Seasonal contrasts in diurnal carbon incorporation by phytoplankton size classes of the coastal plume of Chesapeake Bay

Abstract

Diurnal variations in patterns of carbon fixation by phytoplankton size classes in the coastal plume of the Chesapeake Bay were compared during seasonal extremes in freshwater flow and nutrient discharge. The diurnal mean chlorophyll-specific productivity (PB) for net and nanoplankton was correlated closely to temperature with the amplitude of diurnal variation in PB being minimal in April, higher in June, and maximal in August for both size classes. However, nanoplankton PB increased more rapidly and was significantly higher than netplankton pB in August. Biomass carbohydrate/protein ratios were consistently higher in netplankton than in nanoplankton and from April to August the magnitude of the ratio decreased for both size classes (2.35 to 1.23 and 0.85 to 0.58, respectively). Carbon incorporation into biochemical fractions was significantly correlated with productivity. Carbon incorporation into protein and polysaccharide exhibited greater diurnal periodicity during June and August when PB was correlated to solar radiation, respectively decreasing and increasing with increas- ing radiation during both months and in both size classes. Diurnal polysaccharide synthesis was highest relative to productivity in nanoplankton during June when nanoplankton dominated the phytoplankton biomass, and likewise, netplankton polysaccharide synthesis was highest during August when net- plankton dominated. Results suggest that these high diurnal rates of polysaccharide synthesis set the stage for higher rates of nocturnal protein synthesis and biomass production by nanoplankton during June and by netplankton during August.