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Geomorphic controls on the distribution of
freshwater mussels in the Angelo Coast Range Reserve |
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Jeanette Howard and Kurt Cuffey |
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Department of Geography, UC Berkeley |
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Introduction |
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Over the past decade, freshwater mussels in
many regions of the United States have been the subject of numerous
studies, motivated by observations of rapid declines in their populations.
Yet, little is known about the landscape-scale controls on mussel habitat.
Therefore, the main research objectives of freshwater mussel studies in the
Angelo Coast Range reserve are to better understand how physical conditions
in the Coast Range rivers' channels control the spatial distribution of
mussels. |
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Freshwater mussels in California provide a
unique opportunity for examining links between fluvial processes and biotic
communities, and for recording stream conditions because they are
long-lived (some species can live > 100 years), relatively sedentary,
directly depend on the base of the stream food web, and record annual
growth in their shells. |
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Research Questions |
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How are mussels distributed with respect to
longitudinal variations of channel type (i.e. pools, riffles, runs)? |
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With a given channel type (pool, riffle, run),
are mussels uniformly distributed? |
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Is the proportion of mussels in each habitat
type equal to the proportion of habitat available? |
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Are mussels found in refuge areas where
hydraulic stresses are lowest? |
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A total of ~ 12,000 M. falcata and ~8,000 A.
californiensis were found within the reserve. Mussels were found almost
exclusively in pools, aggregations were completely absent from summer flow
riffles, and 6 of the 114 aggregations
inhabited runs. |
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These graphs show how mussels (individuals
per 50-meter channel sections) are distributed in relation to the distance
downstream. Panel (a) depicts the distinct appearance of Anodonta
californiensis in the study reach; Panel (b) shows the locations of the
widely distributed Margaritifera falcata. |
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Conclusion |
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The spatial distribution of freshwater
mussels in the South Fork Eel River is characterized by high variability.
Mussels are located primarily in pools, with a few in runs, and none in
riffles. Direct measurements and the hydraulic model show that, in all
flows, mussels are found in areas of low boundary shear stresses and low
velocities (in pools, near banks, within root mats). |
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Long-lived species like mussels must
develop strategies to deal with extreme physical conditions of the Northern
California Coast Range Mediterranean environment (high winter flows and
summer droughts). Our study suggests that, at various spatial scales, M.
falcata appear to be distributed in a manner that protects them from the
highest flow-induced stresses where their chance of displacement is
reduced. |
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Notes