I can't give you an answer. It could be one cause or another. The clearcut picture is that there was a severe decline, similar to the overall decline that occurred in the 1930s. You see this when you look at Hydro-Québec's data for their six stations near the mouth of the La Grande River.
There's another source of information on eelgrass, which comes from qualitative evaluations. Some have been done by the Cree people themselves, and others have been done by Hydro-Québec, which covers a larger part of the James Bay coast. But there are no quantitative data to confirm any changes that the qualitative evaluations would have provided. The only hard data we have pertain to the mouth of the La Grande River.