Zooplankton Distribution, Abundance, and Size Structure, as determined by Optical Plankton Counter,

1
Zooplankton Distribution, Abundance, and Size
Structure, as determined by Optical Plankton
Counter, in Relation to Wind Strength and
Upwelling off the Northern California Coast

• Sean R. Avent1,2, Stephen M. Bollens1,2, Dwight
  Peterson1,3 and Newell Garfield1,3

1Romberg Tiburon Center for Environmental
Studies, 3152 Paradise Dr, Tiburon, CA
94920 2Department of Biology, San Francisco State
University, 1600 Holloway Ave, San Francisco, CA
94132 3Department of Geosciences, San Francisco
State University, 1600 Holloway Ave, San
Francisco, CA 94132
2
Abstract
Summary of Results
320 deg 50 deg
320 deg 50 deg
OPC C1-D8 Transect Date
OPC C2-D1 Transect Date
OPC C2-D2 Transect Date
2001
2000
The OPC afforded us the opportunity to make very
extensive, high-resolution (order 1 meter)
observations of zooplankton distribution and
abundance. This provided vastly more spatial
(but less taxonomic) detail than traditional net
sampling. The wind intensities in the early
summer of 2000 were muted in comparison to those
of early summer of 2001 (see Slaughter et al.,
this session). Although the overall wind
intensity three weeks prior to our sample dates
were similar between years, the frequency of
intense winds (gt10 ms) tended to be different. In
the 2000 season, relaxation periods were long and
in continuity, as where in the 2001 season, the
frequency of the intense winds was greater prior
to the sampling dates (Fig. 3). Upwelling can
clearly be seen as the cold, saline water
approaching shoreward along the bottom (Figs.
5-7, A and B). The upwelling appears to be
greater in the 2001 transects with higher
salinities and lower temperatures. Overall
biovolume measurements remain qualitatively
constant throughout the three sampling periods,
although there are shifts in where that biovolume
is centered. Mid- to high-level biovolume
measurements (1.0-4.0 cm3m-3) coincide well with
higher fluorometry values, both inshore and
offshore (Figs 5-7, C and D). The inshore
biovolume patches have a larger particle size and
lower particle density where the offshore patches
are more numerous and smaller on average.
Highest-level biovolume measures are frequently
at greater depths, commonly at the shelf break
away from high fluorometry values and consist of
larger particle sizes. It is likely that these
are patches of large zooplankton, and possibly
migrating from depth at dusk (Fig 5 and 6, D).
Those cases of high-level biovolume patches with
larger particle sizes near high fluorometer
measures tend to be negatively correlated with
the fluorometry. This may be an area of high
grazing rates. This usually occurs on the shelf
break (20km) and offshore of it. Inshore, larger
particle size is correlated with higher density
and high fluorometry values. The relationships
between fluorometry, biovolume, particle density,
and particle sizes reveal that the communities on
the shelf and offshore of the shelf vary .
Inshore of the shelf break, particle densities,
with a lower mean particle size, is distributed
more evenly after a 3-day relaxation and/or
during late night (Figs.5-6, E and F), as opposed
to following a 3-day wind event or during the
late afternoon/early evening, that is
characterized by shallow- oriented particle
densities with larger particle sizes. After a
3 day relaxation period or at late night, the
large biovolume values tend to be inshore rather
than on the shelf break or offshore as in the
case of the afternoon/early evening transects
following a 3-day wind event.
As part of the Coastal Ocean Processes Program -
Wind Events and Shelf Transport (CoOP-WEST)
project, which addresses the effects of upwelling
events on the productivity along the Northern
California coast, we deployed a Focal
Technologies optical plankton counter mounted on
a ScanFish MkII tow body to characterize the
distribution, abundance, and size structure of
zooplankton during May/June of 2000 and 2001.
May/June of 2000 was characterized moreover by
relaxed winds and weak upwelling, whereas
May/June of 2001 was characterized by intense and
extensive periods of wind and upwelling.
Interannual comparisons reveal that in 2000 (weak
upwelling), zooplankton densities were higher and
sizes were smaller than in 2001 (stronger
upwelling), such that overall biomass was higher
in 2000. In addition, zooplankton biomass was
distributed closer to shore in 2000 compared to
2001, most likely due to the surface water
advecting zooplankton offshore during strong
upwelling events in 2001. These data reveal that
while some upwelling is necessary to promote
nutrient injection and primary and secondary
production, too much wind may have a detrimental
effect on zooplankton by advecting them off the
shelf.
Wind Speed (m/s)
Wind Speed (m/s)
140 deg 230 deg
140 deg 230 deg
May 25
May 4
June 5
June 26
Figure 3. Winds during the three weeks preceding
the Scanfish/OPC tows shown below. The blue lines
indicate the alongshore winds with northerly
winds negative and the red lines indicate cross
shelf winds with easterly being positive. Wind
directions are adjusted for a 40 deg CCW
rotation.
May 22, 2001 C2-D1 Transect
1541h
2219h
Introduction
Coastal Ocean Processes (CoOP) Wind Events and
Shelf Transport (WEST) is an interdisciplinary
project designed to determine the effects (both
direct and indirect) of wind-driven transport on
productivity over the shelf off northern
California (Figure 2). This region experiences a
wide variety of wind event lengths (days to
weeks), including strong upwelling-favorable
winds and relaxations, allowing us to observe a
range of responses to different wind forcing
events. For the zooplankton component of this
project, two specific objectives for which the
optical plankton counter is well suited for are
i) What is the cross-shelf variation in the
zooplankton distribution, abundance, and
biomass?, and ii) How are these affected by local
wind events?
Figure 4. Deploying the Scanfish with OPC
Methods
Figure 5. Cross-shelf plots of Scanfish MK II
physical data (A.-C.) and optical plankton
counter data (D.-G.) for the D-Line Transects
(see Figure 2). All times are local. Black dots
trace the Scanfish path.

