Numbers and lipid content of Diporeia collected in 1999 were also determined. The 1999 results confirmed a trend observed in previous years. That is, numbers continued to decline, while lipid content continued to increase. The trend in lipid content is contrary to the theory that a depletion of food resources is the primary cause for the decline in numbers. Diporeia declined, and is now absent from the Grand Haven site, while individuals are still present at the Muskegon site.
Preliminary results indicate no differences in either short term or long term responses.
Both sites have a substrate consisting of silty sand, which contrasts to the silt substrate found at St. Joseph, Saugatuck, and Grand Haven. Diporeia prefers the former substrate, and data collected in 1999 will provide information on the relationship between decline rate and substrate type. We will not take monthly samples at these last three sites since Diporeia is no longer present. In addition to collecting samples for estimates of Diporeia density and production, we will also deploy sediment traps at these sites. Analysis of samples collected in 1997 and 1998 will continue.