NOAA Technical Memorandum GLERL-136

DISAPPEARANCE OF THE AMPHIPOD DIPOREIA SPP. IN THE GREAT LAKES: 

WORKSHOP SUMMARY, DISCUSSION, AND RECOMMENDATIONS

Thomas F. Nalepa1, David C. Rockwell2, and Donald W. Schloesser3

1

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

2

EPA, Great Lakes National Program Office, Chicago, IL 60604

3

USGS, Great Lakes Science Center, Ann Arbor, MI 48105

Great Lakes Environmental Research Laboratory

Ann Arbor, Michigan

March 2006

UNITED STATES

DEPARTMENT OF COMMERCE

Carlos M. Gutierrez

Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

Conrad C. Lautenbacher, Jr.

Administrator


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. 1381.

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.


TABLE OF CONTENTS

EXECUTIVE SUMMARY...........................................................................................................5

INTRODUCTION ........................................................................................................................5

SUMMARY AND DISCUSSION OF WORKSHOP PRESENTATIONS...................................7

RESULTS OF WORKSHOP SURVEY......................................................................................11

GENERAL DISCUSSION .........................................................................................................12

KEY QUESTIONS AND RECOMMENDATIONS...................................................................15

ACKNOWLEGEMENTS...........................................................................................................16

REFERENCES ...........................................................................................................................16

WORKSHOP PRESENTATIONS..............................................................................................19

WORKSHOP PARTICIPANTS..................................................................................................20

LIST OF FIGURES

Figure 1. Areas of the Great Lakes where Diporeia were once present but are now 

completely gone ............................................................................................................................5



Disappearance of the amphipod Diporeia spp. in the Great Lakes: 

Workshop Summary, Discussions, and Recommendations

T.F. Nalepa, D.C. Rockwell, and D.W. Schloesser

EXECUTIVE SUMMARY

A workshop was held in October, 2006 to discuss population status and causes of the 

disappearance of the benthic amphipod Diporeia spp. in the Great Lakes, and to provide 

recommendations for future research. Recent surveys indicate that Diporeia continues to 

decline in Lakes Michigan, Huron, and Ontario, but surveys in Lake Superior are conflicting. One data set shows that the population in Lake Superior is declining in offshore 

regions (> 90 m) but not in nearshore regions (< 90 m). Other data sets show that the 

population is stable throughout the lake. Reasons for this discrepancy were not resolved. 

In the other lakes, Diporeia are now rare or completely gone at depths < 90 m, and are 

declining at depths > 90 m. Declines at the latter depths are preceding the expansion 

of Dreissena bugensis (quagga mussel) from shallow to deeper regions. While there 

is a strong negative relationship between Diporeia and Dreissena in the Great Lakes, 

Diporeia remains abundant in the Finger Lakes, New York, despite the long-term presence of Dreissena. There are areas in the Great Lakes where Diporeia seemed to persist 

but are now declining, indicating that environmental conditions do play a mitigating role 

to some extent. While there was agreement that Dreissena was the cause of the disappearance of Diporeia in the Great Lakes, a survey of workshop participants indicated no clear 

consensus on potential mechanisms for the negative response. The most popular theories 

were food limitation directly or indirectly related to dreissenid filtering activities, a toxic 

by-product associated with dreissenid biodeposits, and an introduced pathogen/disease. 

Based on previous field surveys and laboratory experiments, there are inconsistencies in 

each of these theories, which may imply a multitude of causative factors whose relative 

importance may vary depending on specific environmental conditions. Given the potential for multi-stressors, techniques that provide genomic or protein profiles offer promise 

in defining a specific cause. Included among the recommendations for future research are 

continued monitoring of Diporeia in the Finger Lakes along with an assessment of conditions that allow it to persist, experiments to further define a dreissenid by-product that 

negatively impacts Diporeia, the development of genomic and proteonic approachesspecific for Diporeia, a better definition of taxonomic status, and further efforts to characterize/resolve conflicting trends in offshore populations in Lake Superior. 

INTRODUCTION 

Over the past few decades, one of the most dramatic and enigmatic changes in the biotic community of the Great Lakes has been the decline of the deep-water amphipod Diporeia spp. This organism was once the dominant benthic taxa in offshore waters (> 30 m) of all the lakes. Recently 

however, population declines have been documented in Lakes Michigan, Huron, Erie, and Ontario, and large areas in each of these lakes are now completely devoid of this organism (Dermott 

and Kerec 1997, Nalepa et al. 1998, 2003, Dermott 2001, Lozano et al. 2001) (Figure. 1). 


