential role in the global cycling of nitrogen, carbon, oxygen, phosphorous, iron, sulfur and trace elements (Karl, 2007).

Net community production in the deep euphotic zone of the subtropical North Pacific gyre from glider surveys

Seagliders, deployed through most of 2005 in the subtropical North Pacific gyre, made measurements of temperature, salinity, and dissolved oxygen to quantify net community production (NCP) at Station A Long‐ Term Oligotrophic Habitat Assessment (ALOHA) of the Hawaii Ocean Time‐series (HOT) using an oxygen mass balance approach. A ‘bowtie’‐shaped pattern, 50 km by 50 km in size was repeatedly traversed at two week intervals with the goal of observing the influence of Rossby waves and eddies on the productivity of the study region. Rossby waves and eddies in the region cause a vertical displacement of isopycnal depth of ~±50 m at the base of the euphotic zone. Shoaling of isopycnals is demonstrated to drive productivity in the deep euphotic zone. Four mesoscale shoaling events were observed between February and November in 2005. During each event when isopycnals shoaled, oxygen concentrations on isopycnals increased, fluorescence in the deep euphotic zone was higher, and net community production was elevated. Productivity in the deep euphotic zone was strongly influenced by Rossby waves and eddies, but this influence was not observed to extend into the mixed layer.

Causes for Large-Scale Hydrographic Changes at the Hawaii Ocean Time Series Station

Abstract Results from Estimating the Circulation and Climate of the Ocean (ECCO)–Scripps Institution of Oceanography (SIO) global ocean state estimate, available over the 11-yr period 1992 through 2002, are compared with independent observations available at the Hawaii Ocean time series station ALOHA. The comparison shows that at this position, the estimated temporal variability has some skill in simulating observed ocean variability and that the quality of future syntheses could benefit from additional information available from the Argo network and from the time series observations themselves. On a decadal time scale, the influence radius of the station ALOHA T–S time series covers large parts of the tropical and subtropical Pacific Ocean and reaches even into the Indian Ocean through the Indonesian Throughflow. Estimated changes in sea surface height (SSH) result largely from thermosteric changes; however, nonsteric (barotropic) variations on the order of 1–2 cm also contribute to SSH changes at station ALOHA. Moreover, changes of similar magnitude can be caused by changes in the salinity field because of a quasi-biennial oscillation in the horizontal flow structure and heaving of the mean salinity structure on seasonal and interannual time scales. The adjoint modeling framework confirms westward-propagating Rossby waves (due to wind forcing) and subduction of water-mass anomalies (due to surface buoyancy forcing) as the primary mechanisms leading to observed changes of T–S structures at station ALOHA. Specifically, the analysis identifies surface freshwater fluxes along the wintertime outcrop of intermediate waters as a primary cause for salinity changes at station ALOHA and wind stress forcing east of the station position as another forcing mechanism of salinity variations around the Hawaiian Archipelago.

Net biological oxygen production in the ocean: Remote in situ measurements of O2 and N2 in surface waters

We describe a method for determining net annual biological oxygen production in the euphotic zone of the ocean using remote in situ measurements of oxygen and nitrogen gas. Temperature, salinity, oxygen, and total dissolved gas pressure were measured every 2 hours at 10‐m depth on a mooring at the Hawaii Ocean time series during the year 2005. Since dissolved N2 is effectively inert to biological processes it can be used as a tracer for the physical mechanisms affecting the O2 concentration in an upper ocean model of gas concentrations. We determine a net biological oxygen production in the surface mixed layer of 4.8 ± 2.7 mol m−2 yr−1. The most important term in the mixed‐layer mass balance other than biological oxygen production is the flux of oxygen across the air–water interface to the atmosphere. Simultaneous glider surveys of the O2 field measured in a companion paper (Nicholson et al., 2008) yield net biological oxygen production below the mixed layer of 0.9 ± 0.1 mol O2 m−2 yr−1. The upper‐ocean mass balance also includes a potential contribution from diapycnal mixing of O2 into the pycnocline of 0–0.8 mol O2 m−2 yr−1. Assuming that the net biological oxygen production over a period of a year or longer is stoichiometrically related to net biological carbon production and export via ΔO2/ΔC = 1.45, the biological carbon flux from the euphotic zone at HOT is 4.1 ± 1.9 mol C m−2 yr−1 in 2005, with roughly 80% of the carbon production originating in the mixed layer. Annual estimates of this flux (the ocean's “biological carbon pump”) have been determined experimentally in only a few locations of the ocean because of the labor and expense involved in repeated ship board measurements. With this new in situ method, it may now be possible to better quantify the global distribution of the net annual biological carbon export, a prominent mechanism of carbon cycle feedback in response to climate change, both in the past and future.

