Byssal threads remaining after mussel detachment

Besides modifying nearshore nutrient cycles and enhancing Cladophora growth, zebra mussels may play a role in causing more Cladophora to be released from the bottom than would naturally occur.  There are two detachment points for Cladophora; one is at the base of the filament itself, and the other is by the byssal threads of the mussels that Cladophora is attached to.  Possible mechanisms for mussel detachment are presently being explored.  It is possible that a Cladophora canopy depresses either filter feeding or gas exchange/waste removal by the mussels, and the mussels detach either by deterioration of health or by some intentional mechanism to obtain more nutritional resources.  The algal filaments may promote this detachment by acting as a drogue in the water currents.

Cladophora pulled from rock reveals it is attached to zebra mussels

Regardless of the mechanism that causes the mussel detachment, mussels, both living and dead, invariably wash up on the shore with Cladophora attached to their shells.  While mussels have been washing ashore since they became established in the Great Lakes, washed up mussels do appear to be smaller than they were prior to the increased Cladophora levels.  Also, sometimes it is possible to pull mussels from the substrate by pulling on heavy growths of Cladophora that are attached to them, making the mussel attachment the 'weakest link.' 

Mussel reefs may provide additional hard substratum for Cladophora to attach to, but it has been recently observed that quagga mussel shells may not be as conducive to Cladophora attachment, possibly because they have smoother shells than zebra mussels.  Indeed, it appears that zebra mussels are more likely to detach and wash ashore with Cladophora attached.  The ratio of quagga mussels to zebra mussels in the littoral zone of Lake Michigan appears to be on the rise, and this trend is likely to continue.  These changing ratios could impact Cladophora dynamics.

Rotting vegetation is often blamed for the odor that beached Cladophora is associated with.  We have observed that these odors are likely enhanced by the presence of rotting animal material, such as mussels, crustaceans, fish, etc.  This idea is substantiated by the presence of waterfowl, which congregate at wash up locations for feeding.  Also, there are many reports of washed up Cladophora that does not stink.

Illustration of how zebra mussels may contribute to detachment of Cladophora