Each time a boat moves through water, its wake may be subtly altering the local ecosystem, according to a paper to be published in a forthcoming issue of the Journal of Experimental Marine Biology and Ecology.
Samantha Bickel, Joseph Hammond and Kam Tang have demonstrated that the turbulence generated by boat propellers kills considerable numbers of copepods, microscopic crustaceans that is a major component of zooplankton. This might significantly affect the food webs in areas of high boat traffic, since zooplankton transfers energy up the marine food chain from microscopic plants to fish.
The harmful effects of boat propellers on large marine organisms (most notably dugongs) are well known, but their effects on zooplankton have not been studied.
The authors first sampled three sites in the Hampton River (which flows into the Chesapeake Bay) for zooplankton. Two of the sites had little boat-generated turbulence, and the third was in an area where a high volume of boat traffic frequently travelled at high speed.
They next sampled zooplankton inside and outside the wakes of boats travelling along the lower York River.
Lastly, they subjected unfiltered York River water (which contained zooplankton) in an experimental chamber to turbulence generated with a stirrer made from a mixing paddle attached to a dual-speed motor.
The authors focused on the calanoid copepod Acartia tonsa, which is a dominant member of the zooplankton community in Chesapeake Bay.
They were able to determine the proportion of living and dead copepods from their sample by adding a dye (Neutral Red) to their samples: the live copepods would take up the dye and the copepod carcasses would not.
In the first experiment, the authors found that carcasses comprised a significant proportion (34%) of the copepod population in the site with the high boat-generated turbulence; carcasses only made up 5–6% of the copepod population for the other two sites.
In the second experiment, the authors found that the proportion of copepod carcasses increased from 8% outside the wakes to 14% inside the wakes.
Lastly, the authors found that the proportion of copepod carcasses increased with increasing turbulence in their third experiment.
Given that areas with high boat traffic constitute only a very small fraction of the marine environment, it is thought that the boat-induced zooplankton mortality would only affect the ecosystem at very local scales.
For more information, see the paper: Bickel, SL, JDM Hammond and KW Tang (2011) Boat-generated turbulence as a potential source of mortality among copepods. Journal of Experimental Marine Biology and Ecology DOI:10.1016/j.jembe.2011.02.038