Export stoichiometry and migrant-mediated flux of phosphorus in the North Pacific Subtropical Gyre

Export processes play a major role in regulating global marine primary production by reducing the efficiency of nutrient cycling and turnover in surface waters. Most studies of euphotic zone export focus on passive fluxes, that is, sinking particles. However, active transport, the vertical transfer of material by migrating zooplankton, can also be an important component of carbon (C) and nitrogen (N) removal from the surface ocean. Here we demonstrate that active transport is an especially important mechanism for phosphorus (P) removal from the euphotic zone at Station ALOHA (Hawaii Ocean Time-series program; 22°45′N, 158°W), a P-stressed site in the North Pacific Subtropical Gyre. Migrant excretions in this region are P-rich (C 51:N 12:P 1) relative to sinking particles (C 250:N 31:P 1), and migrant-mediated P fluxes are almost equal in magnitude (82%) to P fluxes from sediment traps. Migrant zooplankton biomass and therefore the importance of this P removal pathway relative to sinking fluxes has increased significantly over the past 12 years, suggesting that active transport may be a major driving force for enhanced P-limitation of biological production in the NPSG. We further assess the C:N:P composition of zooplankton size fractions at Station ALOHA (C 88:N 18:P 1, on average) and discuss migrant-mediated P export in light of the balance between zooplankton and suspended particle stoichiometries. We conclude that, because active transport is such a large component of the total P flux and significantly impacts ecosystem stoichiometry, export processes involving migrant zooplankton must be included in large-scale efforts to understand biogeochemical cycles.

Changes in fecal pellet characteristics with depth as indicators of zooplankton repackaging of particles in the mesopelagic zone of the subtropical and subarctic North Pacific Ocean

We investigated how fecal pellet characteristics change with depth in order to quantify the extent of particle repackaging by mesopelagic zooplankton in two contrasting open-ocean systems. Material from neutrally buoyant sediment traps deployed in the summer of 2004 and 2005 at 150, 300, and 500 m was analyzed from both a mesotrophic (Japanese time-series station K2) and an oligotrophic (Hawaii Ocean Time series—HOT station ALOHA) environment in the Pacific Ocean as part of the VERtical Transport In the Global Ocean (VERTIGO) project. We quantified changes in the flux, size, shape, and color of particles recognizable as zooplankton fecal pellets to determine how these parameters varied with depth and location. Flux of K2 fecal pellet particulate organic carbon (POC) at 150 and 300 m was four to five times higher than at ALOHA, and at all depths, fecal pellets were two to five times larger at K2, reflective of the disparate zooplankton community structure at the two sites. At K2, the proportion of POC flux that consisted of fecal pellets generally decreased with depth from 20% at 150 m to 5% at 500 m, whereas at ALOHA this proportion increased with depth (and was more variable) from 14% to 35%. This difference in the fecal fraction of POC with increasing depth is hypothesized to be due to differences in the extent of zooplankton-mediated fragmentation (coprohexy) and in zooplankton community structure between the two locations. Both regions provided indications of sinking particle repackaging and zooplankton carnivory in the mesopelagic. At ALOHA, this was reflected in a significant increase in the mean flux of larvacean fecal pellets from 150 to 500 m of 3–46m gCm � 2 d � 1 , respectively, and at K2 a large peak in

Mesozooplankton biomass and grazing responses to Cyclone Opal, a subtropical mesoscale eddy

