NOAA Technical Memorandum GLERL-143

SELECTING OPTICAL PLANKTON COUNTER SIZE BINS TO 

OPTIMIZE ZOOPLANKTON INFORMATION IN GREAT LAKES 

STUDIES

James R. Liebig and Henry A. Vanderploeg

NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105

August 2008

UNITED STATES

DEPARTMENT OF COMMERCE

Carlos M. Gutierrez

Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

Conrad C. Lautenbacher, Jr.

Administrator
2

NOTICE

Mention of a commercial company or product does not constitute an endorsement 

by the NOAA. Use of information from this publication concerning proprietary 

products or the tests of such products for publicity or advertising purposes is not 

authorized. This is GLERL Contribution No. 1479.

This publication is available as a PDF file and can be downloaded from GLERL’s 

web site: www.glerl.noaa.gov. Hard copies can be requested from GLERL, 

Information Services, 2205 Commonwealth Blvd., Ann Arbor, MI 48105.

pubs.glerl@noaa.gov.
3

TABLE OF CONTENTS

ABSTRACT..................................................................................................................................5

1. INTRODUCTION....................................................................................................................5

2. METHODS ...............................................................................................................................5

Field Procedures.......................................................................................................................5

Instruments...............................................................................................................................6

Estimation of Zooplankton ESD/L Ratios................................................................................6

Bin Size Selection ....................................................................................................................7

Applying Species Information to OPC Depth Profile Data .....................................................8

3. RESULTS AND DISCUSSION................................................................................................8

Lake Michigan Example ..........................................................................................................8

Lake Erie Example .................................................................................................................12

4. CONCLUSIONS.....................................................................................................................15

5. ACKNOWLEDGEMENTS ....................................................................................................15

4. REFERENCES .......................................................................................................................16

LIST OF TABLES

Table 1. Relationship of optical plankton counter (OPC) equivalent spherical diameter (ESD) 

to microscope measured length (L) ratios for major taxa of different length/width (L/W) ratios. 

The L/W ratios are approximations for a taxonomic group and meant to depict the general shape 

of an animal, with W representing, in this case, a rough mean of width and depth .................................. 7

Table 2. OPC size bins for Lake Michigan and taxa likely to fall within those bins................................. 9

Table 3. Derived OPC size bins for Lake Erie and taxa likely to fall within those bins.......................... 14

LIST OF FIGURES

Figure 1. Size spectra (relative biomass at each ESD within each taxon) of Bosmina spp. 

(n=351), Daphnia spp. (n=952), Leptodiaptomus sicilis (n=44), and Bythotrephes 

longimanus (n=1396) from a composite of net samples in Lake Michigan. The yellow 

triangles are bin divisions and were placed so as to separate species........................................................ 9

Figure 2. Mean ESD (mm) estimated from mean individual biomass of taxa found in a 

plankton net sample from station M60 in Lake Michigan on August 3, 2004. The yellow 

lines are the previously designated OPC bin divisions to show approximately how the taxa 

would be binned....................................................................................................................................... 10
4

Figure 3. Total dry biomass (μg L-1) of the major species (colored squares) and the remaining 

other taxa (black squares) species found in plankton net samples from station M60 in Lake 

Michigan on August 3, 2004.....................................................................................................................11

Figure 4. Size biomass distribution of Bythotrephes longimanus, Daphnia mendotae, 

Limnocalanus macrurus, Leptodiaptomus sicilis, and Diaptomus copepodites relative to 

total biomass of a plankton net sample from station M60 in Lake Michigan on August 3, 2004. 

Note that these are the dominant species, which comprise most of the biomass at this station. 

The yellow triangles show the previously determined bin divisions........................................................11

Figure 5. Comparison of OPC biomass spectrum (red) from M60 on August 3, 2004 with 

total relative biomass from a corresponding plankton net sample (black, i.e. sum of individual 

species curves from Figure 4). The yellow triangles show the previously determined bin divisions. .... 12

Figure 6. PSS depth profile (mean data for one meter intervals) of temperature (o

C), 

chlorophyll (μg L-1), and zooplankton wet biomass (mg L-1) in bins 3 and 4 from OPC 

survey at ~20:00 EDT on August 3, 2004 at station M60 in Lake Michigan .......................................... 13

