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 River estuary, North Carolina, USA during summer. Chl a concentratlons were highest in the late afternoon (mean 22 pg I-1) and least around dawn (mean 12 pg 1-1) 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 was also observed, due to temporal vanations in uptake relative to A diel cycle in NH,' concentrat~on water column remlneralization Ammonium concentrations reached a maxlmum (> 2 uiv several hours after dawn, then decllned to a minimum (<0 5 pm 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 NH4+, 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 appear- ance accounted for 50% of the chl a loss.