As part of E-Flux III cruise studies in March 2005, plankton net collections were made to assess the effects of a cyclonic cold-core eddy (Cyclone Opal) on the biomass and grazing of mesozooplankton. Mesozooplankton biomass in the central region of Cyclone Opal, an area of uplifted nutricline and a subsurface diatom bloom, averaged 0.8070.24 and 1.5170.59 g DW m � 2 , for day and night tows, respectively. These biomass estimates were about 80% higher than control (OUT) stations, with increases more or less proportionately distributed among size classes from 0.2 to 45 mm. Though elevated relative to surrounding waters south of the Hawaiian Islands (Hawai’i lee), total biomass and size distribution in Cyclone Opal were almost exactly the same as contemporary measurements made at Stn. ALOHA, 100 km north of the islands, by the HOT (Hawaii Ocean Time-series) Program. Mesozooplankton biomass and community composition at the OUT stations were also similar to ALOHA values from 1994 to 1996, preceding a recent decadal increase. These comparisons may therefore provide insight into production characteristics or biomass gradients associated with decadal changes at Stn. ALOHA. Gut fluorescence estimates were higher in Opal than in ambient waters,

Design and testing of ‘genome‐proxy’ microarrays to profile marine microbial communities

Microarrays are useful tools for detecting and quantifying specific functional and phylogenetic genes in natural microbial communities. In order to track uncultivated microbial genotypes and their close relatives in an environmental context, we designed and implemented a 'genome-proxy' microarray that targets microbial genome fragments recovered directly from the environment. Fragments consisted of sequenced clones from large-insert genomic libraries from microbial communities in Monterey Bay, the Hawaii Ocean Time-series station ALOHA, and Antarctic coastal waters. In a prototype array, we designed probe sets to 13 of the sequenced genome fragments and to genomic regions of the cultivated cyanobacterium Prochlorococcus MED4. Each probe set consisted of multiple 70-mers, each targeting an individual open reading frame, and distributed along each approximately 40-160 kbp contiguous genomic region. The targeted organisms or clones, and close relatives, were hybridized to the array both as pure DNA mixtures and as additions of cells to a background of coastal seawater. This prototype array correctly identified the presence or absence of the target organisms and their relatives in laboratory mixes, with negligible cross-hybridization to organisms having <or= approximately 75% genomic identity. In addition, the array correctly identified target cells added to a background of environmental DNA, with a limit of detection of approximately 0.1% of the community, corresponding to approximately 10(3) cells ml(-1) in these samples. Signal correlated to cell concentration with an R(2) of 1.0 across six orders of magnitude. In addition, the array could track a related strain (at 86% genomic identity to that targeted) with a linearity of R(2) = 0.9999 and a limit of detection of approximately 1% of the community. Closely related genotypes were distinguishable by differing hybridization patterns across each probe set. This array's multiple-probe, 'genome-proxy' approach and consequent ability to track both target genotypes and their close relatives is important for the array's environmental application given the recent discoveries of considerable intrapopulation diversity within marine microbial communities.

Observational Evidence of Winter Spice Injection

Abstract Temperature and salinity (T–S) profiles from the global array of Argo floats support the existence of spice-formation regions in the subtropics of each ocean basin where large, destabilizing vertical salinity gradients coincide with weak stratification in winter. In these characteristic regions, convective boundary layer mixing generates a strongly density-compensated (SDC) layer at the base of the well-mixed layer. The degree of density compensation of the T–S gradients of an upper-ocean water column is quantified using a bulk vertical Turner angle (Tub) between the surface and upper pycnocline. The winter generation of the SDC layer in spice-formation zones is clearly seen in Argo data as a large-amplitude seasonal cycle of Tub in regions of the subtropical oceans characterized by high mean Tub. In formation regions, Argo floats provide ample evidence of large, abrupt spice injection (T–S increase on subducted isopycnals due to vertical mixing) associated with the winter increase in Tub. A simple conceptual model of the spice-injection mechanism is presented that is based on known behavior of convective boundary layers and supported by numerical model results. It suggests that penetrative convective mixing of a partially density-compensated water column will enhance the Turner angle within a transition layer between the mixed layer and the upper pycnocline, generating seasonal T–S increases on density surfaces below the mixed layer. Observations are consistent with this hypothesis. In OGCMs, regions showing high Tub mean and seasonal amplitude are also the sources of significant interannual spice variability in the permanent pycnocline. Decadal changes in the North Pacific of a model hindcast simulation show qualitative resemblance to the observed multiyear time series from the Hawaii Ocean Time series (HOT) station ALOHA. Modeled pycnocline variations near Hawaii can be linked to high Tub seasonality and winter spice injection within a formation region upstream of ALOHA, suggesting that spice injection may explain the origins of observed large, interannual variations on isopycnals in the ocean interior.

