R.F.C. Mantoura

Pigment signatures of the phytoplankton composition in the northeastern Atlantic during the 1990 spring bloom

Abstract Pigment signatures were used to track the development and composition of a phytoplankton bloom in the northeastern Atlantic during May/June 1990 using reversed-phase high-performance liquid chromatography. Chlorophyll a concentrations at 5 m increased from 1.2 to 3.7 μg 1 −1 during the first half of May, and decreased progressively thereafter in the post-bloom stage. Multiple regression analysis of chlorophyll a and selected accessory pigments indicate that diatoms (fucoxanthin; 23–70%) and prymnesiophytes (19′-hexanoyloxyfucoxanthin; 40-20%) dominated the chlorophyll a biomass in the development phase, with prymnesiophytes dominating the post-bloom stage (45–55%). Dinoflagellates (peridinin; 5–25%) and “green” algae (chlorophyll b ; 5–10%) were secondary components of the microalgal community. Depth distributions revealed that the pigment maxima occurred near the surface at 5–15 m, with concentrations decreasing rapidly below 15 m. At the peak of the bloom, diatoms (fucoxanthin) were dominant throughout the water column down to 300 m, while in the post-bloom phase, prymnesiophytes (19′-hexanoyloxyfucoxanthin) dominated the community in the upper 20 m with diatoms accumulating in deeper water. Concomitant measurements of nutrients and downwelling irradiance suggest that nitrate availability limited the growth of the phytoplankton in the upper 15 m and below this depth limitation was due to low irradiance levels.

The fate of added iron during a mesoscale fertilisation experiment in the Southern Ocean

The first Southern Ocean Iron RElease Experiment (SOIREE) was performed during February 1999 in Antarctic waters south of Australia (61°S, 140°E), in order to verify whether iron supply controls the magnitude of phytoplankton production in this high nutrient low chlorophyll (HNLC) region. This paper describes iron distributions in the upper ocean during our 13-day site occupation, and presents a pelagic iron budget to account for the observed losses of dissolved and total iron from waters of the fertilised patch. Iron concentrations were measured underway during daily transects through the patch and in vertical profiles of the 65-m mixed layer. High internal consistency was noted between data obtained using contrasting sampling and analytical techniques. A pre-infusion survey confirmed the extremely low ambient dissolved (0.1 nM) and total (0.4 nM) iron concentrations. The initial enrichment elevated the dissolved iron concentration to 2.7 nM. Thereafter, dissolved iron was rapidly depleted inside the patch to 0.2-0.3 nM, necessitating three re-infusions. A distinct biological response was observed in iron-fertilised waters, relative to outside the patch, unequivocally confirming that iron limits phytoplankton growth rates and biomass at this site in summer. Our budget describing the fate of the added iron demonstrates that horizontal dispersion of fertilised waters (resulting in a quadrupling of the areal extent of the patch) and abiotic particle scavenging accounted for most of the decreases in iron concentrations inside the patch (31-58 and 12-49 of added iron, respectively). The magnitude of these loss processes altered towards the end of SOIREE, and on days 12-13 dissolved (1.1 nM) and total (2.3 nM) iron concentrations remained elevated compared to surrounding waters. At this time, the biogenic iron pool (0.1 nM) accounted for only 1-2 of the total added iron. Large pennate diatoms (> 20 μm) and autotrophic flagellates (2-20 μm) were the dominant algal groups in the patch, taking up the added iron and representing 13 and 39 of the biogenic iron pool, respectively. Iron regeneration by grazers was tightly coupled to uptake by phytoplankton and bacteria, indicating that biological Fe cycling within the bloom was self-sustaining. A concurrent increase in the concentration of iron-binding ligands on days 11-12 probably retained dissolved iron within the mixed layer. Ocean colour satellite images in late March suggest that the bloom was still actively growing 42 days after the onset of SOIREE, and hence by inference that sufficient iron was maintained in the patch for this period to meet algal requirements. This raises fundamental questions regarding the biogeochemical cycling of iron in the Southern Ocean and, in particular, how bioavailable iron was retained in surface waters and/or within the biota to sustain algal growth.

Determination of iron in seawater

Iron plays an important role in oceanic biogeochemistry and is known to limit biological activity in certain ocean regions. Such regions have a replete complement of major nutrients but low primary production of phytoplankton due to low ambient iron concentrations. The determination of iron in seawater is a major challenge, although much progress has been made during the last two decades. Techniques for total dissolved iron and iron speciation have been developed in order to rationalise its biogeochemical cycling and better understand its role in limiting phytoplankton growth. In this paper, a critical review of historical and current analytical methods for the determination of iron in seawater is presented and their capabilities evaluated. The need for standard protocols for the clean sampling and storage of low-level (<1 nM) iron seawater in order to maintain sample integrity is emphasised. The importance of laboratory and shipboard intercomparison exercises to distinguish between environmental variability and operationally measured fractions is also considered.

