Stanford B. Hooker

Vertical distribution of phytoplankton communities in open ocean: An assessment based on surface chlorophyll

[1] The present study examines the potential of using the near-surface chlorophyll a concentration ([Chla]surf), as it can be derived from ocean color observation, to infer the column-integrated phytoplankton biomass, its vertical distribution, and ultimately the community composition. Within this context, a large High-Performance Liquid Chromatography (HPLC) pigment database was analyzed. It includes 2419 vertical pigment profiles, sampled in case 1 waters with various trophic states (0.03–6 mg Chla m � 3 ). The relationships between [Chla]surf and the chlorophyll a vertical distribution, as previously derived by Morel and Berthon (1989), are fully confirmed. This agreement makes it possible to go further and to examine if similar relationships between [Chla]surf and the phytoplankton assemblage composition along the vertical can be derived. Thanks to the detailed pigment composition, and use of specific pigment biomarkers, the contribution to the local chlorophyll a concentration of three phytoplankton groups can be assessed. With some cautions, these groups coincide with three size classes, i.e., microplankton, nanoplankton and picoplankton. Corroborating previous regional findings (e.g., large species dominate in eutrophic environments, whereas tiny phytoplankton prevail in oligotrophic zones), the present results lead to an empirical parameterization applicable to most oceanic waters. The predictive skill of this parameterization is satisfactorily tested on a separate data set. With such a tool, the vertical chlorophyll a profiles of each group can be inferred solely from the knowledge of [Chla]surf. By combining this tool with satellite ocean color data, it becomes possible to quantify on a global scale the phytoplankton biomass associated with each of the three algal assemblages.

The calibration and validation of SeaWiFS data

Abstract The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) is the successor ocean color imaging system to the Nimbus-7 Coastal Zone Color Scanner (CZCS). The SeaWiFS calibration and validation effort includes spacecraft, atmospheric, sea surface, subsurface (or in situ), plus laboratory and data analysis components which require pre- and postlaunch activities. The most important goals of this effort are to produce water-leaving radiances with an uncertainty of 5% in clear-water regions and chlorophyll a concentrations within ±35% over the range of 0.05–50 mg m−3. The first objective requires field instruments with a calibration and measurement capability on the order of 1%; because these challenging in situ measurements will be acquired from a variety of field instruments over the five-year mission interval, a measurement assurance program is required. This program consists of several activities: an accurate prelaunch characterization and calibration of the SeaWiFS instrument; a Marine Optical Buoy (MOBY) rotation in clear water to provide a water-leaving radiance time series for postlaunch vicarious calibration; the SeaWiFS Bio-Optical Archive and Storage System (SeaBASS) to hold the relevant data; clearly defined SeaWiFS Ocean Optics Protocols (SOOP) for established data collection methodologies; annual SeaWiFS Intercalibration Round-Robin Experiments (SIRREXs) for intercomparing field and calibration equipment, and training scientific personnel; direct comparison to a national standard laboratory using the SeaWiFS Transfer Radiometer (SXR); a portable field source, called the SeaWiFS Quality Monitor (SQM), for monitoring the temporal stability of the calibration of field instruments; a highly accurate atmospheric correction algorithm designed for the SeaWiFS instrument response functions; bio-optical algorithms that encompass a broad range of bio-optical provinces; and satellite data processing, quality control, and analysis procedures for monitoring the postlaunch performance of the sensor and the validity of the derived products. The culmination of many of these activities is the deployment of the instruments and methodologies on Atlantic Meridional Transect (AMT) cruises between England and the Falkland Islands, a 13 000 km voyage spanning more than 100° of latitude, with a calibration and measurement accuracy that is on the order of 1%. The AMT Program is the primary product validation activity supported by the SeaWiFS Project. The AMT cruises also serve as a testbed for new technology development and have demonstrated that high quality bio-optical data can be routinely provided to the Project in near-real time.