What factors are driving summer phytoplankton blooms in the North Pacific Subtropical Gyre?

Annually recurrent summer to fall surface blooms of the dinitrogen (N2) fixing genera Trichodesmium and Richelia have a significant impact on biogeochemical cycling in the North Pacific Subtropical Gyre (NPSG). Yet the environmental determinants of these blooms have not been thoroughly resolved. Here, we combine remote sensing of ocean color, sea surface temperature (SST), sea surface height anomalies (SSHa), wind forcing, and integrated irradiance with the vessel‐based time series of the Hawaii Ocean Time‐series (HOT) program at Station ALOHA (22.75°N, 158.00°W) and mooring data derived from the National Data Buoy Center (NDBC) buoy 51001 (23.42°N, 162.2°W). With these data sets we attempt to constrain the environmental window under which blooms of large cell‐sized N2 fixing organisms increase in abundance in NPSG surface waters using phycoerythrin (PE) as a proxy. For identified blooms, our analyses indicate that these events are confined to the months of June–October, SST in the range of 25°–27°C, and mixed layer depths less than 70 m. Neither wind forcing nor SSHa are correlated (directly or time‐lagged) with increases in PE concentrations. Furthermore, blooms do not consistently result in increases of in situ or remotely sensed chlorophyll a. Additional higher‐resolution data sets of physical forcing, diazotroph abundance, and biochemical properties, sampled on the timescale of bloom development (days‐weeks), will be necessary to the environmental conditions supporting annual summer‐fall blooms.

Summer phytoplankton blooms in the oligotrophic North Pacific Subtropical Gyre: Historical perspective and recent observations

The export of organic matter from the oceanic euphotic zone is a critical process in the global biogeochemical cycling of bioelements (C, N, P, Si). Much of this export occurs in the form of sinking particles, which rain down into the unlit waters of the deep sea. Classical models of oceanic production and export balance this gravitational loss of particulate bioelements with an upward flux of dissolved nutrients, and they describe reasonably well those areas of the ocean where deep winter mixing occurs. The surface waters of the North Pacific Subtropical Gyre (NPSG), however, are strongly stratified and chronically nutrient-depleted, especially in summer. Nevertheless, there is ample evidence that blooms of phytoplankton and subsequent pulses of particle export occur during the height of summer stratification in these waters, especially to the northeast of the Hawaiian Islands. These blooms impact regional bioelemental cycling and act as a food source to the deep-sea benthos. We review here numerous published observations of these events in the NPSG, and present new data collected at Station ALOHA (22.75°N, 158°W) during the first 176 cruises of the Hawaii Ocean Time-series program (1988–2005), along with results from transect cruises conducted in the region in 1996 and 2005. We suggest that the summer phytoplankton bloom can be considered a frequent, perhaps annual feature in the northeastern NPSG, and that its perceived stochastic nature is a manifestation of chronic undersampling in time and space. The bloom is typically dominated by only a few genera of large diatoms and the cyanobacterium Trichodesmium. It appears to be consistently supported by dinitrogen fixation, but the fate of the organic matter produced during the summer depends critically on the species composition of the responsible diazotrophs. We estimate that the summer bloom is responsible for up to 38% of N 2 fixation and up to 18% of N-based new production annually at Station ALOHA. We hypothesize that the spatial distribution, timing and magnitude of the bloom may be determined largely by the physical and biological processes controlling new phosphorus delivery into the euphotic zone during the summer and the preceding winter.

Impact of climate forcing on ecosystem processes in the North Pacific Subtropical Gyre

Measurements at the Hawaii Ocean Time‐series (HOT) Station ALOHA (22°45′N, 158°W) have revealed a significant, approximately 50% increase in euphotic zone depth‐integrated rates of primary production (PP; mol C fixed m−2 d−1) based on in situ 14C experiments. The character of the nearly two‐decade increasing trend in PP was punctuated by several abrupt episodes that coincided with changes in the El Niño/Southern Oscillation (ENSO), and the Pacific Decadal Oscillation (PDO) climate indices, or both. In contrast to the observed increase in rates of PP, the PP per unit chlorophyll a (mol C fixed mol chl a−1 d−1), a measure of the biomass‐normalized production, was relatively constant, whereas PP per unit solar radiation (mol C fixed mol quanta−1), a measure of the efficiency of light utilization, varied in synchrony with the temporal trend in PP. Coincident with variations in PP, the HOT program core data sets also revealed changes in mixed‐layer depth, upper ocean stratification, inorganic nutrients, phototrophic microbial abundances and pigment inventories. These time‐series data suggest that the ENSO/PDO may control upper ocean stratification and vertical nutrient delivery into the euphotic zone at Sta. ALOHA, thereby influencing the composition of the plankton assemblage and altering rates of PP and particulate matter export.

