Evaluating the combined effects of ballast water management and trade dynamics on transfers of marine organisms by ships

Abstract

Global trade by merchant ships is a leading mechanism for the unintentional transfer of marine organisms, including non-indigenous species, to bays and estuaries worldwide. To reduce the likelihood of new invasions, ships are increasingly being required to manage their ballast water (BW) prior to discharge in coastal waters. In the United States, most overseas arrivals have been required to manage BW discharge since 2004, primarily through ballast water exchange (BWE), which flushes out ballast tanks in the open ocean (>200 miles from shore). Studies have found BWE to generally reduce the abundance of organisms, and the amount of water exchanged has been estimated at 96-100%. Despite its widespread use, the overall effect of this management strategy on net propagule supply through time has not been explored. Here, temporal changes in zooplankton concentrations and the volume of BW discharged in Chesapeake Bay, U.S. were evaluated, comparing pre-management era and post-management era time periods. Chesapeake Bay is a large port system that receives extensive BW discharge, especially from bulk cargo vessels (bulkers) that export coal overseas. For bulkers arriving from overseas, mean zooplankton concentrations of total and coastal indicator taxa in BW did not decline between pre- (1993-2000) and post management (2012-2013) eras, when controlling for season and sampling method. Moreover, bulkers discharged 21 million tonnes (82% of total for Chesapeake Bay) of overseas BW in 2013, representing a 374% increase in volume when compared to 2005. The combination of BW discharge volume and zooplankton concentration data indicates that (a) net propagule supply by bulkers has increased since BWE began in Chesapeake Bay; and (b) changes in vessel behaviour and trade have contributed strongly to this outcome. Specifically, the coal-driven increase in BW discharge volume from 2005-2013, concurrent with the onset of BWE regulations, worked to counteract intended results from BW management. A long-term analysis of bulker arrivals (1994-2013) reveals a 20-year minimum in arrival numbers in 2000, just when the implementation of BWE began. This study underscores the need to consider shifts in trade patterns, in order to advance and evaluate effective management strategies for biological invasions.

Fig 1

Concentrations of zooplankton in pre- and post-management era ballast water samples, for (A) total zooplankton, (B) total zooplankton minus copepod nauplii, and (C) coastal zooplankton designated taxa only. Values are the mean of 20 samples and 30 samples ± SE for pre-management (unexchanged) and post-management (exchanged), respectively. * shows statistically significant data.

Fig 2. Zooplankton concentration (unexchanged, empty-refill and flow through exchange) with respect to voyage length, ballast water age and salinity.

(A) Voyage length (i.e. time from uptake to time of discharge) vs. zooplankton concentration (none, empty-refill and flow through exchange). (B) Ballast water age (i.e. time from BWE to time of discharge) vs. zooplankton concentration (none, empty-refill and flow through exchange). (C) Salinity vs. zooplankton concentration (none, empty-refill and flow through exchange).

Fig 3. Total volume of ballast water discharged from all vessels types (total) and total overseas (coastal uptake location) ballast water discharged by bulkers arriving at Chesapeake Bay ports annually between 2005 and 2013 [45].

Volume of coal exported from the Ports of Baltimore and Norfolk located in Chesapeake Bay between 2005 and 2013 [46].

Fig 4. The number of total vessel arrivals and bulker arrivals to Chesapeake Bay between 1994 and 2013.

(A) Arrivals to Chesapeake Bay based upon reports to the Baltimore and Hampton Roads Maritime Exchanges (MarEx) and MARAD from 1994–2005 and to NVMC from 2005–2013. (B) The number of overseas arrivals by bulk cargo vessels to Chesapeake Bay ports as reported to MARAD and NVMC. (C) The percentage of all arrivals to Chesapeake Bay that were overseas arrivals by bulkers as reported to MARAD and NVMC. n/a: Vessel type data were incomplete in 1994 and were excluded from analysis.