Is desert dust making oligotrophic waters greener

In situ optical measurements provide evidence that oligotrophic waters of the Mediterranean Sea have a greener color than would result from their phytoplankton content alone. This anomaly, detectable in low chlorophyll waters, remains unnoticed in the chlorophyll-rich waters of the nearby waters of the Moroccan upwelling zone. It is due to enhanced absorption in the blue and enhanced backscattering in the green parts of the visible spectrum likely resulting from the presence of submicron Saharan dust in suspension within the upper layer. This result implies that regional estimations of carbon fixation from ocean color images might be significantly overestimated, not only in the Mediterranean Sea, but also in other oligotrophic areas of the Northern hemisphere, subjected to desert dust deposition.

Pan-Arctic distributions of continental runoff in the Arctic Ocean

Continental runoff is a major source of freshwater, nutrients and terrigenous material to the Arctic Ocean. As such, it influences water column stratification, light attenuation, surface heating, gas exchange, biological productivity and carbon sequestration. Increasing river discharge and thawing permafrost suggest that the impacts of continental runoff on these processes are changing. Here, a new optical proxy was developed and implemented with remote sensing to determine the first pan-Arctic distribution of terrigenous dissolved organic matter (tDOM) and continental runoff in the surface Arctic Ocean. Retrospective analyses revealed connections between the routing of North American runoff and the recent freshening of the Canada Basin, and indicated a correspondence between climate-driven changes in river discharge and tDOM inventories in the Kara Sea. By facilitating the real-time, synoptic monitoring of tDOM and freshwater runoff in surface polar waters, this novel approach will help understand the manifestations of climate change in this remote region.

An Evaluation of Above- and In-Water Methods for Determining Water-Leaving Radiances

Abstract A high-quality dataset collected at an oceanographic tower was used to compare water-leaving radiances derived from simultaneous above- and in-water optical measurements. The former involved two different above-water systems and four different surface glint correction methods, while the latter used three different in-water sampling systems and three different methods (one system made measurements a fixed distance from the tower, 7.5 m; another at variable distances up to 29 m away; and the third was a buoy sited 50 m away). Instruments with a common calibration history were used, and to separate differences in methods from changes in instrument performance, the stability (at the 1% level) and intercalibration of the instruments (at the 2%–3% level) was performed in the field with a second generation Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Quality Monitor (SQM-II). The water-leaving radiances estimated from the methods were compared to establish their performance during the field campaign,...

The Atlantic Meridional Transect: overview and synthesis of data

The Atlantic Meridional Transect programme uses the twice-annual passage of the RRS James Clark Ross between the UK and the Falkland Islands, before and after the Antarctic research programme in the Austral Summer (see Aiken, J., & Bale, A. J. (2000). An introduction to the Atlantic Meridional Transect (AMT) Programme. Progress in Oceanography, this issue). This paper examines the scientific rationale for a spatially-extensive time and space series programme and reviews the relevant physical and biological oceanography of the Atlantic Ocean. The main scientific observations from the research programme are reported. These are set in the context of historical and contemporary observations pertinent to the principal objectives of the cruise, notably the satellite remotely sensed observations of ocean properties. The extent to which the programme goals have been realised by the research to date is assessed and discussed. New bio-optical signatures, which can be related to productivity parameters, have been derived. These can be used to interpret remotely sensed observations of ocean colour in terms of productivity and production processes such as the air/sea exchange of biogenic gases, which relate to the issues of climate change and the sustainability of marine ecosystems.

An autonomous above-water system for the validation of ocean color radiance data

An operational system for autonomous above-water radiance measurements, called the SeaWiFS Photometer Revision for Incident Surface Measurements (SeaPRISM), was deployed at the Acqua Alta Oceanographic Tower in the northern Adriatic Sea and used for the validation of remote sensing radiometric products in coastal waters. The SeaPRISM data were compared with simultaneous data collected from an independent in-water system for a wide variety of sun elevations along with different atmospheric, seawater, and sea state conditions. The average absolute differences between the above- and in-water determinations of water-leaving radiances (computed linearly) were less than 4.5% in the 412-555-nm spectral interval. A similar comparison for normalized water-leaving radiances showed average absolute differences less than 5.1%. The comparison between normalized water-leaving radiances computed from remote sensing and SeaPRISM matchup data, showed absolute spectral average (linear) differences of 17.0%, 22.1%, and 20.8% for SeaWiFS, MODIS, and MERIS, respectively. The results, in keeping with those produced by independent in-water systems, suggest the feasibility of operational coastal networks of autonomous above-water radiometers deployed on fixed platforms (towers, lighthouses, navigation aids, etc.) to support ocean color validation activities.