Figure 7. Maximum, minimum, and mean (±1 SD) ESD (mm) of taxa found in plankton net 

samples from several stations in Lake Erie, June through September 2005. ESDs of individuals 

were determined from ESD/L ratios. The yellow lines are the designated OPC bin divisions ............... 13

Figure 8. PSS depth profile (mean data for one meter intervals) of zooplankton wet biomass 

(μg L-1), chlorophyll (μg L-1), dissolved oxygen (μg L-1), and temperature (oC) in bins 3 

(0.50 - 1.00 mm) and 4 (1.00 - 4.00 mm) from OPC survey at ~21:00 EDT on August 16, 2005 

at site B in the deepest part of the central basin of Lake Erie.................................................................. 14
5

Selecting Optical Plankton Counter Size Bins to Optimize 

Zooplankton Information in Great Lakes Studies

James R. Liebig and Henry A. Vanderploeg

Abstract. Equivalent spherical diameter (ESD) size bins for the optical plankton counter (OPC) 

were determined so as to optimize the OPC for providing abundance of Great Lakes zooplankton 

species. By determining OPC ESDs for animals of known lengths (L) pumped through laboratory 

OPC, we derived the ESD/L ratio for a variety of species. Based on the size spectra of individual 

species lengths determined from net tows and application of previously determined ESD/L ratios, 

divisions for OPC size bins were selected so as to separate species or groups of species. Since 

there are differences in zooplankton composition and size within species between Lakes Michigan 

and Erie, the OPC size bins designated were not the same in the two lakes. This report describes 

the process used to determine OPC bin sizes and presents the results of an example from Lake 

Michigan and one from Lake Erie. In addition, ESDs calculated from zooplankton lengths may 

provide a size metric more useful than length because an ESD more closely resembles visual 

target strength than length.

1. INTRODUCTION

Developed for marine systems more than 20 years ago (Vanderploeg and Roman 2006), the optical 

plankton counter (OPC) has been used in the Great Lakes for more than 10 years (Stockwell and Sprules 

1995) to provide information on spatial distribution of zooplankton biomass and zooplankton community 

size structure, but has not revealed much about zooplankton species composition (e.g. Sprules et al. 

1998, Zhou et al. 2001, Yurista et al. 2006). Individual species usually cannot be distinguished in OPC 

size-frequency distributions because there is often a significant size overlap among species. The OPC 

can be used as a tool for estimating the spatial distribution of specific taxa if appropriate size bins are 

selected to separate those taxa. By determining OPC ESDs for animals of known lengths (L) pumped 

through laboratory OPC, we determined the ESD/L ratio for a variety of species. There is no single 

universal ESD/L ratio because of the many different shapes of zooplankton in the Great Lakes. Based 

on the size spectra of individual species OPC ESDs, calculated from experimentally determined ESD/L 

ratios or estimated from measured dimensions, divisions for OPC ESD size bins were selected so as to 

separate species or groups of species. These binning techniques can be used in various studies to help 

analyze data collected by the OPC. Selective binning of OPC data from a particular lake allows us to 

produce depth profiles of the major zooplankton species, and correlate various taxa with temperature 

and other environmental data. Gathering the OPC data over long transects and over 24 hour periods can 

reveal insights about spatial distribution and diel vertical migration. Results from specific studies will 

be presented in other papers. This report focuses on methods used to determine bin sizes and presents an 

example from Lake Michigan and Lake Erie.

2. METHODS 

Field Procedures

To illustrate the process and application of bin sizing, data from 153-μm and 64-μm mesh zooplankton 

net samples from Lake Michigan and Lake Erie respectively and OPC tows in both lakes were utilized. 

Lake Michigan data was collected along an inshore/offshore transect (10-m to 60-m deep) west of 

Muskegon, Michigan in August 2004. Lake Erie data were from the central basin at offshore sites up to 

24-m deep north of Cleveland, Ohio in August 2005.
6

Instruments

The OPC (Model 2T, Focal Technologies, Inc.) used in this study is mounted on a V-fin (Endeco/YSI) as 

part of a plankton survey system (PSS) (Ruberg et al. 2001), which also includes a mechanical flow meter 

(General Oceanics), an Aquatracka III fluorometer (Chelsea Technology Group), an OS200 CTD (Ocean 

Sensors), and a photosynthetically active radiation (PAR) sensor. To obtain vertical as well as horizontal 

spatial structure, the PSS was continuously lowered and raised at ~0.25 m s-1 in a sinusoidal path between 

1-2 m beneath the lake surface and 1-2 m above the bottom with the OPC logging data every 0.5 s as the 

boat moved at ~2.5 m s-1 along the transect.

