Late Winter Biogeochemical Conditions Under Sea Ice in the Canadian High Arctic

With the Arctic summer sea-ice extent in decline, questions are arising as to how changes in sea-ice dynamics might affect biogeochemical cycling and phenomena such as carbon dioxide (CO2) uptake and ocean acidification. Recent field research in these areas has concentrated on biogeochemical and CO2 measurements during spring, summer or autumn, but there are few data for the winter or winter–spring transition, particularly in the High Arctic. Here, we present carbon and nutrient data within and under sea ice measured during the Catlin Arctic Survey, over 40 days in March and April 2010, off Ellef Ringnes Island (78° 43.11′ N, 104° 47.44′ W) in the Canadian High Arctic. Results show relatively low surface water (1–10 m) nitrate (<1.3 µM) and total inorganic carbon concentrations (mean±SD=2015±5.83 µmol kg−1), total alkalinity (mean±SD=2134±11.09 µmol kg−1) and under-ice pCO2sw (mean±SD=286±17 µatm). These surprisingly low wintertime carbon and nutrient conditions suggest that the outer Canadian Arctic Archipelago region is nitrate-limited on account of sluggish mixing among the multi-year ice regions of the High Arctic, which could temper the potential of widespread under-ice and open-water phytoplankton blooms later in the season

Nutrient budget for the Eastern Mediterranean: Implications for phosphorus limitation

The eastern Mediterranean has a high nitrate to phosphate (N : P) ratio (~28 : 1) in the deep water and a highly unusual P limitation of the primary productivity. We present a detailed nutrient budget of inputs to the basin, which shows that there is a high N: P ratio (>16 : 1) in all the input sources, particularly from the atmospheric source, where the N: P ratio was 117 : 1. The high N: P ratio is retained within the system because there is no significant denitrification in either the sediments or intermediate water. This is because of the extreme oligotrophic nature of the system, which is caused by the unusual anti-estuarine flow at the Straits of Sicily. Support for this conclusion is provided by the observation that the only area of the eastern Mediterranean where the N: P ratio in deeper water is ~16 : 1 is the northern Adriatic, which is also the only area with significant denitrification. The N budget (total input to basin vs. net output at the straits of Sicily) balances closely. This N balance suggests that N fixation is an insignificant process in this P-limited system. The unusually light 15N values in the deep water nitrate and particulate organic nitrogen can be explained by processes other than nitrogen fixation. These processes include a lack of significant denitrification in the basin and by particulate organic matter exported from surface waters during the Plimited winter plankton bloom

Flow cytometry and pigment analyses as tools to investigate the toxicity of herbicides to natural phytoplankton communities

Characterisation of natural phytoplanktonic communities is currently being advanced through flow cytometry and high resolution pigment analyses. To date, toxicological methods to assess impacts of herbicides on natural phytoplankton populations are lacking. Here, we report the novel use of these techniques in combination to study changes in phytoplankton populations exposed to 2-methylthio-4-tertiary-butylamino-6-cyclopropylaminos-triazine (Irgarol 1051), a herbicide used in antifouling paints. Flow cytometry results revealed that following a 72-h exposure to approximately 100 ng L -1, eukaryote abundance was less than half that in the controls. High performance liquid chromatographic analyses of pigments demonstrated that 190-hexanoyloxyfucoxanthin was selectively lost relative to the control. This carotenoid is specific to the prymnesiophytes which are key constituents of phytoplanktonic communities within temperate marine waters. Values of EC50 (72 h) as low as 70 ng L -1 were calculated from the selective reduction in this compound. Concentrations substantially exceeding this level have been reported in UK and other European coastal waters

Response of East Mediterranean surface water to Saharan dust: On-board microcosm experiment and field observations

