8.12 Case Study No. 12Reference: Feldman, G.C., 1986,Variability of the productive habitat in the Eastern Equatorial Pacific. EOS Transactions, American Geophysical Union, 67(9):106–8Remote Sensing Technique: Satellite Remote Sensing:Platform - NIMBUS-7;Sensor - CZCS.Objective: To show that satellite ocean colour data can be used to define the spatial extent of the region of enhanced biological production (productive habitat) in the eastern equatorial Pacific (refer to Figure 8.23). To determine the degree of interannual variability in the areal extent of the productive habitat and in the estimated primary production of the region.Experimental Rationale: The changes in ocean colour detected by the CZCS provide a quantitative measure of phytoplankton pigment concentrations in the surface layer of the ocean. These concentrations are an index of phytoplankton biomass and may be empirically related to primary production. Examination of a series of large-scale images, covering the entire eastern equatorial Pacific, allows the determination of the temporal and spatial scales of oceanic processes and of the resulting variability in the distribution and abundance of phytoplankton. Phytoplankton represent the first link in the food chain and their patterns of distribution in time and space may indicate how oceanographic processes regulate primary production.Method: A sequence of CZCS scenes was processed to derive images of chlorophyll-like pigment concentration coregistered to a uniform spatial grid covering the eastern equatorial Pacific region. Subsequently, these images were composited to produce seasonal mean pigment maps for the 1978–79, 1979–80 and 1982–83 winter periods. Finally a comparison of the results obtained for each of these three periods was made with reference to previous descriptive and modelling studies of the eastern equatorial Pacific environment.Results: Significant coherence in the distribution and abundance of phytoplankton was found, in both time and space, within each of the three periods considered. The primary production estimates from the CZCS data show a close agreement with those from ship sampling obtained in the same periods. The time/space composite images retain the major features observed during each period and appear to be the best means for quantifying the high degree of interannual variability evident from the imagery. This interannual signal was found to be greater than that observed over the shorter time scales involved in constructing the seasonal composites. Surprisingly, the largest variability occurred between the 79–80 period and the other two periods, i.e. 78–79 and 82–83, which actually have similar characteristics in spite of the El Nino event of the 82–83 period (refer to Figure 8.24). In the 79–80 winter, the area classified as productive habitat (pigment concentrations greater than 1 mg/cu.m) was about one order of magnitude larger than in the other two winters, reaching almost 30% of the study area versus 3–10% in 78–79 and 82–83 respectively. Therefore, the major question raised by this study does not revolve around El Nino, but rather in trying to understand the reasons for the variability between 78–79 and 79–80, since in these periods the conditions throughout the region have been characterized as being close to normal (refer to Figure 8.25).
Conclusion: This work demonstrates the potential of remotely sensed pigment measurements for the assessment of primary production and productive habitat extent on a regional or even global scale. For the eastern equatorial Pacific, there is evidence that there may be significant large-scale oceanic and atmospheric differences even when El Nino type phenomena are not active. Variations in the strength, location and timing of intensified undercurrent flows (e.g, the Equatorial and Peru Undercurrents) could alter the large-scale patterns of vertical mixing and nutrient input, i.e. upwelling along the Peru coast, thereby influencing phytoplankton production and the fish population sustained by this production. Perhaps the system is periodically purged. Low primary production during certain periods of time results in a significant reduction in the abundance of herbivores such as copepods and anchovies. The associated reduction in grazing pressure would then allow a large increase in planktonic abundance if accompanied by sufficient nutrient levels. A “boom and bust” type of cycle could then be established in the ecosystem of the region. The quantitative information derived from the satellite images allows the primary production to be estimated for the entire study area, as well as the production arising from specific regions. The repetitive character of the information makes it possible to follow the evolution of this production. Marine resource exploitation strategies may be identified byøm such analyses.ates the potential of remotely sensed pigment measurements for the assessment of primary production and productive habitat extent on a regional or even global scale. For the eastern equatorial Pacific, there is evidence that there may be significant large-scale oceanic and atmospheric differences even when El Nino type phenomena are not active. Variations in the strength, location and timing of intensified undercurrent flows (e.g, the Equatorial and Peru Undercurrents) could alter the
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