Michigan Department of Natural Resources
Fisheries Research Report No. 2038, 1998.
A Thermal Habitat Classification for Lower Michigan Rivers
Kevin E. Wehrly, Michael J. Wiley
School of Natural Resources and Environment
The University of Michigan
Ann Arbor, Michigan 48109
and
Paul W. Seelbach
Institute for Fisheries Research
Michigan Department of Natural Resources
212 Museums Annex Building
Ann Arbor, Michigan 48109
Abstract.-We developed a thermal classification for Lower Michigan rivers that characterizes the spatial variation in summer (July) temperatures in terms of both mean temperatures and temperature fluctuations. We used patterns of change in community composition, species richness, and abundance of key species to partition continuous gradients of mean temperature and temperature fluctuation to identify discrete thermal categories.
We identified three mean temperature categories (cold <19 C; cool 19 to <22 C; and warm ³22 C) and three temperature fluctuation categories (stable <5 C; moderate 5 to 10 C; and extreme ³10 C). These categories were combined to create a 3 x 3 matrix with 9 discrete thermal regimes. Species distribution data were plotted on this 3 x 3 matrix to examine how selected species were distributed across thermal regimes. In order to quantify patterns of distribution and abundance, we calculated both the average density of a species within each thermal regime and the proportion of sites within each thermal regime where that species was present. We also generated habitat suitability scores within each thermal regime for each species in order to identify appropriate thermal habitats for individual species.
Within the MRI database, 92% of 667 sites occurred in the categories exhibiting moderately fluctuating temperatures. Relatively few sites occurred in the stable (3%) or extreme fluctuation categories (5%). The total percentages of sites were evenly distributed in cold (36%), cool (36%), and warm (28%) mean categories.
We found a continuous increase in species richness from sites with cold to warm mean temperatures. Species richness ranged from 6 in the cold-stable regime to 31 in the warm-stable regime. Within the cold and cool mean categories, species richness increased with increasing levels of temperature fluctuation. The opposite trend was observed within the warm categories with the lowest richness occurring at high fluctuations.
Distributions of fish representing distinct thermal guilds showed considerable overlap and in all cases species were distributed across more than one thermal category. Differences in species distribution patterns were also observed for fish within each thermal guild, indicating that.individual species within a guild occupied different thermal habitats. In general, there was poor correspondence between guild membership and fish presence within a thermal category.
The classification developed in this study provides a framework to describe the summer thermal distribution of stream fishes, and can be used to generate expectations of species assemblage structure and standing stocks of key species at sites having similar thermal characteristics. Biologically meaningful patterns in fish species assemblage and abundance of selected species were observed across gradients. This suggests that summer thermal regime may be an important factor structuring fish communities in Lower Michigan rivers.