Zooplankton size and abundance data were collected by the OPC component of the PSS. Herman (1992) 

has described the OPC-1T that operates in the same manner as our OPC-2T. The OPC-2T is different in 

that the sampling tunnel, instead of being 25-cm wide, is only 10-cm wide to reduce coincidence (Focal 

1996). Each particle between 0.25 and 14 mm equivalent spherical diameter (ESD) going through the 

OPC generates a signal proportional to the area of shadow cast as it passes a light beam and the OPC 

software converts the digital output to ESD. The generated ESD is actually the same as equivalent circular 

diameter (ECD) based on the area of a zooplankter’s silhouette and, for non-spherical particles such as 

zooplankton, is not necessarily equal to the zooplankter’s true ESD (Sprules et al. 1998, Beaulieu et al. 

1999). Depending on the orientation of zooplankton as they pass the detector, ESD, and thus biomass 

(wet, based on volume assuming a density of 1.0 g cm-3), calculated by the standard OPC software may 

be underestimated or overestimated (Liebig et al. 2006). However, zooplankton appear to go through the 

OPC in random orientation, which produces an approximation of their true ESD and biomass on average 

(Wieland et al. 1997, Sprules et al. 1998, Liebig et al. 2006).

Estimation of Zooplankton ESD/L Ratios

To determine ESD/L ratios of the different species, the laboratory OPC (Model 1L, Focal Technologies 

Inc.) was used to generate ESDs. The basic operating principle of the laboratory unit is the same as 

for the field unit (Model 2T), but the sensing zone is 2-cm wide (1.6 ml) instead of the 10-cm wide 

(8 ml) for the OPC-2T (Focal 1996). To perform tests in the laboratory, we modified the design of a 

circulator system suggested by Focal Technologies (MacKay 1996) and circulated through the OPC 

animals from a single taxonomic group, individually and in aliquots of 100-200 animals per test (Liebig 

et al. 2006). Zooplankters were added gradually to keep the concentration low so that coincidence 

would not be a factor, and they were recovered using a 100-μm sieve after passing once through the 

OPC. They were counted and measured under a microscope with a video camera using Image-Pro Plus 

(Media Cybernetics) imaging software. By measuring the length of each zooplankter in the test group 

and comparing the lengths to their OPC ESD spectrum, ESD/L ratios were obtained for Daphnia sp., 

Bythotrephes longimanus, Limnocalanus macrurus, Diaptomus spp., and mixed cyclopoids. Since it 

was not always possible to match a particular individual’s length with its individual OPC signature, a 

simple ratio of the means of ESDs and lengths for each taxon was calculated (Table 1). For other taxa not 

included in lab OPC tests, an OPC ESD/L ratio was approximated by calculating an individual’s ESD 

from its volume since calculated ESDs are nearly the same as OPC measured ESDs (Liebig et al. 2006), 

and the assumption is that animals of a similar size and shape have the same ESD/L ratio. In these cases, 

the volume (V) and ESD of an individual was calculated using the following equations assuming an 

ellipsoid shape using its measured length, width (either measured or calculated using length/width ratios 

determined from published taxon-specific lengths and widths (Malley et al. 1989)), and depth (calculated 

using length/depth ratios determined from published taxon-specific lengths and depths (Malley et al. 

1989) or, if not available, assumed to be the same as width):
7

(1) V = (π/6) * L * W * D 

where V = volume, L = length, W = width, D = depth, and

(2) ESD = (6V/π)1/3, 

where ESD = equivalent spherical diameter, and V = volume.

Bin Size Selection

To determine what zooplankton taxa were detected by the OPC in a particular lake or study region, the 

species composition was identified from net collections. The objective was to separate major taxa into 

different OPC size bins. Vertical plankton net tows of the full water column were used to determine 

zooplankton species composition in the lake or region of interest. Using the methods described in the 

section above, length measurements of at least 25 random individuals from each of the most abundant 

taxonomic groups were used to calculate ESDs for size frequency distributions. If widths and depths 

could not be obtained, the volume (and associated ESD) was estimated from calculated mass using 

published taxon-specific length-mass regressions (Culver 1985, Malley et al. 1989). There was usually 

much size overlap among all zooplankton species from the community, especially among smaller ones. 