Diporeia have long been considered a keystone species in the food web of offshore waters. It 

resides in the upper sediments and feeds on fresh organic material settled from the water column and, in turn, are fed upon by many fish species. Thus, it plays an important role in cycling 

energy from lower to upper trophic levels. Diporeia are high in lipids and therefore rich in calories, making it a valued food resource for fish. Recent changes in the condition, distribution, and 

abundances of several fish species have been attributed to the loss of this organism (Madenjian 

et al. 2003, Hondorp et al. 2005). A recent workshop explored the impact of declining Diporeia

populations on the relative health of the commercially-important lake whitefish (Coregonus clupeaformis) (Mohr and Nalepa 2005). 

While a number of studies have documented the disappearance of Diporeia, few have specifically examined reasons for its decline, and cause-and-effect mechanisms are currently unknown. 

Population declines were first noted in the early 1990s and, in all lake areas, these declines were 

coincident with the introduction and spread of the invading species Dreissena polymorpha (zebra mussel) and Dreissena bugensis (quagga mussel). On a broad scale, the decline of Diporeia

is linked to the expansion of dreissenids, but on a more local scale inconsistencies are apparent. 

For instance, Diporeia has disappeared from lake areas that were far-removed from dreissenid 

colonies, yet it persists in other areas despite the long-term presence of dreissenids (Nalepa et al. 

2006).

Figure 1. Area of the Great Lakes where Diporeia were once present but are now completely gone 

(hatched area). There is some evidence that Diporeia are now gone from portions of Georgian Bay 

and North Channel, Lake Huron, but the spatial extent of its disappearance is not known. Because 

of warm water temperatures, Diporeia are naturally not found in Green Bay, Saginaw Bay, Lake St. 

Clair, and in the western and central basins of Lake Erie. 


Because of growing concerns over ecological impacts, a workshop was held October 20-21, 2005 

in Ann Arbor, MI to address the issue of Diporeia declines and potential causes. The objective 

of the workshop was to bring together researchers currently studying Diporeia to (1) assess the 

current status of populations; (2) discuss potential causes of the decline based on previous and 

ongoing studies, and (3) develop a list of research recommendations that may further our understanding of causes. By better understanding reasons for Diporeia’s decline relative to increased 

numbers of dreissenids, we may better predict the eventual extent and ultimate consequences of 

the population loss. Also, by better understanding the cause, we may better assess the potential 

for population recovery if and when driessenid populations stabilize or decline. The workshop 

was co-sponsored by the Great Lakes National Program Office, EPA, and the Great Lakes Environmental Research Laboratory, NOAA.

SUMMARY AND DISCUSSION OF WORKSHOP PRESENTATIONS

Initial presentations focused on population trends in Lake Superior. Ongoing yearly collections (1997-2005) by the Great Lakes National Program Office of EPA (GLNPO) indicated that 

populations at some offshore sites were in a downward trend (Balcer et al. Workshop Presentation; hereafter all workshop presentations will be abbreviated as WP). There was a statistically 

significant decline in densities at sites > 90 m (n = 9) between 1997 and 2005. Mean densities 

were 300 m-2 in 1997, but only about 40 m-2 in 2005. Nearshore sites (< 65 m; n = 2) did not 

show a similar downward trend. Since dreissenids are not abundant and mainly found in the far 

western end of the lake (Duluth Harbor), a decline in offshore waters of the main basin would 

indicate a cause potentially unrelated to Dreissena. In contrast, other surveys in Lake Superior 

have not shown population declines in either nearshore or offshore areas. In a survey of nearshore sites (n = 270) along the southern shoreline, densities did not decline but were actually 

higher in 2003 than in 1973 or 2000 (Scharold WP). Further, yearly densities at offshore sites in 

the western portion of the lake (n = 5) did not show a consistent trend between 1993 and 2001; 

however, densities were highly variable from year-to-year. In another Lake Superior study, densities at nearshore and offshore sites located off the Keweenaw Peninsula and sampled over the 

past several years did not show any consistent temporal trends (Urban et al. WP). Reasons for 

the discrepancy between the various studies were not clear. Naturally low numbers and substrate 

variability may contribute to some variation in yearly density estimates, but reasons for the statistically significant downward trend in densities at the offshore GLNPO sites could not be readily reconciled relative to the stable densities indicated by other studies. 

A number of recent surveys confirmed the continued decline of Diporeia in Lakes Michigan, 

Huron, and Ontario (Nalepa WP, Watkins WP, Dittman WP). In each of these lakes, Diporeia

are now completely gone or rare at depths < 90 m, and slowly declining at depths > 90 m. In 

contrast, quagga mussel populations are expanding in each of these three lakes. In Lake Ontario, 

the quagga mussel population has apparently stabilized at depths < 90 m, but is slowly increasing at deeper depths. In Lakes Michigan and Huron, where quagga mussels became established 

about 7 years later than in Lake Ontario, populations are presently increasing most rapidly at the 

30-50 m interval. While presently not abundant at depths > 90 m in Lakes Michigan and Huron, 

quagga mussels are expected to gradually increase at these deeper depths in both lakes as they 

did in Lake Ontario. In all three lakes, the decline of Diporeia at depths > 90 m preceded the ex-


pansion of quagga mussels from shallow to deeper depths. Thus, mussels in shallow areas were 

remotely affecting Diporeia in deeper areas. Another example of remote effects was the pattern 

of Diporeia declines at a 45-m site in Lake Michigan (Nalepa WP). Diporeia densities at this site 

declined to near zero in just 6 months in 1992 and, although Dreissena was present at shallower 

depths, it was not present at the site itself. The site was located in an area with high rates of sediment deposition and strong nearshore-offshore sediment transport. Rapid declines have also been 

noted in depositional areas in northeast Lake Ontario (Dermott 2001) and in southern Lake Huron (Nalepa unpublished data). 

