Complex sentence: Pamlico Sound is part of a large interconnected network of lagoon estuaries, which as a whole is the second largest estuary in the United States (Chesapeake Bay being the largest).
Split into simpler sentences: Pamlico Sound is part of a large, interconnected network of lagoon estuaries. As a whole it is the second largest estuary in the United States; (Chesapeake Bay is the largest).