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Relationships between Standing Crops at Three Successive Trophic Levels in the Eastern Tropical Pacific
|Title:||Relationships between Standing Crops at Three Successive Trophic Levels in the Eastern Tropical Pacific|
|Issue Date:||Jan 1966|
|Publisher:||University of Hawai'i Press|
|Citation:||Blackburn M. 1966. Relationships between standing crops at three successive trophic levels in the eastern tropical Pacific. Pac Sci 20(1): 36-59.|
|Abstract:||Measurements of the following standing crops were made at each of
several pairs of stations on various cruises in the eastern tropical Pacific: (a) chlorophyll
a, mg/m^2, 0-100 m; (b) zooplankton (total, and, for some cruises, copepods
separately), ml/10^3m^3, 0-300 m; (c) small fish and cephalopods, combined, from
net-caught micronekron, ml/10^3m^3, 0-90 m. These were considered to represent
plants, herbivores, and primary carnivores. It was estimated that most of the zooplankton
was located at 0-140 m. The stations of each station-pair were separated
by less than 120 miles and 36 hr.
Relationships between the logarithms of crops were investigated by simple correlations, partial correlations, and structural two-variable linear regressions. In
the 36 station-pairs available from cruises made in the northern spring, both zooplankton
and carnivores had a significant positive regression on chlorophyll a; the
points for 11 of these pairs fell within or close to the 95% confidence limits of each
of the regressions. For these 11 "statistically selected" pairs all simple correlation
coefficients were positive and significant, the partial correlation coefficient of chlorophyll
a and zooplankton was positive and significant, and the other two partial
correlation coefficients were non-significant. These results were considered to be
consistent with steady-state conditions between the three standing crops. A similar
analysis using copepods instead of total zooplankton gave a generally similar result.
Chlorophyll a and primary productivty (by the C14 method) were positively and
significantly correlated at 19 stations where both measurements were taken.
Most of the station-pairs for which these results were obtained were located in the
area bounded by 5oN, 95oW, 12oN, and the American coast (excluding the Costa
Rica Dome). This is a moderately eutrophic area, where a steady state might
not have been expected; however, there are indications that the process of eutrophication, which probably is vertical mixing of the upp er part of the very shoal thermocline
(<30 m) by wind, is itself fairly steady throughout the year. No definite
indications of a steady state were obtained from any other area at any season; however,
the possibility of obtaining them from more copious material is not denied .
The regression (slope) coefficients showed that standing crop of herbivores varied
as some power <1.0 of standing crop of chlorophyll a, suggesting increasingly inefficient
utilization of plants by herbivores with increase of plant standing crop. On
the other hand the crop of carnivores varied in an approximately linear way with
that of herbivores. The standing crop ratios, copepods/plants (by weight of carbon)
and carnivores/zooplankton (by displacement volume), were both roughly estimated
at 0.04 under steady-state conditions; for various reasons the corresponding
food-chain efficiency ratios, for standing crops of all material at the appropriate
trophic levels, would be higher.
|Appears in Collections:||Pacific Science Volume 20, Number 1, 1966|
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