Based on spatial distributions in earlier surveys, some lake areas were thought to be a “refugia” 

for Diporeia. For example, in Lake Ontario Diporeia persisted at a 130-m site despite disappearing at other sites of similar depth (Dittman WP). This site was located between depositional 

basins and is characterized by thin sand lags of postglacial sediment, an indication of winnowing 

and re-suspension of post glacial muds. Recent data, however, indicated that the Diporeia population at this site is now declining (Dittman WP). Lake areas subject to frequent upwelling were 

also thought to be a refugia for populations. In Lake Michigan, upwelling is far more frequent on 

the west side compared to the east side (Plattner et al. in press). Consequently, Diporeia persisted on the west side but declined rapidly on the east side (Nalepa WP). Lower temperatures and a 

greater abundance of food in upwelling areas likely provided a more favorable (or less stressful) 

habitat for Diporeia. Recent data collected in 2005 showed that Diporeia are now also disappearing from the west side of the lake (Nalepa WP). Declines on the west side appear to be temporally well-correlated to the recent expansion of quagga mussels throughout the lake. Although 

Diporeia have now declined in areas thought to be refugia for populations, some persistence in 

these areas would indicate that environmental conditions do play a mitigating role.

A number of deep lakes in North America outside the Great Lakes were re-sampled to assess 

long-term trends in Diporeia populations (Dermott et al. WP). Densities of Diporeia in lakes 

without Dreissena increased slightly over a 30-year period, indicating the decline in the Great 

Lakes was not part of a widespread regional problem associated with perhaps global warming or 

atmospheric inputs of contaminants. Also, densities did not decline in all lakes with Dreissena

(i.e., some Finger Lakes), suggesting that other factors may be involved beyond the mere presence/absence of Dreissena. This finding further suggests that environmental conditions play a 

mitigating role. 

A common theory for the decline is that dreissenids are outcompeting Diporeia for available 

food. Dreissenids filter feed near the sediment surface, and this theory assumes that settling organic material is intercepted by dreissenids before it reaches the upper sediments and becomes 

available to Diporeia. If true, then Diporeia should show some physiological signs of food limitation prior to and during population declines. Data collected at a 45-m site in Lake Michigan 

when the population was declining showed no corresponding decreases in length-weight, lipid 

content, and eggs per brood (Nalepa WP). Based on previous studies of cold-water amphipods, 

these three variables are directly linked to food availability and therefore should have declined 

if food was limiting. At this 45-m site, abundances gradually decreased over a 3-4 year period 

before the population totally disappeared. During this time, recruitment occurred, but the youngof-year (YOY) were not surviving to become juveniles (Nalepa WP). Adults did not decline until 


later in the 3-4 year period. This finding may indicate food was limiting since the young, because of higher metabolic rates, are more susceptible to declines in food availability compared to 

adults. Thus, at this Lake Michigan site, indicators of food limitation at the organism level and at 

the population level did not appear to be consistent. 

As a component of the food-limitation theory, food quality as well as food quantity can be an 

important contributing factor. Previous studies have shown that Diporeia are heavily dependent 

upon the spring diatom bloom as a major source of nutrition (Gardner et al. 1985, Quigley 1988). 

Diatoms are rich in lipids and long-chain polyunsaturated fatty acids (PUFAs). Besides playing 

an important role in growth and reproduction, these fatty acids are needed by organisms such as 

Diporeia to maintain cell elasticity in cold-water environments. Because of Dreissena, diatom 

availability to Diporeia would likely be greatly diminished. One way to characterize shifts in the 

diet of Diporeia is through the use of phytoplankton pigments (Alben WP). Each of the major 

phytoplankton groups (including diatoms) have a unique carotenoid signature, and comparing 

signatures in populations from different locations within and between lakes (stable vs declining 

populations) may provide insights into the role of food composition on population trends. 