Phytoplankton pigment and absorption characteristics along meridional transects in the Atlantic Ocean

Pigment patterns and associated absorption properties of phytoplankton were investigated in the euphotic zone along two meridional transects in the Atlantic Ocean, between the UK and the Falkland Islands, and between South Africa and the UK. Total chlorophyll a (TChla=MVChla+DVChla+chlorophyllide a) concentrations and the biomarker pigments for diatoms (fucoxanthin), nanoflagellates and cyanobacteria (zeaxanthin) appeared to have similar distribution patterns in the spring and in the autumn in the temperate NE Atlantic and the northern oligotrophic gyre. Divinyl chlorophyll a levels (prochlorophytes) were greater in spring at the deep chlorophyll maximum in the oligotrophic gyre, however. Marked seasonal differences were observed in the NW African upwelling region. TChla concentrations were twice as high in the upper mixed layer in the spring, with the community dominated by diatoms and prymnesiophytes (19?-hexanoyloxyfucoxanthin). A layered structure was prevalent in the autumn where cyanobacteria, diatoms and prymnesiophytes were located in the upper water column and diatoms and mixed nanoflagellates at the sub-surface maximum. In the South Atlantic, the Benguela upwelling ecosystem and the Brazil-Falklands Current Confluence Zone (BFCCZ) were the most productive regions with the TChla levels being twice as high in the Benguela. Diatoms dominated the Benguela system, while nanoflagellates were the most ubiquitous group in the BFCCZ. Pigment concentrations were greater along the eastern boundary of the southern oligotrophic gyre and distributed at shallower depths. Deep chlorophyll maxima were a feature of the western boundary oligotrophic waters, and cyanobacteria tended to dominate the upper water column along both transects with a mixed group of nanoflagellates at the chlorophyll maximum. Absorption coefficients were estimated from spectra reconstructed from pigment data. Although absorption was greater in the productive areas, the TChla-specific coefficients were higher in oligotrophic regions. In communities that were dominated by diatoms or nanoflagellates, pigment absorption was generally uniform with depth and attenuating irradiance, with TChla being the major absorbing pigment at 440 nm and photosynthetic carotenoids (PSC) at 490 nm. Absorption by chlorophyll c and photoprotective carotenoids (PPC) was much lower. Populations where cyanobacteria were prevalent were characterized by high PPC absorption, particularly at 490 nm, throughout most of the euphotic zone. The data suggested that the effect of pigments on the variability of phytoplankton absorption was due primarily to the variations in absorption by PPC.

An overview of the SeaWiFS Project

It is apparent to the oceanographic community that due to the dynamic nature of the worlds oceans and climate and the importance of the oceans role in global change, a follow-on sensor to the Coastal Zone Color Scanner (CZCS) should be flown [Ocean Color Science Working Group, 1982; Joint EOSAT/NASA SeaWiFS Working Group, 1987]. Acquisition of ocean-color data from space in the early 1990s is a high-priority goal that has been recognized in National Academy of Sciences reports. NASAs Office of Space Science and Applications (OSSA) and Goddard Space Flight Center (GSFC) have designated the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) Project to develop and operate an ocean-color research data system.

Satellite data for ocean biology, biogeochemistry, and climate research

Understanding how the effects of increasing atmospheric carbon dioxide (CO2) concentrations and higher surface temperatures cascade through the oceans physical and biogeochemical systems is a major theme of NASAs Ocean Biology and Biogeochemistry Program, and this requires decadal time series of accurate global satellite measurements of key marine biological properties, such as phytoplankton chlorophyll a concentration. Such time series have been a primary NASA objective since the mid-1980s when the results from the Coastal Zone Color Scanner (CZCS),a proof-of-concept program (sparse global coverage with a limited post-launch validation program), demonstrated that space-based retrievals of ocean water-leaving radiances (Lws) and chlorophyll a concentrations were sufficiently accurate for quantitative research.