Hawaii Ocean Time-series Station ALOHA Research Articles

Models for estimating photosynthesis parameters from in situ production profiles

The rate of carbon assimilation in phytoplankton primary production models is mathematically prescribed with photosynthesis irradiance functions, which convert a light flux (energy) into a material flux (carbon). Information on this rate is contained in photosynthesis parameters: the initial slope and the assimilation number. The exactness of parameter values is crucial for precise calculation of primary production. Here we use a model of the daily production profile based on a suite of photosynthesis irradiance functions and extract photosynthesis parameters from in situ measured daily production profiles at the Hawaii Ocean Time-series station Aloha. For each function we recover parameter values, establish parameter distributions and quantify model skill. We observe that the choice of the photosynthesis irradiance function to estimate the photosynthesis parameters affects the magnitudes of parameter values as recovered from in situ profiles. We also tackle the problem of parameter exchange amongst the models and the effect it has on model performance. All models displayed little or no bias prior to parameter exchange, but significant bias following parameter exchange. The best model performance resulted from using optimal parameter values. Model formulation was extended further by accounting for spectral effects and deriving a spectral analytical solution for the daily production profile. The daily production profile was also formulated with time dependent growing biomass governed by a growth equation. The work on parameter recovery was further extended by exploring how to extract photosynthesis parameters from information on watercolumn production. It was demonstrated how to estimate parameter values based on a linearization of the full analytical solution for normalized watercolumn production and from the solution itself, without linearization. The paper complements previous works on photosynthesis irradiance models by analysing the skill and consistency of photosynthesis irradiance functions and parameters for modeling in situ production profiles. In light of the results obtained in this work we argue that the choice of the primary production model should reflect the available data and these models should be data driven regarding parameter estimation.

Hawaiian imprint on dissolved Nd and Ra isotopes and rare earth elements in the central North Pacific: Local survey and seasonal variability

Dissolved neodymium isotopes (143Nd/144Nd, expressed as εNd) and rare earth elements (REEs) have the potential to trace the provenance of lithogenic material as well as water masses. The central North Pacific is poorly investigated with respect to its Nd isotope signature and REE cycling, and little is known about the contributions of volcanic islands, such as Hawaii, relative to dust input from Asian deserts to the surface water REE budgets. Here we present dissolved Nd isotope and REE data along with long-lived radium isotope activities from Hawaii Ocean Time-Series Station ALOHA and coastal waters from Oahu, sampled for a GEOTRACES process study in February 2011. The data are supplemented with seasonal samples from ALOHA. Our results show a clear influence of the Hawaiian Islands on the coastal ocean and surface waters at ALOHA during February, expressed by higher surface water Ra activities, radiogenic surface εNd (εNd=+1.4 to −1.0), and elevated Eu anomalies (Eu/Eu∗⩾1.3). Seasonal cycles of Asian dust deposition most likely contribute to the seasonal εNd variability of surface waters at ALOHA, as suggested by more negative εNd and the lack of Eu anomalies in summer. Neodymium isotopes in the intermediate and deep water column at ALOHA trace typical North Pacific water masses, such as North Pacific Intermediate Water and North Pacific Deep Water. We suggest that a radiogenic εNd excursion in 1000–2000m water depth, observed in various North Pacific profiles, is controlled by advection of a modified Upper Circumpolar Deep Water or North Equatorial Pacific Intermediate Water. We further present an updated average εNd signature of −3.5±0.5 for North Pacific Deep Water and show that REE patterns of deep waters at ALOHA are dominantly controlled by vertical processes.

Wind and sunlight shape microbial diversity in surface waters of the North Pacific Subtropical Gyre.

Few microbial time-series studies have been conducted in open ocean habitats having low seasonal variability such as the North Pacific Subtropical Gyre (NPSG), where surface waters experience comparatively mild seasonal variation. To better describe microbial seasonal variability in this habitat, we analyzed rRNA amplicon and shotgun metagenomic data over two years at the Hawaii Ocean Time-series Station ALOHA. We postulated that this relatively stable habitat might reveal different environmental factors that influence planktonic microbial community diversity than those previously observed in more seasonally dynamic habitats. Unexpectedly, the data showed that microbial diversity at 25 m was positively correlated with average wind speed 3 to 10 days prior to sampling. In addition, microbial community composition at 25 m exhibited significant correlations with solar irradiance. Many bacterial groups whose relative abundances varied with solar radiation corresponded to taxa known to exhibit strong seasonality in other oceanic regions. Network co-correlation analysis of 25 m communities showed seasonal transitions in composition, and distinct successional cohorts of co-occurring phylogenetic groups. Similar network analyses of metagenomic data also indicated distinct seasonality in genes originating from cyanophage, and several bacterial clades including SAR116 and SAR324. At 500 m, microbial community diversity and composition did not vary significantly with any measured environmental parameters. The minimal seasonal variability in the NPSG facilitated detection of more subtle environmental influences, such as episodic wind variation, on surface water microbial diversity. Community composition in NPSG surface waters varied in response to solar irradiance, but less dramatically than reported in other ocean provinces.

