Introduction

Surveillance sampling of benthic invertebrates is the newest element of GLNPO's biological monitoring program. Sampling was initiated in all five lakes in 1997 . Here we present results from 1998, the second year of GLNPO’s benthic invertebrate monitoring program.

Site description

Summer sampling for benthos was expanded slightly in 1998 by the addition of sites in Green Bay, Lake Michigan, Saginaw Bay, Lake Huron, and several sites in the western basin of Lake Erie. 

Sampling depths ranged from 12 m to 257 m, averaging just over 100 m, and substrates were characterized by varying proportions of silt, clay and fine sand. 

Sites in Lakes Erie and Superior tended to have a slightly lower percentage of fine sand; otherwise substantial differences did not exist from lake to lake.  The tendency towards finer substrates with increasing depth seen in 1997 was also seen in 1998; silt, clay and F (the inverse log of sediment grain size) all tended to increase with depth, while sand tended to decrease. 

Sediment nutrient concentrations varied between 2.1-83.0 (_X = 26.2) mg C gm DW^-1 for carbon; 0.05-4.9 (  X  = 1.3) mg N gm DW^-1 for nitrogen and 0.2-1.9 (X  = 1.0) mg P gm DW-1 for phosphorus, and were generally comparable to those found in 1997.  High values occurred at Green Bay, Lake Michigan and Saginaw Bay, Lake Huron.

 In general, though, large lake to lake differences were not found in these parameters.  Sediment nutrient content, most notably phosphorus, and percent water exhibited a tendency to increase with depth.

Benthic community

Most sites supported a very limited number of taxa, with maximum numbers of taxa per site ranging from 4 - 19 for the five lakes, and minimum numbers of taxa per site between 2 and 6. 

A larger number of taxa were found in 1998 compared to 1997 [link], though this increase was most likely the result of a change in analysts between those two years, rather than the result of any real changes in species richness.  There was a clear trend of greater species richness associated with higher trophic state; numbers of taxa increased along the sequence Superior->Huron/Michigan->Ontario->Erie.  Lake Erie supported the greatest number of taxa overall (35), while benthic invertebrate communities in Lake Superior were extremely taxa poor, with a total of only 6 taxa being found in the lake. 

Areal abundances of benthic organisms varied greatly within each lake, and to a lesser extent from lake to lake. 

Abundances varied from site to site within each lake by at least an order of magnitude, two orders of magnitude in the case of Lake Superior.  Particularly high abundances were found at sites in northern Michigan and western Ontario.  Lake-wide median abundances were somewhat more similar, varying from 1,400 to 3,700 organisms m^-2 with the exception of Lake Superior, where the median abundance was only 424 organisms m^-2. 

As in 1997, the amphipod Diporeia was by far the dominant benthic invertebrate in the upper three lakes; although it was notably absent from Lake Erie, nearshore sites in Lake Ontario, and Saginaw Bay in Lake Huron. 

Oligochaetes were the second most dominant group, and made up a majority of individuals at those sites where Diporeia did not.  They were the most diverse group, with a total of 26 different species identified in 1998.  Members of the oligochaete family Tubificidae made up at least 50% of the oligochaete communities in all lakes except Lake Superior, where members of the family Lumbriculidae were the most common.

The proportion of lumbriculids increased along the sequence Erie->Ontario->Huron/Michigan->Superior, which is in keeping with their preference for lower productivity environments.  Over a dozen genera of Chironomidae were also found in the lakes. 

The oligotrophic genus Heterotrissocladius was the only chironomid found in Lake Superior, and also dominated the off-shore sites of Lakes Michigan and Huron.  Communities in the lower lakes were more diverse, supporting notable populations of Chironomus, Procladius and Micropsectra, among other genera.

The most notable change in the benthos seen between 1997 and 1998 was the decline in the abundances of Diporeia across all lakes where it was found.  Of the 32 stations that were sampled in both years, 26 experienced declines in Diporeia populations, of which 17 were statistically significant declines. 

These declines were not confined to any one lake, but were distributed across all four lakes supporting Diporeia.  Declines in Diporeia populations have been studied in detail in southern Lake Michigan [http://www.glerl.noaa.gov/pubs/brochures/dipoflyer/dipo.pdf ] and western Ontario  our data suggest this phenomenon could be more widespread.  These declines do not appear to be due to sampling variability between the two years, but it is hard to draw conclusions regarding possible causal factors.  While it is possible that these changes might be 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 benthic communities to establish ranges of natural variation in population sizes, which should allow us to use these communities as more sensitive indicators of environmental changes in the Great Lakes.

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