Hawaii Ocean Time-series Research Articles

Role of late winter mesoscale events in the biogeochemical variability of the upper water column of the North Pacific Subtropical Gyre

Continuous records of upper water column (0–150 m) temperature profiles, spectral distribution of down welling irradiance, and phytoplankton solar‐induced fluorescence at 25 m depth were obtained during the inaugural deployment of the Hawaii Air‐sea Logging Experiment, A Long‐term Oligotrophic Habitat Assessment (HALE ALOHA) mooring, near the Hawaii Ocean Time‐series (HOT) Station ALOHA (22°45′N; 158°00′W). The temperature record showed a strong upwelling event in March–April 1997, displacing the thermocline by 120 m. Remote sensing satellite (NSCAT and TOPEX/ERS 2) analyses suggest that the observed upwelling was a result of strong wind divergence and the passage of a cyclonic eddy through the HOT program study area. At the onset of the upwelling event increases in colored dissolved organic matter (CDOM) and chlorophyll fluorescence efficiency in the upper water column were detected by changes in the spectral distribution of the down welling irradiance. The 0–25 m mean chlorophyll a (chl a) concentration increased threefold toward the end of the upwelling period. Water column samples collected during the monthly HOT cruises also indicate that the relative contribution of diatoms to total chl a increased twofold inside the eddy. The long‐term temporal variability in frequency and intensity of these poorly resolved mesoscale events might be key factors determining the structure of the pelagic ecosystem in the North Pacific Subtropical Gyre. Integrating multi‐year remote sensing satellite, moored, and vessel‐based time series records permits a quantification of the spatial and temporal scale of upper water column perturbations and the characterization of the pelagic ecosystem response at various timescales.

Ocean province classification using ocean colour data: observing biological signatures of variations in physical dynamics

SummaryWe have used satellite colour data to classify ocean environments for monitoring interannual changes in the ocean. The unsupervised classification method is based on our observation that the frequency distributions of Coastal Zone Color Scanner (CZCS) annual pigment means and standard deviations are nonuniform and contain distinct clusters. The frequency distributions are used to objectively determine ocean areas with similar pigment statistical characteristics. A major separation between high variance, high pigment and lower variance, lower pigment waters is observed in terms of global ocean area. The ocean areas determined with our method reflect different bio‐logical responses to variations in ocean physical dynamics. Pigment means and variances around the Joint Global Ocean Flux Study (JGOFS) Time Series stations are used as fiducial characteristics. Hawaii Ocean Time‐series (HOT) station is associated with the low‐variance portion of the global annual pigment distribution characteristic of the central gyres, but shows slightly higher mean and variance than the minima in the central Pacific gyre. The Bermuda Atlantic Time Series (BATS) pigment associations comprise a transitional region between the gyres and high‐variance pigment areas, and circumscribe the HOT pigment associations. Together, these associations encompass 23% (HOT‐like) and 48% (BATS‐like) of the Northern Hemisphere open ocean. The Pacific regions delineated by the JGOFS station pigment‐based patterns are similar to distributions described historically for Pacific zooplankton communities. Interannual variation for the northern hemisphere gyre area is on the order of by 10% for the 11/78–10/81 period.

Remineralization ratios in the subtropical North Pacific gyre

Based on a new mixing model of two end-members, the water column remineralization ratios of P/N/Corg - O2 = 1/13 ± 1/135 ± 18/170 ± 9 are obtained for the Hawaii Ocean Time-series (HOT) data set at station ALOHA. The traditional Redfield ratios of P/N/Corg/–O2 = 1/16/106/138 have standard deviations of more than 50%, when they are based on the average composition of phytoplankton. Apparently, the remineralization processes in the water column have smoothed out the observed large variability of plankton compositions. A new molar formula for the remineralized plankton may be written as 135H280O105N13P or C25(CH2O)101(CH4)9(NH3)13(H3PO4). Oxidation of this formula results in

Temporal variations in diatom abundance and downward vertical flux in the oligotrophic North Pacific gyre

