Chlorophyll budgets: Zooplankton grazing and phytoplankton growth in a temperate fjord and the Central Pacific Gyres1

Abstract

The processes controlling the daily production and fate ofphytoplankton pigments were evaluated in a moderately productive fjord (Dabob Bay, Washington) and the oligotrophic open ocean (North and South Central Pacific Gyres). The processes included phytoplankton growth, macrozooplankton grazing, microzooplankton grazing, the downward vertical flux of pheopigments, photodegradation, dark degradation, cell sinking, cell senescence, and physical mixing. A model was constructed describing the dynamic budget of chlorophyll and pheopigments within the euphotic zone. The model provides estimates of phytoplankton growth, macrozooplankton grazing, and microzooplankton grazing based on field measurements of the vertical distribution of pigments, the vertical flux of pigments out of the euphotic zone, and the flux of solar radiation through the water column. The pigment-specific rates (d-l) provided by the model are derived in situ without manipulation of either the phytoplankton or zooplankton, and the experiments are largely container-free. Growth rates, averaged over the whole euphotic zone, varied seasonally in Dabob Bay (0.0549 d-l) and ca. 67% of the total daily grazing rate was due to macrozooplankton herbivores. Phytoplankton growth rates, averaged over the whole euphotic zone, were ca. 0.2 d-' in the oceanic gyres, regardless of season or location, and ca. 95% of daily grazing was due to microzooplankton herbivores. The model suggests that grazing and growth are in balance.Several processes affect the concentrations of chlorophyll and pheopigments in the euphotic zone. These include phytoplankton growth, zooplankton grazing, cell sinking, cell senescence, photodegradation, fecal pellet sinking, physical mixing, and advective transport. The rates of these processes are of broad general interest and knowledge of them is fundamental to understanding the causative factors underlying the dynamics of planktonic ecosystems.

A simple assay for chitin application to sediment trap samples from the subarctic Pacific

“…Sediment traps were spaced throughout the upper 500 m water column on floating trap arrays (Lorenzen et al 1983, Welschmeyer & Lorenzen 1985 and deployed normally for 3 d, although there was one experiment with a 3 h deployment. Sediment traps were closed by electronic messenger release and retrieved for sampling.…”

The origin of sedimenting detrital matter in a coastal system

“…An independent estimate of fecal pellet sedimentation was based on concentrations of pheopigments (Pha) in the sediment traps, as these pigments are considered to be associated with sinking fecal pellets (Welschmeyer and Lorenzen 1985). Pigments were extracted in 96% ethanol and measured spectrophotometrically according to Lorenzen (1967) by using an absorption coefficient for Chl a in ethanol of 83.4 1 g-l cm-' (Wintermans and DeMots 1965).…”

The effect of food concentration on fecal pellet size in marine copepods1

“…The size composition of the grazer community can therefore affect the transport rate of material out of the euphotic zone; small pellets have a higher probability of being recycled in the euphotic zone because they sink slowly. In addition, small pellets are in the right size range to be captured by particle-grazing copepods (Paffenhiifer and Knowles 1979 sis of these general relationships between body size and pellet size, and pellet size and sinking rate, information on the size composition of the grazer community has been deduced from the size composition of the fecal material collected in the water column and in sediment traps (Welschmeyer and Lorenzen 1985).…”