CAMERA: A Community Resource for Metagenomics

Microbes are responsible for most of the chemical transformations that are crucial to sustaining life on Earth. Their ability to inhabit almost any environmental niche suggests that they possess an incredible diversity of physiological capabilities. However, we have little to no information on a majority of the millions of microbial species that are predicted to exist, mainly because of our inability to culture them in the laboratory. A growing discipline called metagenomics allows us to study these uncultured organisms by deciphering their genetic information from DNA that is extracted directly from their environment, thus effectively bypassing the laboratory culture step. Metagenomics allows us to address the questions “who's there?”, “what are they doing?”, and “how are they doing it?”, offering insights into the evolutionary history as well as previously unrecognized physiological abilities of uncultured communities. Studies such as the J. Craig Venter Institute's Global Ocean Sampling (GOS) expedition (in this issue) reveal a remarkable breadth and depth of microbial diversity in the oceans. To date, researchers have made significant but largely preliminary inroads into understanding the biogeography of microbial populations across ecosystems. We know even less about the dynamic physiological processes and complex interactions that impact global carbon cycles and ocean productivity. Marine microbes are thought to act as part of the biological conduit that transports carbon dioxide from the surface to the deep oceanic realms. By removing carbon from the atmosphere and sequestering it (in the form of organic matter), marine microorganisms may significantly affect global climate. Although we now have numerous global and real-time methods to measure physical and chemical parameters within the ocean, few methods or concepts have been developed to measure important microbial processes on a global scale. Even if the technology to make such measurements existed, we would presently not know what to measure or how to interpret those measurements. We invite the research community to submit its metagenomics data to CAMERA. We need a systematic way to explore the structure and function of ocean ecosystems, and their impact on global carbon processing and climate. Metagenomics has the potential to shed light on the genetic controls of these processes by investigating the key players, their roles, and community compositions that may change as a function of time, climate, nutrients, carbon dioxide, and anthropogenic factors. These studies include a substantial informatics component, requiring researchers to take on complex computational and mathematical challenges. Nonetheless, microbiologists have been quick to seize upon this modern technique, resulting in a deluge of sequence data, and an ever-widening gap between the rates of collecting data and interpreting it. The Community Cyberinfrastructure for Advanced Marine Microbial Ecology Research and Analysis (CAMERA) project [1] is an important first step in attempting to bridge these gaps and in developing global methods for monitoring microbial communities in the ocean and their response to environmental changes. The aim is to create a rich, distinctive data repository and bioinformatics tools resource that will address many of the unique challenges of metagenomics and enable researchers to unravel the biology of environmental microorganisms (Figure 1). CAMERA's database includes environmental metagenomic and genomic sequence data, associated environmental parameters (“metadata”), precomputed search results, and software tools to support powerful cross-analysis of environmental samples. Figure 1 Schematic of Intended Core Functions of the CAMERA Project The initial release will include data and tools associated with the companion set of GOS expedition publications [2–4]; metagenome data from the Hawaii Ocean Time Series Station ALOHA [5] and marine viromes from four different oceanic regions[6]; standard nonredundant sequence databases (e.g., nrnt for nucleotides and nraa for amino acids[7]); and collections of microbial genome sequences, including a set of 155 marine microbial genomes funded by the Gordon and Betty Moore Foundation. The focal point for the CAMERA project is its Web site: http://camera.calit2.net. We invite the research community to submit its metagenomics data to CAMERA, and are establishing mechanisms to streamline this process. Here we describe some of the key challenges and features of the CAMERA project.