Determination of sub-nanomolar levels of iron in seawater using flow injection with chemiluminescence detection

The development of a highly sensitive system for the shipboard determination of dissolved iron at the sub-nM level is presented. The technique is based on a flow injection method coupled with luminol chemiluminescence detection. Dissolved Fe(II+lII) levels are determined after Fe(III) reduction using sulphite and in-line matrix elimination/preconcentration on an 8-hydroxyquinoline (8-quinolinol) chelating resin column. The detection limit (3s) is 40 pM when 1.5 ml of sample is loaded onto the column, and the relative standard deviation is 3.2 (n=5) for a 1.0 nM Fe sample. One analytical cycle can be completed in 3 min. The automated method proved reliable when employed on-board the RRS James Clark Ross during Autumn 1996, mapping dissolvable Fe(II+III) levels along the Atlantic Meridional Transect from 50°N to 50°S. Data from vertical profiles through the upper water column are presented.

Biogeochemistry of Fe and other trace elements (Al, Co, Ni) in the upper Atlantic Ocean

Iron and other trace metals (Al, Co, Ni) were measured through the upper water column during two north-south transects of the Atlantic Ocean (approximately 50°N-50°S), from the United Kingdom (UK) to the Falkland Islands (September/October 1996) and from South Africa to the UK (May/June 1998). Total dissolvable iron (TD-Fe) concentrations in the surface layers ( 2.2 nM observed in surface waters in these regions. Benthic fluxes provided a significant amount of Fe (2-38 nM) to the base of the water column in Coastal zones. In addition, samples collected from one Atlantic Meridional Transect (AMT) expedition were re-analysed after a 16 month acidification period and showed significant increases over shipboard analyses (average values increasing to 2.26±1.50nM), indicating the extended release of Fe from leachable particulate material in the stored samples. Detailed profiling through the euphotic zone revealed TD-Fe distributions that exhibited strong relationships with biological uptake, regeneration and water column hydrography. In equatorial and tropical North Atlantic waters, trace elemental distributions showed evidence of recent atmospheric deposition through a history of stratified mixed layers.

Synechococcus And Its Importance To The Microbial Foodweb Of The Northwestern Indian-Ocean

Abstract The abundance, distribution, size, biomass, growth and grazing-induced mortality of phycoerythrin (PE) rich chrococcoid cyanobacteria were studied during September-October 1986 in the Arabian Sea, the Gulf of Oman and the monsoonal upwelling region off the South East Arabian coast. Cyanobacteria were abundant (>107 cells 1−1) through the region and particularly so (>108 cells 1−1) in oligotrophic waters where they exhbited distinct subsurface concentration maxima that were situated above, but related to the depth of the chlorophyll maxima. Cell diameter increased from 0.7 μm in surface waters to 1.2 μm at depth. Standing stocks of cyanobacteria ranged up to 50μgC 1−1, and accounted for up to 40% of the POC in oligotrophic stations indicating that Synechococcus constitutes an important trophic resource. Experimental investigations showed that cyanobacterial populations were growing fast, with specific growth rates of 0.5–1.0 day−1, while simultaneously experiencing high mortality due to microzooplankton grazing. Grazing rates varied between 0.3 and 1.2 day−1, indicating that 31–71% of the cyanobacteria were predated daily. Grazing and cyanobacterial growth were correlated, suggesting that Synechococcus production and its fate by microbial grazing activity were tightly coupled. Cyanobacteria are clearly a major component of a dynamic but well-balanced microbial foodweb present in oligotrophic regions of the northwest Indian Ocean.

Size-fractionated primary production and nitrogen assimilation in the northwestern Indian Ocean

Abstract Rates of phytoplankton production and nitrogen assimilation were measured at various stations along a transect in the northwestern Indian Ocean, from near the equator, northwards into the upwelling system off the Arabian peninsula, during September–October 1986. The measurements were made using in situ incubation techniques with the simultaneous use of 14C and 15N isotopes. Samples were fractionated after the incubations into three size classes: 0.2–0.8μm, 0.8–5.0 μm, and >5.0μm for the 14C incubations; and 5μm for the 15N incubations. The assimilation of nitrate and ammonium was measured. These measurements were supported by a detailed description of the horizontal and vertical distributions of chlorophyll, temperature and underwater light field, by the deployment of a towed undulating oceanographic recorder. Rates of primary production ranged from approximately 0.5 g C m−2 day−1 at the equator, reducing to 2.5 in the upwelling region off the coast of Oman; total nitrogen assimilation followed a similar pattern. Very significant variations in the size distribution of the activity of the plankton were observed. Over 75% of the carbon and nitrogen assimilation was in the

