Pelagic-Benthic Coupling in Nearshore Lake Michigan: Linking Pelagic Inputs to Benthic Productivity

Primary Investigator:

Thomas Nalepa () - NOAA/GLERL

Co-Investigators:

David Fanslow () - NOAA/GLERL

Gretchen Messick ()
Research Biologist - NOS-Cooperative Oxford Laboratory*

John Robbins () - NOAA/GLERL

Eugene Stoermer ()
Professor of Natural Resources and Environment, SNRE - University of Michigan*

Executive Summary of Rationale

The amphipod Diporeia is declining throughout the Great Lakes with the exception of Lake Superior. This project closely examines the population dynamics and physiological condition of Diporeia at selected sites in the southeastern portion of Lake Michigan to determine potential causes for the decline and total loss of Diporeia in this region. In addition to exploring reasons for population decline, this project examines population dynamics of Diporeia related to environmental influences such as zebra mussels and potential food availability.

Proposed Work

Current/Ongoing
In 2007, we planned to analyze data from laboratory experiments conducted in 2003; however, other projects were given higher priority. The experiments were designed to compare the susceptibility of juveniles and adult Diporeia to food limitation. The following variables were measured after a period of starvation at different animal densities: carbon: nitrogen ratio, lipid content, and protein. Project collaborators are analyzing experimental results in 2008.

Past Accomplishments
This project began in 1997 to examine reasons for the decline of Diporeia in Lake Michigan. Physiological characteristics and population dynamics of organisms from various sites were examined. Selected physical and chemical variables at each site were also recorded. It was hypothesized that the decline of Diporeia was related to a decline in available food resulting from filtering activity of dreissenid mussels (zebra and quagga mussels).

1. Between 1997 and 2002, population densities of Diporeia were measured monthly (April-November) at four sites off St. Joseph, Saugatuck, Grand Haven, and Muskegon. Table 1 provides a summary of Diporeia decline in these four areas.

**Diporeia Population Densities**
	1997-98	1998	1999-2000	2002
St. Joseph	0 m ^-2
Saugatuck	no data	0 m ^-2
Grand Haven	10,000 m ^-2	0 m ^-2
Muskegon	10,000 m ^-2	4,000 m ^-2	‹ 500 m ^-2	0 m ^-2

Table 1. Diporeia population densities at varying locations (per m²).

2. Diporeia lipid content was also analyzed from 1997-2002. As population densities declined, lipid content of both juveniles and adults increased. If food was a limiting factor, it might be expected that lipid content would decline. An examination of abundances of young-of-year (0-3 mm), juveniles (3-5 mm), and adults (> 5 mm) before and after the establishment of dreissenids in 1989 indicated that reproduction was still occurring, but juveniles were not surviving to become adults (see Figure 1).

Figure depicting Diporeia densities in SE Lake Michigan

Figure 1. Densities of young-of-year (0-3 mm), juveniles (3-5 mm), and adults (>5 mm) at several 45-m sites before (1986-1989) and after (1997-2001) establishment of Dreissena.

3. Pelagic inputs, an indicator of potential food, were measured with sediment traps deployed from late March to mid October/November. Traps were deployed at all four sites in 1997 and 1998, but only at Grand Haven and Muskegon in 1999. Material collected was analyzed for total carbon and biogenic silica. Beginning in 1998, we also collected surface sediment samples for carbon content, biogenic silica, and chlorophyll.

4. In 2001 and 2002, we continued to collect monthly samples at the Muskegon site, and also added monthly collections at two other sites, a 45-m site off Little Sable Point, and a 95-m site off Muskegon. Diporeia were still abundant at both these sites. In addition to the previously mentioned biological variables, we also measured:

ETS (a growth-related transcription factor)
Gut-fullness
Carbon: Nitrogen

As seen in Table 1, densities at the 45-m site off Muskegon declined to zero in 2002. Densities at the 45-m site off Little Sable Point declined to zero in 2003. Thus, X-2 was the only site where animals were available to measure physiological variables in 2003. In addition to collecting animals on a monthly basis at X-2, laboratory experiments were conducted to determine if reduced food supplies were a potential cause of the decline. Juveniles and adults from X-2 were placed into sediments from X-2 and M-45 at various densities for a 90-day period. Mortality and various physiological variables were monitored. We also examined the incidence rate of epibionts, organisms which depend on a host for survival, attached to Diporeia at each of these sites over a seasonal period. The hypothesis was that perhaps a change in conditions induced by dreissenids may favor an increase of epibionts.

In 2006, we published a paper that summarizes density trends, lipid content, length-weight, and size-frequency of Diporeia populations at the various sites, and also presents trends in chemical variables from the sedimentation traps and the sediments. In 2007 we organized data collected from the laboratory experiments and the epibiont evaluation.

