Crayfish are very interesting creatures to me and in retrospect ... as is often stated around here ... "it depends" probably applies to them as well. They are a habitat modifier whose effects can be keystone to shaping a community. They denuded the vegetation in the lake that esshup mentioned. This probably impacts pan fish population structure where crayfish replace pan fish as prey. To be sure, changing where the nutrients are utilized have an impact on water. When they aren't going into macrophytes, nutrients support competitors like phytoplankton or other types of macrophytes (for example the grass blade blooms wbuffetjr is observing). Their contribution to nutrient cycling is noteworthy but I can see where this effect could be excessive (possibly detrimental) in water with excessive nutrients.
