Benthic Community Health

Benthic invertebrate communities are well suited for use as biomonitoring tools, because the various benthic organisms have differing sensitivities to environmental stressors. By measuring the diversity of the benthic community, we can gain some insight into the level of human impacts on the aquatic system

Benthic invertebrates are longer lived than most planktonic organisms, and thus will indicate the effects of environmental conditions over time. They are relatively sedentary, therefore easy to sample and can serve as indicators of specific areas. In addition to serving as indicators of ecosystem condition, many benthic invertebrates are also important components of fish diets and provide an important link in the food chain.

A number of different approaches can be employed when using benthic invertebrates to monitor aquatic systems. Here, two complementary approaches were used: assessment of the population of the sensitive species Diporeia, and use of an index of oligochaete community make-up.

Diporeia: The amphipod Diporeia has historically been one of the most abundant and widespread organisms in the Great Lakes (Dermott and Corning, 1988). This surface-feeding detritivore is important to the diet of many fish, (Scott and Crossman, 1973) and is particularly important in assessing open lake conditions. Diporeia is sensitive to low oxygen concentrations and to many toxicants (Nalepa and Landrum, 1988), and due to its high lipid content and absence of biotransformation capability, has a high bioaccumulation potential for organic contaminants (Landrum and Nalepa, 1998). These characteristics make it an appropriate organism for biomonitoring, both for its inherent ecological importance, and for its potential usefulness as an indicator of overall system health.

The State of Diporeia: Provisional target abundances of Diporeia have been established for different depth ranges in the Great Lakes (SOLEC, 1998). When applied to GLNPO's benthos data from 1997, all stations in the upper lakes either met or exceeded these criteria. While historically present there, Diporeia is not currently found in Lake Erie. Diporeia was absent from half the sites examined in Lake Ontario in 1997, and met the criteria in four of the remaining five sites.

In 1998, numbers were substantially lower in the upper lakes, with the result that four sites in Lake Superior that had exceeded abundance criteria in 1997 simply met the criteria, while two sites in Lake Michigan that had exceeded the criteria slipped below criteria.

Two sites were added in 1998, one in northern Green Bay and one in Saginaw Bay; both were below criteria. In contrast, abundances of Diporeia at one site in Lake Superior increased sufficiently to exceed the criteria, when in 1997 it had merely met criteria. Results from Lakes Erie and Ontario were identical to the previous year.

The Future of Diporeia: The declines noted in the upper lakes were statistically significant, and thus do not appear to be due to sampling variability between the two years. However, it is hard to draw conclusions regarding possible causal factors, particularly from only two years of data. While it is possible that these changes might reflect changes in water quality in the lakes, it is also possible that they merely represent natural annual fluctuations in recruitment or mortality. It will be necessary to continue to monitor these populations to establish ranges of natural variation.

Milbrink Oligochaete Indicator: The association of oligochaetes with organic enrichment of water was first noted by Aristotle (Hynes, 1960). A number of classification systems have since been developed to try and quantify that relationship. Howmiller and Scott (1977) introduced an index based on community structure, where species were assigned to categories depending on their preference for, or tolerance of, oligotrophic, mesotrophic, or eutrophic conditions.

Milbrink (1983) modified this index to account for differences oligochaete abundance and to accommodate the ecological affinities of Tubifex tubifex, which can be abundant in areas of high, moderate, and low pollution levels. Additionally, he added a fourth species to Howmiller and Scott's original three, Limnodrilus hoffmeisteri, or T. tubifex in instances where total numbers are high and L. hoffmeisteri is a co-dominant.

We have adopted Milbrink's modifications of Howmiller and Scott's original index, while retaining the latter's original classification of species on the basis of these being more appropriate to the Great Lakes. The index is calculated as:

where n0, n1, n2 and n3 are the total numbers of individuals belonging to each of the four ecological groups. Species that are characteristic of oligotrophic waters are assigned to Group 0, those of mesotrophic waters to Group 1, and those of eutrophic waters to Group 2. L. hoffmeisteri and T. tubifex (under the conditions stated above) comprise Group 3.

The coefficient c depends upon total oligochaete number as outlined below:

c=1 n > 3 600 c=3/4 1 200 < n < 3 600 c=1/2 400 < n < 1 200 c=1/4 130 < n < 400 c=0 < n < 130

Milbrink considered index values between 0.6 and 1.0 suggestive of mesotrophic conditions, while higher and lower values indicated eutrophic and oligotrophic conditions, respectively.

Milbrink Oligochaete Indicator Accuracy: There are a number of limitations to this approach, such as the training of the oligochaete sampler, the presence of (or lack of) oligochaete communities, the seasonality of certain species (Limnodrilus hoffmeisteri), and other unknown stressors that may be effecting these communities. Finally, information about ecological tolerances of oligochaetes continues to be refined, therefore it is expected that changes in the classification of constituent species will occur.

In general, however, when the index is applied to data generated from GLNPO's monitoring program, it appears to give a reasonable evaluation of trophic conditions in the lakes. Most sites in the upper lakes fall into the oligotrophic category, with areas of known higher productivity (nearshore northern Lake Michigan; Saginaw Bay, Lake Huron) exhibiting higher index values. Sites in Lake Erie generally fall in the mesotrophic range, while in Lake Ontario nearshore sites were classified as mesotrophic, and offshore sites are oligotrophic.

Future Actions: We believe these two approaches complement each other, and should provide a robust indicator of benthic community health. While the primary strength of Howmiller and Scott's Environmental Index is in assessing organic enrichment, Diporeia provides a more sensitive indicator of other environmental stressors, such as toxics, and can provide an indicator of benthic community health in environments that don't support substantial oligochaete communities. It should be noted, however, that this would be the first benthic index routinely applied to the open waters of all the Great Lakes. Refinements in interpretation of this index should be expected.

Acknowledgements

Dermott, R. and K. Corning. 1988. Seasonal ingestion rates of Pontoporeia hoy (Amphipoda) in Lake Ontario. Can. J. Fish. Aquat. Sci. 45:1886-1895.

Howmiller, R.P. and M.A. Scott. 1977. An environmental index based on relative abundance of oligochaete species. J. Water Pollut. Control Fed. 49:809-815.

Hynes, H.B.N. 1960. The Biology of Polluted Waters. Liverpool Univ. Press, Liverpool, 202 pp.

Landrum, P.F. and T.F. Nalepa. 1998. A review of the factors affecting the ecotoxicology of Diporeia spp. J. Great Lakes Res. 24:889-904.

Milbrink, G. 1983. An improved environmental index based on the relative abundance of oligochaete species. Hydrobiologia 102:89-97.

Nalepa, T.F. and P.F. Landrum. 1988. Benthic invertebrates and contaminant levels in the Great Lakes: Effect, fates, and role in cycling. In Toxic Contaminants and Ecosystem Health: A Great Lakes Focus. ed. M.S. Evans, pp. 77-102. John Wiley & Sons, New York, NY.

Scott, W.B. and E.J. Crossman. 1973. Freshwater Fishes of Canada. Bull. Fish. Res Board Can. 184.

SOLEC (State of the Lakes Ecosystem Conference). 1999. Selection of Indicators for Great Lakes Basin Ecosystem Health. Draft for Review, v. 3. U.S. EPA, Chicago, IL.