Metabolic activities of dreissenids can affect algal composition and thus the availability of essential nutrients. Examples of direct effects include selection/rejection of specific phytoplankton 

groups during filtration, and shifts in nutrient availability through metabolic excretion. An indirect effect includes the change in light climate resulting from dreissenid particle removal. Increased light penetration in the water column can lead to biochemical and physiological changes 

in phytoplankton and lead to subsequent changes in the presence of essential nutrients (Arts and 

Rai 1997). An example of the effects of nutrient limitation promulgated through the food web is 

the connection between thiamine deficiency and early mortality syndrome (EMS) in juvenile salmonids (Fitzsimmons et al. WP). Thiamine deficiency is caused by thiaminase, which is found in 

varying amounts in alewife and other clupeids. When adult salmonids feed on alewife, thiamine 

is catabolized, creating a deficiency and leading ultimately to mortality in the young. Changes in 

phytoplankton composition could potentially lead to similar deficiencies in Diporeia. 

In natural systems, food availability is one of the key factors in defining densities since it influences fecundity and survival. Sources and sinks of benthic food supplies (carbon) were examined along a depth gradient off the Keweenaw Peninsula in Lake Superior (Urban et al. WP). 

Pathways of carbon flow through components of the benthic system, including Diporeia, were 

quantified. The resulting model indicated a strong relationship between food inputs and standing 

stocks of Diporeia. The development of similar models in areas where populations are declining 

may provide insights into food availability relative to density trends. 

To investigate whether Diporeia are responding to a toxic agent associated with Dreissena, individuals were exposed to dreissenid biodeposits (pseudofeces) in a 90-day laboratory experiment (Dermott et al. WP). The percent survival of Diporeia exposed to pseudofeces was only 

75% compared to 100% in controls, which suggests that some unknown component associated 

with dreissenid biodeposits was having a negative impact on Diporeia. While these results may 

explain the loss of Diporeia in lakes with Dreissena, they are inconsistent with the persistence 

of Diporeia in other lakes also with Dreissena (i.e., Finger Lakes). The rapid loss of Diporeia in 
10

depositional areas of the Great Lakes does appear consistent with the toxic-agent theory. Biodeposits (and associated by-products) that are re-suspended from nearshore mussel beds would accumulate at high rates in these depositional areas. Also, the most severe population declines tend 

to occur after the spring period when deposition is at a seasonal maximum (Nalepa WP). Any 

toxicity associated with biodeposits is apparently short-lived. Sediments from depositional areas 

where Diporeia had disappeared were not acutely toxic to Diporeia in laboratory experiments 

(Landrum et al. 2000). 

Another theory for the decline is that mortality rates have increased because of a pathogen or disease. To investigate this possibility, Diporeia were collected over a wide area in Lakes Michigan 

and Huron and incidence rates of parasites and pathogens were determined (Messick WP). Many 

different types of pathogens were found, including virus-like infections, rickettsia-like organisms, fungi, haplosporidian-like organisms, microsporidians, putative epibiont ciliates, gregarians, cestodes, and acanthacephalans. However, except for epibiont ciliates, incidence rates were 

uniformly low (< 5%) and not higher in populations that were declining compared to populations 

that were stable. This would indicate that these pathogens were not a likely cause of declines. 

For epibiont ciliates, incident rates were high, but were similar to rates in the 1980s prior to the 

introduction of dreissenids and population declines.

Studies have shown that Diporeia are sensitive to various contaminants. This taxa exhibited high 

mortality rates compared to other amphipod taxa when exposed to riverine sediments containing various organic and inorganic contaminants (Gannon and Beeton 1969). Also, densities were 

lower in lake areas subjected to anthropogenic inputs (Vander Wal 1977, Kraft 1979). One theory 

for current declines is that populations are responding negatively to persistent contaminants accumulating in the Great Lakes. An example is the herbicide athrazine and associated metabolites, 

which in Lake Michigan are now near concentrations shown to lower survival of Diporeia in the 

laboratory (Ralston-Hooper et al. WP). An argument against this theory is that Diporeia has disappeared in areas far-removed from major sources of contaminants, including atrazine. 

Several developing technologies were put forth that may help define reasons for the decline. 

Genomic tools such as cDNA micro-arrays are useful in separating environmental factors that 

may influence organism health (Klapper et al. WP). An organism under stress will activate or 

deactivate suites of genes specific to the particular stress type. Thus, these gene “fingerprints” 

can be related to different environmental stressors such as food limitation, low oxygen, exposure 

to contaminants, etc. A similar approach involves the use of protein expression technologies 

(proteomics) (Ralston-Hooper et al. WP). Proteins serve as the functional effectors of cellular 

processes and the abundance, structure, and function of various protein complexes can be used as 

diagnostic tools to identify altered physiological processes at various levels of organization (molecular, tissue, and organism). Proteomics offers a method by which these stressors can be better 

characterized. Relevant to Diporeia, the functional protein complexes of most interest would be 

those associated with metabolic and nutritional pathways, protein destination and storage, metabolic transport, and disease-defense mechanisms (immune system). Both the genomic and proteomic approaches would compare profiles in individuals from declining and stable populations. 
11

RESULTS OF WORKSHOP SURVEY

After all presentations, workshop participants were asked to consider the most common theories 

for the disappearance of Diporeia and then choose and prioritize the top three. The first choice 

was to be given 3 points, the second choice 2 points, and the third choice 1 point. Theories included:

Decrease in Food Availability –This theory assumes that there has been a decline in the quantity 

and/or quality of food available to Diporeia as a result of the filtering activity of dreissenids. The 

underlying assumption is that energetic requirements are no longer being met, and/or an essential 

nutrient is no longer available. 