During the early summers of 2000 (June 1-30) and
2001 (May 17-June 15), the top 100m of the water
column was surveyed using a Scanfish MKII
undulating tow platform with CTD, fluorometer,
and Focal Optical Plankton Counter (OPC) model 2T
mounted on top (Fig. 1). Data were processed with
the Focal OPC-dat software into text files which
were then manipulated and merged with the
Scanfish data to provide for salinity,
temperature, and fluorometry data using
customized MATLAB routines. The data for both
years was gridded by maximum range from shore and
maximum depth of tow and then plotted in MATLAB
6.1. Equivalent Spherical Diameter (ESD) was
calculated using methods in Herman (1992). A
representative paths of the Scanfish and OPC are
shown in Figure 2.
June 26, 2000
May 25, 2001 D2 Transect
1525h
1912h
2321h
0318h
Acknowledgements
The authors wish to thank the crew of the R/V Pt
Sur,Anne Slaughter, Jeff Dorman, Tessa Johnson,
Kathy Papastephanou (SFSU) for their assistance
in the field, and Dave Nelson (URI) for his
assistance with ship-board operations of the OPC.
This project is funded by the National Science
Foundation (NSF 99-08072) to S.M. Bollens.
References
Figure 2. CoOP/WEST study region (all within
California) showing a representative D-Line OPC
tow transect in Figure 5.
Figure 1. Optical Plankton Counter (OPC) mounted
on a Scan Fish MK II.
Herman, A.W. 1992. Design and calibration of a
new optical plankton counter capable of sizing
small zooplankton. Deep-Sea Research. (39)
395-415.
3
Abstract
As part of the Coastal Ocean Processes Program -
Wind Events and Shelf Transport (CoOP-WEST)
project, which addresses the effects of upwelling
events on the productivity along the Northern
California coast, we deployed a Focal
Technologies optical plankton counter mounted on
a ScanFish MkII tow body to characterize the
distribution, abundance, and size structure of
zooplankton during May/June of 2000 and 2001.
May/June of 2000 was characterized moreover by
relaxed winds and weak upwelling, whereas
May/June of 2001 was characterized by intense and
extensive periods of wind and upwelling.
Interannual comparisons reveal that in 2000 (weak
upwelling), zooplankton densities were higher and
sizes were smaller than in 2001 (stronger
upwelling), such that overall biomass was higher
in 2000. In addition, zooplankton biomass was
distributed closer to shore in 2000 compared to
2001, most likely due to the surface water
advecting zooplankton offshore during strong
upwelling events in 2001. These data reveal that
while some upwelling is necessary to promote
nutrient injection and primary and secondary
production, too much wind may have a detrimental
effect on zooplankton by advecting them off the
shelf.
4
Introduction
Coastal Ocean Processes (CoOP) Wind Events and
Shelf Transport (WEST) is an interdisciplinary
project designed to determine the effects (both
direct and indirect) of wind-driven transport on
productivity over the shelf off northern
California (Figure 2). This region experiences a
wide variety of wind event lengths (days to
weeks), including strong upwelling-favorable
winds and relaxations, allowing us to observe a
range of responses to different wind forcing
events. For the zooplankton component of this
project, two specific objectives for which the
optical plankton counter is well suited for are
i) What is the cross-shelf variation in the
zooplankton distribution, abundance, and
biomass?, and ii) How are these affected by local
wind events?
5
Figure 1. Optical Plankton Counter (OPC) mounted
on a Scan Fish MK II.
6
Figure 2. CoOP/WEST study region (all within
California) showing a representative D-Line OPC
tow transect in Figure 5.
7
320 deg 50 deg
320 deg 50 deg
OPC C2-D1 Transect Date
OPC C2-D2 Transect Date
2001
2000
OPC C1-D8 Transect Date
Wind Speed (m/s)
Wind Speed (m/s)
140 deg 230 deg
140 deg 230 deg
May 25
June 26
May 4
June 5
Figure 3. Winds during the three weeks preceding
the Scanfish/OPC tows shown below. The blue lines
indicate the alongshore winds with northerly
winds negative and the red lines indicate cross
shelf winds with easterly being positive. Wind
directions are adjusted for a 40 deg CCW
rotation.
8
Figure 4. Deploying the Scanfish with OPC
9
A.
A.
A.
B.
B.
B.
C.
C.
C.
D.
D.
D.
E.
E.
E.
F.
F.
F.
G.
G.
G.
Figures 5-7. Cross-shelf plots of Scanfish MK II
physical data (A.-C.) and optical plankton
counter data (D.-G.) for the D-Line Transects
(see Figure 2). All times are local. Black dots
trace the Scanfish path.
10
Summary of Results
The OPC afforded us the opportunity to make very
extensive, high-resolution (order 1 meter)
observations of zooplankton distribution and
abundance. This provided vastly more spatial
(but less taxonomic) detail than traditional net
sampling. The wind intensities in the early
summer of 2000 were muted in comparison to those
of early summer of 2001 (see Slaughter et al.,
this session). Although the overall wind
intensity three weeks prior to our sample dates
were similar between years, the frequency of
intense winds (gt10 ms) tended to be different. In
the 2000 season, relaxation periods were long and
in continuity, as where in the 2001 season, the
frequency of the intense winds was greater prior
to the sampling dates (Fig. 3).
11
Summary of Results (cont.)