To make it easier to determine boundaries for size bins, instead of using all of the species, only selected 

Table 1. Relationship of optical plankton counter (OPC) equivalent spherical diameter (ESD) to 

microscope measured length (L) ratios for major taxa of different length/width (L/W) ratios. The 

L/W ratios are approximations for a taxonomic group and meant to depict the general shape of an 

animal, with W representing, in this case, a rough mean of width and depth.

Taxon L/W ratio ESD/L Length measurement criteria

Bosminidae 1.5:1 0.76a Anterior margin of head to posterior margin of valves

Bythotrephes 2.0:1 0.603b Distal margin of head to base of posterior spine

Cercopagis 2.0:1 0.603c Distal margin of head to base of posterior spine

Calanoid copepods 3.8:1 0.404b Anterior margin of head to posterior margin of caudal rami

Cyclopoid copepods 3.5:1 0.423b Anterior margin of head to posterior margin of caudal rami

Daphnia spp. 2.0:1 0.566b Anterior margin of head to base of posterior spine

Diaphanosoma 2.5:1 0.54a Anterior margin of head to posterior margin of valves

Leptodora 6.0:1 0.30a Anterior margin of head to fork in postabdomen

Lg. Chydoridae 1.5:1 0.76a Distal margin of head to posterior margin of valves

Sm. Chydoridae 1.3:1 0.84a Distal margin of head to posterior margin of valves

a

 estimate, based on ESD calculated from ellipsoid volume determined by length and L/W ratio.

b OPC-determined ESD and microscope-measured length from lab OPC test.

c used same ratio as Bythotrephes.
8

major taxa were used. Furthermore, the major taxa were the ones in which we were interested in tracking, 

and they contained most of the zooplankton biomass. Based on the calculated ESD size spectra of 

individual species or groups of species, divisions for OPC size bins were selected at points where there 

was minimal overlap of major taxa. These size bins were applied to OPC data for that lake or region.

Applying Species Information to OPC Depth Profile Data

At specific sites, the objective was to identify species or taxonomic groups at different depth strata using 

the OPC. Knowledge of the species present, relative abundance, size, and preferred habitat are required 

to optimize zooplankton information obtained from OPC data to construct a species depth profile at a 

specific site and time. From a full water column net tow sample collected at the same time (within an 

hour or two) and station as the OPC data, ESDs for each species were calculated. ESD size biomass 

distributions were produced for dominant taxa (i.e. taxa with the highest relative biomass). Using that 

information, we determined which species or taxonomic group best represented each OPC size bin at 

any particular time and depth. There are usually more species at the small end of the size spectrum, and 

the bin containing the smallest sizes often could not be identified as any particular one or two species or 

taxonomic groups.

3. RESULTS AND DISCUSSION

Lake Michigan Example

Overall, in zooplankton net samples collected along the inshore/offshore transect in Lake Michigan in 

August 2004, there were four main taxonomic groups with high relative biomass that had reasonable 

size separation: Bythotrephes sp., Daphnia spp., Bosmina sp., and Leptodiaptomus sicilis. Based on the 

calculated ESD size distribution of these groups (Figure 1), we selected bin divisions to separate the taxa 

for associated OPC data. The bin sizes were 0.25 - 0.50 mm (Bosmina), 0.50–0.75 mm (Leptodiaptomus

and small Daphnia), 0.75–1.50 mm (large Daphnia), and 1.50–3.50 mm (Bythotrephes) for bins 1, 2, 3, 

and 4 respectively (Table 2). Since there is a complete overlap in their sizes, the OPC cannot be used to 

distinguish Leptodiaptomus sicilis from small Daphnia. Note also that much of the Bosmina biomass fell 

below 0.25 mm, the minimum size threshold of detection for the OPC.

To examine a more specific place and date, in this case station M60 (offshore, 60 m depth) in Lake 

Michigan on August 3, 2004, ESDs for the taxa found in a full water column plankton net samples were 

determined. Biomass was calculated from measured lengths for abundant species and for less abundant 

species, previously determined default biomass values (unpublished data) were used. To get a quick 

overview of the site, mean ESDs were derived from mean individual biomass since that had already been 

generated from a prior analysis. Figure 2 shows mean ESD of the taxa and in what bins they would likely 

be detected based on the previously determined bin divisions. The variance around the mean ESD of each 

taxon could not be determined because mean ESD was calculated from mean biomass, not from each 

individual. However, it is obvious that some species would fall into two size bins and many taxa overlap, 

especially when you consider that there is variation in size around the mean, as evidenced in Figure 1 

showing the size spectra of dominant species from composite samples. The point is that more information 

is needed in order to give a size bin any kind of taxonomic identity. Depending on the total biomass 

present for each species, bins 3 and 4 and possibly 2 may each represent a single taxon, however bin 1 

contains many taxa which overlap in size and is less likely to have a clear dominant in that range. Table 2

shows the bins with their size ranges and the taxa that are likely to occur in those bins for Lake Michigan.

