Ecology and Vulnerability

Brook Floater

Background
The brook floater is a freshwater mussel species that inhabits streams and rivers with low to moderate flows¹.  When present in fast water, they will often be found in protected pockets behind boulders.  Brook floater are never found in lakes or reservoirs.  They appear to require relatively undisturbed stream reaches and have little tolerance for stressors such as dams, urban areas, and poor water quality.  Brook floaters are found in Atlantic coastal rivers from South Carolina to Nova Scotia and New Brunswick.  As with other mussel species, the larval phase is a parasite that requires a fish host for development, after which it will drop off to continue development as a benthic adult.  Several potential host fish species for brook floaters have been identified in lab settings, but it is unknown if these are all hosts under natural conditions.  Adults live partially buried in the sediment where they filter algae, bacteria, zooplankton, and sediment from the water.

Freshwater mussels in general have experienced large declines² and the brook floater is no exception.  It is one of the most endangered mussels in northeastern North America and is listed as endangered in Massachusetts¹.  Remaining populations in Massachusetts are fragmented and appear to consist mostly of older adults showing little evidence of recent reproduction.

Climate Impacts
Brook floaters have a high thermal tolerance so even significant warming of Massachusetts waters is not likely to reach lethal temperatures.  Lethal temperature for the most sensitive developmental stage is approximately 35°C/95°F³, which is about 10°C/18°F warmer than for the coldwater fish species, brook trout, currently found in Massachusetts⁴.  It is unknown how sublethal temperatures may affect brook floaters, although water temperature directly relates to water quality such as dissolved oxygen.  As water temperature increases, the amount of dissolved oxygen that water can hold decreases⁵.  Additionally, a lengthened growing season and increased evapotranspiration are projected to increase the frequency of short-term drought conditions⁶.  Potentially, such changes could impact water quality in ways that affect brook floater populations⁷. 

Because mussels require a host fish species for larval development^1,7, impacts on these fish could also impact mussels.  Species identified as potential hosts for brook floaters include cold to cool water species, such as blacknose dace and slimy sculpin. These fish require cooler temparatures and may be impacted by significant warming of stream water ⁸.