Microbial food web structure in the Arabian Sea: a US JGOFS study

One of the main objectives of the Joint Global Ocean Flux Studies (JGOFS) p rogram is to develop an understanding of the factors controlling organic ca rbon production in the ocean and the time-varying vertical flux of carbon f rom surface waters (US JGOFS (1990) US JGOFS Planning Report Number 11; Sar miento and Armstrong (1997) US JGOFS Synthesis and Modeling Project Impleme ntation Plan). A considerable amount of evidence suggests that carbon cycli ng and the potential for exporting carbon from ocean systems is a function of food web structure. As part of the US JGOFS Arabian Sea Studies, the bio mass of planktonic organisms, ranging from heterotrophic bacteria through m icroplankton-sized organisms, was estimated using a variety of methods incl uding flow cytometry and microscopy. This is a first attempt to combine bio mass data from a number of sources, evaluate the structure of the food web, examine changes in food web structure in relation to seasonal or spatial f eatures of the study area, and look for indications of how changing structu re affects carbon-cycling processes. Biomass in the upper 100 m of the wate r column ranged from approximately 1.5 to > 5.2 gC m(-2). Heterotrophic bac teria (Hbac) made up from 16 and 44% of the biomass; autotrophs comprised 4 3-64%; and the remainder was made up of nano- and microheterotrophs. Autotr ophs and nano- and microheterotrophs showed a general pattern of higher val ues at coastal stations, with the lowest values offshore. Heterotrophic bac teria (Hbac) showed no significant spatial variations. The Spring Intermons oon and early NE Monsoon were dominated by autotrophic picoplankton, Prochl orococcus and Synechococcus. The late NE Monsoon and late SW Monsoon period s showed an increase in the larger size fractions of the primary producers. At several stations during the SW Monsoon, autotrophic microplankton, prim arily diatoms and Phaeocystis colonies, predominated. Increases in the size of autotrophs were also reflected in increasing sizes of nano- and microhe terotrophs. The biomass estimates based on cytometry and microscopy are con sistent with measurement of pigments, POC and PON. Changes in community str ucture were assessed using the percent similarity index (PSI) in conjunctio n with multidimensional scaling (MDS) or single-linkage clustering analysis to show how assemblages differed among cruises and stations. Station clust ering reflected environmental heterogeneity, and many of the conspicuous ch anges could be associated with changes in temperature, salinity and nutrien t concentrations. Despite inherent problems in combining data from a variet y of sources, the present community biomass estimates were well constrained by bulk measurements such as Chi a, POC and PON, and by comparisons with o ther quantitative and qualitative studies. The most striking correlation be tween Food web structure and carbon cycling was the dominance of large phyt oplankton, primarily diatoms, and the seasonal maxima of mass flux during t he SW Monsoon. High nutrient conditions associated with upwelling during th e SW Monsoon would explain the predominance of diatoms during this season. The sinking of large, ungrazed diatom cells is one possible explanation for the flux observations, but may not be consistent with the observation of c oncurrent increases in larger microzooplankton consumers (heterotrophic din oflagellates and ciliates) and mesozooplankton during this season. Food-web structure during the early NE Monsoon and Intermonsoons suggests carbon cy cling by the microbial community predominated. (C) 2000 Elsevier Science Lt d. All rights reserved.