Jim Aiken

The atmospheric correction of water colour and the quantitative retrieval of suspended particulate matter in Case II waters: Application to MERIS

The remote sensing of turbid waters (Case II) using the Medium Resolution Imaging Spectrometer (MERIS) requires new approaches for atmospheric correction of the data. Unlike the open ocean (Case I waters) there are no wavelengths where the water-leaving radiance is zero. A coupled hydrological atmospheric model is described here. The model solves the water-leaving radiance and atmospheric path radiance in the near-infrared (NIR) over Case II turbid waters. The theoretical basis of this model is described, together with its place in the proposed MERIS processing architecture. Flagging procedures are presented that allow seamless correction of both Case I waters, using conventional models, and Case II waters using the proposed model. Preliminary validation of the model over turbid waters in the Humber estuary, UK is presented using Compact Airborne Spectrographic Imager (CASI) imagery to simulate the MERIS satellite sensor. The results presented show that the atmospheric correction scheme has superior perfo...

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.

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.

Size-fractionated primary production and nitrogen assimilation in the northwestern Indian Ocean

Abstract Rates of phytoplankton production and nitrogen assimilation were measured at various stations along a transect in the northwestern Indian Ocean, from near the equator, northwards into the upwelling system off the Arabian peninsula, during September–October 1986. The measurements were made using in situ incubation techniques with the simultaneous use of 14C and 15N isotopes. Samples were fractionated after the incubations into three size classes: 0.2–0.8μm, 0.8–5.0 μm, and >5.0μm for the 14C incubations; and 5μm for the 15N incubations. The assimilation of nitrate and ammonium was measured. These measurements were supported by a detailed description of the horizontal and vertical distributions of chlorophyll, temperature and underwater light field, by the deployment of a towed undulating oceanographic recorder. Rates of primary production ranged from approximately 0.5 g C m−2 day−1 at the equator, reducing to 2.5 in the upwelling region off the coast of Oman; total nitrogen assimilation followed a similar pattern. Very significant variations in the size distribution of the activity of the plankton were observed. Over 75% of the carbon and nitrogen assimilation was in the

Validation of MERIS reflectance and chlorophyll during the BENCAL cruise October 2002: preliminary validation of new demonstration products for phytoplankton functional types and photosynthetic parameters

We measured water leaving reflectance, phytoplankton pigments, optical properties and photosynthetic parameters in the southern Benguela ecosystem in October 2002. These data were used to validate MERIS standard products: reflectance (MERIS wavelengths) and Case 1 Chlorophyll‐a. In this heterogeneous area, accurate validation required sampling within a few minutes of the satellite overpass. Inter‐pigment relationships e.g. Total Chlorophyll (TChla) to Total Pigment (TP) were robust (R2∼0.99) yet pigment ratios (TChla/TP) were not constant (range 0.44 to 0.62) increasing log‐linearly with biomass (R2∼0.7). Photosynthetic parameters (e.g. Photosynthetic Quantum Efficiency, PQE) and optical ratios (a676/a440) also increased log‐linearly with biomass (R2∼0.8). PQE, pigment and optical ratios were linearly inter‐correlated (R2∼0.7 to 0.8). From these data we derived the bio‐optical traits for several phytoplankton functional types (PFTs): micro‐plankton (diatoms and dinoflagellates) had high biomass, pigment ratios and PQE; nano‐flagellates had low to intermediate biomass, pigment ratios and PQE; prokaryotes had very low biomass, pigment ratios and PQE. We present MERIS data analysed for PFTs and new products (PQE).

Remote-Sensing Of Oceanic Biology In Relation To Global Climate Change

The oceans have a fundamental role in the global climate system because of their capacity to store and transport heat and absorb and emit trace gases which affect the earths radiation budget. Although good progress has been made with issues such as carbon and sulfur cycling, feedback responses related to the impact of climate change on biological systems, and links between plankton ecology and climate, there is a lack of information on the distributions of biological properties on a global scale. This article reviews the potential contribution of ocean color measurements for biological studies within the context of climate change. The remote sensing of oceanic phytoplankton from satellites measuring radiance at visible and near infrared wavelenghts has produced a wealth of new information on biomass distributions and has provided a basis for new approaches to estimation of global marine primary productivity.

