EPA/600/F-19/059
2016 Lake Superior Cooperative Science and Monitoring Initiative
Lower Food Web Assessment: Status of the amphipod Diporeia spp.
Jill Scharold and Tim Corry
U.S. Environmental Protection Agency Mid-continent Ecology Division
6201 Congdon Blvd., Duluth MN 55804
One of the priorities of the 2016 Lake Superior Cooperative Science and Monitoring Initiative
(CSMI) was monitoring and assessment of the lower food web. Historically, the amphipod
Diporeia spp. has been the dominant component of the benthic macroinvertebrate assemblage in
deeper waters of the Laurentian Great Lakes (Cook and Johnson, 1974). As an important prey
item in the diets of numerous species of benthivorous fish, Diporeia plays a key role in the Lake
Superior food web. Because of the importance of Diporeia in the ecosystem, its density was
selected as an indicator of ecological condition for Lake Superior in the 1978 revision of the
Great Lakes Water Quality Agreement (GLWQA) (UC 1978). The ecosystem objectives for
Diporeia in Lake Superior were set at then-current densities of 220-320m"2 in nearshore areas
(depths < 100 m) and 30-160 m"2 in offshore areas (depths >100 m) (UC 1978). These values
correspond to a lake-wide survey conducted by the Canada Centre for Inland Waters in 1973,
using a grid-based design of 382 stations covering the entire lake (Cook, 1975). The mean lake-
wide density in 1973 was 140 + 10 (SE) m"2, with nearshore and offshore means of 265 + 33 m"2
and 106 + 9 m"2 (Cook, 1975). Diporeia populations in the lower Great Lakes have experienced
severe declines following invasion and establishment of dreissenid mussels, raising concerns for
its status in Lake Superior. To assess the status of Diporeia in Lake Superior, we conducted a
probability-based survey as part of the CSMI effort in 2016. Specific objectives of the survey
were: 1) to obtain lake-wide estimates of Diporeia density and biomass in Lake Superior, 2) to
compare densities in nearshore and offshore regions with respect to GLWQA objectives, and 3)
to examine trends by comparing results for 2016 with results from similar lake-wide surveys
conducted in 2006 and 2011.
Methods
Site selection used a spatially-balanced probability survey design. A generalized random
tessellation stratified (GRTS) design for an area resource, stratified by inshore (< 100 m) and
offshore (> 100 m) regions, was used to select 54 sites from Lake Superior. Sampling was
1
-------
EPA/600/F-19/059
conducted during August-September of 2016. At each site, three sediment grabs were collected
using a PONAR sampler (area 0.052 m2). Each grab was rinsed in an elutriation device fitted
with a 500-u mesh sleeve, and the material retained in the sleeve was collected in a jar and
preserved with 5% formalin. In the laboratory, organisms were picked from samples under a
dissecting microscope, and Diporeia were identified and enumerated. Biomass was determined
by measuring blotted wet weights of Diporeia from each sample. Counts and weights of
Diporeia from the three grabs collected at each site were averaged to obtain a mean count and
biomass for that site. Weighted means for the entire lake area and for nearshore and offshore
strata were calculated using R statistical software and analysis packages.
Results
Sediment samples were collected from 52 sites; 25 in the nearshore stratum and 27 in the
offshore stratum (Fig. 1). Sediment could not be obtained at 2 sites, due to hard substrate. Site
depths ranged from 17 to 313 m.
The lake-wide weighted mean density of Diporeia was 502 + 60 (SE) m"2, with a range of 0 to
2314 m"2. Mean densities in nearshore and offshore strata were 955 + 91 m"2 and 377 + 67 m"2,
respectively (Fig. 2A). These values are well above the GLWQA objectives of 220-320 m"2 for
nearshore and 60-160 m"2 for offshore. The lake-wide weighted mean biomass of Diporeia was
0.764 + 0.091 g-m"2, with values of 1.811 + 0.172 gm"2 and 0.476 + 0.086 gm"2 in nearshore and
offshore strata (Fig. 2B). Both density and biomass were higher in the nearshore stratum
compared to offshore (Z-test, p < 0.05).
We previously conducted lake-wide probability-based surveys of Diporeia in 2006 (Kelly et al.,
2011) and 2011. The 2006 survey utilized a similar GRTS survey design, with 50 sites sampled.
The 2011 survey used the same survey design and set of sites as the 2016 survey, with 53 sites
sampled. Inability to access some sites and use of replacement sites resulted in only 50 of the
sites being sampled both years. Lake-wide weighted mean densities for all three surveys (2006,
2011, and 2016) were above the GLWQA nearshore objective of 220-320 m"2 (Fig. 3A). In
2016, values of both density and biomass were intermediate between lower values observed in
2006 and higher values in 2011 (Fig. 3 A, B). Lake-wide weighted mean density and biomass in
2016 were not significantly different from densities and biomasses in 2011 or 2006, based on Z-
tests (p > 0.05). However, paired sample t-tests of the sites sampled in both 2006 and 2011
2
-------
EPA/600/F-19/059
showed significant differences in both density and biomass between the two years (p < 0.05).
Lake-wide weighted mean densities for 2006, 2011, and 2016 are all substantially higher than the
mean density in 1973 reported by Cook (1975) (Fig 3A). These results do not indicate a long-
term decline of Diporeia in Lake Superior during this time frame.
