Estimating the grazing pressure of heterotrophic nanoplankton on Synechococcus spp. using the sea water dilution and selective inhibitor techniques
Marine Ecology Progress Series
Marine, phycoerythrin-containing Synechococcus spp. are now recognized as a major component of the photosynthetic biomass in oceanic waters. Although these picoplankters potentially are important food sources for heterotrophic nanoplankton, the cycling of Synechococcus biomass has not yet been well described. Two approaches recently proposed for measuring grazing pressure by heterotrophic nanoplankton (2 to 20 vm) were used to measure specific grazing rates on Synechococcus spp.: (1) the sea
... pp.: (1) the sea water dllution culture technique of Landry & Hassett (Mar. Biol. 67: 283-288); (2) the selective inhibitor method of Fuhrrnan & McManus (Science 224: 1257-1260). Several samples were collected at coastal and oceanic Northwest Atlantic sites and grazing estimates by the 2 methods were compared. Grazing coefficients, g (d-l), ranged from 0 to 0.35 d -' at an oceanic warm core eddy (WCE) station, but were greater (up to 0.83 d-l) at the coastal stations. At the coastal station, both methods yielded similar rates, but at the WCE station, where Synechococcus spp. abundance was low, only the selective inhibitor produced significant grazing estimates. Sample dlution may have reduced cell numbers below a grazing threshold. In such cases, the inhibitor method may provide more reliable measurements. Using the grazing coefficients from these experiments, consumption of Synechococcus biomass was compared w t h production by Synechococcus populahons. At the WCE and Long lsland Sound stations, 37 to 52 % of the Synechococcus biomass produced was consunled by grazers, whereas at the coastal Gulf of Maine station the biomass produced appeared to be balanced by the biornass consumed.