RRS Discovery Cruise 306, 23 Jun-06 Jul 2006. Pelagic biogeochemistry of the PAP Site

The aim of this cruise was to develop a better understanding of carbon cycling in the pelagic waters of the Porcupine Abyssal Plain (PAP). There were three objectives 1) Turnaround moorings at the PAP Observatory; 2) Conduct a 1-D time series on the central station of a wide range of biogeochemical processes and to back this up with a mesoscale survey of key variables; 3) To trial the use of Autosub for mesoscale surveys in conjunction with the ship. All objectives were met, although the tops of the moorings were found to be missing probably due to fishing activity and the Autosub trials were incomplete due to vehicle failure. A full mesoscale survey was carried out using the ship and an eleven day time series at the central station was achieved

Bacterioplankton of low and high DNA content in the suboxic waters of the Arabian Sea and the Gulf of Oman: abundance and amino acid uptake

Amino acid uptakes by bacterioplankton of low DNA (LNA) and high DNA (HNA) content, populating the oxygen minimum zone (OMZ: <5 µM O2) and adjacent oxygen-depleted waters (5 to 50 µM O2), were determined using a 35S-methionine precursor and flow cytometric sorting. The HNA cells were further differentiated into low light scatter (HNA-ls) and high cell light scatter (HNA-hs) groups. Total bacterioplankton methionine uptake strongly correlated (r > 0.998, p < 0.0001) with leucine incorporation into protein and with microbial glucose uptake, suggesting that bacterioplankton growth was controlled by dissolved organic matter, and that methionine uptake could be used as a general estimate for the metabolic activity of bacterioplankton. The variation in methionine uptake depended on the prokaryote group rather than on ambient oxygen concentration, e.g. the numerically dominant LNA cells took 3 to 5 times less precursor than the HNA cells. A percentage of the LNA cells with double the amount of DNA was proposed as an incubation-independent index of growth of the cells in the G2 stage of the cell cycle. The vertical profiles of the percentage of LNA cells in G2 showed pronounced peaks at 300 to 600 m in the OMZ that did not correlate with peaks of either total bacterioplankton abundance or productivity. The present paper underlines the importance of bacterioplankton group studies in the OMZ since high microbial cell abundance does not necessarily mean high metabolic activity and other mechanisms, such as resilience to mortality pressure, have to be investigated

Syringe pumped high speed flow cytometry of oceanic phytoplankton

Nanophytoplankton (2–20 µm) are less numerous than picophytoplankton (<2 µm) in the oceans but their biomass and production are comparable and sometimes higher. The accuracy of cytometry-based enumeration of phytoplankton ultimately depends on cell abundance and sample flow rate. Commercial flow cytometers in which sheath and core streams are driven by air pressure cannot produce sufficiently high, stable sample flow rate. The present study demonstrates the applicability of a syringe pump for flow cytometric enumeration of oceanic nanophytoplankton on two meridional transects across the Atlantic Ocean.Methods: Commercially available syringe pumps were used to deliver live phytoplankton samples into a flow cell of standard flow cytometers (FACSort, FACSCalibur, BD) with increased flow rate of > 1.0 ml min-1 compared to the normal air pressure sample delivery of < 0.1 ml min-1. An auxiliary application of syringe pump flow cytometry for calibrating 0.5 µm bead concentration standards is also discussed.Results: The results demonstrated that flow cytometry of samples injected at rates above 0.1 ml min-1 is achievable and worthwhile. Counts of phytoplankton in air and syringe pumped samples agreed closely. Syringe pumping of samples offered a broader range of flow rates up to 0.8–1.0 ml min-1 without detrimental effect on flow cytometric enumeration of cells. The increased number of coincidences at high flow rates led to an approximate 10% decrease of Cyanobacteria counts when the acquisition rate approached 1,000 particles s-1, but seemed to have a lesser effect on counting rarer phytoplankton. The syringe pump flow cytometry allowed enumeration of phytoplankton groups at concentrations of 5–100 cells ml-1, cell concentrations equivalent to those of Cyanobacteria in the twilight deep ocean.Conclusion: The proposed syringe pump modification of a FACS instrument represents a significant improvement for accurate enumeration of the less abundant phytoplankton and so gives better estimations of phytoplankton distribution and standing stocks

Planktonic community structure and carbon cycling in the Arabian Sea as a result of monsoonal forcing: the application of a generic model

