The North Atlantic spring bloom is one of the largest annual biological events in the ocean, and is characterized by dominance transitions from siliceous (diatoms) to calcareous (coccolithophores) algal groups. To study the effects of future global change on these phytoplankton and the biogeochemical cycles they mediate, a shipboard continuous culture experiment (Ecostat) was conducted in June 2005 during this transition period. Four treatments were examined: (1) 12°C and 390 ppm CO 2 (ambient control), (2) 12°C and 690 ppm CO 2 (high pCO 2 ), (3) 16°C and 390 ppm CO 2 (high temperature), and (4) 16°C and 690 ppm CO 2 ('greenhouse'). Nutrient availability in all treatments was designed to reproduce the low silicate conditions typical of this late stage of the bloom. Both elevated pCO 2 and temperature resulted in changes in phytoplankton community structure. Increased temperature promoted whole community photosynthesis and particulate organic carbon (POC) production rates per unit chlorophyll a. Despite much higher coccolithophore abundance in the greenhouse treatment, particulate inorganic carbon production (calcification) was significantly decreased by the combination of increased pCO 2 and temperature. Our experiments suggest that future trends during the bloom could include greatly reduced export of calcium carbonate relative to POC, thus providing a potential negative feedback to atmospheric CO 2 concentration. Other trends with potential climate feedback effects include decreased community biogenic silica to POC ratios at higher temperature. These shipboard experiments suggest the need to examine whether future pCO 2 and temperature increases on longer decadal timescales will similarly alter the biological and biogeochemical dynamics of the North Atlantic spring bloom.
[1] The FeCycle experiment provided an SF 6 labeled mesoscale patch of high-nitrate low-chlorophyll (HNLC) water in austral summer 2003. These labeled waters enabled a comparison of the inventory of particulate iron (PFe) in the 45-m-deep surface mixed layer with the concurrent downward export flux of PFe at depths of 80 and 120 m. The partitioning of PFe between four size fractions (0.2-2, 2-5, 5-20, and >20 mm) was assessed, and PFe was mainly found in the >20-mm size fraction throughout FeCycle. Estimates of the relative contribution of the biogenic and lithogenic components to PFe were based on an Al:Fe molar ratio (0.18) derived following analysis of dust/soil from the nearest source of aerosol Fe: the semi-arid regions of Australia. The lithogenic component dominated each of the four PFe size fractions, with medians ranging from 68 to 97% of PFe during the 10-day experiment. The Fe:C ratios for mixed-layer particles were $40 mmol/mol. PFe export was $300 nmol m À2 d À1 at 80 m depth representing a daily loss of $1% from the mixed-layer PFe inventory. There were pronounced increases in the Fe:C particulate ratios with depth, with a five-fold increase from the surface mixed layer to 80 m depth, consistent with scavenging of the remineralized Fe by sinking particles and concurrent solubilization and loss of particulate organic carbon. Significantly, the lithogenic fraction of the sinking PFe intercepted at both 80 m and 120 m was >40%; that is, there was an approximately twofold decrease in the proportion of lithogenic iron exported relative to that in the mixed-layer lithogenic iron inventory. This indicates that the transformation of lithogenic to biogenic PFe takes place in the mixed layer, prior to particles settling to depth. Moreover, the magnitude of lithogenic Fe supply from dust deposition into the waters southeast of New Zealand is comparable to that of the export of PFe from the mixed layer, suggesting that a large proportion of the deposited dust eventually exits the surface mixed layer as biogenic PFe in this HNLC region.
Global climate change is predicted to have large effects on the ocean that could cause shifts in current algal community structure, major nutrient cycles, and carbon export. The Bering Sea is already experiencing changes in sea surface temperature (SST), unprecedented algal blooms, and alterations to trophic level dynamics. We incubated phytoplankton communities from 2 Bering Sea regimes under conditions of elevated SST and/or partial pressure of carbon dioxide (pCO 2 ) similar to predicted values for 2100. In our 'greenhouse ocean' simulations, maximum biomass-normalized photosynthetic rates increased 2.6 to 3.5 times and community composition shifted away from diatoms and towards nanophytoplankton. These changes were driven largely by elevated temperature, with secondary effects from increased pCO 2 . If these results are indicative of future climate responses, community shifts towards nanophytoplankton dominance could reduce the ability of the Bering Sea to maintain the productive diatom-based food webs that currently support one of the world's most productive fisheries.