Daily to decadal variability of size-fractionated iron and iron-binding ligands at the Hawaii Ocean Time-series Station ALOHA

Time-series studies of trace metals in the ocean are rare, but they are critical for evaluating both the residence times of the metals themselves and also the timescales over which the marine ecosystems that depend on micronutrient metals can change. In this paper we present two new time-series of the essential micronutrient iron (Fe) taken from the Hawaii Ocean Time-series (HOT) site, Station ALOHA (22.75°N, 158°W): a set of intermittent monthly surface samples taken from ∼50 dates between 1999 and 2011 by the HOT program, and a daily-resolved sample set from summer 2012 and 2013 containing ∼80 surface samples and 7 profiles to 1500m depth. The long-term monthly climatology of surface total dissolvable Fe (TDFe) concentrations covaried with the seasonal cycle of continental Asian dust deposition at Hawaii, indicating dust as the major source of TDFe to ALOHA surface waters and a short residence time for TDFe (order∼months). During the daily summer time-series, surface Fe was most variable in the larger size fractions (>0.4μm particulate and 0.02–0.4μm colloidal) and nearly constant in the smallest (<0.02μm) soluble size fractions, confirming that the larger size fractions have shorter residence times with respect to scavenging/settling. The most significant Fe event lasted three days in early August and “cascaded” through the Fe size fractions from largest to smallest; lacking evidence that dust triggered this event, we correlated it with the arrival of the edge of an anticyclonic eddy and an increase in diatom abundance at ALOHA. The surface Fe-binding ligand daily time-series showed that excess ligand concentrations lagged dFe by 1–2days, revealing a short residence time of ligands in the central North Pacific likely due to photochemical degradation. In the ferricline, the dissolved Fe (dFe) linear relationship with apparent oxygen utilization was used to establish a water column dFe:C ratio of 3.12±0.11μmol/mol and a pre-formed dFe concentration of 0.067±0.009nmol/kg that defines the central/mode waters of Station ALOHA. Finally, in deep waters near 1200m, where minimal temporal variation in dFe might be expected, dFe instead ranged over a factor of two in concentration (0.72–1.44nmol/kg), driven by the intermittent passing of the Loihi hydrothermal plume through Station ALOHA. This study not only provides the largest number of Fe measurements made at a single location anywhere in the global ocean to date but also reveals the strength of time-series measurements for exploring mechanisms and timescales of biogeochemical events.

Thorium isotopes tracing the iron cycle at the Hawaii Ocean Time-series Station ALOHA

The role of iron as a limiting micronutrient motivates an effort to understand the supply and removal of lithogenic trace metals in the ocean. The long-lived thorium isotopes (232Th and 230Th) in seawater can be used to quantify the input of lithogenic metals attributable to the partial dissolution of aerosol dust. Thus, Th can help in disentangling the Fe cycle by providing an estimate of its ultimate supply and turnover rate. Here we present time-series (1994–2014) data on thorium isotopes and iron concentrations in seawater from the Hawaii Ocean Time-series Station ALOHA. By comparing Th-based dissolved Fe fluxes with measured dissolved Fe inventories, we derive Fe residence times of 6–12months for the surface ocean. Therefore, Fe inventories in the surface ocean are sensitive to seasonal changes in dust input. Ultrafiltration results further reveal that Th has a much lower colloidal content than Fe does, despite a common source. On this basis, we suggest Fe colloids may be predominantly organic in composition, at least at Station ALOHA. In the deep ocean (>2km), Fe approaches a solubility limit while Th, surprisingly, is continually leached from lithogenic particles. This distinction has implications for the relevance of Fe ligand availability in the deep ocean, but also suggests Th is not a good tracer for Fe in deep waters. While uncovering divergent behavior of these elements in the water column, this study finds that dissolved Th flux is a suitable proxy for the supply of Fe from dust in the remote surface ocean.

