Photosynthesis in an acre of Chesapeake Bay salt marsh can easily surpass the productivity of any Virginia farm in the Shenandoah Valley. Humans prefer to eat corn on the cob rather than marsh grasses, of course, so few of us normally think of salt water marshes as more valuable than corn fields. We may be picky eaters, but the decaying grasses are a great food source for the tiny zooplanton in the water that we can only see with a microscope. The submerged grasses create especially valuable habitat - and are sensitive to pollution. There was natural band of SAV surrounding the points of land and islands, but little SAV on the bottom of most of the Bay.
Light can only go through several feet of water naturally before being absorbed.
The algae also block the light from reaching the plant leaves, and compound the problem when it dies. Life adapts, over thousands of years of time - but in the short run we have come close to dramatically lowering the productivity and biodiversity in the Chesapeake Bay. If Powhatan somehow magically returned, he'd probably be awed by modern cars and houses and electricity - but he'd be stunned by how little food he could gather easily from the waters today. There's no guarantee that he'd consider the tradeoff to have been a good one. The Bay can be saved.