Photobiological studies of Ross Sea phytoplankton

The Ross Sea polynya is characterized by high spatial and temporal variability and by an annual cycle of sea ice retreat, water column stratification, large phytoplankton blooms, and months of complete darkness. This region is also highly susceptible to increasingly changing climatic conditions that will significantly affect the hydrography, iron supply, primary production patterns and carbon cycling. This project focused on analyzing how differences in photosynthetic traits between the two major bloom-forming functional groups in the polynya, diatoms and the prymnesiophyte Phaeocystis antarctica, and investigate if these differences can explain their dominance and succession. The study was conducted as part of the Controls on Ross Sea Algal Community Structure (CORSAC) program during two cruises in December 2005-January, 2006, and November-December, 2006. A fast repetition rate fluorometer (FRRF) was used to assess photochemical efficiency on natural phytoplankton assemblages and on monoclonal cultures. Measurements were made on cultures to determinate differences in photorecovery kinetics, as well on a suite of experiments performed to test the effects of temperature, iron, CO2 and micronutrients had on natural assemblages. In addition, FRRF measurements were made on 1,182 discrete samples representative of 98 profiles collected over the two cruises. Phaeocystis antarctica consistently photorecovered faster than the diatoms Pseudo-nitzschia sp., indicating different photosynthetic strategies and ecological niches; in addition, temperature and iron significantly promoted photosynthetic quantum yields, indicating a diffuse iron limitation of the natural assemblages used for the experiments and a high susceptibility to forecasted temperature increases in the region. Experiments also demonstrated that the Ross Sea phytoplankton is capable of maintaining high photosynthetic capacity after extensive periods in the dark. The dominance and successions in the blooms appears to be controlled by a combination of hydrography and in particular by the relative depths of the mixed layer and euphotic zone, as well by the water temperatures and possibly by iron concentrations. Diatom-dominated blooms were found in shallow mixed water layers characterized by higher temperature and fresher waters in the summer in the western part of the polynya, while Phaeocystis antarctica prevailed in colder regions with deeper mixed layer depths in the eastern part of the polynya. The dominance in the bloom significantly affected the relative macronutrient drawdown. Photosynthetic characteristic of natural assemblages were also modeled based on variable fluorescence rapid light curves (RLCs), and photophysiological differences were found between diatoms and Phaeocystis antarctica, with the latter having higher Ek and lower functional absorption cross sections (sigmaPSII) and alpha values, but similar maximum electron transport rates (ETRs). Lastly, correlation between RLC-based modeled photosynthetic rates and 14C based primary production presented some discrepancies due to limitations and differences in methodologies

Contrasting Photo-physiological Responses of the Haptophyte Phaeocystis Antarctica and the Diatom Pseudonitzschia sp. in the Ross Sea (Antarctica)

The Antarctic is a unique environment in which substantial variations in irradiance occur over a number of time scales, and as a result phytoplankton need to acclimate and adapt to these changes. We conducted field and laboratory manipulations in the Ross Sea, Antarctica to examine photophysiological differences between Phaeocystis antarctica and Pseudonitzschia sp. a diatom that commonly occurrs in the Ross Sea, since these are the two functional groups that dominate abundance and productivity. Both exhibited reduced quantum yields due to high irradiances. P. antarctica, a haptophyte, displays a distinct photophysiological response to irradiance when compared to diatoms. P. antarctica showed a rapid recovery from high light exposure, as indicated by the rapid return to initial, high quantum yields, in contrast to diatoms, which responded more slowly. Absorption cross sections were high in both forms, but those in P. antarctica were significantly higher. Both organisms recovered within 24 h to initial quantum yields, suggesting that high irradiance exposure does not have a permanent effect on these organisms. Among all micronutrient additions (iron, cobalt, zinc and vitamin B-12), only iron additions resulted in rapid impacts on quantum yields. Iron limitation also can result in reduced photosynthetic efficiency. Understanding these photophysiologial responses and the impact of oceanographic conditions provides constraints on modeling efforts of photosynthesis and primary productivity in the Antarctic

Does eddy-eddy interaction control surface phytoplankton distribution and carbon export in the North Pacific Subtropical Gyre?

