Meereswissenschaftliche Berichte No 1 1990 - Marine Science Reports No 1 1990
http://doi.io-warnemuende.de/10.12754/msr-1990-0001
doi:10.12754/msr-1990-0001
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Die Reaktion des Mesozooplanktons, speziell der Biomasse, auf küstennahen Auftrieb vor Westafrika = The Mesozooplankton Response to Coastal Upwelling off West Africa with Particular Regard to Biomass

Postel, Lutz

Abstract. The coastal upwelling regions in the paths of the Canaries and Benguela Currents are characterized by large amounts of zooplankton. Economic considerations relating particularly to fisheries and marine geology led to an upsurge of exploratory activity in these areas during the seventies. The CINECA (Cooperative Investigations of the Northern Part of the Eastern Central Atlantic) programme alone involved 14 countries and around 100 research cruises between 1970 and 1977, including eight by the r/v 'Alexander von Humboldt' of the GDR's Academy of Sciences. In 1976 and 1979, the region investigated by this vessel was extended to include the sea area off Namibia During the 'Meteor' expedition from 1925 to 1927, HENTSCHEL attempted to concentrate on the quatitative apsects of plankton research in order to identify factors governing the variations in time and space of the 'properties of life'. Complete comparatibility of the methods used was regarded as an indispensible condition for such studies. Owing to the large number of separate studies undertaken by differen institutions, this requirement has still not been fulfilled in the West African upwelling regions. Since 1970, the r/v 'Alexander von Humboldt' has used the WP-2-standard net type to collect mesozooplankton (0.2 to 10 mm body length) distribution data based on dry weight. Such data are now available in space-time scales ranging from hundreds of metres and minutes to several thousand kilometres and years. In the present work, these data are used to investigate the characteristic effects of coastal upwelling on zooplankton biomass (ZPB) after a single upwelling event off Namibia and a whole upwelling season off Northwest Africa and their dependence on water depth and distance from the shore. Measurements in the Central Atlantic north of the equator permitted comparison with conditions in which the absence of a biomass surplus indicated the climax of a planktonic community. The ageing of the water in the direction of the current permitted the development of the plankton succession and biomass to be observed during upwelling events. The transformation of the space axis into a time axis and the estimation of the cumulative seasonal biomass increase allowed the calculation of net growth rates. Initial considerations were devoted to the filter effects influencing the objective identification of distribution patters, such as: - net selectivity in terms of the biomass of the small mesozooplankton relative to that of the plankton as a whole, as taxonomic groups, as feeding types and their development times, and the importance of the net fraction as prey for commercial fish species, - errors associated with field and laboratory conditions, and - sampling errors. The main results are: 1. The samples yielded by the standard WP-2 UNESCO net represent about on third of the total plankton in the top 200 m of the water column in the coastal upwelling region off West Africa. This fraction consists of almost identical proportions of fine filter feeders, coarse filter feeders and carnivores with development times ob between 20 and 40 days. It reflects the food spectrum of commercial fish species to varying degrees, i.e. to 70 % in the case of Scomber colias, 60 % for Trachurus spec. and 50 % for Sardinella spec. These estimates are based on a comparison of plankton body lengths with the length ranges within which the organisms were quantitatively represented in the catches. 2. Measuring errors can be broken down into the categories 'deterministic (constant and quasi-constant)' and 'random'. The deterministic errors varied between -48 % and 21 %. The values were used with opposite signs to ensure the homogeneity of the data. The highest losses arose when cold-stored samples were defrozen and the supernatants decanted before drying. Random errors varied between ±2 and ±13 %. They were highest when a plankton splitter was used. 3. Sampling errors may stem from the use of finite measuring series and discrete sampling. In the former case, interanual variations had to be considered off Northwest Africa, but these are probably minimal because all investigations were performed during a positive Ekman transport anomaly (e.g. off Cap Blanc, ø = 20° 15' N). The diurnal vertical plankton migration was regarded as a sampling error of the latter category and assumed to be a fixed phase process. Except in two cases, its influence could be ignored in the oceanic region off Northwest Africa, possibly owing to the damping effect of the vertically integrated sampling method. A negative anomaly existed for the whole duration of the study. Other meso and coarse scale energy rich signals were attenuated during the consideration of mean values. In the cross-shore vertical distributions, the eddy influence on the zooplankton biomass was of the order of magnitude of the upwelling signal. 