Diel chl a and phaeopigment cycles in a shallow tidal estuary: potential role of microzooplankton grazing

Abstract

A persistent diel cycle in chl a biomass was observed In the Newport Fbver estuary, North Carolina, USA d u n n g summer Chl a concentratlons were highest in the late afternoon (mean 22 pg I-') and least around dawn (mean 12 pg 1-l) The day to night change In chl a was paralleled by a commensurate decllne In volumetnc cell count Thls change In cell number was not correlated with wind resuspension, tldal dilution, or species composition differences, but rather is attributed to grazing A diel cycle in NH,' concentrat~on was also observed, due to temporal vanations in uptake relative to water column remlneralization Ammonium concentrations reached a maxlmum (> 2 u i v several hours after dawn, then decllned to a minimum ( < 0 5 p m by early afternoon as light-dependent uptake exceeded remlnerahzabon By late afternoon remineralization again exceeded uptake and NH,+ concentrahon increased continually throughout the rught In vltro ennchments demonstrated that the dayhme chl a increase was dependent on the early morrung uptake of nitrogen, whether In the form of N H 4 + , NO3-or urea Phaeopigment concentrahons also followed a diel pattern with an early afternoon minimum and a near-dawn maximum, mlrronng changes in chl a This pattern was due to photodestruction of phaeopigment dunng the day and accumulation at night Phaeopigment production was attributed to microzooplankton grazing When converted to chl a equivalents phaeopigment appearance accounted for 50 % of the chl a loss