Toxic Excretions/Metabolic Wastes – This theory assumes that a metabolic by-product of dreissenid metabolism is having a severe negative impact on Diporeia. By-products can be directly or 

indirectly related to dreissenid excretion or biodeposition (feces/pseudofeces). For example, the 

proteinaceous mucous associated with dreissenid pseudofeces may be harmful to Diporeia when 

ingested, or it somehow interferes with Diporeia’s normal life habits. 

Disease/Pathogen/Infection – This theory assumes that a pathogen was introduced coincident 

with Dreissena and is now affecting Diporeia. This theory also includes the possibility that a 

change in conditions caused by Dreissena is making Diporeia more susceptible to pathogens that 

were present prior to Dreissena, or that a change in conditions has led to an increase in harmful 

pathogens. 

Contaminant Sensitivity – This theory assumes that a change in conditions caused by Dreissena

is making Diporeia more sensitive to resident contaminants, or is making contaminant uptake 

more likely. This theory includes negative impacts of newer contaminants such as atrazine and 

/or old-use persistent organochlorines and heavy metals. 

Fish Predation – This theory assumes that conditions have changed such that Diporeia are now 

more susceptible to fish predation. For instance, perhaps an increase in water clarity allow fish to 

more readily see Diporeia, or a decline in food availability has caused Diporeia to become more 

active at the sediment-water interface or in the water column. 

Diminished Oxygen – Diporeia are sensitive to reduced oxygen, and this theory assumes that 

oxygen at the sediment-water interface has been diminished because of the deposition and accumulation of dreissenid biodeposits. Biodeposits are rich in nutrients and bacteria and would have 

a high oxygen demand during decomposition. 

Results of the survey were: food limitation-29, toxic excretion-28, disease/pathogen-25, contaminants-9, diminished oxygen-6, and fish predation-3. Obviously, there was no clear consensus 

on a single, most probable cause. This may reflect perhaps the sometimes inconsistent results of 

previous studies, and/or the complexity of the issue. At least some aspect of all theories except 

“fish predation” and “diminished oxygen” were examined during workshop presentations. Some 

participants felt that several stressors were involved, and the relative importance of individual 
12

stressors varied depending on physical/chemical conditions associated with the particular lake 

environment. Regardless of exact cause(s), all participants agreed that the introduction and expansion of dreissenids was the event that initiated Diporeia’s decline. 

GENERAL DISCUSSION

Many workshop participants thought it would be useful to review the chronology of initial 

Diporeia declines relative to the expansion of dreissenids in each of the lakes. In Lake Erie, 

Diporeia was historically found only in the eastern basin since it is the only basin deep enough to 

sustain a summer-cold hypolimnion, a condition critical for the existence of Diporeia. Both zebra 

and quagga mussels were first found in the eastern basin in 1989, and the first post-dreissenid 

survey of Diporeia occurred in 1992. Densities were lower in 1992 compared to 1979, and another survey in 1993 indicated densities continued to decline as dreissenid densities increased. A 

further survey in 1998 failed to find any Diporeia. In Lake Ontario, dreissenids were first found 

in the western end of the lake in 1990, and by 1991 had spread across the lake to the east end. 

Declines of Diporeia were first noted in nearshore areas off Olcott, New York (south shoreline) 

in 1992, and in the Bay of Quinte (northeast) in 1993. In Lake Michigan, zebra mussels were 

first reported along the southwestern portion of the lake off Chicago in 1989, and by 1991 had 

spread northward along the eastern shoreline. Initial declines of Diporeia were recorded in the 

southeastern portion of the lake in 1992. A detailed account of initial declines in the main basin 

of Lake Huron is lacking. However, in outer Saginaw Bay, Lake Huron, zebra mussels were first 

recorded in 1991, and subsequent declines in Diporeia were observed in 1992. For all four lakes 

the large-scale pattern is clear – initial declines in Diporeia populations occurred soon (< 3 years) 

after Dreissena first became established. 

Given the strong negative relationship between Diporeia and Dreissena in Lakes Michigan, 

Huron, Erie, and Ontario, the persistence of Diporeia in some lakes with dreissenids outside the 

Great Lakes (i.e., Finger Lakes) is perplexing. In one of the Finger Lakes (Cayuga Lake) the 

density of Diporeia at a 45-m site was 3,750 m-2 in 2001 and 5,100 m-2 in 2002, even though 

Dreissena has been present at this site since 1994, and Dreissena density was 1,440 m-2 in 2001 

(Dermott et al. 2006). In comparison, at a 45-m site in Lake Michigan a clear decline in Diporeia

was evident when Dreissena density exceeded 14 m-2 (Nalepa et al. 2006). The Finger Lakes are 

deep and steeply sloped, and one theory is that coarse organic material from terrestrial sources 

(leaf debris) rapidly settles to the bottom and moves along the steep slopes to deeper regions. 