Total biomass for each species (Figure 3) was examined to determine the dominant taxa (i.e. those 

with highest biomass). The size biomass distribution of Bythotrephes longimanus, Daphnia mendotae, 

Limnocalanus macrurus, Leptodiaptomus sicilis, and Diaptomus copepodites relative to total biomass 
9

Table 2. OPC size bins for Lake Michigan and taxa likely to fall within those bins.

Range (mm) Nearshore zooplankton in range Offshore zooplankton in range

Bin 1 0.25 - 0.50 nauplii, Bosmina, copepodites, small 

copepods, small chydorids

nauplii, copepodites, small 

copepods

Bin 2 0.50 - 0.75 Leptodiaptomus sicilis, small Daphnia

spp., Cercopagis

Leptodiaptomus sicilis, small 

Daphnia spp., Leptodora*

Bin 3 0.75 - 1.50 large Daphnia mendotae, Cercopagis large Daphnia mendotae, 

Limnocalanus, Leptodora*

Bin 4 1.50 - 3.50 Bythotrephes longimanus Bythotrephes, Mysis

*Note: Leptodora were not included in laboratory OPC tests, but adult Leptodora should 

show up mainly in bin 3 based on their size. However, due to their clarity, they may actually be 

detected mainly in the bin 2 size range.

Figure 1. Size spectra 

(relative biomass 

at each ESD within 

each taxon) of 

Bosmina spp. (n=351), 

Daphnia spp. (n=952), 

Leptodiaptomus

sicilis (n=44), and 

Bythotrephes 

longimanus (n=1396) 

from a composite 

of net samples in 

Lake Michigan. The 

yellow triangles are 

bin divisions and 

were placed so as to 

separate species.

0.00

0.05

0.10

0.15

0.20

0.25

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3

ESD (mm)

Relative species biomass 

Bythotrephes longimanus

Daphnia spp.

Leptodiaptomus sicilis

Bosmina spp.

Bin divisions

Relative species biomass
10

of the plankton net sample was plotted (Figure 4) to determine how they were distributed in the 

existing OPC bins. The Leptodiaptomus ashlandi size distribution was not plotted due to lack of length 

measurements. The remaining other species (black squares in Figure 3) were insignificant in terms 

of biomass. Basically, calanoid copepods and small Daphnia spp. dominate bins 1 and 2 (0.25-0.50 

mm and 0.50-0.75 mm), Limnocalanus and large Daphnia comprise bin 3 (0.75-1.5 mm), and bin 4 is 

Bythotrephes (1.5-3.5 mm). The boundaries could have been adjusted at this point to better fit the size 

spectra of certain taxa, but it is nice to have the same size bins for a lake or large region for comparison 

among sites and also to lessen the amount of OPC data processing. The shape of the biomass distribution 

for the major taxa from the net samples is approximated by the shape of the OPC biomass spectrum 

(Figure 5), so it appears that the designated ‘major taxa’ characterized the OPC spectrum fairly well. The 

OPC spectrum is broader than the net sample spectrum because the random orientation of zooplankton 

in the OPC sensing zone causes ESDs to be both underestimated and overestimated (Liebig et al. 2006). 

Note also that some biomass from very small and/or less abundant zooplankton are not included in the net 

sample spectrum.

The OPC zooplankton depth profile for a particular station can be plotted showing the biomass 

distribution for each bin. In the plot of bins 3 and 4 (Figure 6), we figured that Bythotrephes were in 

Figure 2. Mean ESD (mm) estimated from mean individual biomass of taxa found in 

a plankton net sample from station M60 in Lake Michigan on August 3, 2004. The 

yellow lines are the previously designated OPC bin divisions to show approximately 

how the taxa would be binned. 