The results of the probability surveys can be examined in more detail using cumulative
frequency distributions, which plot the proportion of the target population having values at or
below a given level (Fig. 4). In 2016, 4% of the nearshore area of Lake Superior was below the
(lower) GLWQA nearshore objective for Diporeia density of 220 m"2, and approximately 15% of
the offshore area was below the (lower) GLWQA offshore objective of 30 m"2 (Fig. 4A).
The lake-wide cumulative frequency distributions of Diporeia density for 2006, 2011, and 2016
are generally similar, with overlapping 95% confidence intervals (Fig. 5A). They are shifted to
the right of the distribution for 1973 (Cook, 1975), corresponding to higher lake-wide densities
of Diporeia during the 2000's. In 1973, 83% of sites lake-wide were below the GLWQA
nearshore objective, compared to only 42-48%) of the lake area during 2011 and 2016.
Peak densities and biomass of Diporeia were found in the 20-80 m depth range for all three
surveys (Fig. 6). The variability in density observed during 2006, 2011, and 2016 is dwarfed by
the large increase from the 1970s to the 2000s (Fig. 6A). The increases in density between 1973
and 2006-2016 were greatest in the nearshore zone.
Conclusions
Based on 2016 densities, Diporeia populations in Lake Superior continue to be in good
condition. Lake-wide mean densities, as well as nearshore and offshore means, are above the
ecosystem objectives established by the Great Lakes Water Quality Agreement. Furthermore,
96%) of the nearshore and 85% of the offshore area exceed the objectives for these strata. There
is no evidence of a severe lake-wide decline in Diporeia from the 1970's to the 2000s, such as
those observed in the lower Great Lakes. Instead, lake-wide densities during 2006-2016 were
greater than those reported in 1973 by factors of 3-4. The changes observed between 2006,
2011, and 2016 may represent normal variability, however, further research is needed to
characterize variability of, and detect long-term trends in, Diporeia populations in Lake
Superior. The probability survey design used in this study provided an efficient and cost-
3
-------
EPA/600/F-19/059
effective method for obtaining unbiased estimates of Diporeia density and biomass, both lake-
wide and for nearshore and offshore strata.
Acknowledgements
The authors thank T. Olsen and T. Kincaid for assistance with the probability survey design and
analysis, and the scientists and crew who helped with sample collection on board the RZVLake
Guardian.
4
-------
EPA/600/F-19/059
Literature Cited
Cook, D.G. and M.G. Johnson. 1974. Benthic Macroinvertebrates of the St. Lawrence Great
Lakes. Journal of the Fisheries Research Board of Canada, 31:763-782.
Cook, D.G. 1975. A preliminary report on the benthic macroinvertebrates of Lake Superior.
Technical Report No. 572, Fisheries and Marine Service, Environment Canada.
IJC. 1978. Great Lakes Water Quality Agreement of 1978. Agreement, with annexes and terms
of reference, between the United States of America and Canada signed at Ottawa November 22,
1978. International Joint Commission, Windsor, ON.
Kelly J.R., P.M. Yurista, S.E. Miller, A.C. Cotter, T.D. Corry, J.V. Scharold, M.E. Sierszen, E.J.
Isaac & J.D. Stockwell. 2011. Challenges to Lake Superior's condition, assessment, and
management: A few observations across a generation of change, Aquatic Ecosystem Health &
Management, 14:4, 332-344.
5
-------
EPA/600/F-19/059
Figure 1. Lake Superior sites sampled during August-September, 2016.
6
-------
EPA/600/F-19/059
A.
1200 n
r\l
E
1000
o*
5
800
o
CL
Q
600
14—
O
400
1/5
c
100 m).
7
-------
EPA/600/F-19/059
A.
B.
Figure 3. Lake-wide weighted mean density (A) and biomass (B) of Diporeia in Lake Superior
during 2006, 2011, and 2016. Lake-wide mean density shown for 1973 from Cook (1975).
Error bars are standard error. Biomass is formalin-preserved wet weight.
8
-------
EPA/600/F-19/059
A.
Density of Diporeia, nrr2
B.
Biomass of Diporeia, g'rrf2
Figure 4. Cumulative frequency distributions of (A) density and (B) biomass of Diporeia in
nearshore and offshore strata of Lake Superior during 2016. Dashed lines are 95% confidence
intervals. Vertical dotted lines in (A) are lower range values of the Great Lake Water Quality
agreement objectives for nearshore and offshore. Biomass is formalin-preserved wet weight.
9
-------
EPA/600/F-19/059
A.
Density of Diporeia, m~2
B,
Bio mass of Diporeia, g'rrf2
Figure 5. Lake-wide cumulative frequency distributions of (A) density and (B) biomass of
Diporeia in Lake Superior during 2006, 2011 and 2016, compared with density for 1973 (A)
from Cook (1975). 95% confidence intervals not shown due to overlap. Vertical dotted line in
(A) is lower value of the Great Lakes Water Quality agreement objective for the nearshore.
Biomass is formalin-preserved wet weight.
10
-------
EPA/600/F-19/059
A.
2500
r-J
E
2000
a*
L.
o
1500
.Q.
Q
¦J—
o
1000
>•
V)
c
500
------- |