Mary Jane Perry

In situ phytoplankton absorption, fluorescence emission, and particulate backscattering spectra determined from reflectance

An inverse model was developed to extract the absorption and scattering (elastic and inelastic) properties of oceanic constituents from surface spectral reflectance measurements. In particular, phytoplankton spectral absorption coefficients, solar-stimulated chlorophyll a fluorescence spectra, and particle backscattering spectra were modeled. The model was tested on 35 reflectance spectra obtained from irradiance measurements in optically diverse ocean waters (0.07 to 25.35 mg m−3 range in surface chlorophyll a concentrations). The universality of the model was demonstrated by the accurate estimation of the spectral phytoplankton absorption coefficients over a range of 3 orders of magnitude (ρ = 0.94 at 500 nm). Under most oceanic conditions (chlorophyll a<3 mg m−3) the percent difference between measured and modeled phytoplankton absorption coefficients was <35%. Spectral variations in measured phytoplankton absorption spectra were well predicted by the inverse model. Modeled volume fluorescence was weakly correlated with measured chl a; fluorescence quantum yield varied from 0.008 to 0.09 as a function of environment and incident irradiance. Modeled particle backscattering coefficients were linearly related to total particle cross section over a twentyfold range in backscattering coefficients (ρ = 0.996, n = 12).

Eddy-Driven Stratification Initiates North Atlantic Spring Phytoplankton Blooms

Early Bloom Trigger Springtime phytoplankton blooms occur when high nutrient concentrations are combined with abundant sunlight and a stratified upper ocean layer. It has been thought that stratification occurs because in the spring, seasonal warming causes the water to expand, making it less dense, which creates a layer resistant to mixing from below. Now, Mahadevan et al. (p. 54; see the Perspective by Martin) have combined observations of the upper water column from the subpolar North Atlantic with ocean model simulations, which demonstrate that the initial stratification can be triggered by the dynamic effects of passing ocean eddies. These eddies can advance the time of the bloom by 20 to 30 days. Oceans eddies can trigger springtime plankton blooms previously attributed to surface heating. Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming.

Eddy-driven subduction exports particulate organic carbon from the spring bloom

Down with atmospheric carbon dioxide How does the ocean move carbon from surface waters to its deep interior? Current understanding is that carbon dioxide is removed from the atmosphere by phytoplankton that are eaten, and in turn their predators die and sink into deep water and seafloor sediments. In addition to this route, Omand et al. show that downwelling caused by ocean eddies 1 to 10 km across can deliver much of the carbon produced in spring to the deep sea. The eddies entrain small particles and dissolved organic carbon to augment the flux of large sinking particles. Science, this issue p. 222 Ocean eddies can transport appreciable quantities of organic carbon from the surface to depth. The export of particulate organic carbon (POC) from the surface ocean to depth is traditionally ascribed to sinking. Here, we show that a dynamic eddying flow field subducts surface water with high concentrations of nonsinking POC. Autonomous observations made by gliders during the North Atlantic spring bloom reveal anomalous features at depths of 100 to 350 meters with elevated POC, chlorophyll, oxygen, and temperature-salinity characteristics of surface water. High-resolution modeling reveals that during the spring transition, intrusions of POC-rich surface water descend as coherent, 1- to 10-kilometer–scale filamentous features, often along the perimeter of eddies. Such a submesoscale eddy-driven flux of POC is unresolved in global carbon cycle models but can contribute as much as half of the total springtime export of POC from the highly productive subpolar oceans.

Prediction of the Export and Fate of Global Ocean Net Primary Production: The EXPORTS Science Plan

Ocean ecosystems play a critical role in the Earth’s carbon cycle and the quantification of their impacts for both present conditions and for predictions into the future remains one of the greatest challenges in oceanography. The goal of the EXport Processes in the Ocean from Remote Sensing (EXPORTS) Science Plan is to develop a predictive understanding of the export and fate of global ocean net primary production (NPP) and its implications for present and future climates. The achievement of this goal requires a quantification of the mechanisms that control the export of carbon from the euphotic zone as well as its fate in the underlying “twilight zone” where some fraction of exported carbon will be sequestered in the ocean’s interior on time scales of months to millennia. Here we present a measurement / synthesis / modeling framework aimed at quantifying the fates of upper ocean NPP and its impacts on the global carbon cycle based upon the EXPORTS Science Plan. The proposed approach will diagnose relationships among the ecological, biogeochemical and physical oceanographic processes that control carbon cycling across a range of ecosystem and carbon cycling states leading to advances in satellite diagnostic and numerical prognostic models. To collect these data, a combination of ship and robotic field sampling, satellite remote sensing and numerical modeling is proposed which enables the sampling of the many pathways of NPP export and fates. This coordinated, process-oriented approach has the potential to foster new insights on ocean carbon cycling that maximizes its societal relevance through the achievement of research goals of many international research agencies and will be a key step towards our understanding of the Earth as an integrated system.

Fate of chiral and achiral organochlorine pesticides in the North Atlantic Bloom Experiment.