• Upwelling can clearly be seen as the cold, saline
  water approaching shoreward along the bottom
  (Figs. 5-7, A and B). The upwelling appears to
  be greater in the 2001 transects with higher
  salinities and lower temperatures.
• Overall biovolume measurements remain
  qualitatively constant throughout the three
  sampling periods, although there are shifts in
  where that biovolume is centered.
• Mid- to high-level biovolume measurements
  (1.0-4.0 cm3m-3) coincide well with higher
  fluorometry values, both inshore and offshore
  (Figs 5-7, C and D). The inshore biovolume
  patches have a larger particle size and lower
  particle density where the offshore patches are
  more numerous and smaller on average.
• Highest-level biovolume measures are frequently
  at greater depths, commonly at the shelf break
  away from high fluorometry values and consist of
  larger particle sizes. It is likely that these
  are patches of large zooplankton, and possibly
  migrating from depth at dusk (Fig 5 and 6, D).

12
Summary of Results (cont.)

• Those cases of high-level biovolume patches with
  larger particle sizes near high fluorometer
  measures tend to be negatively correlated with
  the fluorometry. This may be an area of high
  grazing rates. This usually occurs on the shelf
  break (20km) and offshore of it. Inshore, larger
  particle size is correlated with higher density
  and high fluorometry values.
• The relationships between fluorometry, biovolume,
  particle density, and particle sizes reveal that
  the communities on the shelf and offshore of the
  shelf vary .
• Inshore of the shelf break, particle densities,
  with a lower mean particle size, is distributed
  more evenly after a 3-day relaxation and/or
  during late night (Figs.5-6, E and F), as opposed
  to following a 3-day wind event or during the
  late afternoon/early evening, that is
  characterized by shallow- oriented particle
  densities with larger particle sizes.
• After a 3 day relaxation period or at late night,
  the large biovolume values tend to be inshore
  rather than on the shelf break or offshore as in
  the case of the afternoon/early evening transects
  following a 3-day wind event.

13
Acknowledgements
The authors wish to thank the crew of the R/V Pt
Sur,Anne Slaughter, Jeff Dorman, Tessa Johnson,
Kathy Papastephanou (SFSU) for their assistance
in the field, and Dave Nelson (URI) for his
assistance with ship-board operations of the OPC.
This project is funded by the National Science
Foundation (NSF 99-08072) to S.M. Bollens.
References
Herman, A.W. 1992. Design and calibration of a
new optical plankton counter capable of sizing
small zooplankton. Deep-Sea Research. (39)
395-415.