The dynamics of the microbial plankton community of Kaneohe Bay, Hawaii were investigated in September 1982 using in situ diffusion chambers and dilution manipulations. Total community carbon at the time of the experiments was estimated at 86 pg C 1-' of which Chlorella sp. accounted for 47 %, autotrophic microflagellates 14 %, chroococcoid cyanobacteria 11 %, and heterotrophic microflagellates and bacteria each 9 %. Instantaneous growth rates ranged from 1.2 to 1.9 d-' and 1.4 to 2.0 d-l, and mortality rates varied from 0.5 to 1

No abstract

Plankton biomass, material fluxes, e.g. 14C uptake, and specific growth rates are related quantities. In the course of comparing various methods of measuring these properties in September 1982 off Oahu, Hawaii, we found specific growth rates of l-2.d-I. Such rates approach the maximum expected values observed in laboratory cultures.

The contribution of picoplankton (0.2 to 3.0 pm) to phytoplankton biomass and production was examined in a tributary of the Chesapeake Bay, USA, during summer 1985. The water column in thls tributary oscillates with the spring-neap tidal cycle (homogeneously mixed and stratified, respectively) whlch permits observation of the effects of water column stabdity on phytoplankton dynamics. Picoplankton made up 7 4b of the seasonal autotrophic biomass, with a peak contribution of 10 to 14 % in the first half of July. Phycocyanin-rich chroococcoid cyanobacteria, visible only with fluorescence rnicroscopy using green light excitation (510 to 560 nm), numerically dominated the picophytoplankton. being 8 x more abundant than the phycoerythrin-containing cyanobacteria. Together these 2 cyanobacteria types represented 51 % of the picophytoplankton biomass, and exhibited a pronounced fortnightly cycle in abundance which coincided with the spring-neap tidal cycle. Picoplankton were responsible for 9 O/ O of the primary production at 293 pE m-2 S-' and 13 % at 28 to 43 pE m-2 S-', averaged over the study period. Chlorophyll-specific uptake rates by the larger autotrophs (seasonal mean, 5.2 pg C pg chl a-' h-') were significantly higher than the picoplankton (seasonal mean, 2.5 pg C kg chl a-' h-')at 293 pE m-' S-', but not at 28 to 43 pE m-' S-'. A simple model based on spring-neap, tidally-induced oscillations in mixed layer depth, and its regulation of light availablhty, produced cycles in cyanobacterial growth rates which could partially explain the observed cycles in abundance. These results emphasize the importance of physlcal processes occurring on time scales of days or weeks in regulating plankton biomass and primary production in estuarine environments.

The deep chlorophyll maxlmum (DCM) IS a widespread feature In the subtropical gyre of the North Paclflc In mid-August 1985 chlorophyll concentration increased In the DCM, then dechned over time at 95 to 110 m depth near 28"N 155"W During the higher chlorophyll perlod the DCM lay w~t h i n the top of the nitracline Later, as the chlorophyll concentratlon decllned and the DCM deepened, following the 24 9 slgma-t surface, the maxlmum was above the nitracllne The DCM was also a maxlmum In abundance of autotrophlc flagellates Particulate nltrogen was retained over time in the euphotic zone relative to particulate carbon and chlorophyll Phytoplankton growth rate at 90 to 110 m depth averaged 0 11 d-' and phytoplankton carbon was about 10 pg I-', about 50 % of the total paruculate carbon INTRODUCTIONThis is a report of observations on the deep chlorophyll maximum (DCM) region mid-August to early September, 1985, at 28"N 155"W DCM's are thought to result from in situ growth of phytoplankton at low Light levels near the base of the euphotic zone where growth is dependent both upon regenerated nutrients and on nitrate diffusing upward across the nutricline (Fasham et al. 1985). The DCM in the North Pacific is continuous across the ocean basin at depth above the nutricline (Venrick et al. 1972, Venrick 1979. The feature has been s t u d e d in considerable detail at the CLIMAX (S10 1974) station, 28"N 155" W, where it is found at about 100 m depth. Studies in the 1970's suggested it was neither a phytoplankton biomass (Beers et al. 1975(Beers et al. , 1982) nor a primary production maximum. Apparently the elevated chlorophyll reflected a photoadaptive response of the phytoplankton, altering the chlorophyll/biomass ratio (Eppley et al. 1973