Optical measurements of phytoplankton size and volume concentration (LISST-100X): Applications and limitations
Lee Karp-Boss, Laura Azevedo and Emmanuel Boss
University of Maine, School of Marine Sciences
Contact:lee.karp-boss@maine.edu
Cell size is a fundamental property of phytoplankton
• Affects metabolic rates
• Determines predator-prey interactions and hence the structure of
aquatic food webs
• Determines sinking rates and hence the fate of primary production.
• Phytoplankton span a wide range of cell sizes (few mm-few mm)
Information on cell size distribution is central to the understanding of
many ecological processes, yet in-situ measurements of particle size
distribution (PSD) remains a challenge in oceanography. Recent
developments of laser scattering techniques open new opportunities to
study PSD in-situ.
Measurements of size and volume concentration of phytoplankton cultures: a comparison between LISST and microscopy
• LISST inversion to size was first tested with beads of known sizes (data not shown); filtered seawater (0.2mm) was used as a blank.
• Scattering measurements of phytoplankton cultures were inverted to obtain size and volume concentration, using the factory algorithm.
• Cells were counted and sized, using a microscope. Microscopy data were then used to calculate volumes and cross-sectional areas of cells.
• Diameters of spheres of equal volume and spheres of equal cross-sectional areas were then calculated for a comparison with the LISST inversion.
These diameter were than used in the microscopy based PSD.
Good agreement between LISST and microscopy for species of nearly spherical shape (within a factor of 2 for concentration)
Figure 1a
Figure 1a-e: Size distributions of phytoplankton
cultures. Red line represents sizes obtained by the
LISST-100X inversion. Microscopy derived size
distribution is plotted for two cases: (1) equal volume
spheres (blue line) and (2) spheres of equal crosssectional area (black line). Since scattering in the
geometric-optics limit is proportional to the particle’s
cross-sectional area one would expect the latter to show
a better fit to the LISST inversion. The choice of size
parameter becomes more important as particles deviate
from a spherical shape (Figure 1d).
Figure 1c
Figure 1b
Evaluation of in-situ Laser Diffraction Particle Size Analyzer
(LISST-100X) for size and concentration measurements of
phytoplankton
LISST measures near-forward light scattering by particles and
uses a Mie-theory based inversion to obtain the PSD. It does not
measure size directly.
The relationship between the measured scattered light and the
final PSD depends on inversion assumptions with respect to the
physical and optical properties of the particles
For complex cells and chains LISST and microscopy measurements deviate
Species
LISST “sees” cross-sectional areas of all cell orientations
Figure 1d
Figure 1e
G. simplex
# cells/ml
# cells/ml
(microscope) (LISST)
2330 ± 782
1584 (8<d<16 mm)
A. tamarense 96 ± 11.2
110 (25<d<35 mm)
241 ± 15
124 (35<d<70 mm)
L. polyedra
Table 1: a summary of cell concentrations obtained by
microscopy and the LISST inversion. The range of
particle diameters (d) for which cell concentration was
obtained is shown in brackets.
Physical measurement (scattering)
Inversion
(Mie theory)
Property descriptive of size (crosssectional area, volume)
Applications of the LISST-100X to phytoplankton
ecology: phytoplankton life histories
A.
B.
translation
Single length scale (SED)
• During vegetative cell division mean cell size of populations of diatoms
decreases over time.
LISST-100x, Sequoia Scientific, Inc. photo
courtesy of Jim Loftin.
• The underlying assumption in converting LISST measurements into PSD is
that particles are homogeneous spheres.
• Near-forward light scattering by particles is dominated by diffraction and
therefore is relatively insensitive to particle composition (e.g., index of
refraction)
• The LISST-100, which was originally designed for sediment studies, has been
used successfully to detect purple sulfur bacteria in lakes (Serra et al.
2001).
B
a
• Phytoplankton
display a wide range of cell shapes. Patterns of light
s
scattered
by non-spherical particles differ from those of spheres (Clavano
i
et al.
c submitted). Therefore, PSD obtained from inversions computed for
spheres may give poor estimate of the PSD and volume concentration of
phytoplankton.
References:
Serra et al. 2001. Journal of Environmental Engineering 127:1023.
Calvano et. al., Optical properties of non-spherical particles: from theory to observations. Submitted to
Oceanography and Marine Biology, an annual review.
Acknowledgements: we thank Jim Loftin, LeAnn Pritchard and Maura Thomas for assistance in the lab
and Dave Townsand for the use of his microscope. The Gulf of Maine foundation provided summer
support for L. Azavedo. This study was funded by NASA.
• Diatoms restore cell size via sexual reproduction or vegetative cell
enlargement
LISST shows great promise for life histories studies; it
provides a rapid, non-disruptive measurement - important
when delicate life stages are involved.
Same magnification (20X)
was used for all photos
Figure 2: Changes in the mean cell diameter in a culture of
Coscinodiscus radiatus as obtained from LISST inversion (A) and
microscopy (B).
Field studies and future prospect
• Phytoplankton of different cell sizes can be distinguished by the LISST
(Figure 3)
• Based on laboratory studies, the LISST shows great potential in
monitoring HABs, especially those of Gymnodinium breve and L. polyedra
for which blooms are often mono-specific.
• The LISST may be particularly useful in lake studies, for which coulter
counters are not applicable, and where Dinoflagellates (e.g., Peridinium)
often form mono-species blooms.
Further studies are required to evaluate its utility for monitoring
phytoplankton in the field.
Figure 3: A mixed culture
of G. simplex and A.
tamarense. LISST
inversion is shown by the
black line. Blue (G.
simplex) and red (A.
tamarense) lines show size
distribution based on
microscopy. Note: volume
concentration of
microscopy is based on
stock cultures.