This material, along with associated bacteria, would not be filtered by Dreissena and would thus 

be available as a food source for Diporeia.

A topic of discussion was the uncertainty associated with Diporeia taxonomy. In the 1980s, the 

genus of freshwater pontoporeiid amphipods found in North America was changed from Pontoporeia to Diporeia, while the genus of its Eurasian counterpart was changed from Pontoporeia to 

Monoporeia (Bousfield 1989). Based on levels of genetic divergence using enzyme electrophoresis, the two organisms were more genetically diverse than previously believed, thus dismissing 

the theory of recent marine origins. In addition, the genus Diporeia was characterized as being a 

species complex, consisting of three, and possibly up to eight species (Bousfield 1989). Several 

of the species have yet to be described. Some researchers consider the distribution of genetic 
13

variation in the genus far too complex and too undefined at this point to even consider establishing how many species may be present (J. Witt, University of Waterloo, personal communication, 

post workshop). Because of the uncertainty regarding taxonomic status, field-collected specimens in North America are simply referred to as “Diporeia spp.” 

During subsequent discussions, it was argued that, if the different species have different distribution patterns and have different tolerance levels to an unknown toxic agent, observed differences 

in decline rates relative to Dreissena may simply be a function of species composition. Knowing 

the species present is therefore fundamental to understanding the cause of declines. There has 

been some preliminary work to define genetic variation in North American Diporeia. DNA analysis indicates that there are two distinct phylogroups of the genus that can be tentatively referred 

to as the western and eastern groups (J. Witt, University of Waterloo, personal communication, 

post workshop). The western group occurs in lakes in western North America and Lake Superior, 

and also occurs, but is rare, in Lake Huron. The eastern group is found in Lakes Michigan, Huron, Ontario, and in a number of deep lakes in the east, including the Finger Lakes, New York. 

Despite this differentiation, it is premature to assume that these phylogroups have unique adaptations that predispose them to different levels of success or failure relative to Dreissena. Genetic 

variation even in these phylogroups is substantial, and the only way to determine if there is a correlation between genetic variation and population declines is to analyze archived material from 

populations that have disappeared and compare it to material from populations that appear stable. 

Most likely, different rates of decline are a result of environmental conditions modifying responses, but it was agreed that more analysis is worthwhile to define genetic variation in Diporeia. 

Since preservation in formalin renders specimens useless for genetic analysis, participants were 

encouraged to preserve all specimens in alcohol rather than in formalin. 

The two deepwater amphipods Diporeia and Monoporeia may be farther removed on the evolutionary scale than previously believed, but the two genera apparently share a common sensitivity 

to environmental change. Monoporeia occurs in deep, freshwater lakes in Eurasia and also in the 

brackish waters of the Baltic Sea. Like Diporeia in the Great Lakes, Monoporeia is declining in 

the Baltic Sea (Perus and Bonsdorff 2004). Declines were first observed in the late 1990s and, 

interestingly, circumstances associated with Monoporeia’s decline, as well as uncertainties of potential causes, closely parallel those for Diporeia. The decline of Monoporeia in most areas was 

temporally coincident with the invasion of the Baltic by the polychaete Marenzellaria viridis. 

This polychaete is a detritivore and believed to be competing directly with Monoporeia for available food resources. Yet the decline in Monoporeia was greater in offshore areas (>30 m) than 

in nearshore areas, even though densities of Marenzellaria were greater in the latter (Cederwall 

et al. 1999). Low oxygen levels have been suggested as the reason for the decline in offshore 

waters, but others suggest it is food limitation (Cederwall et al. 1999; Kotta and Olafsson 2003). 

Also, in some parts of the Baltic, the collapse of Monoporeia occurred prior to the establishment 

of Marenzellaria. Another theory suggests that recent run-off events have shifted the phytoplankton community from diatoms to dinoflagellates (B. Sundelin, Institute of Applied Environmental 

Research, Stockholm University, Stockholm, Sweden, personal communication). Dinoflagellates do not sink as readily as diatoms, and therefore food inputs to the benthic region would be 

greatly diminished. 
14

Fish predation was the least likely cause of the decline as voted by participants, but some argued 

strongly it may be more important than many realize. Fish predation can greatly depress Diporeia

abundances (Johnson and McNeil 1986, McDonald et al. 1990), and there are many examples 

in the literature of a prey becoming more susceptible to a predator because of a sudden shift in 

environmental conditions, resulting in the total elimination of the prey. Several Dreissena-related 

changes may have caused Diporeia to become more susceptible to fish predation. The filtering 

activities of dreissenids increases light penetration and thus would make Diporeia more visible 

to fish predators. Dreissena filtering also decreases food availability, perhaps causing Diporeia 

to become more active in search of food, thereby also increasing its exposure to predators. These 

interactions are complex, and while impacts of light climate and the presence/absence of dreissenids on predator-prey relationships have been studied in some amphipod taxa (Gonzalez and 

