Relationships between Standing Crops at Three Successive Trophic Levels in the Eastern Tropical Pacific

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.