Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1995
Description:
A number of investigators have observed diel variations in the bulk water
inherent optical property beam attenuation, with a minimum near dawn and a
maximum near dusk, and have assumed them to be caused by the phytoplankton. In
an attempt to understand these patterns, mean forward light scatter of different
populations of phytoplankton, from flow cytometric analysis of individual cells, was
determined and found to show similar diel patterns for populations of pico- and
nanophytoplankton measured in the equatorial Pacific and the Sargasso Sea. The cell
concentration patterns do not generally correspond to those of beam attenuation.
Laboratory experiments, combined with theoretical calculations, were undertaken to
attempt to account for the diel variations in beam attenuation observed at sea and to
estimate in situ phytoplankton group-specific growth and loss rates.
To investigate how cell growth and division affect the optical properties of
phytoplankton, cultures of the chlorophyte Nannochloris sp. were sampled over a diel
cycle to measure cell size and concentration, light attenuation and absorption, flow
cytometric light scattering (in forward and side directions), and carbon content. In
addition, the refractive index was calculated using the anomalous diffraction
approximation of Mie theory. At six different light levels ranging from 60 - 1500
μmol photons m-2 s-1 (specific growth rates from ~0.2 to ~0.7 d-1), cell division was
tightly phased to the light:dark cycle, occurring soon after dark. There were
pronounced diel patterns, with a minimum near dawn and a maximum near dusk, in
cell size and in cell-specific beam attenuation, absorption, flow cytometric light
scatter and carbon. The diel variations in the attenuation cross section were
primarily influenced by the changes in cell size due to growth and division, while
changes in refractive index had only a small effect. Because eukaryotic cells in the
size range of Nannochloris are major constituents of many phytoplankton
communities, these results have important consequences for the interpretation of diel
variations in optical properties observed in the ocean.
To interpret field data of diel variations in red beam attenuation, the
relationships between cell optical properties determined in the laboratory were
applied to three diel sampling experiments in the equatorial Pacific and four in the
Sargasso Sea. In the equatorial Pacific in April and October 1992, the phytoplankton
biomass was dominated by picophytoplankton (Prochlorococcus and Synechococcus),
and by mixed populations of ultraphytoplankton (1-2 μm diameter) and
nanophytoplankton (2-20 μm, mostly 2-3 μm). Flow cytometric measurements of mean forward light scatter of each of these populations showed typical diel patterns
which were similar to those of bulk beam attenuation due to particles, whereas cell
concentration changes were not. Using a combination of empirical calibrations
relating beam attenuation to flow cytometric measurements of pure cultures of
phytoplankton in the laboratory, and Mie theory, the contributions of different groups
of phytoplankton to the diel variations in beam attenuation were estimated. The
results indicate that the phytoplankton assemblage measured by flow cytometry can
account for essentially all of the diel variation in the beam attenuation signal. In
mo.st instances the nanophytoplankton were the largest contributor to total beam
attenuation due to phytoplankton, but the ultraphytoplankton usually were more
important in determining the diel variations in this property. Prochlorococcus were a
smaller but appreciable contributor to beam attenuation changes, and Synechococcus
were much less important.
A similar analysis was performed for diel sampling experiments in the
Sargasso Sea in January 1992, July 1993, and May 1994. The bulk beam attenuation
due to particles was strongly correlated with calculated beam attenuation due to
phytoplankton. During the July 1993 diel sampling, when pico- and nanophytoplankton
populations were analyzed, in most instances the nanophytoplankton
were the largest contributor to total beam attenuation due to phytoplankton, but
Prochlorococcus were equally important at 70 m for some time points over the diel
cycle. In the upper 40 m, Prochlorococcus were a smaller contributor to beam
attenuation changes than the nanophytoplankton, and Synechococcus were even less
important. These findings emphasize the need to characterize the composition of the
phytoplankton community in order to use beam attenuation to monitor productivity.
Flow cytometric measurements of phytoplankton light scattering and cell
concentration over the diel light cycle were used to estimate in situ phytoplankton
group-specific growth and loss rates in the equatorial Pacific and the Sargasso Sea.
Measurements of forward light scatter were converted to cell volume and carbon
using laboratory and theoretically derived calibration factors for specific groups of
phytoplankton, including Prochlorococcus, Synechococcus, ultraphytoplankton,
nan~phytoplankton, and coccolithophores. Assuming that division was phased,
specific growth rates were estimated based on volume and carbon changes between
minimum and maximum values over the day. Phytoplankton group-specific loss
rates, and also separate day and night loss rates, were estimated from the calculated
growth rates and measured cell concentrations over time. The method used to
estimate growth rates works well for Prochlorococcus and appears to work for small
eukaryotic phytoplankton, but leads to underestimates for Synechococcus and larger
eukaryotes. Estimated growth rates for Prochlorococcus reached one division per
day in the upper waters of the equatorial Pacific, but were about half that in the
Sargasso Sea. For the eukaryotic phytoplankton, growth rates in the equatorial
Pacific were near one division per day in the upper waters; in the Sargasso Sea, the
growth rates approached that, but were more often lower. In general, phytoplankton
growth rates were closely matched by in situ loss rates over the course of a day.
Description:
This work has been supported in part by: NSF (OCE-9012147 and OCE-
9024380), DOE (DE-FG02-93ER61693), ONR (N00014-83-K-0661 and 89-1-1110),
the Vetlesen Foundation, a WHOI Ocean Ventures Award, and the Education Office.
Keywords:
Phytoplankton
;
Circadian rhythms
;
Flow cytometry
;
Thomas G. Thompson (Ship) Cruise TN8
;
Thomas G. Thompson (Ship) Cruise TN12
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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