JOURNAL OF PLANKTON RESEARCH | VOLUME 9 | NUMBER 1 | PAGES 215-234 | 1987
© Oxford University Press
research-article |
Water-column chlorophyll in an oligotrophic environment: correction for the sampling depths and variations of the vertical structure of density, and observation of a growth period
Centre ORSTOM BP A5, Noumea, New Caledonia
Received on April 1, 1986; accepted on October 1, 1986 The chlorophyll content of a water column (WCC), which is commonly used as an index of the phytoplankton abundance, is affected by the choice of the sampling depths and by the variations of the vertical structure of density. For instance, the thickness of the water layer, between two sigma-t values, which contains the deep chlorophyll maximum, can vary with internal waves. The resulting noise often dominates the mesoscale variations of the observed water-column chlorophyll (OWCC). Sigma-t dependent statistics (mean, standard deviation) of the chlorophyll concentration are computed using the observations at 29 casts from a 22-day-long fixed station in an oligotrophic environment at 15•S, 173°E. For each cast, these statistics, the sampling depths, and the water density at these sampling depths, allow the estimation of a station-dependent ‘expected water-column chlorophyll’ (EWCC). The ratio of EWCC to the overall likelihood of WCC during the fixed station (i.e. the mean of all OWCC) is a measure of the effect of sampling and variable density structure at each cast. When this effect is removed, the noise in WCC estimates decreases significantly. The time variations of WCC during the fixed station then show a trend with relatively high values during the first days, followed by a 12-day-long period with low values. A regular increase occurred from 1 October, which was accompanied by high carbon fixation rates and was mainly due to an increase of the chlorophyll concentration between the surface and the deep chlorophyll maximum. New production during this active phase was estimated to be 535 mgC m–2 day–1, corresponding to 62% of the total production. Breaking of internal waves which were recorded at the beginning of the growth phase and vertical mixing of nutrients can explain the observation.