Characteristics of global port phytoplankton and implications for current ballast water regulations.

The International Maritime Organization and U.S. Coast Guard have implemented regulations to reduce introductions of non-indigenous species via ballast water (BW). For phytoplankton, regulations limit discharges to <10 live/viable cells mL (size: 10-50 μm), ignoring other size fractions. Additionally, challenge conditions of 100 (shipboard) and 1000 (land-based) cells mL are required in BW management system certification testing. How these requirements correspond to natural phytoplankton populations is poorly resolved. We analyzed phytoplankton samples from 31 major ports to evaluate: a) how natural communities compare to challenge requirements and b) abundances of unregulated size fractions (i.e., <10 and ≥50 μm). None of the ports met land-based challenge conditions, and only 32% met requirements for shipboard testing. Approximately 71% of organisms ≥50 μm were centric diatoms, also unregulated by current protocols. This study demonstrates that current regulations do not consider natural phytoplankton populations, limiting control efforts for potentially harmful non-indigenous species.