0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

2.25

Mean ESD (mm)

Taxa ESD

Bin division

Copepod nauplii

Asplanchna

Bosmina longirostris

Mesocyclops C1-C5

Cyclops C1-C5

Epischura C1-C5

Ceriodaphnia spp.

Cyclops bicuspidatus

Diaptomus C1-C5

Leptodiaptomus minutus

Leptodiaptomus ashlandi

Cercopagis pengoi

Leptodiaptomus sicilis

Epischura lacustris

Daphnia mendotae

Limnocalanus macrurus

Bythotrephes longimanus

Mean ESD (mm)
11

Figure 4. Size 

biomass distribution 

of Bythotrephes 

longimanus, 

Daphnia mendotae, 

Limnocalanus 

macrurus, 

Leptodiaptomus 

sicilis, and Diaptomus 

copepodites relative 

to total biomass of a 

plankton net sample 

from station M60 in 

Lake Michigan on 

August 3, 2004. Note 

that these are the 

dominant species, 

which comprise most 

of the biomass at this 

station. The yellow 

triangles show the 

previously determined 

bin divisions.

Figure 3. Total dry 

biomass (μg L-1) of 

the major species 

(colored squares) and 

the remaining other 

taxa (black squares) 

species found in 

plankton net samples 

from station M60 in 

Lake Michigan on 

August 3, 2004.

Daphnia

mendotae

Bythotrephes

Diaptomus

C1-C5

Leptodiaptomus

sicilis

Limnocalanus

macrurus

Leptodiaptomus

ashlandi

0

5

10

15

20

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5

Mean ESD (mm)

Other taxa

Dry biomass (

μg L-1)

0.00

0.05

0.10

0.15

0.20

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3

ESD (mm)

Relative total biomass

Bythotrephes longimanus

Daphnia mendotae

Limnocalanus macrurus

Leptodiaptomus sicilis

Diaptomus C1-C5

Bin divisions

Relative total biomass
12

Figure 5. Comparison of 

OPC biomass spectrum 

(red) from M60 on 

Aug. 3, 2004 with total 

relative biomass from a 

corresponding plankton 

net sample (black, i.e. 

sum of individual species 

curves from Figure 4). 

The yellow triangles 

show the previously 

determined bin divisions.

groups throughout the epilimnion and metalimnion, and there is a concentration of large Daphnia at 

the bottom of the metalimnion with some perhaps into the upper hypolimnion. Limnocalanus were 

throughout the hypolimnion. Since Limnocalanus and large Daphnia fell into the same OPC size bin, their 

locations were inferred based on previous knowledge of their habits. This was confirmed with separate 

net samples from the epilimnion, metalimnion, and hypolimnion. However, OPC data are much finer 

depth scale than data obtained from net samples. Biomass in bins 1 and 2 (not shown) is fairly uniform 

throughout the water column and, given the overlap in sizes of several species, no species information 

could be inferred.

Lake Erie Example

The zooplankton community of Lake Erie was more diverse than that of Lake Michigan, and zooplankton 

as a whole were smaller than those of Lake Michigan (Figure 7). The size difference was due to the 

presence of small species and because mean sizes of individuals of a given species present in both 

systems were smaller (e.g. D. mendotae and B. longimanus) in Lake Erie. Thus, different OPC size bins 

were selected for Lake Erie (Figure 7, Table 3).

The OPC zooplankton depth profile for a station in Lake Erie shows the biomass distribution for 

bins 3 and 4 (Figure 8). Net samples contained no Limnocalanus or Leptodora, so we concluded that 

Bythotrephes were in bin 3 and occurred mainly in the metalimnion. Bin 2 biomass was mostly Daphnia

and occurred in most of the water column with biomass peaks at the middle of the epilimnion and at 

the bottom half of the metalimnion into the upper hypolimnion. Biomass in bin 1 (not shown) is fairly 

uniform throughout the water column and, given the overlap in size of several species, no species 

information could be inferred. 

0

0.05

0.1

0.15

0.2

0.25

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3

ESD (mm)

Relative biomass

OPC

Net

Bin divisions

Relative biomass
13

Figure 6. PSS depth profile 

(mean data for 1 meter 

intervals) of temperature 

(

oC), chlorophyll (μg L-1), 

and zooplankton wet 

biomass (mg L-1) in bins 

3 and 4 from OPC survey 

at ~20:00 EDT on August 

3, 2004 at station M60 in 

Lake Michigan.

