S. Groom

A biogeochemical study of the coccolithophore, Emiliania huxleyi, in the North Atlantic

The biogeochemical properties of an extensive bloom (∼250,000 km2) of the coccolithophore, Emiliania huxleyi, in the north east Atlantic Ocean were investigated in June 1991. Satellite (NOAA-AVHRR) imagery showed that the bloom was centered initially at 60°–63°N by 13°–28°W and lasted approximately 3 weeks. Spatial variations in satellite-measured reflectance were well correlated with surface measurements of the beam attenuation coefficient, levels of particulate inorganic carbon, and coccolith density. Rates of both photosynthesis and calcification were typically relatively low within the coccolithophore-rich waters, suggesting the population was in a late stage of development at the time of the field observations. Levels of dimethyl sulphide (DMS) in surface waters were high compared to average ocean values, with the greatest concentrations in localized areas characterized by relatively high rates of photosynthesis, calcification, and grazing by microzooplankton. The estimated spatially averaged flux of DMS to the atmosphere was 1122 nmol m−2 h−1, somewhat greater than that determined for the same region in June-July 1987. Coccolith production (1 × 106 tonnes calcite-C) had a significant impact on the state of the CO2 system, causing relative increases of up to 50 μatm in surface pCO2 in association with alkalinity and water temperature changes. Gradients in pCO2 were as great as 100 μatm over horizontal distances of 20–40 km. The environmental implications of these findings are discussed in relation to the spatial and temporal distributions of E. huxleyi.

Estimation of phytoplankton production from space: current status and future potential of satellite remote sensing.

A new generation of ocean colour satellites is now operational, with frequent observation of the global ocean. This paper reviews the potential to estimate marine primary production from satellite images. The procedures involved in retrieving estimates of phytoplankton biomass, as pigment concentrations, are discussed. Algorithms are applied to SeaWiFS ocean colour data to indicate seasonal variations in phytoplankton biomass in the Celtic Sea, on the continental shelf to the south west of the UK. Algorithms to estimate primary production rates from chlorophyll concentration are compared and the advantages and disadvantage discussed. The simplest algorithms utilise correlations between chlorophyll concentration and production rate and one equation is used to estimate daily primary production rates for the western English Channel and Celtic Sea; these estimates compare favourably with published values. Primary production for the central Celtic Sea in the period April to September inclusive is estimated from SeaWiFS data to be 102 gC m(-2) in 1998 and 93 gC m(-2) in 1999; published estimates, based on in situ incubations, are ca. 80 gC m(-2). The satellite data demonstrate large variations in primary production between 1998 and 1999, with a significant increase in late summer in 1998 which did not occur in 1999. Errors are quantified for the estimation of primary production from simple algorithms based on satellite-derived chlorophyll concentration. These data show the potential to obtain better estimates of marine primary production than are possible with ship-based methods, with the ability to detect short-lived phytoplankton blooms. In addition, the potential to estimate new production from satellite data is discussed.

Retrieval of coccolithophore calcite concentration from SeaWiFS Imagery

We examined blooms of the coccolithophorid E. huxleyi, observed in SeaWiFS imagery, with a new algorithm for the retrieval of detached coccolith concentration. The algorithm uses only SeaWiFS bands in the red and near infrared (NIR) to minimize the influence of the absorption by chlorophyll and dissolved organic material. We used published experimental determinations of the calcite specific backscattering and its spectral dependence, and assumed that the absorption coefficient of the medium was that of pure water, to estimate the marine contribution to the SeaWiFS radiance. The aerosol (and Rayleigh-aerosol interaction) contribution to the radiance was modeled as an exponential function of wavelength. These allow derivation of the coccolith concentration on a pixel-by-pixel basis from SeaWiFS imagery. Application to a July 30, 1999 SeaWiFS image of a bloom south of Plymouth, UK indicates that the SeaWiFS estimates are in good agreement with surface measurements of coccolith concentration.

Sea-surface signatures of the island mass effect phenomena around Madeira Island Northeast Atlantic

Abstract This is an introductory work that describes the manifestation of the island mass effect phenomena in the atmosphere and at the sea surface for a region of the ocean depleted from oceanographic work—the Madeira Archipelago (33°N, 17°W). The use of remote sensing tools becomes essential in recognizing some of the sea-surface features that characterize the island mass effect phenomena. AVHRR, coastal zone color scanner (CZCS), and SeaWiFS ancillary National Center for Environmental Prediction (NCEP) data, together with highly developed processing tools from PANORAMA (Processing and Automatic Navigation Of ReAl-time iMAges) allowed the detection of atmospheric Von Karman Vortex Streets, warm water surface wakes, leeward eddies, Azorean front, and localized upwelling. Results from the remote sensing analysis also helped to promote the reanalysis of historical in situ data for the manifestation of these island effects. IPIMAR cruises carried out between 1979 and 1982 collected temperature, salinity, density, and chlorophyll data, which were reinterpolated at the sea surface to study the island mass effect phenomena for the first time. Results have shown the formation of a warm water wake south of Madeira Island expanding 400 km offshore. The surface signature of the Azorean front, the so-called subtropical front, seems to take place within the Madeira latitude (33°N). North waters were consistently colder than the warm and salty southern waters. AVHRR data also revealed eddies and fronts being formed in the region. Leeward eddies were often observed in the flanks of the islands; CZCS data showed highly productive eddies in the west of Madeira Island. Localized cold water with high chlorophyll concentrations was also observed around the islands coast. A particularly dynamic area was the underwater ridge that connected Madeira and Desertas Islands. Future work should continue to sample these regions of interest with the simultaneous use of different satellite sensors. Vertical characterization of the different phenomena is needed.

