Theory suggests that variation in the attractive solvation force associated with cell-surface hydrophobicity can significantly affect contact rates among small cells in aqueous environments and consequently may influence rates and selective impacts of marine nanoflagellate grazers feeding on picoplankton assemblages. To investigate this hypothesis, we assayed the natural range in hydrophobic characteristics of subtropical picoplankton from the oligotrophic subtropical Pacific (Station Aloha, 22Њ45ЈN, 158ЊW) and mesotrophic Kaneohe Bay, Hawaii, using hydrophobic interaction chromatography (HIC) in conjunction with analytical flow cytometry. Variability in a relative index of cell-surface hydrophobicity (HIC index) for heterotrophic bacteria, Prochlorococcus and Synechococcus, exhibited some consistent spatial patterns. The HIC index for Prochlorococcus at Station Aloha varied about threefold, being consistently more hydrophobic in the upper 80 m of the water column and dropping abruptly below this depth. Heterotrophic bacteria were more hydrophobic near the surface and decreased slightly, but steadily, with increasing depth. The hydrophobicity of heterotrophic bacteria steadily increased along a Kaneohe Bay transect extending from oligotrophic to mesotrophic conditions. In experiments involving nanoflagellates grazing on laboratory cultures of Prochlorococcus, cell cultures exhibiting the highest HIC indices were grazed upon at the highest rates. An additional experiment involving mixtures of Prochlorococcus cells exhibiting high and low hydrophobicities showed that the average hydrophobicity of the uningested prey mixture was driven progressively toward lower hydrophobicity as the more hydrophobic cells were selectively removed through time. If these laboratory grazing results hold in nature, the rate at which picoplankton cells are cleared from suspension by nanoflagellates could vary by as much as twofold due solely to natural variation in cell surface hydrophobicity.

Decadal-scale regime shifts in Northwest Atlantic shelf ecosystems can be remotely forced by climateassociated atmosphere-ocean interactions in the North Atlantic and Arctic Ocean Basins. This remote climate forcing is mediated primarily by basin-and hemispheric-scale changes in ocean circulation. We review and synthesize results from process-oriented field studies and retrospective analyses of time-series data to document the linkages between climate, ocean circulation, and ecosystem dynamics. Bottom-up forcing associated with climate plays a prominent role in the dynamics of these ecosystems, comparable in importance to that of topdown forcing associated with commercial fishing. A broad perspective, one encompassing the effects of basin-and hemispheric-scale climate processes on marine ecosystems, will be critical to the sustainable management of marine living resources in the Northwest Atlantic.

We investigated the accuracy and precision of flow cytometric (FCM) estimates of bacterial abundances using 4',6-diamidino-2-phenylindole (DAPI) and Hoechst 33342 (HO342, a bisbenzamide derivative) on paraformaldehyde-fixed seawater samples collected from two stations near Oahu, Hawaii. The accuracy of FCM estimates was assessed against direct counts by using epifluorescence microscopy. DAPI and H0342 differ in two aspects of their chemistry that make H0342 better suited for staining marine heterotrophic bacteria for FCM analysis. These differences are most important in studies of open-ocean ecosystems that require dual-beam FCM analysis to clearly separate heterotrophic bacterial populations from populations of photosynthetic Prochlorococcus spp. Bacterial populations were easier to distinguish from background fluorescence when stained with H0342 than when stained with DAPI, because H0342 has a higher relative fluorescence quantum yield. A substantially higher coefficient of variation of blue fluorescence, which was probably due to fluorescent complexes formed by DAPI with double-stranded RNA, was observed for DAPI-stained populations. FCM estimates averaged 2.0 and 12% higher than corresponding epifluorescence microscopy direct counts for H0342 and DAPI-stained samples, respectively. A paired-sample t test between FCM estimates and direct counts found no significant difference for H0342-stained samples but a significant difference for DAPI-stained samples. Coefficients of variation of replicate FCM abundance estimates ranged from 0.63 to 2.9%o (average, 1.5%) for natural bacterial concentrations of 6 x 105 to 15 x 105 cells ml-'.

We examined the interannual variability in the timing and magnitude of seasonal phytoplankton blooms in the North Atlantic (70ЊN-10ЊN, 90ЊW-10ЊE) in relation to variability in wind forcing during the bloom period using satellite data from 1998 through 2004. When averaged over the period extending from 1998 to 2004, seasonal increases in phytoplankton in the subpolar North Atlantic were observed predominantly during the spring, while the increases occurred between autumn and winter in the subtropical region. The major modes of interannual variability in bloom timing and magnitude from empirical orthogonal function analysis exhibited large spatial coherency across the North Atlantic. These patterns of variability can be explained, in large part, by the pattern of interannual variability in bloom-period wind mixing. Although convection is important in the seasonal development of blooms, wind-induced mixing during the bloom period appeared to be the key forcing agent contributing to interannual variability in the intensity of blooms. Increased wind-induced mixing during the bloom period reduced bloom magnitude over the subpolar and northern subtropical regions while enhancing it over the southern subtropical region. Atmospheric variations associated with interannual variability in wind mixing during the bloom period also appeared to affect variability in the timing of bloom onset. The major mode of interannual variability in the timing of North Atlantic blooms indicates a possible link to the North Atlantic Oscillation.

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