The abundance of diatoms in the water column and the downward vertical flux of diatom cells from the euphotic zone were investigated during a time series of 11 monthly cruises (June 1994–July 1995) to Station ALOHA (22°45′N, 158°00′W) as one component of the Hawaii Ocean Time-series (HOT) Program. The diatom community was studied using light microscopy and by high-performance liquid chromatographic (HPLC) pigment analyses. Distinct diatom assemblages were found in the mixed-layer and in the Deep Chlorophyll Maximum Layer (DCML). Diatom cell abundances in the water column were generally low during the year, except in July 1994, when they increased in the upper euphotic layer. Two lightly silicified species ( Hemiaulus hauckii [Grunow] and Mastogloia woodiana [Taylor]) were mainly responsible for this increase. Other less abundant diatom species present in the mixed-layer assemblage showed a similar temporal pattern. H. hauckii contained Richelia-type endosymbionts with heterocysts and was presumably able to fix dinitrogen. Both species of diatoms also were an important component of the vertical diatom flux out of the euphotic zone, which, likewise, was highest in July 1994. During this maximum export period, aggregates of these two species were collected in the drifting sediment traps. In the DCML, diatom abundances and export were low throughout the year, with the exception of one genus ( Pseudonitzschia) for which a slight concentration increase was observed in spring. Reflecting the observed diatom cell abundance and vertical flux, fucoxanthin concentrations (a pigment marker for diatoms) did not indicate any significant increase of diatom pigment biomass in the DCML during the year. Ratios of diadinoxanthin to chromophyte pigments suggested that the phytoplankton cells sinking out of the euphotic zone in midsummer originated from the mixed-layer. The attenuation of the pigment vertical fluxes with depth was significantly lower for fucoxanthin, indicating a generally slower decay of diatom flux with depth compared with other phytoplankton groups. Our findings suggest that, in the subtropical North Pacific Ocean, summer conditions seem to favor the development of selected species of diatoms in the mixed-layer and that these assemblages appear to be more important with regard to export production than those in the DCML.

A pelagic ecosystem model calibrated with BATS and OWSI data

Ocean ecosystems play an important role in the global carbon cycle. In this paper, we develop and calibrate a pelagic ocean ecosystem model by simultaneously fitting data from two sites in the North Atlantic: the site of the Bermuda Atlantic Time-series Study (BATS, 32°N, 64°W) and Ocean Weather Ship Station “I” (OWSI, 59°N, 20°W). These sites differ dramatically in the magnitude and timing of primary production, chlorophyll concentrations, and light and nutrient availabilities. We find that a relatively simple model can be simultaneously fit to ecosystem data at both sites. As in the model of Hurtt and Armstrong (1996, Deep-Sea Research II 43(2–3): 653–683), the fit largely depends on the inclusion of multiple size classes of phytoplankton and detritus and a variable chlorophyll-to-biomass (nitrogen) ratio for phyto-plankton. The inclusion of these features in the model enables it to adjust to predict important biological differences temporally within sites and spatially between sites. In addition, the model also includes a phytoplankton self-shading parameter to capture differences in water clarity between sites, and the parameterization of iron limitation at OWSI as a hypothesis for the moderately HNLC condition there. Finally, we suggest that the model performs reasonably well at the Hawaii Ocean Time-series (HOT, 23°N, 158°W), a site where no model calibration was done.

Diatom fluxes to the deep sea in the oligotrophic North Pacific gyre at Station ALOHA