The response of phytoplankton production to periodic upwelling and relaxation events at the Iberian shelf break: estimates by the 14C method and by satellite remote sensing

Abstract The primary productivity of the NW Iberian margin has been determined by a combination of 14 C incorporation experiments on natural phytoplankton assemblages and satellite remote sensing with the ocean colour sensor, SeaWiFS. The approach to the field determination of phytoplankton production was Eulerian, involving measurements on a fixed grid of stations covering the region from 41°30′N to 43°00′N and 09°00′W to 10°30′W. Three different laboratories made primary production measurements over a 3-year period. Each group used variations of the 14 C method—in situ incubations and two modelling approaches based on P – E parameter derivation. An intercalibration experiment showed acceptable agreement between the three estimates. Measurements by the three groups on eight cruises have been merged to provide estimates of regional productivity. However, the temporal coverage of the data was too limited to provide robust seasonal estimates since few experiments were done in the autumn and winter months. A second approach to the estimation of primary production utilised surface pigment concentrations derived from satellite remote sensing. Two models were used. Firstly, an empirical relationship was found between depth-integrated primary production and surface chlorophyll concentration, which explained 78% of the variance in the production estimates. The second approach used a semi-analytical model which incorporates irradiance and temperature as well as chlorophyll-derived parameters. Both models have been applied to SeaWiFS-derived, surface chlorophyll concentrations to estimate primary production during 1998, 1999 and 2000 for three regions—the shelf, the slope and the open ocean of the region of study. Using the semi-analytical model, the mean primary production for the 3 years is estimated to be 319 g C m −2 year −1 for the shelf, 280 g C m −2 year −1 for the slope and 217 g C m −2 year −1 for the adjacent open ocean. Interannual variation in production on the shelf ranged from 334 g C m −2 year −1 in 1998 to 301 g C m −2 year −1 in 2000. It is estimated that upwelling increased primary production, relative to that in the open ocean, by ∼50%. Satellite-derived estimates of the ratio of new production to primary production were 0.37, 0.28 and 0.40 for the shelf region during the upwelling seasons of 1998, 1999 and 2000.

Analysis of satellite imagery for Emiliania huxleyi blooms in the Bering Sea before 1997

The presence of blooms of the coccolithophore Emiliania huxleyi in the Bering Sea shelf has been studied using satellite imagery in order to ascertain whether its first reported appearance in 1997 is really a new phenomenon for the area. Examination for Emiliania huxleyi blooms in Coastal Zone Color Scanner (CZCS) and Advanced Very High Resolution Radiometer (AVHRR) imagery dating from 1978 to 1996 was performed and the relationship between the presence of Emiliania huxleyi and the Pacific Decadal Oscillation and the El Nino Southern Oscillation was investigated. No evidence of the presence of this species was found in CZCS or AVHRR imagery between 1978 and 1995. AVHRR images reveal that a small coccolithophore bloom was present in summer 1996. Although the blooms of 1997 were unprecedented in extension and intensity, it appears that the Bering Sea ecosystem did not respond as abruptly to atmospheric anomalies as initially reported.

Optical modeling and measurements of a coccolithophore bloom

Blooms of the phytoplankton coccolithophorid Emiliania huxleyi can cause significant changes to both the inherent and the apparent optical properties within an oceanic column. Measurements made within such a bloom off the southwestern coast of England during July 1999 are reported. The multiple scattering properties of the bloom prevented accurate retrieval of absorption (a) and attenuation (c) coefficients with a WETLabs ac-9. Upwelling radiance measurements were similarly affected by the bloom, which caused the sensors to saturate. An optical model has been developed that gives close agreement with the in situ optics when it is used as input to the Hydrolight radiative-transfer model.

Spatial and temporal variability of particle flux at the N.W. European continental margin

A synopsis of results from two sediment trap moorings deployed at the mid- and outer slope (water depths 1450 and 3660 m, respectively) of the Goban Spur (N.E. Atlantic Margin) is presented. Fluxes increase with trap deployment depth; below 1000 m resuspended and advected material contributes increasingly to bulk flux. Fluxes of dry weight, POC and diatoms in the traps 400 m above bottom (mab) are smaller than those recorded at the sediment surface due to lateral fluxes in the benthic nepheloid layer. These near-bottom fluxes are larger at shallower water depths. Pa-231/Th-230 ratios in sedimenting material suggest that boundary scavenging is not significant at the Goban Spur. Fluxes of Pb-210 in the intermediate and deep traps are comparable to the Pb-210 supply rate at this site. At the outer slope, sediment Pb-210 fluxes are similar to those measured in the traps 400 mab; at the mid- slope they are a factor of 2 higher, once again indicating large near-bottom lateral particle input. Based on POC- normalised biomarkers in sedimenting material, we followed changes in the quality of sedimenting material with differing trap depth and on seasonal and event-related time scales. In spring fresh, diatom-dominated sedimentation occurs, with progressive degradation of POC with time (to winter) and depth (from 600 to 3220 m). Deeper traps are distinguished on the basis of opal and aluminium fluxes that are dominant in lateral input. A storm event during late September 1993 was clearly reflected in the delta N-15 isotope ratio of sedimenting material, with a time lag of 2-3 weeks. Diatom and opal fluxes were elevated in this storm-related signal, and its biomarker composition in the 600-m trap was similar to that during spring. An estimate made of upward nitrate flux (new production) at the shelf break and at the outer slope indicated a 2-fold higher new (export) production at the shelf break. Particulate organic carbon export from the shelf break to below the depth of maximal seasonal mixing ranges between 3 and 9% of primary production. [KEYWORDS: Isotopic composition; north-atlantic; goban spur; organic-matter; open-ocean; sediments; nitrogen; carbon; phytoplankton; 20-degrees-w]