The Arabian Sea exhibits a complex pattern of biogeochemical and ecological dynamics, which vary both seasonally and spatially. These dynamics have been studied using a one-dimensional vertical hydrodynamic model coupled to a complex ecosystem model, simulating the annual cycle at three contrasting stations. These stations are characterised by seasonally upwelling, mixed-layer-deepening and a-seasonal oligotrophic conditions, respectively, and coincide with extensively measured stations on the two JGOFS ARABESQUE cruises in 1994. The model reproduces many spatial and temporal trends in production, biomass, physical and chemical properties, both qualitatively and quantitatively and so gives insight into the main mechanisms responsible for the biogeochemical and ecological complexity. Monsoonal systems are typified by classical food web dynamics, whilst intermonsoonal and oligotrophic systems are dominated by the microbial loop. The ecosystem model (ERSEM), developed for temperate regions, is found to be applicable to the Arabian Sea system with little reparameterisation. Differences in in-situ physical forcing are sufficient to recreate contrasting eutrophic and oligotrophic systems, although the lack of lateral terms are probably the greatest source of error in the model. Physics, nutrients, light and grazing are all shown to play a role in controlling production and community structure. Small-celled phytoplanktons are predicted to be dominant and sub-surface chlorophyll maxima are robust centers of production during intermonsoon periods. Analysis of carbon fluxes indicate that physically driven outgassing of CO2 predominates in monsoonal upwelling systems but ecological activity may significantly moderate CO2 outgassing in the Arabian Sea interior.<br/

Heterotrophic bacterial turnover along the 20°W meridian between 59°N and 37°N in July 1996

Heterotrophic bacteria were enumerated by flow cytometry in samples from 200 m depth profiles along the 20°W meridian between 59°N and 37°N during June and July 1996. Bacterial volume estimates made by size fractionation were used in the determination of heterotrophic bacterial biomass. Concentrations of heterotrophic bacteria in the surface mixed layer were close to 106 ml?1 along most of the transect, but decreased to about 0.2×106 ml?1 at the southern end; this corresponded with a biomass range between about 2 and 15 mg C m?3. Concentrations also decreased to 0.1–0.3×106 ml?1 below the top mixed layer. Production of heterotrophic bacteria was measured in samples from eight profiles by following the simultaneous incorporation of isotopically labelled thymidine and leucine; in the mixed layer it ranged between a maximum of 1.5–2 mg C m?3 d?1 in the region of a frontal system near 50°N and 0.2–0.25 mg C m?3 d?1 at the southern end of the transect, with bacterial growth rates generally about 0.1 d?1. The daily production of heterotrophic bacteria integrated for the euphotic layer ranged between about 5 and 15% of the daily 14C primary production of phytoplankton. Heterotrophic nanoplankton biomass reflected the bacterial biomass and was about half as large; it showed a strong correlation with the production of heterotrophic bacteria. At the station with the highest bacterial biomass, the rate of grazing of bacteria by heterotrophic nanoplankton was 5–6% d?1, but at all other stations it was too low (&lt;5% d?1) to be measured reliably by the techniques used. The data suggest that the nanoplankton grazed at least half of the bacterial production.<br/

Flow cytometric enumeration of DNA-stained oceanic planktonic protists

The aim of this study was to test the practicality of enumerating fixed, DNA-stained heterotrophic protists (H) and phototrophic protists (P) in contrasting regions of the Atlantic Ocean. Oceanic protists were enumerated using a standard flow cytometer (FACSort, BD) at an enhanced flow rate of up to 1.0 mL min–1 to increase numbers of counted cells. The enumeration error of protists decreased hyperbolically from 30–40 to &lt; 5% corresponding to the number (&lt;100 to &gt; 2000) of enumerated cells. H and P were discriminated using the extra red chlorophyll-derived plastidic fluorescence of the latter. The relationship between counts of stained and unstained fixed and unfixed P was statistically close to 1:1, confirming the accuracy of stained protist counting by flow cytometry and adequate discrimination of P from H cells. The estimated average abundance of H in the surface mixed layer of the southern and northern oligotrophic gyres was remarkably similar, with 400 ± 140 and 450 ± 60 cells mL–1, respectively, adding further evidence to the suggestion that these regions are in steady state. In agreement with earlier studies in more productive aquatic environments, a significant correlation (correlation coefficient 0.84, P &lt; 0.0001) was found between the H and the total bacterioplankton numbers, with an average ratio of 1300 prokaryotes to 1 H cell, suggesting a relatively constant trophic interaction between these two groups. This study demonstrates that flow cytometric enumeration of protists is 100 times faster compared with microscopy and, thus, represents a major improvement for quantifying protists in ocean waters, including oligotrophic gyres