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 182:55-67 (1999) - doi:10.3354/meps182055 Diatom fluxes to the deep sea in the oligotrophic North Pacific gyre at Station ALOHA Renate Scharek*, Luis M. Tupas, David M. Karl Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA *Present address: Institut de Ciències del Mar, Passeig Joan de Borbó s/n, E-08039 Barcelona, Spain. E-mail: rscharek@cucafera.icm.csic.es ABSTRACT: Planktonic diatoms are important agents of vertical transport of photosynthetically fixed organic carbon to the ocean's interior and seafloor. Diatom fluxes to the deep sea were studied for 2 yr using bottom-moored sequencing sediment traps located in the vicinity of the Hawaii Ocean Time-series (HOT) program station 'ALOHA' (22°45'N, 158°W). The average flux of empty diatom frustules was around 2.8 x 105 cells m-2 d-1 in both years, except in late summer when it increased approximately 30-fold. Flux of cytoplasm-containing diatom cells was much lower (about 8 x 103 cells m-2 d-1) but increased 500-fold in late July 1992 and 1250-fold in August 1994. Mastogloia woodiana Taylor, Hemiaulus hauckii Grunow and Rhizosolenia cf. clevei var. communis Sundström were the dominant diatom species observed during the July 1992 event, with the former 2 species again dominant in August 1994. The 1994 summer flux event occurred about 3 wk after a documented bloom of H. hauckii and M. woodiana in the mixed-layer and a simultaneous increase in vertical flux of these species. This surface flux signal was clearly detectable at 4000 m, suggesting rapid settling rates. A further indication of very high sinking speeds of the diatoms was the much larger proportion of cytoplasm-containing cells in the bottom-moored traps during the 2 summer events. Cells of H. hauckii and R. cf. clevei var. communis frequently contained endosymbiotic cyanobacteria with heterocysts (cf. Richelia), similar to the cells of these species in the mixed-layer. Our data show for the first time that diatoms containing nitrogen-fixing cyanobacteria contribute directly to the vertical flux of organic matter to the deep sea in the oligotrophic regions. The peak of diatom flux coincided with a significant flux increase of biogenic silica in both years. During periods of rapid sinking, the vertical flux of diatom assemblages out of the uppermost water column seems to be more important than the diatom flux out of the deep chlorophyll maximum layer (DCML). Aggregate formation may be responsible for the fast sinking of the diatoms. KEY WORDS: Biogenic silica · Bottom-moored sediment traps · Deep sea · Diatoms · Vertical particle flux Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 182. Publication date: June 11, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

The role of dissolved organic matter release in the productivity of the oligotrophic North Pacific Ocean

Based on a long‐term set of observations and measurements at a station in the subtropical North Pacific Ocean, it now appears that contemporaneous rates of primary production in low‐nutrient open ocean regions and perhaps in the ocean as a whole may be greater than had been considered in field studies conducted in previous decades. Data collected at the Hawaii Ocean Time‐Series (HOT) Station ALOHA from October 1988 to July 1997 indicate that daytime particulate organic carbon (POC) production, based on 12‐h 14C in situ incubations, averages 472 mg C m−2 d−1 (SD = 125 mg C m−2 d−1; n = 70). This carbon production rate is two‐ to three‐fold greater than most of the pre‐1980 estimates. We present evidence that particulate production rates may have been overestimated by up to 30% as a result of 14C‐labeled dissolved organic carbon (14C‐DOC) adsorption onto glass fiber filters. More importantly, when one considers the 14C‐DOC that is produced but not adsorbed onto the filters, gross primary production rates (14C‐POC plus 14C‐DOC) in the subtropical North Pacific Ocean may approach 1 g C m−2 d−1. We hypothesize that the large flux of 14C‐DOC may be a manifestation of decade‐scale habitat changes resulting from variations in climate. The balance between POC and DOC production will ultimately influence the structure of the food web, especially the interactions of phytoplankton and heterotrophic bacterial populations, and the mechanisms and rates of carbon sequestration by the biological pump.

Rising surface ocean dissolved inorganic carbon at the Hawaii Ocean Time-series site

Abstract Surface ocean dissolved inorganic carbon (DIC) and titration alkalinity have been measured for 7 years as a part of the Hawaii Ocean Time-series (HOT) program. The time-series data set displays an interannual increase in the inventory of surface ocean DIC which we interpret as a response to increasing atmospheric carbon dioxide concentrations. The rate of increase in surface ocean DIC at the open ocean HOT site is approximately 1 μ mol kg −1 yr −1 with a 95% confidence interval of 0.72 to 1.37 μ mol kg −1 yr −1 . This accumulation rate is consistent with the rate of increase predicted from the rise in boundary layer p CO 2 .

The Hawaii Ocean Time-series (HOT) program: Background, rationale and field implementation

Long-term ocean observations are needed to gain a comprehensive understanding of natural habitat variability as well as global environmental change that might arise from human activities. In 1988, a multidisciplinary deep-water oceanographic station was established at a site north of Oahu, Hawaii, with the intent of establishing a long-term ( > 20 years) data base on oceanic variability. The primary objective of the Hawaii Ocean Time-series (HOT) program is to obtain high-quality time-series measurements of selected oceanographic properties, including: water mass structure, dynamic height, currents, dissolved and particulate chemical constituents, biological processes and particulate matter fluxes. These data will be used, in part, to help achieve the goals of the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS) research programs. More importantly, these data sets will be used to improve our description and understanding of ocean circulation and ocean climatology, to elucidate further the processes that govern the fluxes of carbon into and from the oceans, and to generate novel hypotheses. These are necessary prerequisites for developing a predictive capability for global environmental change.