We investigated phytoplankton Fe limitation using shipboard incubation experiments in the high-nutrient South American eastern boundary current regime. Low ambient Fe concentrations (ϳ0.1 nM) in water collected from the Humboldt and Peru Currents were supplemented with a range of added Fe levels up to 2.5 nM. Phytoplankton chlorophyll a, photosystem II photosynthetic efficiency, and nitrate and phosphate drawdown increased in proportion to the amount of Fe added. The Humboldt Current algal community after Fe additions included colonial and flagellated Phaeocystis globosa and large pennate diatoms, whereas the Peru Upwelling assemblage was dominated by coccolithophorids and small pennate diatoms. Apparent half-saturation constants for growth of the two communities were 0.17 nM Fe (Humboldt Current) and 0.26 nM Fe (Peru Upwelling). Net molar dissolved Si(OH) 4 : NO drawdown ratios were low in both experiments (ϳ0.2-0.7), but net particulate silica to nitrogen production Ϫ 3 ratios were higher. Fe limitation decreased net NO : PO utilization ratios in the Humboldt Current incubation to
[1] An improved knowledge of iron biogeochemistry is needed to better understand key controls on the functioning of high-nitrate low-chlorophyll (HNLC) oceanic regions. Iron budgets for HNLC waters have been constructed using data from disparate sources ranging from laboratory algal cultures to ocean physics. In summer 2003 we conducted FeCycle, a 10-day mesoscale tracer release in HNLC waters SE of New Zealand, and measured concurrently all sources (with the exception of aerosol deposition) to, sinks of iron from, and rates of iron recycling within, the surface mixed layer. A pelagic iron budget (timescale of days) indicated that oceanic supply terms (lateral advection and vertical diffusion) were relatively small compared to the main sink (downward particulate export). Remote sensing and terrestrial monitoring reveal 13 dust or wildfire events in Australia, prior to and during FeCycle, one of which may have deposited iron at the study location. However, iron deposition rates cannot be derived from such observations, illustrating the difficulties in closing iron budgets without quantification of episodic atmospheric supply. Despite the threefold uncertainties reported for rates of aerosol deposition (Duce et al., 1991), published atmospheric iron supply for the New Zealand region is $50-fold (i.e., 7-to 150-fold) greater than the oceanic iron supply measured in our budget, and thus was comparable (i.e., a third to threefold) to our estimates of downward export of particulate iron. During FeCycle, the fluxes due to short term (hours) biological iron uptake and regeneration were indicative of rapid recycling and were tenfold greater than for new iron (i.e. estimated atmospheric and measured oceanic supply), giving an ''fe'' ratio (uptake of new iron/uptake of new + regenerated iron) of 0.17 (i.e., a range of 0.06 to 0.51 due to uncertainties on aerosol iron supply), and an ''Fe'' ratio (biogenic Fe export/uptake of new + regenerated iron) of 0.09 (i.e., 0.03 to 0.24).Citation: Boyd, P. W., et al. (2005), FeCycle: Attempting an iron biogeochemical budget from a mesoscale SF 6 tracer experiment in unperturbed low iron waters, Global Biogeochem. Cycles, 19, GB4S20,
The toxic dinoflagellate Pfiesteria piscicida has been identified in coastal waters and estuaries along the Atlantic coast of the United States. Estuaries in the mid-Atlantic region, in particular, have been targeted as high-risk areas for toxic blooms since reports of Pfiesteria-related fish kills in the Pocomoke River, Maryland, in 1997. The development of monitoring strategies for these areas requires that the presence of Pfiesteria be rapidly and accurately assessed. Routine monitoring by light microscopy lacks both the sensitivity and accuracy required for species-specific detection and enumeration of Pfiesteria, especially at the low levels normally found in non-bloom conditions. In this study, we developed 2 molecular techniques to identify and enumerate P. piscicida in the Delaware Inland Bays and the Pocomoke River. The first technique, denaturing gradient gel electrophoresis (DGGE), was used to identify several similar but distinct strains of Pfiesteria in water and their benthic stages (cysts or amoebae) in sediment samples. A comparison of DGGE analyses of Pfiesteria community structure in the Pocomoke River and the Delaware Inland Bays revealed subtle differences in strain composition. A second technique, PCR-fluorescent fragment detection (PCR-FFD), was designed for quantitative enumeration of Pfiesteria in water samples. This technique offers a 1000-fold