Remineralization of phytoplankton-derived organic matter by natural populations of heterotrophic bacteria

The relative lability, elemental stoichiometry, and remineralization rates of various particulate organic matter (POM) substrates by natural heterotrophic marine microorganisms were investigated. POM was harvested from laboratory cultures of a marine diazotroph (Trichodesmium IMS101), a cosmopolitan diatom (Thalassiosira weissflogii), a common marine cyanobacteria (Prochlorococcus MED4), and from surface waters off the Oregon coast. These POM resources were used as inoculants in a field experiment conducted at the Hawaii Ocean Time-series Station ALOHA in the North Pacific Subtropical Gyre. POM from these various sources was added to seawater collected from below the surface mixed layer, incubated in the dark, and remineralization rates were quantified via high-frequency measurement of soluble phosphorus (P) and nitrogen (N) concentrations over a 6-d period. Rapid solubilization and near complete remineralization of particulate P (PP) occurred in all treatments where cultured POM was used, with lesser relative mobilization of P from a ‘natural’ POM sample isolated from surface seawater off the Oregon coast. Soluble P pools, likely consisting of surface-adsorbed inorganic P and inorganic P liberated from cells during harvesting of biomass accounted for 28% of natural PP pools and 80±32% of cultured PP. 31P nuclear magnetic resonance (NMR) confirmed that PP was predominately present as orthophosphate in all POM types. By the end of the incubation period, all added P from cultured material had been converted to dissolved inorganic P. This finding may be a caveat of our utilization of laboratory cultures and natural POM which has been exposed to high inorganic P concentrations (0.8–5.0μmolL−1), albeit it is consistent with previous reports of significant contributions of surface-adsorbed P to total particulate P. In contrast, over the course of these experiments, only 37–40% of added N had been remineralized to ammonium (NH4+). In general, N remineralization rates of cultured material increased with the amount of N added (per gram of dry material). The net yield of bacterial cells was also positively correlated to the initial amount of C and N added. Most notably, when corrected for non-biological turnover (i.e. removal of soluble pools), the N:P remineralization ratio of cultured material (8.5±1.3) was independent of the N:P of added organic material (5–23).

Biogenic silica cycling during summer phytoplankton blooms in the North Pacific subtropical gyre

Biogenic silica (bSiO2) cycling, diatom abundance and floristics were examined within summer-period diatom blooms in the North Pacific Subtropical Gyre (NPSG) in 2008 and 2009. Hemiaulus hauckii was the most abundant diatom observed in an expansive (100,000km2) bloom near the subtropical front in the northeastern NPSG in 2008 and the small pennate diatom Mastogloia woodiana dominated a smaller (30,000km2) bloom sampled in 2009 in the gyre interior. In both blooms, the bSiO2 stock and production rates were up to an order of magnitude higher relative to non-bloom areas. Remnants of a bSiO2 export event was sampled in the H. hauckii bloom area where the export rate at 300m exceeded that at 150m, and was among the highest values recorded in the NPSG. The M. woodiana bloom was very active with specific bSiO2 production rates of 0.50–0.75d−1 and net bSiO2 production rates were among the highest observed in any subtropical-gyre diatom bloom to date. Net silica production rates in the euphotic zone were strongly positive within blooms and near zero outside of blooms, consistent with an important role for blooms in bSiO2 export. The difference in the areal extent of the H. hauckii and M. woodiana blooms was consistent with remote-sensing observations that blooms in the northeastern portion of the NPSG, near the subtropical front, are typically more extensive than those in the gyre interior near Hawaii Ocean Time-series station ALOHA. Initial estimates suggest that blooms in the northeast region produced 3–25 times more bSiO2 in 2008 and 2009, respectively, than did blooms in the gyre interior; and due to the large areal extent these blooms, their area-integrated production of bSiO2 is similar to intense diatom blooms coastal upwelling systems (e.g. Monterey Bay, Santa Barbara Channel) despite significantly lower production rates and standing stock.

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).

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.

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.

A 9-year increasing trend in mesozooplankton biomass at the Hawaii Ocean Time-series Station ALOHA

A 9-year increasing trend in mesozooplankton biomass at the Hawaii Ocean Time-series Station ALOHA

A 9-year increasing trend in mesozooplankton biomass at the Hawaii Ocean Time-series Station ALOHA