A comparison of dissolved iodine cycling at the Bermuda Atlantic Time-series Station and Hawaii Ocean Time-series Station

The results of analyses of samples from the Bermuda Atlantic Time Series (BATS) and Hawaii Ocean Time-series (HOT) for dissolved iodine speciation are presented. At both sites iodate is reduced to iodide in surface waters by biological processes, but the iodide concentrations in the upper 100 m at the HOT site are approximately twice those at the BATS site. Using published estimates of surface water residence times and primary productivity at each station we use the differences between the two sites to derive an estimate of the in situ rate of iodide oxidation and the relationship between iodate reduction and primary production using a simple mass balance model. The model results suggest that on an annual basis iodide oxidation is relatively fast (half-life about 70 days) and that biological iodate reduction to iodide is about 100 times faster than iodide incorporation into particulate organic carbon.

Seasonal cycles of temperature, salinity and dissolved oxygen observed in the Hawaii Ocean Time-series

Profiles of temperature, salinity and dissolved oxygen observed during the first 5 years of the Hawaii Ocean Time-series (HOT) were fit with annual and semi-annual harmonics to describe the annual variation of water mass properties in the North Pacific subtropical gyre. The fit was made to data on pressure and density surfaces. Generally, there are three areas in the water column where the fit is best, one near the surface, another near 500 m/26.6 σ φ and a third at about 800 m/27.2 σ φ . For temperature, the fit is best within 120 m of the surface, as the surface layer warms and cools with the local seasonal forcing. Minimum surface temperature is in mid-March, while minimum temperature at 100 m is in mid-May. For salinity, the most significant seasonal cycle is found in the core of the North Pacific Intermediate Water near 500 m. This seasonal cycle is associated with oscillations in the salinity of the main salinity minimum of 50 mpsu, with vertical displacements of the salinity minimum of 40 m. For oxygen, the most significant fits are at the surface and in the Antarctic Intermediate Water oxygen minimum near 800 m, where vertical displacements of the oxygen minimum of 25 m and variations of 2 μM kg −1 were observed. Examination of the residuals left after removing the annual cycle indicates that the remaining variability generally has timescales of less than 6 months with little interannual variability. The exception to this is in the near-surface waters, where larger interannual variability is observed. Residuals show strong vertical coherence as non-seasonal variation tend to extend throughout a large portion of the water column.

Interannual variability of Pacific deep- and bottom-waters observed in the Hawaii Ocean Time-series

The deep ocean temperatures from 6 years of CTD profiling at the Hawaii Ocean Time-series series exhibit robust interannual variations. A total of 63 (62) casts were made which reached to at least 3600 (4500) dbar between October 1988 and November 1994. In the North Pacific Deep Water between 3500 and 3600 dbar, potential temperature (θ) had a mean value of 1.2°C, with variations over a range of 54.0 m°C and a standard deviation of 10.8 m°C. In the Pacific Bottom Water between 4400 and 4500 dbar, the mean θ was 1.115°C, with variations over a range of 11.0 m°C, and a standard deviation of 2.1 m°C. Smoothing splines fitted to the time series in each pressure interval are used to separate interannual variability from instrumental and natural high frequency noise. In the Deep Water, the interannual variability had an RMS value of 7.4 m°C, with a signal-to-noise ( S N ) ratio of 1.1. In the Bottom Water, interannual variations had an RMS value of 1.5 m°C with an S N ratio of 1.3. The interannual variations of 0 were not coherent between the Deep Water and the Bottom Water. The apparent dominant time scale in the Deep Water was near 2 years, and close to 3 years in the Bottom Water. Previous observations of interannual baroclinic Rossby waves in the North Pacific are suggested as an explanation for the deep ocean interannual temperature variability.