increase in sensitivity over microscopic techniques. To demonstrate the utility of PCR-FFD, we conducted a study of Pfiesteria at the Roosevelt Inlet, Lewes, Delaware. Pfiesteria concentrations over 2 tidal cycles were correlated to other physical, biological and chemical variables. Overall, our data establish the presence of Pfiesteria as a minor but prevalent member of the phytoplankton community in mid-Atlantic estuaries.KEY WORDS: Pfiesteria · Harmful algal blooms · PCR-FFD · DGGE · Delaware Inland Bays · Broadkill River · Pocomoke River · Sediments · Cysts Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 24: [275][276][277][278][279][280][281][282][283][284][285] 2001 fied in coastal waters and estuaries ranging from the Long Island Sound, New York, south to Mobile Bay, Alabama , Lewitus et al. 1995, Rublee et al. 1999. The discovery of other closely related Pfiesteria species or strains suggests that toxic blooms could involve several members of a 'Pfiesteria species complex ' (Burkholder & Glasgow 1997a). Although the environmental factors that contribute to toxic Pfiesteria outbreaks are unclear (Burkholder & Glasgow 1997a, Pinckney et al. 1997, the distribution and proliferation of specific strains are most likely regulated by biological, physical and chemical factors in the environment. An analysis of these factors and their relationships to the Pfiesteria community composition is essential to the prediction and prevention of toxic blooms.Development of monitoring strategies for Pfiesteria requires that the presence of the dinoflagellate be rapidly and accurately assessed. Low-...
The North Atlantic spring bloom is one of the largest annually occurring phytoplankton blooms in the world ocean. The present study investigated the potential effects of climate change variables (temperature and pCO 2 ) on trophic dynamics during the bloom using a shipboard continuous culture system. The treatments examined were (1) 12°C and 390 ppm CO 2 (ambient), (2) 12°C and 690 ppm CO 2 (high pCO 2 ), (3) 16°C and 390 ppm CO 2 (high temperature), and (4) 16 °C and 690 ppm CO 2 (greenhouse). Individually, increasing temperature and pCO 2 initially resulted in significantly higher total microzooplankton abundance and grazing rates over the ambient treatment mid-experiment, with significantly greater increases still in the greenhouse treatment. By the end of the experiment, microzooplankton abundance was highest in the 2 low temperature treatments, which were dominated by small taxa, while the larger ciliate Strombidium sp. numerically dominated the high-temperature treatment. Microzooplankton community composition was dominated by small taxa in the greenhouse treatment, but total abundance declined significantly by the end after peaking mid-experiment. This decrease occurred concurrently with the growth of a potentially unpalatable phytoplankton assemblage dominated by coccolithophores. Our results suggest that indirect effects on microzooplankton community structure from changes in phytoplankton community composition as a result of changing temperature or pCO 2 were likely more important than direct effects on microzooplankton physiology. Similar changes in trophic dynamics and whole plankton community composition may also be important for future climate-driven changes in the North Atlantic spring bloom assemblage.KEY WORDS: Microzooplankton · Herbivory · Temperature · pCO 2 · North Atlantic spring bloom · Climate change Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 388: [27][28][29][30][31][32][33][34][35][36][37][38][39][40] 2009 also be an important food source for mesozooplankton and, as such, can contribute to carbon transfer to higher trophic levels (Sherr et al. 1986, Atkinson 1996, Schnetzer & Caron 2005.The annually occurring and spatially extensive spring bloom of phytoplankton in the North Atlantic Ocean has been extensively studied since the early days of oceanography (e.g. Sverdrup 1953). The US Joint Global Ocean Flux Study (JGOFS;Ducklow & Harris 1993), US Marine Light Mixed Layer research initiative (MLML;Plueddemann et al. 1995), UK Biogeochemical Ocean Flux Study (BOFS;Savidge et al. 1992), UK Plankton Reactivity in the Marine Environment program (PRIME; Savidge & Williams 2001), and French 'Programme Océan Multidisciplinaire Méso Echelle' (POMME; Memery et al. 2005) have all been large-scale efforts to combine physical, chemical, biological, and modeling approaches to understand ecosystem dynamics throughout the stages of the bloom. This phytoplankton bloom is initiated by a reduction in mixed layer depth and is typified by an i...
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