That depends on the intensity of the disaster, if you will. For example, there was a major decline in the 1930s. The rhizomes were torn out or destroyed, and died out. It took about 30 years before they grew back. As well, we are talking about a very large area. There were no beds in the vicinity and no great likelihood that new seeds could be reintroduced into the area.
I really have no idea what is currently happening in James Bay. We do not know what the situation really is. Are there rhizomes dormant in the sediment? That seems possible because I heard that there were still some stems to be found. There probably still are rhizomes living in the sediment. If that is the case, it will take time to reconstitute the larger beds. The smaller the rhizomes, the more sensitive they are to the hydrodynamics. If there are strong currents or waves, then the eelgrass is located in areas with a very high flow characterization. Small rhizomes have a harder time settling and growing in areas where there is a lot of current. Once they have settled and gained strength, the process accelerates.