Saline intrusions in the intermediate waters north of Oahu, Hawaii

The Hawaii Ocean Time-series (HOT) hydrographic observations at station ALOHA (22°45′N, 158°W) reveal that the region is sometimes the site of intrusive interleaving of different water types near the salinity minimum of the North Pacific Intermediate Waters. This interleaving is associated with saline laminae, or lenses, 5–30 km long (submesoscale) and 5–100 m thick (finestructure). The temporal variability of these intermediate waters is investigated statistically, and a background water type is defined against which to measure the strength of intrusions. The salinity minimum shows a relative maximum in variability, caused mainly by the episodic appearance of the saline, finestructure intrusions, which are correlated with low oxygen and silicate concentrations. The intermediate salinity minimum has its southern boundary near the latitude of the Hawaiian Islands, where salinity increases towards the south and oxygen gradually decreases. Thus, stirring processes local to the Hawaiian Ridge may explain the observations. Possible mechanisms are suggested, including fluctuations of the subtropical gyre southern boundary and a hydrodynamically unstable flow at intermediate depths along the ridge. Either process could transport relatively saline water into the ALOHA region, and would lead to interleaving, driven by double diffusion on scales of the order of 10 m in the vertical.

On using satellite altimetric heights to provide a spatial context for the Hawaii Ocean Time-series measurements

It is assumed that the Hawaiian Ocean Time-series (HOT) measurements would be usefully complemented by datasets capable of providing a spatial context for the variability observed at the HOT site. The potential usefulness of high quality sea surface heights from satellite altimeters is evaluated by analyzing data from the Topex/Poseidon ( T P ) mission. Three signals are examined: an intra-annual oscillation, the annual variability, and an interannual sea level drop observed at the HOT site during 1993. The intra-annual signal observed at the HOT site is shown to be part of a propagating disturbance that moves energy to the northwest, but propagates phase to the southwest. Farther to the northwest, however, the energy and phase propagate nearly due west. At the annual period, the small amplitude observed at the HOT site is confirmed, but this is not true of the larger scale, where annual amplitudes reach 5–10 cm. The annual phase is largely consistent with the sea surface response to annual heating. Although the interannual event is barely resolved by the T P series, the drop of dynamic height observed at the HOT site is shown to be part of a larger-scale event. This event is stationary in space and is initially characterized by positive height anomlies near the ridge and negative anomalies further offshore. The signs of these two anomalies reverse in just over 1 year. The observed pattern gives rise to significant meridional sea surface height gradients, and the associated mass transport anomalies are estimated to be 8 Sv. In all three cases the T P time series, although less than 2 years in duration, provide information about the spatial structure of the signals that extends the descriptions that would be possible using the HOT data alone.

Currents observed north of Oahu during the first five years of HOT

Shipboard ADCP sections from 30 Hawaii Ocean Time-series cruises over a 5-year period show a mean flow 15 degrees north of west, reaching a maximum of 17 cm s − 1 about 50 km north of Oahu. This North Hawaiian Ridge Current is highly variable from cruise to cruise and from year to year. It was generally strong in the sections 1988–1991 and weak or absent in 1992 and 1993. There is no prominent annual cycle. Unresolved short-period oscillations are so energetic as to obscure the mean current in individual sections. The 5-year mean is highly significant, however, at five times the estimated standard error. Currents also were measured with a shipboard ADCP during the nominally 3-day occupations of the HOT Station ALOHA. The mean over 32 cruises was near 6 cm s −1 in magnitude at all depths from 20 to 200 m, but the median was near zero. Hence, the northern boundary of the North Hawaiian Ridge Current was usually south of ALOHA. The inter-cruise standard deviation of velocity varied from 22 cm s −1 at 20 m to 11 cm s −1 at 200 m for the zonal component, and 13 cm s −1 to 8 cm s − 1 for the meridional component. Intra-cruise variability during the 17 cruises with the best sampling in depth and time was dominated by a linear trend, near-inertial oscillations, and diurnal and semidiurnal tides. Each of these four elements had an rms value near 6 cm s −1 at 20 m, decreasing to 2–4 cm s −1 at 250 m. The vertical shear field was dominated by the diurnal tide and near-inertial oscillations; the semidiurnal tide and the trend had larger vertical scales of variability and consequently made smaller contributions to the shear.