Pigment chemotaxonomic distributions of phytoplankton during summer in the western Mediterranean

Pigment distributions were investigated in the western Mediterranean basin during July 1993 to document the trophic status of the summer phytoplankton community. The characteristic deep chlorophyll maximum (DCM) was observed at all oceanic stations, and chlorophyll a concentrations of up to 1700 ng 1−1 were measured in the DCM in the northern regions. High chlorophyll a levels (2000–3000 ng 1−1) were determined in the lower reaches of the Rhone River, accompanied by high fucoxanthin levels. Fucoxanthin was also the dominant accessory pigment at the inshore stations influenced by the Rhone, while hexanoyloxyfucoxanthin was the major carotenoid at all other northern sites. Divinyl chlorophyll a concentrations were very low in the north (<30 ng 1−1) and only accounted for a maximum of 8% of the total chlorophyll a. Chlorophyll a levels were much lower in the southwestern Mediterranean; we estimate that divinyl chlorophyll a contributed 11–40% to the total chlorophyll a. Fucoxanthin was the prominent accessory pigment at Gibraltar, but hexanoyloxyfucoxanthin, chlorophyll b, zeaxanthin and divinyl chlorophyll a were more important at the other southern stations. The pigment data were used to estimate the contributions of prokaryotes (cyanobacteria and prochlorophytes) and eukaryotes to the total chlorophyll a at the surface and in the DCM. Overall, we determined that eukaryotes accounted for most of the chlorophyll a biomass, contributing 53–98%, and the prokaryote proportion was 2–47%. The pigment pattern revealed that the phytoplankton assemblage was not homogeneous and trophic conditions ranged from eutrophic in coastal and frontal regions where fucoxanthin containing diatoms dominated, to oligotrophic throughout most of the basin. Higher chlorophyll a biomass and dominant hexanoyloxyfucoxanthin containing prymnesiophytes were observed in the northern sector, while an increased prominence of prokaryotes in the south suggested that the southern sector was more oligotrophic.

Determination of atrazine and simazine in estuarine samples by high-resolution gas chromatography and nitrogen selective detection

A method for the determination of ultra trace concentrations of triazine herbicides in natural water using silica C18 solid-phase extraction (SPE) and high-resolution gas chromatography with nitrogen-selective detection (GC-NDP) has been developed and optimized for estuarine conditions. Recoveries of spiked dissolved simazine and atrazine at a concentration of 100 ng l−1 were 82–85% and 87–97%, respectively and were onl slightly affected by the range of salinities and pH encountered for estuarine samples. The sediment-bound (particulate) triazines were ultrasonically extracted with high recoveries (86–91%) using dichloromethane. The water sample extracts were analysed without selective detection provided interference-free detection of simazine and atrazine in the extracts. The detection limits for simazine and atrazine in water and sediment were 1 ng l−1 for 1 l of water and 5 ng g−1 200 mg of suspended sediments. The reproducibility of the determination of atrazine and simazine in real water samples at low ng l−1 concentrations as shown by %R.S.D., was better than 10%. A comparison of the GC-NPD determination with an alternative method using ion selective detection (GC-ITD) showed very good agreement and indicated superior (ca. 5 fold) sensitivity of the NPD. The method was applied to a study of triazine herbicides in estuarine waters of the Rivers, Tamar, Thames and Mersey.

Vertical profiles of pigments, fatty acids and amino acids: Evidence for undegraded diatomaceous material sedimenting to the deep ocean in the Bellingshausen Sea, Antarctica

The organic carbon content and biochemical composition of suspended particulate material was investigated at five stations in the marginal ice zone of the Bellingshausen Sea during the austral spring of 1992. Stations, each consisting of profiles of between four and eight depths, were sampled along longitude 85°W from fast ice conditions to open water. Samples were collected using large volume in situ filtration systems. The horizontal and vertical distribution of organic carbon, fatty acids, pigments and amino acids reflected strongly the physical environment and planktonic species composition. Concentrations of total hydrolysable amino acids, total fatty acids and photosynthetic pigments all exhibited marked reductions with depth. At an open water station, significant levels of labile fatty acids (16 : 4n−1 and 20 : 5n−3) and the xanthophyll fucoxanthin were present at a depth of 3900 m, indicating the sedimentation of undegraded, diatom derived material into the deep ocean. Amino acid, fatty acid and pigment concentrations suggest that degradation rates of particulate material below 500–1000 m were very low. The results show that in some circumstances undegraded material of photosynthetic origin reaches the deep ocean. However, the significance and contribution of this material to the nutrition of deep water pelagic and benthic communities remains to be established. The results are discussed in terms of the transfer of biogenic material from the euphotic zone into the deep ocean and the implications for deep water ecosystems.