Freshwater ecosystems are threatened by a host of environmental stressors from human activities. Among the most insidious and impactful of these is invasion by non-native species.
October 13, 2022
Freshwater ecosystems are threatened by a host of environmental stressors from human activities. Among the most insidious and impactful of these is invasion by non-native species.
Over the past two centuries, established populations of nearly 190 non-native species of invertebrates, fishes, plants and microbes have been discovered in the Great Lakes basin. They were introduced through several sources and pathways including canals, pet release, bait bucket dumping, aquaculture escapes and—most notably—ballast water discharge from transoceanic ships.
Ballast tanks of ships may contain various life stages of invertebrates, larval fishes and enormous numbers of phytoplankton and zooplankton. The discharge of ballast water is responsible for thousands of coastal invasions worldwide. It has been the most important cause of invasion in the Great Lakes since 1959, when the modern St. Lawrence Seaway was opened to accommodate larger ocean vessels carrying cargo.
In the mid-1980s a cargo ship from eastern Europe began an overseas voyage to the Great Lakes. Before the ship left its home port, water was pumped into its ballast tanks to add weight and stability to safely cross the Atlantic Ocean. The water contained larvae of an invasive freshwater mussel. After the ship passed through the St Lawrence Seaway, it discharged some of the water—and mussel larvae—at a Great Lakes port, thus introducing the zebra mussel to North America. This scenario has occurred for many other species.
Effective management of the ballast water pathway is crucial to slowing the rate of invasion in the Great Lakes and protecting its resources. Here, I describe an intervention that has apparently achieved this goal.
Gaps in ballast water management
From 1959 to 2006, one new invader was discovered established in the Great Lakes basin every six to seven months, on average. No other freshwater system on the planet has been invaded as frequently.
Nearly two-thirds of these species were delivered in ballast water. They include invaders that have reduced native biodiversity, impaired fisheries and caused other ecological and socioeconomic impacts in the Great Lakes.
In 1993, Canada and the United States attempted to control ballast-water invasions by requiring inbound ships to exchange their freshwater ballast with saltwater before entering the Great Lakes.