Abstract Mesozooplankton biomass in the North Pacific Subtropical Gyre (NPSG), as measured by the Hawaii Ocean Time-series program at Station ALOHA (22.45°N 158°W), increased significantly from 1994 to 2002. The changes occurred at a rate of 60 mg DW m−2 yr−1 for night-time collections and 45 mg DW m−2 yr−1 for daytime collections. Principal components analysis indicates that the 9-year trend was driven by an increase in small (0.5–2.0 mm) zooplankton that do not migrate on a diel cycle. This plankton class is known to increase during the summer at Station ALOHA when the water column is most stratified, and a strong summertime response is also apparent within the long-term trend from 1998 through 2002. Both long-term and seasonal changes in zooplankton biomass at Station ALOHA can be linked to an enhanced role of nitrogen fixation in ecosystem productivity. Climate forcing from El Niño–Southern Oscillation (ENSO) events may have influenced nitrogen fixation, general ecosystem productivity, and thus zooplankton biomass in the NPSG. However, it is difficult to evaluate the effect of climate cycles in this region without the benefit of a longer time-series at Station ALOHA. Because biomass trends in higher-level consumers like mesozooplankton can have cascading influences on lower levels, understanding the relative roles of bottom-up climate influences and top-down trophic processes will be important in resolving long-term trends in community composition and structure in the subtropical North Pacific Ocean.

Iodine chemistry reflects productivity and denitrification in the Arabian Sea: evidence for flux of dissolved species from sediments of western India into the OMZ

Abstract Dissolved iodine species and total iodine concentrations were measured in the Arabian Sea during the Spring Intermonsoon of 1995. Two separate regimes of iodine chemistry are highlighted in this study: (1) the well-oxygenated surface layer (WOSL) where iodide concentrations were in the range of 158–558 nM, and (2) the oxygen minimum zone (OMZ) where total iodine concentrations [primarily as iodide and in excess to the oceanic iodine/salinity ratio of ∼13] varied from ∼200 to 950 nM. Iodine data in the WOSL of the Arabian Sea are contrasted with data from the Bermuda Atlantic Time-series Station (BATS), the Hawaii Ocean Time-series Station ALOHA (HOT), VERTEX in the Pacific and the Black Sea. Total iodine concentrations in excess of 400 nM were observed in eastern portions of the OMZ. The eastern portion of the basin has a permanent denitrification zone as well as high concentrations of dissolved Mn2+ (d-Mn2+) and iodide. While there is precedent for high values of iodide and total iodine in several other isolated basins, this is the first report of such values in open-ocean waters. Potential sources of excess total iodine to the OMZ include advection along isopycnals, from hydrothermal vents or margin sediments; atmospheric deposition; and remineralization of sinking particulate organic iodine (POI) associated with elevated productivity in surface waters. We estimate that only 3.6% of the excess total iodine can result from remineralization of sinking POI from the WOSL to the OMZ. Advection from margin sediments off of India is the most plausible source of iodine to the OMZ and contributes ∼96% of the total excess iodine to the OMZ. I− is maintained as the dominant form of iodine via in situ reduction of iodate by bacteria.

Photosynthetic picoplankton community structure in the subtropical North Pacific Ocean near Hawaii (station ALOHA)

The structure of the picoplankton community in the subtropical Pacific was examined on four depth profiles, one from each season, sampled at the Hawaii Ocean Time-series station ALOHA (22°45′N, 158°W). Three cell populations were discriminated by flow cytometry: Prochlorococcus prochlorophytes, Synechococcus cyanobacteria, and picoeukaryotes. Prochlorococcus were the most abundant component (maximum ca 2 × 10 5 cells ml −1). Unlike previous reports, their concentration was almost constant down to roughly 100 m, with a slight maximum at the surface or near the chlorophyll maximum. Cellular chlorophyll fluorescence increased 50-fold between surface and deep populations. One distinguishing feature of the community off Hawaii was the co-occurrence near the chlorophyll maximum of at least two distinct Prochlorococcus populations with different chlorophyll and DNA contents. Throughout the year, Synechococcus abundance was two orders of magnitude lower and there was no seasonal alteration between Prochlorococcus and Synechococcus, as observed in the northern Sargasso Sea. Synechococcus populations did not extend below 120 m and were dominated by high phycourobilin cell types. Picoeukaryote abundance was quite similar to that of Synechococcus, but these cells extended deeper in the water column. Their chlorophyll fluorescence exhibited much less depth variation than Prochlorococcus or Synechococcus. Seasonal variability was small (<2- to 3-fold) for all three components of the picoplankton, not only for cell abundance but also for cellular parameters such as light scatter or pigment fluorescence. Synechococcus populations exhibited the largest seasonal changes (e.g. abundance maximum and chlorophyll fluorescence varied 3-fold). Picoplankton community structure in the Pacific Ocean appears to be distinct from previous reports for other areas. In comparing station ALOHA to the Atlantic Ocean (especially the Sargasso Sea) and the Mediterranean Sea, depth-integrated abundances of Prochlorococcus were higher, that of Synechococcus were lower, and that of picoeukaryotes were similar. We believe this structure, dominated by Prochlorococcus, may be typical for subtropical open-ocean regions.