Sulfur hexafluoride as a transient tracer in the North Pacific Ocean

The atmospheric concentration of sulfur hexafluoride (SF6) has increased steadily during the past 30 years, making it potentially a valuable transient tracer of oceanic circulation and mixing processes on decadal timescales. Simultaneous measurements of dissolved SF6 with chlorofluorocarbons (CFCs), which have longer atmospheric histories but different growth rates, provides additional information over the use of each tracer alone. Concentrations of dissolved SF6, CFC11 and CFC12 were measured at the Hawaii Ocean Time‐Series (HOT) site in 2005. Concentrations were highest in the upper water column, with pronounced CFC11 and CFC12 maxima at a depth of ∼400 meters. Apparent water mass ages calculated from SF6 concentrations tend to be younger than those calculated from CFC12 concentrations. An isopycnal pipe model is used to estimate the effects of mixing on SF6 and CFC12 derived ages. Combining SF6 and CFC12 ages allows improved estimates of ideal ages and oceanic uptake of anthropogenic CO2.

Methods for the Reconstruction of Vertical Profiles from Surface Data: Multivariate Analyses, Residual GEM, and Variable Temporal Signals in the North Pacific Ocean

AbstractDifferent methods for the extrapolation of vertical profiles from sea surface measurements have been tested on 14 yr of conductivity–temperature–depth (CTD) data collected within the Hawaii Ocean Time-series (HOT) program at A Long-Term Oligotrophic Habitat Assessment (ALOHA) station in the North Pacific Ocean. A new technique, called multivariate EOF reconstruction (mEOF-R), has been proposed. The mEOF-R technique is similar to the previously developed coupled pattern reconstruction (CPR) technique and relies on the availability of surface measurements and historical profiles of salinity, temperature, and steric heights. The method is based on the multivariate EOF analysis of the vertical profiles of the three parameters and on the assumption that only a few modes are needed to explain most of the variance/covariance of the fields. The performances of CPR, single EOF reconstruction (sEOF-R), and mEOF-R have been compared with the results of residual GEM techniques and with ad hoc climatologies, stressing the potential of each method in relation to the length of the time series used to train the models and to the accuracy expected from planned satellite missions for the measurement of surface salinity, sea level, and temperature. The mEOF-R method generally produces the most reliable estimates (in the worst cases comparable to the climatologies) and seems to be slightly less susceptible to errors in the surface input. Multivariate EOF analysis of HOT data also gave by itself interesting results, being able to discriminate the three major signals driving the temporal variability in the area.

In vitro and in situ gross primary and net community production in the North Pacific Subtropical Gyre using labeled and natural abundance isotopes of dissolved O2

We measured gross primary productivity (GPP) in vitro and GPP and net community production (NCP) in situ on four cruises to the Hawaii Ocean Time series (HOT) station ALOHA during 2002–2003. In vitro GPP, determined by 18O labeling, yielded integrated production (0–100 m) that was on average 1.5 times the 14C integrated production. Mean integrated productivity from two winter and two summer cruises was 575 mg C m−2d−1 and 930 mg C m−2d−1, respectively. In situ GPP, determined from the triple‐isotope composition of dissolved O2, averaged 910 mg C m−2d−1 in the winter and 1225 mg C m−2d−1 in the summer/fall, with an uncertainty of ±40%. The NCP/GPP ratio, determined using O2/Ar gas ratio and oxygen isotope measurements, was around 0.1 in the summer, close to the canonical f‐ratio for the open ocean, indicating station ALOHA is a net autotrophic system during summer months. The consistently higher gross carbon production measured by the in situ method, which integrates production over the ∼2‐week residence time of O2 in the mixed layer, suggests that aperiodic bursts of production contribute significantly to time‐averaged mean productivity at station ALOHA.

Biogeochemical cycling in the oligotrophic ocean: Redfield and non-Redfield models

The assumption of fixed elemental ratios in ocean biogeochemical models was tested using the Hawaii Ocean Time-Series data set from the subtropical North Pacific Ocean, where nutrient-starvation is a permanent condition for near-surface phytoplankton populations. Two three-element (C, N, P) ecosystem models were coupled to a mixedȁlayer model, an inorganic carbon chemistry model, and dynamic pools of dissolved organic C, N, and P. One model has fixed Redfield ratios for phytoplankton (constant ratio model, CRM), whereas the other has varying ratios (variable ratio model, VRM). The results suggest that the ecosystem is strongly phosphorus limited, but would be nitrogen limited in the absence of dinitrogen fixation (DNF), and may be nitrogen limited in the lower part of the euphotic zone. Known sources of phosphorus appear to be close to those required to sustain observed levels of export, given plasticity of elemental ratios. The vertical profile and seasonal time course of primary production of organic carbon are simulated well by the VRM but poorly by the CRM, as are surface concentrations of dissolved organic carbon. The seasonal cycles of air-sea CO2 flux and export of organic carbon by sedimentation are similar in the two models, but there is a slight but persistent bias toward greater downward fluxes in the VRM. The ability of oceanic phytoplankton to adapt to P stress by reducing cellular requirements can therefore potentially enhance the oceanic sink for atmospheric carbon over vast areas of low-latitude ocean, but there is potential for saturation of this enhancement if DNF increases.

