534
Limnol.
Oceanogr.,
25(3), 1980,534-537
@ 1980, by the American
Society of Limnology
The effects of transect
patterns of chlorophyll
Notes
and Oceanography,
Inc.
direction on observed
in Lake Tahoe1
Abstract-Horizontal
transects of fluorescence measurements
have been used extensively to investigate
phytoplankton
patchiness. Variance sDectra have been calculated
from these data,thus
quantifying
spatial heterogeneity as a function of length scale. Analysis of such fieldwork
and associated theoretical investigations
is based on the assumntion
that horizontal
patchiness is isotropic. Three
transects done in Lake Tahoe (California-Nevada) in different directions
on the same day
resulted in variance spectra that were significantly different
from each other. The possibility of windrows appears consistent wi;h the
spectra. Although the exact cause of the differences is unclear, the results do violate the
assumption
of isotropic
horizontal
phytoplankton patterns.
Fluorometric
techniques
have been
used to study spatial p&terns of phytoplankton in a wide range of aquatic systems (e.g. Powell et al. 1975; Denman
and Platt 1975; Fasham and Pugh 1976;
Lekan and Wilson 1978). The technique
involves
constant-depth
transects,
recording fluorescence
(as an indicator of
chlorophyll
a concentration)
continuously. These transects routinely
include
thousands of points. Spectral analysis has
been the primary method of reducing
such data by partitioning
the variance in
the record to different length scales (for
a short description of spectral analysis see
Platt and Denman 1975; Denman 1975).
Information
like this is important in understanding the way biological and physical processes interact to produce phytoplankton
patches of varying sizes and
intensities (Steele 1978).
The resulting variance spectrum is a
particularly
useful tool since it quantifies
spatial variability
as a function of length
scale. Theoretical
work by Kierstead and
Slobodkin
(1953) and Denman
et al.
(1977) indicates that at scales less than
several hundred
meters phytoplankton
l This work was supported by NSF-DEB76-20341
(Ecology Section, National Science Foundation).
spatial
distributions
are governed by turbulent
water motions, while at larger scales biological processes are important. The effect of these processes occurring at different scales is to affect the spectral
slopes at length scales where a particular
process is dominant. This is analogous to
studies of the turbulent
kinetic energy
spectrum, where particular slopes of the
spectrum are associated with particular
physical
processes
(e.g. Gibson
and
Schwarz 1963). Fasham (1978) has developed spectral
models
that describe
changes in biological
conditions.
Under
bloom conditions, the spectrum has a different slope at large scales than under
postbloom conditions. The predictions of
this model are generally consistent with
data collected
in the ocean (Horwood
1978) and in lakes (Leigh-Abbott
1978).
The slope of the variance spectrum is
thus important in interpreting
both field
and theoretical results.
One of the assumptions of this field and
theoretical
work is that the horizontal
spatial patterns are isotropic.
That is,
plankton patchiness is a one-dimensional
phenomenon.
Thus, transects done in
different directions
should produce the
same variance spectra. However, as Haury et al. (1978) suggest, patchiness is twodimensional
(neglecting
vertical variation). Bainbridge
(1957) noted that most
phytoplankton
patches are long ellipses
and are thus two-dimensional.
When this
is coupled with spatial and temporal variation in phytoplankton
growth rates and
turbulent intensity, there may not be any
“universal”
spectral shapes (Denman et
al. 1977).
The effects of transect direction may be
particularly
important
in small basins,
where the influence
of topography
and
stream flow can be pronounced.
For example, spatial patterns of chlorophyll
are
significantly
different nearshore than in
midlake (Leigh-Abbott
et al. 1978). A
Notes
535
k ,(-j-44
kE
10-3
E
INVERSE
WAVELENGTH
(me’)
INVERSE
Fig. 1. Normalized
fluorescence
variance spectrum from transect done 285” from north. Error bars
are 80% confidence intervals.
transect made parallel to shore in the
coastal region might give different results
from one starting at shore and proceeding
to midlake. The effects of river plumes or
coastal jets in estuaries (Bowman and
Esaias 1977) may create apparently different spatial patterns depending on tow
direction.
In the open ocean, such processes are not as apparent so phytoplankton patterns may be relatively
isotropic.
Work in Lake Tahoe (Leigh-Abbott
et al.
1978) has indicated that the midlake region is isolated from effects of stream and
bottom topography by a distinct coastal
zone. The midlake region behaves like
the open ocean, although
local topographic effects should be more important
in Lake Tahoe than in the ocean. However, if processes that are aligned with
the wind are important to spatial heterogeneity (Stavn 1971), one might expect
their influence to be seen in any regime
where the winds are predominantly
from
one direction.
Thus, the isotropic
assumption
may not be entirely
correct
even in the open ocean.
