Spatial and temporal variability in plankton size structure along biogeochemical gradients in the Pacific Ocean

T. M. Clemente, M. J. Church, D. M. Karl

Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

Abstract

The size structure of upper ocean plankton assemblages appears to play an important role in determining the efficiency of the biological carbon pump and the resulting magnitude of biologically-mediated carbon export to the deep sea. To evaluate temporal dynamics in the size structure of plankton populations in the upper ocean of the Pacific Ocean, we have examined time series measurements on size fractionated chlorophyll and primary production at Station ALOHA in the ollgotrophic subtropical North Pacific Ocean. Based on these monthly observations, photoautotrophic biomass and productivity in the upper ocean waters at Station ALOHA appear to be consistently dominated by picoplankton (<2 µm in diameter). Overall, the contribution of nano- and microplankton (>10 µm diameter) to photoautotrophic biomass and production was relatively low, but demonstrated considerable temporal variability. Normalization of the size fractionated rates of primary production to size fractionated chlorophyll a suggests that on average, the larger plankton size classes (>10 µm) grow more rapidly than the numerically dominant picoplankton assemblages. As such, these larger plankton populations may be able to respond more rapidly to nutrient input to the upper ocean, thereby exerting a large influence on new production at Station ALOHA. To evaluate spatial variability in upper ocean plankton size structure, measurements of size fractionated chlorophyll and ATP concentrations were assessed during a transect cruise from American Samoa to Honolulu, Hawaii in March 2005. Sampling for this study spanned several biogeochemical provinces including the oligotrophic South Pacific subtropical gyre, the relatively nutrient-enriched Equatorial upwelling regions, and the oligotrophic North Pacific subtropical gyre. In general, the contribution of larger plankton size classes increased along nutrient gradients, with smaller plankton assemblages dominating the oligotrophic open ocean ecosystems. Such results further support the hypothesis that larger, presumably eukaryotic plankton populations are regulated by nutrient availability and thereby constitute an important source of carbon export in open ocean ecosystems.