Figure 7. Maximum, 

minimum, and mean 

(±1 SD) ESD (mm) 

of taxa found in 

plankton net samples 

from several stations 

in Lake Erie, June 

through September 

2005. ESDs of 

individuals were 

determined from 

ESD/L ratios. The 

yellow lines are the 

designated OPC bin 

divisions.

0

10

20

30

40

50

60

0 5 10 15 20 25

Depth (m)

0 0.5 1 1.5 2 2.5 3

Temperature

Chlorophyll a

Bin 3 (0.75-1.50 mm)

Bin 4 (1.50-3.50 mm)

Lim nocalanus

Large D aphnia

B ythotrephes

Depth (m)

Temperature (oC)

Zooplankton (mg L-1, Chl (μg L-1)

ESD (mm)

0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

2.25

2.5

ESD (mm)

Mean + 1SD

Maximum

Minimum

Bin division

Copepod nauplii

Bosmina longirostris

Mesocyclops C1-C5

Skistodiaptomus oregonensis

Epischura C1-C5

Leptodora

Bythotrephes

Diaptomids C1-C5

Leptodiaptomus ashlandi

Daphnia mendotae

Tropocyclops C1-C5

Chydorus

Tropocyclops prasinus

Cyclopoids C1-C5

Eubosmina

Diaphanosoma

Diacyclops thomasi

Mesocyclops edax

Daphnia longiremus

Daphnia retrocurva
14

Table 3. Derived OPC size bins for Lake Erie and taxa likely to fall within those bins.

Range (mm) Zooplankton taxa in range

Bin 1 0.25 - 0.50 nauplii, Bosmina, copepodites, copepods, Chydorus

Bin 2 0.50 - 1.00 large Daphnia, Limnocalanus, Leptodora*

Bin 3 1.00 - 4.00 Bythotrephes, Leptodora*

*Note: Leptodora were not included in laboratory OPC tests, but adult Leptodora should 

show up mainly in bin 3 based on their size. However, due to their clarity, they may actually be 

detected mainly in the bin 2 size range.

Figure 8. PSS depth 

profile (mean data 

for 1 meter intervals) 

of zooplankton wet 

biomass (μg L-1), 

chlorophyll (μg L-1), 

dissolved oxygen (μg 

L-1), and temperature 

(

oC) in bins 3 (0.50 - 1.00 

mm) and 4 (1.00 - 4.00 

mm) from OPC survey at 

~21:00 EDT on August 

16, 2005 at site B in 

the deepest part of the 

central basin of Lake 

Erie.

Depth (m)

0

5

10

15

20

25

0 100 200 300 400 500 600 700 800

Depth (m)

0 5 10 15 20 25

Daphnia (Bin 2)

Bythotrephes (Bin 3)

Chlorophyll a

Dissolved oxygen

Temperature

Zooplankton (μg L-1)

Chl (μg L-1), DO (mgL-1), Temp (oC)
15

4. CONCLUSIONS

Plankton net samples are very important in determining the zooplankton species that are represented 

by a particular OPC size bin. Generally, without additional information, individual species cannot be 

distinguished from an OPC size spectrum because there is too much overlap in the size spectra of each 

species, and the OPC tends to smear the data. However, proper size binning can help determine where 

particular taxa are within the spectrum. Using species information from net samples in conjunction with 

OPC tows can help determine spatial and temporal distribution of specific or groups of taxa. In our 

studies, dominant species could be tracked by the OPC because they were known (based on net samples) 

to be far more abundant than other species within a size bin, or because they are known to live in a 

different part of the water column from another species. In some cases, we could only say that a certain 

size bin contained one or more of a several species of similar size. Where there were high concentrations 

of zooplankton or other large particles, it was not possible to differentiate taxa because coincidence often 

caused species to appear larger than they really were and shifted biomass to larger size bins (Liebig et al. 

2006). This is a bigger problem in Lake Erie than Lake Michigan.

The bin divisions for the Lake Michigan example were determined based on the data shown in Figure 1. 

The 0.5 mm division was designated in order to separate Leptodiaptomus sicilis from smaller copepods. 

Based on the net sample data from our example site, it may have been better to place that division at 

about 0.4 mm because then it would include all or most calanoid copepods as well as all or most of the 

small Daphnia. We were trying to make the bin sizes generic for a lake, but depending on how much 

custom data processing you want to do and how much change there is over the length of a transect, bin 

sizes could be custom fit to a site or transect. 