In the North Pacific Subtropical Gyre (NPSG), the regular occurrence of summer phytoplankton blooms contributes to marine ecosystem productivity and the annual carbon export. The mechanisms underlying the formation, maintenance, and decay of these blooms remain largely unknown; nitrogen fixation, episodic vertical mixing of nutrients, and meso- (<100 km) and submesoscale (<10 km) physical processes are all hypothesized to contribute to bloom dynamics. In addition, zones of convergence in the ocean's surface layers are known to generate downwelling and/or converging currents that affect plankton distributions. It has been difficult to quantify the importance of these convergence zones in the export flux of particulate organic carbon (POC) in the open ocean. Here we use two high-resolution ocean transects across a pair of mesoscale eddies in the vicinity of Station ALOHA (22° 45′N, 158° 00′W) to show that horizontal turbulent stirring may have been a dominant control on the spatial distribution of the nitrogen fixing cyanobacterium Trichodesmium spp. Fast repetition rate fluorometry measurements suggested that this distribution stimulated new primary production; this conclusion was not confirmed by ¹⁴C-based measurements, possibly because of different sampling scales for the two methods. Our observations of particle size distributions along the two transects showed that stretching by the mesoscale eddy field produced submesoscale features that mediated POC export via frontogenetically generated downwelling currents. This study highlights the need to combine high-resolution biogeochemical and physical data sets to understand the links between Trichodesmium spp. surface distribution and POC export in the NPSG at the submesoscale level

Variability in Measured and Modelled Remote Sensing Reflectance for Coastal Waters at LEO-5

A large database of in situ bio-optical measurements were collected at the LEO-15 (Long-term Ecosystem Observatory) off the southern coast of New Jersey, USA. The data were used to quantify the impact of coastal upwelling on near-shore bulk apparent (AOP) and inherent (IOP) optical properties. There was good qualitative agreement between the AOPs and IOPs in space and time. The measured IOPs were used as inputs to the Hydrolight radiative transfer model (RTE). Estimated spectral AOPs from the RTE were strongly correlated (generally R2\u3e0.80) to measured AOPs. If optical closure between in-water measurements was achieved then the RTE was used to construct the spectral remote sensing reflectance. The modelled remote sensing reflectances were compared to satellite-derived reflectance estimates from four different algorithms. Quantitative agreement between the satellite-measured and in-water modelled remote sensing reflectance was good but results were variable between the different models. The strength of the correlation and spectral consistency was variable with space and time. Correlations were strongest in clear offshore waters and lowest in the near-shore turbid waters. In the near-shore waters, the correlation was strongest for blue wavelengths (400-555 nm) but lower for the red wavelengths of light

Variability in Spectral Backscatter Estimated from Satellites and its Relation to \u3cem\u3ein situ\u3c/em\u3e Measurements in Optically Complex Coastal Waters

A large database of in situ bio-optical measurements was collected at the Long-term Ecosystem Observatory off the southern coast of New Jersey, USA. In part, the research effort focused on reconciling in situ estimates with satellite-derived estimates of the inherent optical properties (IOP). At 442 nm, in situ absorption values ranged from less than 0.2 to over 1.5 inverse metres. Satellite estimates of backscatter ranged from 0.002 to 0.03 inverse metres at 442 nm and showed significant variability in time and space during July 1999, reflecting the recurrent high frequency events that characterize the region—wind-mixing, storms and coastal upwelling. Despite this variability, there was good qualitative agreement between the satellite derived IOP estimates and in situ IOP measurements. Both absorption and backscatter values increased near-shore, reflecting enhanced concentrations of phytoplankton, sediments and dissolved organic matter

Surface Layer Variability in the Ross Sea, Antarctica as Assessed by In Situ Fluorescence Measurements

Phytoplankton fluorescence, temperature and salinity were measured from December through February using in situ instruments deployed at two locations in the southern Ross Sea, Antarctica during the austral summers of three consecutive years (2003-2004, 2004-2005, and 2005-2006) to assess the short-term, seasonal and interannual variations in phytoplankton biomass and oceanographic conditions. The seasonal climatologies of physical forcing variables were also determined from satellite measurements, and the data from the two sites compared to the 2000-2009 mean. In situ fluorometers were deployed at three depths at 77 degrees S, 172.7 degrees E and 77.5 degrees S, 180 degrees. Significant differences between the two sites were consistently observed, confirming the anticipated high level of spatial and temporal heterogeneity. Chlorophyll fluorescence was maximal in late December, and generally decreased rapidly to modest levels in January and February. However, during 1 year (2003-2004) a secondary bloom was found, with summer maxima being similar to those observed during spring. Fluorescence displayed a strong diel cycle, with strong quenching during periods of maximum irradiance. The magnitude of this reduction was large (the minimum average fluorescence was 25% of the daily mean) and decreased with depth. Fluorescence varied interannually, with the absolute levels and temporal patterns being different among years. The two sites had different temperature/salinity properties as measured at 24 m, and both variables changed with time. During 2004-2005 we were able to continuously measure the photosynthetic quantum efficiency of PSII (Fv/Fm) at 11 m, which revealed a minimum in December, and an increase in January, whereas the absolute fluorescence (Fo) decreased simultaneously. We suggest that this reflected a mixing event, whereby available irradiance increased, allowing a short period of growth in a more favorable optical environment. While substantial variations from the mean physical forcing were observed, the linkage of these physical variations with fluorescence was not always clear. Short-term (over 24-h) changes in fluorescence occurred, and were likely related to advective events. Wind events altered fluorescence in the surface layer, and these redistributed phytoplankton in the surface. The variability in chlorophyll fluorescence and physical forcing over a variety of scales in the Ross Sea provides insights into temporal-spatial coupling of phytoplankton. (C) 2010 Elsevier Ltd. All rights reserved