Iron, manganese, and lead at Hawaii Ocean Time-series station ALOHA: Temporal variability and an intermediate water hydrothermal plume

Trace metal clean techniques were used to sample Hawaii Ocean Time-series (HOT) station ALOHA on seven occasions between November 1998 and October 2002. On three occasions, full water-column profile samples were obtained; on the other four occasions, surface and near-surface euphotic zone profiles were obtained. Together with three other published samplings, this site may have been monitored for “dissolved” (≤0.4 or ≤0.2 μm) Fe more frequently than any other open ocean site in the world. Low Fe concentrations (<0.1 nmol kg −1) are seen in the lower euphotic zone, and Fe concentrations increase to a maximum in intermediate waters. In the deepwaters (>2500 m), the concentrations we observe (0.4–0.5 nmol kg −1) are significantly lower than some other deep North Pacific stations but are similar to values that have been reported for a station 350 miles to the northeast. We attribute these low deepwater values to transport of low-Fe Antarctic Bottom Water into the basin and a balance between Fe regeneration and scavenging in the deep water. Near-surface waters have higher Fe levels than observed in the lower euphotic zone. Significant temporal variability is seen in near-surface Fe concentrations (ranging from 0.2–0.7 nmol kg −1); we attribute these surface Fe fluctuations to variable dust deposition, biological uptake, and changes in the mixed layer depth. This variability could occur only if the surface layer Fe residence time is less than a few years, and based on that constraint, it appears that a higher percentage of the total Fe must be released from North Pacific aerosols compared to North Atlantic aerosols. Surprisingly, significant temporal variability and high particulate Fe concentrations are observed for intermediate waters (1000–1500 m). These features are seen in the depth interval where high δ 3He from the nearby Loihi Seamount hydrothermal fields has been observed; the total Fe/ 3He ratio implies that the hydrothermal vents are the source of the high and variable Fe. The vertical profile of Mn at ALOHA qualitatively resembles other North Pacific Mn profiles with surface and intermediate water maxima, but there are some significant quantitative differences from other reported profiles. The ≤0.4 μm Mn concentration is highest near the surface, decreases sharply in the upper 500 m, then shows an intermediate water maximum at 800 m and then decreases in the deepest waters; these concentrations are higher than observed at a station 350 miles to the northeast that shows similar vertical variations. It appears that there is a significant Mn gradient (throughout the water column) from HOT towards the northeast. Compared to the first valid oceanic Pb data for samples collected in 1976, Pb at ALOHA in 1997–1999 shows decreases in surface waters and waters shallower than 200 m. Pb concentrations in central North Pacific surface waters have decreased by a factor of 2 during the past 25 yr (from ∼65 to ∼30 pmol kg −1); surface water Pb concentrations in the central North Atlantic and central North Pacific are now comparable. We attribute the surface water Pb decrease to the elimination of leaded gasoline in Japan and to some extent by the U.S. and Canada. We attribute most of the remaining Pb in Pacific surface waters to Asian emissions, more likely due to high-temperature industrial activities such as coal burning rather than to leaded gasoline consumption. A 3-year mixed-layer time series from the nearby HALE-ALOHA mooring site (1997–1999) shows that there is an annual cycle in Pb with concentrations ∼20% higher in winter months; this rise may be created by downward mixing of the winter mixed layer into the steep gradient of higher Pb in the upper thermocline (Pb concentrations double between the surface and 200 m). From 200 m to the bottom, Pb concentrations decrease to levels of 5–9 pmol kg −1 near the bottom; for most of the water column, thermocline and deepwater Pb concentrations do not appear to have changed significantly during the 23-yr interval.