James Aiken

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.

Long-term oceanographic and ecological research in the western English Channel

Long-term research in the western English Channel, undertaken by the marine laboratories in Plymouth, is described and details of survey methods, sites, and time series given in this chapter. Major findings are summarized and their limitations outlined. Current research, with recent reestablishment and expansion of many sampling programmes, is presented, and possible future approaches are indicated. These unique long-term data sets provide an environmental baseline for predicting complex ecological responses to local, regional, and global environmental change. Between 1888 and the present, investigations have been carried out into the physical, chemical, and biological components (ranging from plankton and fish to benthic and intertidal assemblages) of the western English Channel ecosystem. The Marine Biological Association of the United Kingdom has performed the main body of these observations. More recent contributions come from the Continuous Plankton Recorder Survey, now the Sir Alister Hardy Foundation for Ocean Science, dating from 1957; the Institute for Marine Environmental Research, from 1974 to 1987; and the Plymouth Marine Laboratory, which was formed by amalgamation of the Institute for Marine Environmental Research and part of the Marine Biological Association, from 1988. Together, these contributions constitute a unique data series-one of the longest and most comprehensive samplings of environmental and marine biological variables in the world. Since the termination of many of these time series in 1987-1988 during a reorganisation of UK marine research, there has been a resurgence of interest in long-term environmental change. Many programmes have been restarted and expanded with support from several agencies. The observations span significant periods of warming (1921-1961; 1985-present) and cooling (1962-1980). During these periods of change, the abundance of key species underwent dramatic shifts. The first period of warming saw changes in zooplankton, pelagic fish, and larval fish, including the collapse of an important herring fishery. During later periods of change, shifts in species abundances have been reflected in other assemblages, such as the intertidal zone and the benthic fauna. Many of these changes appear to be related to climate, manifested as temperature changes, acting directly or indirectly. The hypothesis that climate is a forcing factor is widely supported today and has been reinforced by recent studies that show responses of marine organisms to climatic attributes such as the strength of the North Atlantic Oscillation. The long-term data also yield important insights into the effects of anthropogenic disturbances such as fisheries exploitation and pollution. Comparison of demersal fish hauls over time highlights fisheries effects not only on commercially important species but also on the entire demersal community. The effects of acute (Torrey Canyon oil spill) and chronic (tributyltin [TBT] antifoulants) pollution are clearly seen in the intertidal records. Significant advances in diverse scientific disciplines have been generated from research undertaken alongside the long-term data series. Many concepts in marine biological textbooks have originated in part from this work (e.g. the seasonal cycle of plankton, the cycling of nutrients, the pelagic food web trophic interactions, and the influence of hydrography on pelagic communities). Associated projects currently range from studies of marine viruses and bacterial ecology to zooplankton feeding dynamics and validation of ocean colour satellite sensors. Recent advances in technology mean these long-term programmes are more valuable than ever before. New technology collects data on finer temporal and spatial scales and can be used to capture processes that operate on multiple scales and help determine their influence in the marine environment. The MBA has been in the forefront of environmental modelling of shelf seas since the early 1970s. Future directions being pursued include the continued development of coupled physical-ecosystem models using western English Channel time-series data. These models will include both the recent high-resolution data and the long-term time-series information to predict effects of future climate change scenarios. It would be beneficial to provide more spatial and high-resolution temporal context to these data, which are fundamental for capturing processes that operate at multiple scales and understanding how they operate within the marine environment. This is being achieved through employment of technologies such as satellite-derived information and advanced telemetry instruments that provide real-time in situ profile data from the water column.

Semianalytical model for the derivation of ocean color inherent optical properties: description, implementation, and performance assessment

A semianalytical approach to the problem of determining inherent optical properties from satellite and in situ ocean color data is presented. The model uses empirically derived spectral slopes between neighboring wavebands in combination with radiative transfer modeling to determine the spectral absorption (alpha) and backscatter (b(b)); these values are then further decomposed into absorption due to phytoplankton, detrital, and colored dissolved organic matter components. When compared with over 400 in situ data points the model makes good retrievals of the total absorption and backscatter across the entire spectrum, with regression slopes close to unity, little or no bias, high percentage of variance explained, and low rms errors.