Time series sampling and data assimilation in a simple marine ecosystem model

Simulated distributions of nutrients, phytoplankton and zooplankton were obtained from a simple marine ecosystem model that included nutrient inputs from episodic events. These distributions were then used in numerical identical twin experiments to test the ability of an adjoint data assimilation method to recover rate parameters, such as population growth and death rates, component initial conditions, and the amplitude of episodic events. Data were assimilated into the marine ecosystem model at monthly, bi-weekly and weekly intervals over a period of about 2 months. The ability to recover rate parameters and component initial conditions was determined primarily by the frequency and type of data that were assimilated. Assimilation of data at monthly intervals proved to be adequate for recovery of most of the rate parameters and some of the initial conditions. Bi-weekly data yielded better recoveries; however, increasing the data availability to weekly intervals did not significantly improve the results relative to the bi-weekly cases. The ability to recover biological rates with only monthly data suggests that these are fundamental aspects of marine ecosystems and can be resolved with only a few measurements. The availability of zooplankton information, even at a reduced frequency relative to phytoplankton or nutrient information, improved the ability to recover rate parameters with data more widely spaced in time. Recovery of component initial conditions was related to the timescales of the biological processes; faster processes required more frequent data. The recovery of the amplitude of the episodic events was related to the timing of the sampling relative to the event, rather than to the frequency at which data were available. The number of iterations needed for convergence when using data assimilation with the marine ecosystem model was dependent not only on the frequency and type of the input data series, but also on the structure of the marine ecosystem model. These results have implications for designing sampling strategies for measurement programs, such as the U.S. Joint Global Ocean Flux Study Hawaii Ocean Time-series and Bermuda Atlantic Time-series sites, so that these multidisciplinary data sets can be used with data-assimilative marine ecosystem models.

Geochemistry of carbon dioxide in seawater at the Hawaii Ocean Time Series Station, ALOHA

This paper examines the fundamental biological and physical processes controlling the distribution of the inorganic carbon species throughout the water column at the Hawaii Ocean Time series station, ALOHA. Profiles of alkalinity and calcium are strongly influenced by the distribution and movement of the six water masses at this location. The semiconservative behavior of these parameters can be useful in identifying water masses. The dissolution of carbonate particles, however, contributes up to 21% (49 μmol C kg−1) of the dissolved inorganic carbon added to the intermediate waters between the time the waters enter the Pacific and the time they reach Station ALOHA. A sharp increase in carbonate‐derived carbon observed between 800 and 2200 m is due to the dissolution of the more soluble forms of carbonate such as aragonite. The distribution of TCO2 and pH at this site are significantly affected by biological processes as indicated by the apparent nonlinear mixing curves between water masses, the similarity of the measured ratios of C:N and C:P to the Redfield ratios in the upper 700 m of the water column, and the results of a carbon system model. The nonconservative nature of these properties demonstrates that both physical and biological factors must be considered when evaluating temporal variability. The carbon system evaluation techniques presented here are similar to those used on several previous trans‐Pacific cruises allowing the results from this station to be compared to the results of other cruises in this area.

A micro-hole potentiostatic oxygen sensor for oceanic CTDs

A non-membrane, micro-hole, potentiostatic oxygen sensor was designed and tested for use on an oceanic CTD. The sensor consists of three electrodes: a carbon-fiber cathode, a Ag/ AgC1 reference and a platinum anode; all are mounted in a plexiglass flow duct. The carbon-fiber cathode is a bundle of 1000 carbon fibers recessed in epoxy and electroplated with platinum. The sensor calibrates to oxygen concentration (0 2), not partial pressure. The effect of temperature on sensor output can be modelled with an activation energy term. Pressure decreases current output of these sensors 5% over 5000 dbar. Ten calibration casts were performed as part of the Hawaii Ocean Time Series (HOTS) sampling program in the Pacific Ocean north of Oahu, Hawaii. Vertical profiles of O 2 were calibrated using 18–23 Winkler determinations from each cast. The coefficient of determination ( r 2) for the calibrations ranged from 0.9979 to 1.0000. The standard deviation of the unexplained error between predicted O 2 and Winkler OZ ranged from 0.49 to 3.6 μM, with a mean value of 1.3 μM.

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.