Cellular iron contents of plankton during the Southern Ocean Iron Experiment (SOFeX)

Iron (Fe) availability limits phytoplankton biomass and production in large regions of the Southern Ocean and influences community composition and size structure, which may affect C export and other system-level functions. To improve our understanding of Fe partitioning within communities and the response of different components to fertilization, we assessed the cellular Fe contents of individual diatoms, autotrophic flagellates, and heterotrophic flagellates during the recent Southern Ocean Fe Experiment using synchrotron-based X-ray fluorescence (SXRF). Dual ⁵⁵Fe/¹⁴C radioisotope incubations were also conducted to estimate Fe:C ratios in size-fractionated plankton. Cellular Fe quotas determined by the two techniques were in close agreement when low amounts of ⁵⁵Fe (0.2 nM) were added, but ⁵⁵Fe additions of 2 nM resulted in 2–3-fold higher quotas. SXRF assessments of cellular Fe quotas (normalized to C) were generally in good agreement with prior bulk analyses of natural assemblages, but revealed compositional differences among protistan taxa not previously detected. Mean Fe:C ratios for diatoms, autotrophic flagellates, and heterotrophic flagellates from unfertilized waters were 6.0, 8.7, and 14.1 μmol mol C⁻¹, respectively. Smaller cells had higher Fe:C ratios than larger cells. Fertilization enhanced Fe quotas in all cell types, with mean Fe:C ratios increasing approximately 4-fold (from about 10 to about 40 μmol mol C⁻¹) after two Fe additions. This study provides some of the first measurements of Fe quotas in phytoplankton cells from natural communities and the first measurements of Fe quotas in natural protozoa.     

Temporal variations in the concentration and settling flux of carbon and phytoplankton pigments in a deep fjordlike estuary

The weekly mass flux of C and phytoplankton pigments at five depths in the main basin of Puget Sound, a deep (200 m) fjordlike estuary, was sampled for a year with moored sequentially-sampling sediment traps. Flux measurements were compared with weekly samples of suspended pigments in the euphotic zone and bi-monthly samples of total suspended matter and particulate C throughout the water column at the mooring site.Seasonal changes in the total mass flux at all depths were small; instead, physical (river runoff, bottom resuspension) and biological (phytoplankton blooms) events caused occasional sharp increases on a weekly scale. The dry weight concentration of pigments in the trap samples mirrored the concentration of pigments in the euphotic zone suspended matter, increasing from 0·01% in winter to a maximum of 0·65% in late summer. Bloom-induced changes in the pigment concentration were observed almost simultaneously in the euphotic zone and in the traps to a depth of 160 m, indicating a rapid vertical transfer of surface-originating particles by organic aggregates. In contrast to the strong seasonal signal in the pigment concentration, C concentration varied by only a factor of three during the year.The seasonal trend of C/pigment ratios in the C flux arises from at least two sources: (1) a balance between terrestrial sources of C during the high-runoff winter season and in-situ primary production in spring and summer, and (2) cycling of C through the zooplankton population. Budget calculations suggest that the loss of primary-produced C and pigment from the euphotic zone by settling is 5% regardless of season. On an annual basis, this C flux (16 g m⁻²) is sufficient to support previously measured values of benthic aerobic respiration at the mooring site. To account for other C sinks such as burial, predation and chemical oxidation, however, terrestrial C sources and alternate transport pathways, such as vertical advection and sediment movement down the steep basin walls, are necessary.     

Assessment of a shell bank and associated substrates as nursery habitat of postsettlement red snapper

Trawl surveys were conducted in 2000 and 2001 to examine patterns of distribution and abundance of postsettlement red snapper (Lutjanus campechanus) on a shell bank, Freeport Rocks Bathymetric High (FRBH), in the northwestern Gulf of Mexico. In addition, otolith-based methods were used to determine age, hatch-date, growth and mortality of new recruits associated with FRBH. Date and region were significant factors affecting density of red snapper in 2000. Peak densities of red snapper were observed in July and August, and mean density among habitat types (shell bank, inshore mud, offshore mud) was similar (range: 50–52 ind hectare⁻¹) in 2000. Alternatively, a habitat effect was detected during a limited survey conducted in 2001, with density higher on the shell bank than inshore or offshore mud habitat. Postsettlement red snapper were first detected at approximately 16 mm standard length, and individuals less than 20 mm were present in all habitats. Estimated ages of red snapper ranged from 26 to 121 d, with new settlers (≤20 mm) typically less than 28 d. Predicted hatch dates ranged from early April to mid August with a single peak occurring from late May to early June. Growth rate for the April–May cohort (0.817 mm d⁻¹) was similar to the June–July cohort (0.830 mm d⁻¹). Habitat-specific differences in growth were observed, and rates were highest for individuals from the inshore habitat (0.881 mm d⁻¹). Mortality rates (Z) during the early post-settlement period were approximated using catch curves, and early life mortality of red snapper was 12.1% d⁻¹ (Z=0.129). While the difference in mortality between cohorts was negligible, a habitat-specific difference in mortality was observed. Mortality rate of red snapper inhabiting the inshore mud habitat (Z=0.045, 4.4% d⁻¹) was lower than rates observed for individuals on the shell bank (Z=0.120, 11.9% d⁻¹) or offshore (Z=0.099, 9.3% d⁻¹) habitat. Individuals residing in the inshore habitat had significantly higher growth rates and significantly lower mortality rates, suggesting that recruitment potential was higher for these individuals.     

A new spectroscopic facility at millimetre wavelengths

A new millimeter-wave facility is in operation at the Bordeaux Observatory for spectroscopic observations of interstellar and stratospheric molecules. A cooled receiver has been installed on a 2·5m radio telescope. The overall system temperature is in the range 400 to 600 K (single side band) in the operating frequency range 75 to 115 GHz. The relatively broad beam of the telescope (∼ 5 arcmin) combined with a sensitive receiver will permit studies of extended molecular cloud complexes.