The following is a summary of criteria necessary to apply zooplankton species names or taxonomic 

groups to OPC data:

- Little or no OPC coincidence. 

- Lengths and corresponding ESDs of species in study area.

- Good size separation between at least some of the species present.

- Knowledge of zooplankton species composition and relative abundance from net samples at a particular 

site to determine the significance and dominance of particular taxa.

- Knowledge of a species range in habitat (i.e. where they are usually found in the water column and 

whether they live inshore or offshore) to determine spatial separation.

In addition to knowing ESD/L ratios for choosing bin sizes for zooplankton, these ratios are useful for 

converting L to ESD, which may be a better measure of zooplankton conspicuousness, in regard to 

predators, than length. ESD also has the attractive feature of being easily related to mass (volume).

5. ACKNOWLEDGEMENTS

We thank the crews of the R/V Laurentian and the R/V Lake Guardian for assistance in field operations. 

We thank M. Bundy for use of the OPC-1L and T. Miller for assistance in constructing the OPC circulator 

system. We also thank J. Cavaletto and M. Clouse for analyzing zooplankton samples and G. Lang for 

processing PSS field output.
16

6. REFERENCES

Beaulieu, S.E., M.M. Mullin, V.T. Tang, S.M. Pyne, A.L. King, and B.S. Twining. Using an optical 

plankton counter to determine the size distributions of preserved zooplankton samples. Journal of 

Plankton Research 21:1939-1956 (1999).

Culver, D.A., M.M. Boucherle, D.J. Bean, and J.W. Fletcher. Biomass of freshwater crustacean 

zooplankton from length-weight regressions. Canadian Journal of Fisheries and Aquatic Sciences 42: 

1380-1390 (1985).

Focal Technologies Inc. Optical plankton counter user’s guide. Dartmouth, Nova Scotia, 23 pp. (1996).

Herman, A.W. Design and calibration of a new optical plankton counter capable of sizing small 

zooplankton. Deep-Sea Research 39: 395-415 (1992).

Liebig, J.R., H.A. Vanderploeg, and S.A. Ruberg. Factors affecting the performance of the optical 

plankton counter in large lakes: Insights from Lake Michigan and laboratory studies. Journal of 

Geophysical Research 111:C05S02, 10 pp. (2006).

MacKay, I. Using the OPC-1L laboratory unit, Application Note No. 4, 3-6 pp, Focal Technologies, Inc. 

(1996).

Malley, D.F., S.G. Lawrence, M.A. MacIver, and W.J. Findlay. Range of variation in estimates of dry 

weight for planktonic Crustacea and Rotifera from temperate North American lakes. Canadian Technical 

Report Fisheries and Aquatic Sciences 1666 (1989).

Ruberg, S.A., H.A. Vanderploeg, J.F. Cavaletto, G.A. Lang, J.R. Liebig, T.C. Miller, and M. Agy. 

Plankton survey system, Proceedings of the Oceans 2001 MTS/IEEE Conference, Honolulu, HI, 

November 5-8, 2001, Marine Technology Society, pp. 1899-1903 (2001).

Sprules, W.G., E.H. Jin, A.W. Herman, and J.D. Stockwell. Calibration of an optical plankton counter for 

use in fresh water. Limnology and Oceanography 43:726-733 (1998).

Stockwell, J.D. and W.G. Sprules. Spatial and temporal patterns of zooplankton biomass in Lake Erie, 

ICES Journal of Marine Science 52:557-564 (1995).

Vanderploeg, H.A. and M.R. Roman. Introduction to the special section on analysis of zooplankton 

distributions using the optical plankton counter. Journal of Geophysical Research 111:C05S01, 

doi:10.1029/2006JC003598 (2006).

Wieland, K., D. Peterson, and D. Schnack. Estimates of zooplankton abundance and size distribution with 

the Optical Plankton Counter (OPC). Archive of Fishery and Marine Research 45:271-280 (1997).

Yurista, P. M., J. R. Kelly, and S.E. Miller. Comparisons of zooplankton community size structure in the 

Great Lakes. Journal of Geophysical Research 111:C05S08, 12 pp. (2006).

Zhou, M., Y. Zhu, S. Putman, and J. Peterson. Mesoscale variability of physical and biological fields in 

southeastern Lake Superior. Limnology and Oceanography 46:679-688 (2001).