Scientific Rationale

This project was initiated in 1997 to examine reasons for the decline in Diporeia in southern Lake Michigan. Because of the decline in Diporeia, there will be disturbances in the pelagic food webs throughout the Lake, especially among organisms depending on Diporeia as a food source. The hypothesis for Diporeia decline is that the amount of food available to Diporeia has declined because of the filtering activity of zebra and quagga mussels. Other macrobenthic groups, such as oligochaetes, rely more on detritus and bacteria as a food source and may be utilizing zebra mussel feces and pseudofeces, therefore are not as affected by rising mussel populations.

Governmental/Societal Relevance

The loss of Diporeia in the Great Lakes is temporally linked to the introduction and spread of zebra and quagga mussels, but the exact reason for the decline is not clear. Diporeia has most significantly declined where there are no mussels in the immediate area and where food is seemingly still available. Diporeia is a native species and a key component in the food web. It feeds on organic matter freshly settled from the water column and in turn is fed upon by many fish species. Thus, it cycles energy from the lower to the upper food web. By providing clear links between the loss of Diporeia and dreissenid mussels, the public can be made more aware of the severe and often unexpected consequences of invasive species.

Relevance to Ecosystem Forecasting

Our measures of benthic food inputs and observed changes in physiological well-being of Diporeia have not provided conclusive evidence of a cause. Inconsistencies in results may suggest many causative factors whose relative importance may depend on specific environmental conditions. If food limitation is the sole cause, then a nutritional problem not evident from our measures of gross food inputs or physiological well-being is a possibility.

Our efforts, however, will provide guidance for future research in determining a cause for the decline and the link between the negative response of Diporeia and conditions created by Dreissena. By better understanding potential causes for the loss of Diporeia, we may better assess whether current populations of Diporeia in lake areas might also be at risk. This will lead to better predictions on the potential for population recovery and if other species are at risk. Since recent studies show that populations of some fish species (whitefish, sculpin) have been affected by the loss of Diporeia, knowing the reason for the decline will also lead to better predictions of long-term trends in the fishery.

Products

Presentations:

Nalepa, T. F. 2005. Aquatic invasive species and food web disruptions in the Great Lakes. Invaders of the Great Lakes: Options for Prevention and Management. Conference, Michigan State University, East Lansing, MI. March, 2005.

Nalepa, T. F. 2005. Food web disruptions in the context of invasive species and other stressors. Aquatic Invasive Species Symposium, National Wildlife Foundation, Chicago, IL. May, 2005.

Foley, A. J., Nalepa, T. F., and Walker, G. K. 2005. Associated epibiont populations and the decline of Diporeia spp. (Amphipoda) from Lake Michigan. 48th Conference on Great Lakes Research, International Association for Great Lakes Research, Ann Arbor, MI. June, 2005.

Nalepa, T. F., Fanslow, D. L., and Foley, A. J. 2005. Declines in the benthic amphipod Diporeia spp. in the Great Lakes: is there evidence for food limitation? ASLO 2005 Summer Meeting, American Society of Limnology and Oceanography, Santiago, Spain. June, 2005.

Nalepa, T. F. 2005. The disappearance of Diporeia in the Great Lakes: in search of a cause. NOAA Great Lakes Seminar Series, Ann Arbor, MI. September, 2005.

Nalepa, T. F. 2006. Aquatic invasive species and recent food web disruptions in the Great Lakes. Public Forum and Discussion, Inland Seas Education Association, Traverse City, MI, June, 2006

Nalepa, T. F. 2006. Aquatic invasive species and recent food web disruptions in the Great Lakes. Public Forum and Discussion, Sea Grant and Ann Arbor Public Schools, Ann Arbor, MI, August, 2006

Publications:

Nalepa, T. F., Fanslow, D. L., Messick, G. Characteristics and potential causes of declining Diporeia populations in southern Lake Michigan and Saginaw Bay, Lake Huron. Proceedings of Whitefish-Diporeia Workshop, Gt. Lakes Fish. Comm. Tech. Rep. 66, Ann Arbor, MI, 157-188 (2005).

Dermott, R., Munawar, M., Bonnell, R., Carou, S., Niblock, H., Nalepa, T. F., and Messick, G. Preliminary investigations into causes of the disappearance of Diporeia from Lake Ontario. Proceedings of Whitefish-Diporeia Workshop, Gt. Lakes Fish. Comm. Tech. Rep. 66, Ann Arbor, MI, 203-232 (2005).

Messick, G.A., R.M. Overstreet, T.F. NALEPA, and S. Tyler. Prevalence of parasites in amphipods, Diporeia spp. from Lakes Michigan and Huron, USA. Diseases of Aquatic Organisms 59:159-170 (2004).