To investigate
the effects of transect
direction, we made a series of three tran-
Fig. 2.
10-2
WAVELENGTH
10-l
( m-’
1
As Fig. 1, but done 160” from north.
sects in the midlake region of Lake Tahoe on 23 October 1976. Each was about
4 km long and at the same depth (7 m).
The midpoint
of each transect was the
same, marked by a fixed buoy to aid in
navigation. The transect directions were
k
lo-*:
i
zl
d
c
;
2
Q
I$ lo-3;
>
2
6
Ik
5
E
B ,o-4
8
lO-3
E
I
INVERSE
Fig. 3.
WAizi:NGTH
bn-‘~o-’
As Fig. 1, but done 40” from north.
536
Notes
40”, 160”, and 285” from north. The series
was made during midday in a space of 4
h, minimizing
the effects of temporal
variation. In vivo fluorescence was measured continuously
with a Turner Designs model 10 fluorometer.
Samples
were collected at the beginning,
middle,
and end of each transect for later chlorophyll
extraction
(Strickland
and Parsons 1968). Although there were significant differences in chlorophyll
a content
between end point samples of the transects, the midpoint
concentrations
were
essentially the same so that our assumption of little temporal variation between
transects appears reasonable.
The transects were made under clear,
calm conditions,
but the previous
day
had been stormy with strong, southerly
winds. Figures 1, 2, and 3 are the chlorophyll variance spectra from the three
transects. Spectral estimates have been
normalized
to the total variance in each
transect record to facilitate
comparison
(Denman 1975). The transect in Fig. 1
was nearly perpendicular
to the southerly winds. The spectrum has a definite
peak at a wavelength of 200 m. The spectrum in Fig. 2 does not have a definite
peak but increases steadily from 50 m to
the limit of resolution at 500 m. Figure 3
has an apparent spectral peak at a scale
of 300 m.
One explanation of these spectra might
invoke a simple geometric
pattern of
chlorophyll
concentration.
If the wind
had created organized structures, such as
parallel windrows of equal spacing, chlorophyll might be concentrated
in bands
at regular intervals in the lake. These features might persist if the wind stress
stopped, as it had on the day of the transects. Towing
across such windrows
would lead to a strong spectral peak. The
hypothesis that the peak in Fig. 1 is the
result of chlorophyll
windrows
is supported by the spectra from the other two
transects. If there were definite bands of
chlorophyll,
oriented
approximately
north-south,
the transect shown in Fig.
3 would
have crossed
them
more
obliquely
than the transect in Fig. 1, so
that the apparent spacing would have
been larger (300 m); the transect in Fig.
2 would have been nearly parallel to such
bands, leading to an even larger apparent
spacing. This might have been longer
than the limit of spectral resolution (500
m) so that the spectrum would have a
steady increase up to this limit. Simple
trigonometry
shows that north-south
bands, of spacing 193 m, will lead to apparent spacings of 200, 300, and 565 m
for transects along directions
285”, 40”,
and 160”. This is consistent
with the
spectral
shapes discussed
above, although more complex chlorophyll
patterns might also be consistent. Any conclusions must be made carefully, but the
data do indicate the potential influence
of prevailing
winds on variance spectral
shapes.
The potential effect of Langmuir
circulations on plankton distributions
has
been discussed by Stavn (1971). Since
Langmuir cells are usually separated by
distances about equal to the depth of the
mixed layer (Boyce 1974), we do not believe that the chlorophyll
patterns we observed are the result of such circulations;
the mixed layer was only 20 m deep on
the day of the transects. However, during
other physical regimes, Langmuir circulations may be important in creating twodimensional phytoplankton
patterns, particularly
in areas where zooplankton
grazing is more important than in Lake
Tahoe (avg zooplankton concentration,
1
animal per liter: Richerson 1972).
Whatever physical (or biological)
process generated
these three spectral
shapes, the data indicate that significantly different spectral shapes may be obtained simply by varying the transect direction. Any one of these transects taken
by itself might have led to different conclusions,
since significantly
different
shapes may be interpreted
as being the
result of different
processes. Spectral
shapes are clearly influenced
by the details of physical and biological processes
at medium and large scales which, among
other things, may not be isotropic. A further complicating
factor is the apparent
time-dependence
of chlorophyll
spatial
patterns (Wilson et al. 1979). Both more
Notes
elaborate theory and sampling programs
that control for or take account of anisotropy and temporal variation are needed
for a better understanding
of patchiness.
We thank R. C. Richards for assistance
with vessel operations (partly financed
by NSF-DEB76-11095).
We also thank C.
ddelius for help in preparing the manuscript.
Mark R. Abbott
Thomas M. Powell
Peter J. Richerson
Division of Environmental
Studies
University
of California
Davis 95616
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