Downing 1999, Mayer et al. 2001), studies specifically with Diporeia are lacking. Thus, the 

relative role of fish predation on Diporeia declines is unknown. Some evidence suggests that the 

role of fish, at least initially, may be minimal. If fish were the cause of Diporeia declines because 

of increased susceptibility to predators, then relatively more Diporeia would be found in diets 

just prior to and during population declines. Unfortunately, few studies have assessed shifts in 

fish diet patterns prior to and during declines, and those that have show a decrease of Diporeia

in diets in response to lower abundances (Pothoven et al. 2001, Hondorp et al. 2005). Further, 

as noted earlier, in Lake Michigan initial declines of Diporeia were characterized by a decrease 

in the number of juveniles while adults remained abundant. If fish were the cause, adults might 

be expected to decline prior to juveniles since fish tend to select for larger individuals (Evans 

et al. 1990, McDonald et al. 1990). Of course, selectivity could have changed given dreissenidinduced changes in the environment (i.e., increased light levels). Another consideration is the 

increased structural complexity of the bottom because of dreissenid clusters. Such complexity 

decreases susceptibility to predation for some amphipods (Mayer et al. 2001). Finally, Diporeia

are declining at depths > 100 m in Lakes Michigan, Huron, and Ontario. Since fish abundances 

are minimal and light conditions would have changed little at these depths in the post-dreissenid 

period, declines in this deep, offshore region would not likely be a result of fish predation. 

Profiles of genomic and protein expressions offer promise in characterizing the exact mode of 

negative physiological responses, yet much research is needed to realize the potential of these 

techniques. Individuals from stressed and unstressed populations may display different profiles, 

but the meaning (causes) of such differences need to be defined with carefully designed laboratory experiments. Individuals from unstressed populations need to be exposed to different kinds 

and levels of stress (low oxygen, starvation, etc.). The resulting profile would then be compared 

to profiles from stressed field populations. While stresses have been thus characterized for other 

biota, there has been no previous work on Diporeia. Given the apparent sensitivity of this genus 

to a variety of different environmental stressors, there are risks associated with this approach. 

Also, implications of dealing with an organism with high genetic variation are unknown. 
15

KEY QUESTIONS AND RECOMMENDATIONS:

1) Why do Diporeia populations within the Great Lakes appear to be more susceptible to Dreissena than populations in lakes outside the Great Lakes? 

Given that decline rates are highly variable in the Great Lakes, monitoring of populations in 

lakes outside the Great Lakes (i.e., Finger Lakes) is encouraged. Although populations in these 

lakes appear to be stable despite the presence of dreissenids, populations in areas thought to be 

“refugia” in the Great Lakes eventually declined to zero. Monitoring of dreissenids in these lakes 

is also recommended. The physiological health of Diporeia from these outside lakes should be 

assessed and compared to Great Lakes populations. 

2) How are Diporeia populations remotely affected by dreissenids? 

Evidence suggests an unknown toxic or inhibitory by-product associated with dreissenid biodeposits can be transported from nearshore to offshore areas, probably by normal currents or episodic storm events. This by-product does not accumulate or linger in the sediments since mortality cannot be induced when Diporeia are exposed to sediments from areas where the population 

has disappeared. Further laboratory studies are needed to verify and characterize a potential toxic 

agent(s) associated with fresh dreissenid biodeposits. Such studies should involve specimens of 

Diporeia and Dreissena collected inside and outside the Great Lakes. 

3) Can new biochemical tools be used to determine a cause of declines? 

Genomic and protein expression technologies offer great promise in providing insights into 

potential causes. To be useful, these techniques require studies that carefully coordinate field 

(comparison of stressed and unstressed populations) and laboratory studies (characterize stress 

responses). 

4) Do we need to define genetic variation within the genus Diporeia to fully understand the cause 

of declines? 

Knowing the extent of genetic variation would be useful and should proceed along with other decline-related research. While on a broad scale there are two genetically identifiable phylogroups, 

there is still substantial genetic variation within groups, and populations within a single Great 

Lake do not appear to be genetically uniform. Despite genetic variation, however, it remains that 

the entire population has disappeared in large areas of Lakes Michigan, Huron, Erie, and Ontario, 

and the one common feature in all these lakes is that declines were initiated by the introduction 

of Dreissena. If available, genetic composition of archived material from populations that have 

disappeared should be examined and compared to the composition of populations still present 

and seemingly unaffected by Dreissena. 