Spatial variability in near-surface chlorophyll a fluorescence measured by the Airborne Oceanographic Lidar (AOL)

The primary purpose of the aircraft remote sensing component of the North Atlantic Bloom Experiment (NABE) was to: (1) quantify spatial patterns of surface Chl a variability and co-variability with temperature (T) within the NABE study regions along the 20°W meridian near 48 and 60°N; and (2) determine if the major NABE ship and mooring locations were representative of surrounding ocean waters with respect to large-scale distributions of surface Chl a and T. The sampling platform was a NASA P-3 aircraft equipped with the Airborne Oceanographic Lidar (AOL) system, which measures laser-induced Chl a fluorescence (LICF), upwelling spectral radiance and surface temperature (T). n nResults collected during nine AOL missions conducted between 26 April and 3 June show considerable mesoscale variability in LICF and T. Spatial statistics (structure functions) showed that the dominant scales of LICF and T were significantly correlated in the range 10–290 km. Spectral analysis of the results of long flight lines showed spectral slopes averaging −2 for both LICF and T for spatial scales in the range 1.2–50 km. As for previous investigations of this type, we interpret the correlation between LICF and T as evidence that physical processes such as upwelling and mixing are dominant processes affecting spatial variations in Chl a distributions in the North Atlantic during the period of our sampling. The minimum dominant T and LICF spatial scales (ca 10 km) we determined from structure functions are similar to minimum scales predicted from models (Woods, 1988, In: Toward a theory on biological-physical interactions in the world ocean, Kluwer Academic, Boston, pp. 7–30) of upwelling induced by vortex contraction on the anticyclonic side of mesoscale jets. n nThe NABE experiment was planned with the explicit assumption that major biological and chemical gradients are in the north-south direction in the northeast Atlantic. Our results support this assumption, and we observed no large-scale (>200 km), east-to-west trends in surface Chl a in the two principal study areas. n nOur analyses show that satellite ocean color scanners with pixel resolution of 4 × 4 km will generally detect the major spatial patterns of Chl a distributions (at scales > 0.3 km), in near surface waters during the spring bloom in the North Atlantic.

Validation of SeaWiFS ocean color satellite data using a moored databuoy

[1]xa0The Plymouth Marine Bio-Optical Databuoy (PlyMBODy) was deployed in the western English Channel to measure Lwn over an extended period of time in order to vicariously validate ocean color data from the NASA Sea-Viewing Wide Field-of-View Sensor (SeaWiFS). Several experiments have shown that the buoy had radiometric accuracy similar to standard sea-going optical profiling equipment (about ±8%). There were 15 match-ups of good quality SeaWiFS and PlyMBODy data during 10 months of deployment between May 1997 and September 1998. Differences between PlyMBODy measurements of Lwn and SeaWiFS data (processed using SeaDAS version 4.1) were found to be spectrally variable and sometimes large. The differences were small in the blue-green and green bands (SeaWiFS bands 3–5, 490, 510, and 555 nm) where the biases were <3% and the mean absolute differences were ∼20%. Bands 1, 2, and 6 (412, 443, and 670 nm) were significantly underestimated by SeaWiFS: the average differences were 55%, 19%, and 58%, respectively. The mean absolute differences were also much larger in these bands (30–77%). SeaWiFS radiometric measurements were generally sufficiently accurate to estimate chlorophyll-a concentration using OC4v4 to within the mission target of ±35% but with a high bias of ∼8%, though the performance of the algorithm itself was not tested. Bias and scatter of SeaWiFS Lwn measurements relative to PlyMBODy measurements were much poorer when SeaDAS version 3.3p5, rather than version 4.1, was used. The upgrade from SeaDAS 4.0p3 to 4.1 made little difference to the agreement.

Relationship between the distribution function of ocean nadir radiance and inherent optical properties for oceanic waters

The distribution function of the ocean nadir radiance, defined by the upward radiance-to-irradiance ratio, is investigated as functions of the absorption coefficient and the volume scattering function to understand their relationship rather than to develop a numerical algorithm. It is shown for oceanic waters that the distribution function is directly proportional to the volume scattering function normalized by the backscattering coefficient. The relatively small spectral variation of the distribution function is explained by the small spectral variation of the normalized volume scattering function, as well as by a function that describes the contribution of the backscattering-to-absorption ratio to the distribution function. The interpretation described was verified against in situ data, highlighting factors controlling the distribution function of oceanic waters.