Organochlorine pesticides (OCPs) were measured in the surface seawater and lower atmosphere during the North Atlantic Bloom Experiment in the spring 2008 from samples collected on the R/V Knorr. The gaseous concentration profiles resulted from both long-range transport (LRT) from the Arctic by polar easterlies and local biogeochemical processes. Relatively constant α/γ-hexachlorocyclohexane (HCH) ratios and enantiomer fractions of α-HCH indicated that a single water mass was sampled throughout the cruise. Changes in dissolved phase concentrations were dominated by bloom processes (air-water exchange, partitioning to organic particles, and subsequent sinking) rather than LRT. α-HCH and dissolved phase trans-chlordanes showed depletion of (+) enantiomer, whereas depletion of the (-) enantiomer was observed for heptachlor exo-epoxide (HEPX) and cis-chlordanes. Fugacity ratio calculations suggest that hexachlorobenzene (HCB) and γ-HCH were depositing from air to water whereas heavier OCPs (chlordanes, HEPX) were evaporating. Dissolved phase concentrations did not decrease with time during the three-week bloom period; neither were lipophilic OCPs drawn down from air to water as previous studies hypothesized. Comparison with Arctic measurements suggested that the Arctic returned higher concentrations of α-HCH and HCB through both the atmospheric (polar easterlies) as well as oceanic transport (East Greenland Current) to the lower latitudes.

Three Years of Ocean Data From a Bio‐optical Profiling Float

Ocean color, first measured from space 30 years ago, has provided a revolutionary synoptic view of near-surface fields of phytoplankton pigments. Since 1979, a number of ocean color satellite missions have provided coverage of phytoplankton biomass and other biogeochemical variables on scales of days to years and of kilometers to ocean basin. Because of the nature of visible light and its interaction with absorbing and scattering materials in the ocean and atmosphere, these measurements are biased toward near-surface waters and are obscured by clouds. As a consequence, ocean color satellites miss significant fractions of phytoplankton biomass, marine primary productivity, and particle flux that occur at depths beyond their sensing range. They also miss phytoplankton blooms and other events that occur during periods of extended cloud cover.

Autonomous data describe North Atlantic spring bloom

Each spring, increasing sunlight and associated changes in the ocean structure trigger rapid growth of phytoplankton across most of the North Atlantic Ocean north of 30°N. The bloom, one of the largest in the world, is a major sink for atmospheric carbon dioxide and a prototype for similar blooms around the world. Models of the ocean carbon cycle, a necessary component of climate models, need to accurately reproduce the biological, chemical, and physical processes occurring during these blooms. However, a paucity of detailed observations severely limits efforts to evaluate such models.

Net community production and export from Seaglider measurements in the North Atlantic after the spring bloom

Mean rates of net community production (NCP) and particulate organic carbon (POC) export were estimated from sensor measurements of dissolved oxygen (O2), chlorophyll fluorescence (chl F), and particulate backscatter (bbp700) collected from three Seagliders that surveyed a 20 × 20 km area in the North Atlantic subsequent to a large diatom bloom. Since the Seagliders sampled geographically fixed patterns, care was taken in the calculation of all terms applicable to the Eulerian reference frame, including local rate of change, vertical mixing, air-sea exchange, and horizontal advection. Although similar studies of NCP in the open ocean have generally assumed advection to be insignificant, we have found that this term cannot be ignored when dealing with temporal scales of ≤1 month and/or spatial scales ≤20 km. The overlapping sampling pattern of the Seagliders was sufficiently rapid such that 4–5 day time scales observed in the O2 and POC data were adequately resolved and variations were not a consequence of aliasing spatial variability. During the study period, ratios of chlorophyll fluorescence-to-particulate backscatter (chl:bbp700) were lower than values encountered during the spring diatom bloom, suggesting the phytoplankton community was predominantly composed of smaller cells (picoplankton and nanoplankton) and/or coccolithophorids. Coupled budgets of oxygen and POC indicated a net community production of 1.0 mol C m−2 and carbon export of 0.6 mol C m−2, respectively, over a period of 23 days. Thus, the production and export of carbon that occurred over the month-long experiment period was comparable to that encountered during the spring bloom.

Evidence of small‐scale spatial structuring of phytoplankton alpha‐ and beta‐diversity in the open ocean

Summary Phytoplankton assemblages in the open ocean are usually assumed to be mixed on local scales unless large semi-permanent density discontinuities separating water masses are present. Recent modelling studies have, however, suggested that ephemeral submesoscale oceanographic features leading to only subtle density discontinuities may be important for controlling phytoplankton alpha- and beta-diversity patterns. Until now, no empirical evidence has been presented to support this hypothesis. Using hydrographic and taxonomic composition data collected near Iceland during the period of the 2008 spring bloom, we show that the distribution of phytoplankton alpha- and beta-diversity was related to submesoscale heterogeneity in oceanographic conditions. Distinct phytoplankton communities as well as differences in richness were identified on either side of a front delimiting surface waters of slightly different (˜0.03) salinities. Alpha-diversity was significantly higher on the high salinity side of the front compared to the low salinity side. This difference was primarily driven by the presence of several large diatom species in the high salinity region, especially of the genus Chaetoceros which dominated the biomass here. By investigating beta-diversity in relation to environmental and spatiotemporal variables, we show that the regional distribution of phytoplankton taxa was influenced by both different environmental conditions on either side of the front and dispersal limitation across the front. Changes in beta-diversity were primarily driven by turnover rather than nestedness and were apparently controlled by different processes in each region. Synthesis. This study shows that small-scale and ephemeral density discontinuities created by submesoscale frontal dynamics can play a major role in structuring patterns of phytoplankton diversity. Evidence is presented that they can generate changes in environmental conditions (leading to environmental filtering) and act as physical (dispersal) barriers for phytoplankton transport. The study suggests that dispersal barriers are potentially of much greater importance for phytoplankton diversity at local scales than currently recognized and indicates that drivers of marine phytoplankton diversity are similar to those structuring diversity of land plants.