An objective methodology for identifying oceanic provinces

An objective methodology for identifying oceanic provinces in hydrographic data is presented, although, the technique is sufficiently general so as to be applicable to a variety of data sets. The sub- and near-surface temperature (T) and salinity (S) of the Atlantic Ocean were measured on two Atlantic Meridional Transect (AMT) cruises covering approximately 100° of latitude and 50° of longitude. The cruises covered the boreal autumn and austral spring (AMT-1) and the boreal spring and austral autumn (AMT-2) during 1995 and 1996, respectively. There are distinct patterns of change in T-S from 50°N to 50°S for both sub- and near-surface measurements, although this study concentrates primarily on the latter. Near-surface density (σt) encapsulates the variations in T-S. The position of localized extrema in the σt first spatial derivative locates the change in bulk T-S properties and, hence, the extent of a physical province. The province identification methodology is validated by comparing the results obtained using in situ density, a mixed layer depth space series constructed from expendable bathythermograph profiles, and climatological density. Additional validation of the methodology is achieved by intercomparing different in situ data sources between the two cruise periods. The comparisons show the near-surface measurements are indicative of the mixed layer and that the AMT-1 and AMT-2 cruises are representative of the corresponding climatological months. A simple scheme for investigating how far a province extends away from the cruise track uses the climatological data in concert with a T-S description of each province from the AMT along-track observations to produce a two-dimensional map of the geographical extent of each province. Although exceptions between the province extents and the basic circulation of the Atlantic Ocean occur, the majority of the provinces have the correct shape and extent, that is, their edges follow the flow fields associated with the currents that bound them and they extend over an appropriate area. The province identification methodology is sensitive and reliable enough to investigate temporal differences in oceanic provinces, which means it might be useful for parameterizing global budget calculations in numerical models.

A meeting place of great ocean currents: shipboard observations of a convergent front at 2°N in the Pacific

Abstract We present a synthesis of physical, chemical and biological shipboard observations of a convergent front at 2°N, 140°W and its surrounding environment. The front was a component of a tropical instability wave generated by shear between westward-flowing equatorial waters to the south and warmer equatorial counter current water to the north. Surface waters on the cold side were undersaturated with oxygen, which suggests that the water had only been exposed at the sea surface for a period of a few weeks. Although the atmospheric exposure time was short, the effects of biological activity could be detected in enhanced concentrations of total (dissolved plus suspended particulate) organic carbon concentration, proving that TOC can be produced in the top centimeters of the changing environmental conditions. The front itself was dominated by the accumulation of a “patch” of buoyant diatoms Rhizosolenia castracanei concentrated in the top centimeters of the warm surface water north of the front, and elevated chlorophyll concentrations were observed from the air over a spatial scale of order 10–20 km northward from the front. The nitrogen budget and thorium data suggest that a significant fraction of the elevated POC, and virtually all of the PON, arrived in the patch waters as imported particles rather than in situ photosynthesis. Photosynthetic uptake of carbon appears to have occurred in patch waters, but without corresponding uptake of fixed nitrogen (an uncoupling of the usual Redfield stoichiometry). Solute chemistry of the patch appears to be controlled by turbulent mixing, which flushes out patch waters on a time scale of days

Variability of pCO2 in the tropical Atlantic in 1995

Atmospheric and oceanic partial pressures of CO2 (pCO2) have been recorded automatically along two Atlantic meridional transects in 1995. The tropical Atlantic ocean (20°S–20°N) is generally a source of CO2 for the atmosphere, but in the region of the North Equatorial Countercurrent an undersaturation of CO2 has been observed. Undersaturations previously reported in the literature are explained by the decrease of salinity due to the high precipitations associated with the Intertropical Convergence Zone. In June 1995, strong CO2 undersaturations (ΔpCO2 = −70μatm) were observed near 8°N, which suggests, in addition of the salinity effect, an uptake of CO2 due to biological activity. This undersaturation, although weaker than in spring, also appeared at other periods of the year 1995.

SeaWiFS validation in European coastal waters using optical and bio-geochemical measurements

The National Aeronautics & Space Administration (NASA) Sea viewing Wide Field of view Sensor (SeaWiFS) began operational measurement of ocean colour in September 1997. Upgrades to the SeaWiFS data processing system (SeaDAS) have occurred frequently and the effects of these revisions on the remotely sensed estimates of chlorophyll-a concentration (chl-a) have been significant. Measurements of chl-a from research work in the Bay of Biscay and Gulf of Cadiz during 1998–1999 are used to validate the SeaWiFS chl-a product generated using the current version of SeaDAS (version 4.1). The validation data cover coastal and offshore waters, including those dominated by inorganic suspended sediment, and an intense dinoflagellate bloom where shipboard chl-a measurements exceeded 50 mg m−3. The standard SeaWiFS chlorophyll algorithm (OC4v4) generally performed well, but significantly over-estimated chl-a where inorganic suspended sediment was present. The algorithm is only applicable to chl-a values up to 64 mg m−3, which was less than chl-a at the centre of the bloom. A novel algorithm for chl-a, which first estimates the inherent optical properties of the water, was applied to the SeaWiFS measurements but failed on over 90% of the pixels, perhaps because SeaWiFS is under-estimating water reflectance at the extreme blue end of the visible spectrum.