Interannual Variations In Nutrients, Net Community Production, and Biogeochemical Cycles In the Ross Sea

The Ross Sea continental shelf is dominated by the seasonal appearance of a large phytoplankton bloom. This bloom is regularly dominated by diatoms and the haptophyte Phaeocystis antarctica, and significant nutrient (nitrogen and silicon) reductions within the water column occur during the growing season (early November to late February). Diatoms mediate silicic acid removal, whereas both taxa remove nitrate. Dissolved and particulate nitrogen and silica concentrations were collected from a series of cruises to the southern Ross Sea over 3 years. Simple, one-dimensional nutrient budgets were generated for nitrogen and silica, and estimates of vertical flux were derived from these budgets. Substantial variations among years are observed to occur in seasonal community production, assemblage composition, Si:N uptake ratios, and export, and standard deviations are equal to similar to 30% of the mean. During 2003-2004 a large Phaeocystis antarctica bloom occurred in December, and was followed by a bloom of diatoms. This secondary bloom was equal in magnitude to that of the initial P. antarctica bloom. In contrast, no secondary bloom was observed in 2001-2002. Continuous fluorescence measurements suggested that the spatial-temporal mosaic of phytoplankton dynamics in the Ross Sea is far more complex than previously thought. We hypothesize that variations occur between years not only in terms of both magnitude and composition of the bloom, but also in the controlling mechanisms. (c) 2006 Elsevier Ltd. All rights reserved

Hydrogen Cycling by the Unicellular Marine Diazotroph Crocosphaera watsonii Strain WH8501 ▿

The hydrogen (H2) cycle associated with the dinitrogen (N2) fixation process was studied in laboratory cultures of the marine cyanobacterium Crocosphaera watsonii. The rates of H2 production and acetylene (C2H2) reduction were continuously measured over the diel cycle with simultaneous measurements of fast repetition rate fluorometry and dissolved oxygen. The maximum rate of H2 production was coincident with the maximum rates of C2H2 reduction. Theoretical stoichiometry for N2 fixation predicts an equimolar ratio of H2 produced to N2 fixed. However, the maximum rate of net H2 production observed was 0.09 nmol H2 μg chlorophyll a (chl a)−1 h−1 compared to the N2 fixation rate of 5.5 nmol N2 μg chl a−1 h−1, with an H2 production/N2 fixation ratio of 0.02. The 50-fold discrepancy between expected and observed rates of H2 production was hypothesized to be a result of H2 reassimilation by uptake hydrogenase. This was confirmed by the addition of carbon monoxide (CO), a potent inhibitor of hydrogenase, which increased net H2 production rates ∼40-fold to a maximum rate of 3.5 nmol H2 μg chl a−1 h−1. We conclude that the reassimilation of H2 by C. watsonii is highly efficient (>98%) and hypothesize that the tight coupling between H2 production and consumption is a consequence of fixing N2 at nighttime using a finite pool of respiratory carbon and electrons acquired from daytime solar energy capture. The H2 cycle provides unique insight into N2 fixation and associated metabolic processes in C. watsonii

Does eddy-eddy interaction control surface phytoplankton distribution and carbon export in the North Pacific Subtropical Gyre?

International audienceIn the North Pacific Subtropical Gyre (NPSG), the regular occurrence of summer phytoplankton blooms contributes to marine ecosystem productivity and the annual carbon export. The mechanisms underlying the formation, maintenance, and decay of these blooms remain largely unknown; nitrogen fixation, episodic vertical mixing of nutrients, and meso- (<100 km) and submesoscale (<10 km) physical processes are all hypothesized to contribute to bloom dynamics. In addition, zones of convergence in the ocean's surface layers are known to generate downwelling and/or converging currents that affect plankton distributions. It has been difficult to quantify the importance of these convergence zones in the export flux of particulate organic carbon (POC) in the open ocean. Here we use two high-resolution ocean transects across a pair of mesoscale eddies in the vicinity of Station ALOHA (22 degrees 45'N, 158 degrees 00'W) to show that horizontal turbulent stirring may have been a dominant control on the spatial distribution of the nitrogen fixing cyanobacterium Trichodesmium spp. Fast repetition rate fluorometry measurements suggested that this distribution stimulated new primary production; this conclusion was not confirmed by C-14-based measurements, possibly because of different sampling scales for the two methods. Our observations of particle size distributions along the two transects showed that stretching by the mesoscale eddy field produced submesoscale features that mediated POC export via frontogenetically generated downwelling currents. This study highlights the need to combine high-resolution biogeochemical and physical data sets to understand the links between Trichodesmium spp. surface distribution and POC export in the NPSG at the submesoscale level