An on-board microcosm experiment was performed during the CYCLOPS May 2002 cruise to track the biogeochemical response of Eastern Mediterranean surface seawater to a gradient addition of fresh and pre-leached Saharan dust, mimicking the potential fertilization effect as opposed to the impact of adding particles alone. Response parameters examined were P-turnover time, bacterial production and abundance, chlorophyll a, other phytopigments, abundance of different pico and nanophytoplankton groups, primary production rates, abundance of heterotrophic nanoflagellates and ciliates. The addition of fresh Saharan dust (range: 0.2–4.9mg1(-1)) and the subsequent nutrient release triggered an increase in phytopigments and primary production, while no response was detected for pre-leached dust particles. Most responses were linearly related to the amount of fresh dust added. Synechococcus and prymnesiophytes increased in abundance along with cellular pigment content while Prochlorococcus disappeared, heterotrophic bacteria increased production rates, and ciliates showed a small increase in cell density. A less clear response was recorded by in situ measurements following a Saharan dust storm during a cruise in the Levantine Basin in May 2001. The calculated amount of nutrients and dust particles delivered by such an event to a 15-m thick mixed surface layer is low (~0.3nmolP1(-1),~9nmolN1(-) and 0.06mg dust 1(-1)), falling close to the lowest dust addition in our microcosm experiment. Even so, an enhancement of phosphate turnover time, a sharp decline of Prochlorococcus abundance, and slight increases in chlorophyll a and bacterial activity were observed in response to the dust storm. Considering the linear effect of fresh dust concentrations on the bacterial activity, primary production and pigment concentration (total and per cell), and the likely stimulation of grazing, it is not surprising that changes due to moderate strength dust storms are mostly close to detection limit of either field or remote sensing measurements

The fate of added iron during a mesoscale fertilisation experiment in the polar 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 (&gt;20 m) and autotrophic flagellates (2–20 m) 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

Studies of the microbial P-cycle during a Lagrangian phosphate-addition experiment in the Eastern Mediterranean

Microbial uptake of orthophosphate was studied before and during a Lagrangian experiment where orthophosphate was added to the surface mixed layer in the Cyprus Gyre, Eastern Mediterranean, a region previously hypothesized to be characterized by P-limited growth of both phytoplankton and heterotrophic bacteria. The addition of ca. 110 nM orthophosphate to a ca. 16 km2 patch in situ led, within 1 day, to an increase in particulate-P from 8 to ca. 15 nM, a result in good agreement with a previous microcosm bioassay indicating this system to have a maximum capacity for orthophosphate consumption of between 10 and 25 nM phosphate. In samples of unperturbed water taken before the addition, outside, or below the experimental patch, orthophosphate turnover time (Tt) was <4 h, argued to be consistent with the assumption of diffusion-limited phytoplankton growth. Upon addition, Tt increased to 94 h. Estimates of maximum potential uptake rate (Vmax) for orthophosphate in unperturbed water exceeded by more than one order of magnitude the biological P-requirement (ν) as obtained from stoichiometric conversion of C-based primary and bacterial production values to estimated P-requirement. Upon addition of orthophosphate, Vmax decreased to a level comparable to ν. The observations are consistent with the assumption of P-starved cells before and P-replete cells with excess external orthophosphate after the addition. Orthophosphate uptake in unperturbed water was dominated by<1 μm organisms (mean ±SD between samples 0.56±0.03 μm). In samples with higher turnover time, orthophosphate uptake was shifted towards larger organisms, culminating after 5 days with a near doubling in mean size (1.08 μm). The size distribution of particulate-P standing stock had a mean size of 10 μm, indicating the presence of a substantial biomass of micro-organisms larger than those involved in P-uptake. Comparison of the measured particulate-P with microscope-based biomass estimates indicated a microbial food web dominated by heterotrophic organisms (70% of particulate-P), distributed with ca. 25% of total particulate-P in heterotrophic bacteria, ca. 40% in heterotrophic flagellates, and ca. 5% in ciliates. Concentration of bioavailable phosphate (Sn) estimated from the relationship Sn=νTi indicated Sn values<1 nM PO4 before the addition, increasing afterwards. Estimates of the sum Kt+Sn for the 0.6–0.2 μm size fraction were in the range 1–7 nM PO4 before and outside patch, suggesting this sum to be dominated by the half-saturation constant Kt. Kt+Sn increased to 69 nM after addition, then dropped over the following week back to background levels. As reported elsewhere in this volume, there was a decline in the observed chlorophyll concentrations, but a positive response in copepods. Less clear than the effects at the level of osmotroph physiology were the subsequent responses expected in the food web. Two possible mechanisms are discussed: (1) a positive response in bacterial production and the subsequent food chain of bacterial predators, and (2) a positive response in phytoplankton predators due to a shift in food quality rather than in food quantity