The response of zooplankton and phytoplankton from the North American Great Lakes to filtration

Filtration of ballast water was investigated as a means of minimizing the introduction of nonindigenous zooplankton and phytoplankton by ships visiting the North American Great Lakes-St. Lawrence Seaway system (GLSLSS). An automatic backwash screen filtration (ABSF) system with nominal filtration options of 25, 50 or 100 μm was mounted on the deck of an operating Seaway-sized dry bulk carrier, the MV Algonorth. Water was pumped through the ABSF with a deck mounted pump at 341 m³ hr^-1 during routine ship operations in the GLSLSS, and effectiveness of the various screen pore sizes at removing taxonomic categories of zooplankton and phytoplankton was measured using matched treatment and control ballast tanks. The smallest pore sizes (25 and 50 μm) performed better than the 100 μm pore size at removing biological material. There was no difference in the filtration efficiency of the 25 and 50 μm screens relative to macro- or microzooplankton in these tests, but this result was probably due to low densities of macrozooplankton, and soft-bodied (aloricate) characteristics of the microzooplankton present. The 25 and 50 μm pore sizes were subjected to more controlled tests on board a stationary barge platform equipped with triplicate 700 L catchment bins moored in Duluth Harbor of Lake Superior. In these tests, filter pore size, organism size and rigidity influenced zooplankton removal efficiency by the ABSF. The 25 μm screen reduced both macrozooplankton and microzooplankton significantly more than the 50 μm screen. Zooplankton width was more determinative of filtration performance than length, and both filters removed loricate species of rotifers significantly more efficiently than aloricate species of the same length and width size classes. The 25 and 50 μm ABSF also significantly reduced algal densities, with the exception of colonial and filamentous green algae (50 μm only). Filter efficiency relative to algal particles was influenced by filter pore size, organism morphology and structure, and intake density, while algal particle size was not determinative. This research provides compelling evidence that 25 or 50 μm filtration is a potentially powerful means of reducing densities of organisms discharged by ships operating in the Great Lakes but an additional treatment step would be necessary to effectively minimize risk and meet the International Maritime Organization's discharge standards associated with organisms of all sizes in the water column.