PHYTOPLANKTON PRODUCTION AND CHLOROPHYLL CONCENTRATION IN THE BEAUFORT CHANNEL, NORTH CAROLINA 1

A yearlong study was conducted on phytoplankton at the mouth of a shallow estuary. Photosynthesis had a pronounced seasonal cycle, without major irregularities, that followed the cycle in water temperature. Gross photosynthesis at the surface ranged from 0.12 to 0.72 mg C day −1 . Daily photosynthesis at 50, 25, and 10% of surface illumination averaged 104, 85, and 58% of the surface value. Respiration ranged from 0.01 to 0.29 mg C liter −1 day −1 and averaged 40% of surface gross photosynthesis. Annual gross production was 113 g C/m 2 for a 1.0 m water column (average depth of the estuary) and 225 g C/m 2 for the entire euphotic zone. Annual respiration was 0.39 g C/m 3 . This low production was ascribed to shallowness and turbidity. The phytoplankton was chiefly nannoplankton (centric diatoms) and averaged 2 × 10 6 cells/liter. Nannoplankton also contained most of the chlorophyll a and produced most of the respiration and photosynthesis. Chlorophyll a ranged from 2.0 to 9.3 µ g/liter, and averaged 3.6 µ g/liter from June to October (the period of high water temperatures and high production) and 4.8 µ g/liter the remainder of the year. The ratio of photosynthesis to chlorophyll a (at the surface during daylight hours) ranged from 1.9 to 19.8 mg C (mg Chl a ) −1 hr −1 and followed water temperature. Values were predicted by the equation: yielding a Q 10 of 2.25 for the ratio of photosynthesis to chlorophyll a .