5) Is the Diporeia population in Lake Superior stable or declining?

Data sets are conflicting, but the general consensus is that populations are generally not declining. Intensive monitoring should be continued and all Lake Superior data sets should be com-
16

bined for further statistical analysis. For the GLNPO data set, which shows that densities in Lake 

Superior are trending lower at offshore sites while other Lake Superior data sets show no trend, 

the age structure of Diporeia in collected samples from the monitoring program in all the lakes 

should be determined. Populations at offshore sites in the other lakes (i.e., Lakes Michigan, Huron, Erie, and Ontario) are declining and, since shifts in age structure occur in declining populations, a comparison of age-structure trends between Lake Superior and the other lakes may provide further insights into density trends in Lake Superior. 

ACKNOWLEDGEMENTS

We thank workshop participants for all the insightful discussions and useful comments. We also 

thank the Cooperative Institute for Limnology and Ecosystems Research (CILER), University of 

Michigan, for their cooperation. The workshop was supported with funding from the Great Lakes 

National Program Office, Environmental Protection Agency, and the Great Lakes Environmental 

Research Laboratory, National Oceanic and Atmospheric Administration. This is GLERL Contribution No. 1381.

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WORKSHOP PRESENTATIONS

Alben, K. “Case histories of using algal pigments as biomarkers of food web relationships.”

Balcer, M., Schmude, K., Rockwell, D. C., and Barberio, R. “Diporeia are declining throughout 

the Great Lakes: does this include Lake Superior?” 

Dermott, R., Bonnell, R., Carou, S., and Jarvis, P. “Negative association between Diporeia and 

presence of zebra mussels or their pseudofeces.”

Dittman D. E. “Diporeia and Dreissena bugensis in southern Lake Ontario.”

Fitzsimons, J., Brown, S., Brown, L., Vandenbyllaardt, L., Williston, B., Williston, G., Tillet, 

D., Zajicek, J., and Honeyfield, D. “Possible role of thiaminase in the food chain on declines in 

Diporeia.”

1

Klaper, R., Goetz, R., and Janssen, J. ”Use of micorarrays to assess sources of stress on 

Diporeia.”

Messick, G. “Role of disease in the decline of Diporeia.”

Nalepa, T. F. “In search of a cause for declines of Diporeia populations in the Great Lakes.”

Ralson-Hooper, K. J., Sepúlveda, M., Ochoa-Acuna, H., and Nalepa T. F. “New approaches for 

measuring the effects of multiple stressors on Diporeia survival.”

Scharold, J. “Recent trends in abundance of Diporeia in Lake Superior.”

Watkins, J. “Status of Diporeia in Lake Ontario: results from LOLA benthic survey 2003.”

Urban, N. E., Auer, M. T., Auer, N. A., and Verhamme, E. “Diporeia distribution and carbon 

fluxes in Lake Superior.”

1

presented as a handout.
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Workshop Participants

Name Affiliation Email Address

Katherine Alben State University of New York at Albany alben@wadsworth.org

Nancy Auer Michigan Technological University naauer@mtu.edu

Mary Balcer University of Wisconsin -Superior MBalcer@uwsuper.edu

Paul Bertram Great Lakes National Program Office, EPA Bertram.Paul@epamail.epa.gov

Joann Cavaletto Great Lakes Environmental Research Laboratory, NOAA Joann.Cavaletto@noaa.gov

Ron Dermott Fisheries & Oceans, Canada DermottR@DFO-MPO.GC.CA

Dawn Dittman Tunison Laboratory of Aquatic Science, USGS ddittman@usgs.gov

Randy Eshenroder Great Lakes Fishery Commission randye@glfc.org

John Fitzsimons Fisheries & Oceans, Canada FitzsimonsJ@DFO-MPO.GC.CA

Steve Lozano Great Lakes Environmental Research Laboratory, NOAA Stephen.Lozano@noaa.gov

Thomas Nalepa Great Lakes Environmental Research Laboratory, NOAA Thomas.Nalepa@noaa.gov

Scott Peacor Great Lakes Environmental Research Laboratory, NOAA Scott.Peacor@noaa.gov

Michael Quigley Great Lakes Environmental Research Laboratory, NOAA Michael.A.Quigley@noaa.gov

Kim Ralston-Hooper Purdue University ralstonk@purdue.edu

David Rockwell Great Lakes National Program Office, EPA Rockwell.David@epamail.epa.gov

Jill Scharold Mid-Continent Ecology Division, EPA Scharold.jill@epa.gov

Don Schloesser Great Lakes Science Center, USGS dschloesser@usgs.gov

Marisol Sepúlveda Purdue University mssepulv@purdue.edu

Noel Urban Michigan Technological University nurban@mtu.edu

Glenn Warren Great Lakes National Program Office, EPA Warren.Glenn@epamail.epa.gov

Jim Watkins Cornell University jmw237@cornell.edu