Merrymeeting Bay is a 4330-ha freshwater, tidal ecosystem that historically provided vital spawning and nursery habitat for anadromous fish as well as stopover habitat for migrating waterfowl. Formed by the confluence of six rivers, Merrymeeting Bay contains expansive intertidal flats that support productive communities of wild rice (Zizanea aquatica) and other emergent plant species. To understand a precipitous drop in the abundance of anadromous fish and waterfowl during the mid- and late 20th century, we reconstructed a history of environmental change for Merrymeeting Bay and its tributaries using oral histories, aerial photography, field experiments, and duck harvest and juvenile fish surveys.
Results/Conclusions
Our studies revealed that two centuries of overfishing, deforestation, dam-building, and water pollution led to the collapse of this important ecosystem in the mid-20th century. Anoxia and widespread fish kills were common and the submerged aquatic vegetation disappeared along with the benthic food web. Ironically, the unregulated discharge of industrial and municipal wastes and high rates of sedimentation associated with land use probably enhanced the extent and productivity of the intertidal emergent plant communities in the middle decades of the 20th century, attracting more migrating waterfowl than would have otherwise been the case. Following implementation of environmental regulation, water quality improved dramatically. However, key components of the ecosystem have not recovered. Today, submerged aquatic vegetation covers less than 3% of the total area of shallow subtidal habitat and anadromous fish occur in vestigial abundances. Although the water quality of Merrymeeting Bay has improved and fewer dams hinder upriver passage to potential spawning habitat, alewives, blueback herring, and American shad have not made a strong recovery. Our historical research suggests that the collapse of submerged aquatic vegetation and the benthic food web may undermine the recovery of these important forage fish for Atlantic cod and other nearshore predators.