4. On average, the zooplankton biomass doubled after an upwelling event. In the EKMAN layer off Southwest Africa, a maximum increase from about 40 to 80 mg*m^-3 was recorded from the locality of the upwelling to about 130 - 160 km from the coast across-shore. Off Northwest Africa, the onset of the upwelling season was signified by values of about 20 mg*m^-3 from 75 to 200 m depth. On the shelf, the coming upwelling season was signalled by values exceeding 40 - 50 mg*m^-3. 5. In the water column down to a depth of 200m, the net growth rate off Northwest Africa increased from 14 mg*m^-3*month^-1 in the coastal zone (H < 75 m) to 38 mg*m^-3*month^-1 near the shelf edge and then decreased in the oceanic region <20° W to mg*m^-3*month^-1. There it decreased from 29 (0 < z < 25 m) through 12 (25 < z < 75 m ) to 7 mg*m^-3*month^-1 (75 < z < 200 m) as the depth increased. The analogous vertical break-down is not available for the shelf region. Comparison of the results for Southwest Africa with corresponding Northwest African data taken from the literature, the net growth rates are the same in both regions. Off Namibia, 1.56 mg*m^-3*d^-1 and 47 mg*m^-3*month^-1 in the top 75 m from the upwelling site to the biomass maximum. 6. According to the net growth rates, the year-round upwelling of nutrient-rich water leads to a cumulative increase in mean zooplankton biomass to 250 mg*m^-3 in the coastal zone (H < 75 m) off Northwest Africa, 500 mg*m^-3 near the sheld edge and 100 mg*m^-3 in the oceanic region up to 20° W, where the vertical gradation is 400, 150 and 30 mg*m^-3. Drastic deviations from these values indicate local peculiarities. For instance, a 2.5-fold positive anomaly was found over the broad, shallow shelf off Cabo Verga. An apparent 1.5-fold 'loss' in the shelf edge region off Cap Blanc was accounted for as the calculated food requirement of chub mackrel. 7. Seasonal variations were observed particularly seaward of the shelf. Off Northwest Africa, the elevated biomass zone in this area extended southward for 15 degress of latitude from 24° N, vertically to a depth of 200 m, and seaward for a distance of over 400 km in the first half of the year. In October/November, the zone contracted and was limited to between 20° N and 22° N, the top 25 m of the water column and a distance of 100 to 200 km from the coast. 8. Off Southwest Africa, the biomass in the top 75 m of the water column peaked 23 days after the upwelling event. At greater depths, the lag increased to 8 - 10 weeks at z < 200 m in the oceanic region off Northwest Africa. 9. North of Cap Blanc, the upwelling water consisted predominantly of nutrient-rich North Atlantic central water (NACW). In this area, the ZPB response typical of predominantly nutrient-rich South Atlantic central water (SACW) did not materialize. The biomass differences reflected the position of the internal NACW/NASW water front at the water surface. In the oceanic part of the investigation area, its position changed distinctly from 24° N in the first half-year to 21/22° in the second. 10. A region of considerable biomass variability that was independent of coastal upwelling activity coincided in space with an active eddy zone seaward of the shelf edge north of the Cape Verde Islands. The ZPB in this region was higher than that of the Central Atlantic for almost the whole year. Horizontal gradients were expressed in an extreme case in a decrease from 257 to 3 mg*m^-3 in a distance of less than 40 km. ZPB and nutrient concentrations correlated negatively in sample cyclonic and anticyclonic eddies with diameters of 40 to 70 km. 11. The absolute maximum of 1096 mg*m^-3 was recorded seaward of the shelf in the top 25 m of the water column in the sea region off Cap Blanc (ø = 20° 55' N) during the main upwelling season in May (1974). In February (1976), when the northern boundary of the trade wind zone was reached, oceanic conditions predominated even over the shelf off Bahia de Garnet (ø = 25° N). 12. During the period of most intensive EKMAN transport (June), downwelling in the shelf edge region led to a reduction in biomass to values below those typical of upwelling activity in the area affected by the cross circulation of the Canaries Current (ø > 20° N).

Citation

Dietwart Nehring: Die hydrographisch-chemischen Bedingungen in der westlichen und zentralen Ostsee von 1979 bis 1988 - ein Vergleich. Meereswiss. Ber., Warnemünde, 2 (1990), doi:10.12754/msr-1990-0002 Dietwart Nehring & Wolfgang Matthäus: Aktuelle Trends hydrographischer und chemischer Parameter in der Ostsee, 1958 - 1989. Meereswiss. Ber., Warnemünde, 2 (1990), doi:10.12754/msr-1990-0002

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