A multivariate analysis of planktonic interactions in the eastern tropical North Pacific

Abstract

Biological and hydrographic properties were sampled in the eastern tropical North Pacific (DOMES region) in two seasons, late summer (29 August to 13 October) 1975 and winter (18 February to 13 March) 1976. Differences between cruises and spatial patterns were summarized with factor analysis and non-metric multidimensional scaling and then analyzed in detail by other statistical techniques. Significant between-cruise difference were detected in the concentrations of nitrate and suspended particulate matter in the surface mixed layer and in vertically integrated standing stocks of microzooplankton and day macrozooplankton. Integrated chlorophyll a also increased in winter, but the productivity: chlorophyll a ratio decreased. The near-surface phytoplankton community appears to be more shade-adapted in winter. Pronounced differences in correlation patterns were observed in 1975 and 1976. In winter 1976, integrated primary production rates varied inversely with the nitracline depth and positively with mixed-layer nitrate concentration, sea-surface irradiance (PAR), and integrated chlorophyll a, while zooplankton standing stock covaried with mixed-layer nitrate concentration ( P ⩽ 0.05). In contrast, none of the correlations was significant in summer 1975 ( P > 0.10). In 1975, short-term (30 to 60 h) variations in surface chemical properties and in integrated chlorophyll a were greater than in 1976. The weaker correlations observed in 1975 may have been influenced by the apparent increase in variability. The vertical supply of nitrate, phytoplankton growth, and zooplankton grazing appear to have been more closely coupled in winter 1976 than in summer 1975.