Mussels and Their Habitat
Mussels and Their Habitat
How does sediment in a river system affect mussels?
Human activity has greatly altered the water quality of the Cannon River and many other waterways throughout North America in the past two centuries. Activities such as row cropping and widespread construction have lead to vast amounts of erosion which greatly affect the turbidity and overall quality of a river's water (Robbins, 1996). A large number of mussel species have gone extinct or have suffered severe decline in numbers in the past century. A principle cause of such population loss was over-harvesting of mussels for the pearl industry in the U.S. that began in the late 1800's and continued until World War two (Sparks). Another contributor to the reduction in mussel populations since then has been extreme buildup of suspended and deposited sediment in America's river systems, including the Cannon (Williams et al., 1993).
What are the ecological impacts of dam removal?
A dam creates a pool of slowly moving water in the river immediately upstream. Sediment particles in the river slow in this pool and are deposited along the riverbed, forming a large plume just behind the dam. A prime concern when removing a dam is the release of this sediment. Depending on the span of time during which the sediment has accumulated, this plume could cause problems downstream by significantly increasing turbidity. Such a drastic change in water quality could jeopardize the survival of organisms that filter-feed or rely on light to penetrate the water (Alexander et al,1986).
Does sediment build-up affect mussel growth?
Growth rate is greater in mussels living in sand/gravel sediment than those found in conditions of higher silt concentrations (Kat 1982). Kat attributed the mussels' higher growth in less turbid environs to silt occluding their siphons, interfering with the mussel's ability to filter feed the surrounding waters. Similarly, mussels located in waters of heavy siltation and erosion, such as downstream from dredging areas also show reduced growth rates (Marking and Bills, 1979). However, at least one species, Lampsilis radiata siliquoidea, prefers muddy substrates over gravel or sand surfaces (Baily 1989). Baily's study indicates variation in substrate preference among species of mussels.
In some cases, mussels may be buried by silt build-up. One study showed that mussels buried up to 15 cm under silt have a 50% chance of emerging over 6 hours before death (Marking and Bills, 1979). Some species such as the Lampsilis ventricosa, were more adept at emergence. The study attributed the superior motility of L. ventricosa to its relatively large, broad foot. Others species cannot emerge from silt as effectively. Pleurobema sintoxia, for example, showed rates of 45% emergence at 10 cm. These findings indicate differences among mussel species in their susceptibility to being buried by silt.