Compensatory effects of boat wake and dredge spoil disposal on assemblages of macroinvertebrates |
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Authors: | Melanie J Bishop |
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Institution: | (1) Centre for Research on Ecological Impacts of Coastal Cities, Marine Ecology Laboratories, A11, University of Sydney, NSW 2006, Australia;(2) present address: University of North Carolina at Chapell Hill, Institute of Marine Sciences, 3431 Arendell St, Mureherd City, NC 28557, USA |
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Abstract: | Estuarine assemblages are exposed to multiple disturbances that overlap in time and space. Along the Atlantic Intracoastal
Waterway (east coast, United States), two disturbances that frequently co-occur are the production of wake by boats and the
disposal of sediment dredged from boat channels. Boat wake generally coarsens sediments by eroding finer particles while deposition
of dredge spoil decreases mean grain size. If previously demonstrated effects of boat wake on infauna are due to coarsening
of grain size, deposition of dredge spoil on wake affected sites may, through compensatory effects, prevent an effect of wake
from being detected. Epifaunal assemblages associated with seagrass blades that are more likely to be structured by hydrodynamic
forces than granulometry may instead be affected by boat wake irrespective of the previous deposition of fine materials. To
test these hypotheses, in fauna and epifauna were sampled in patchy seagrass habitat at sites with and without boat wake that
were affected by historic deposition of dredge spoil and at sites without wake that had not received dredge spoil. Sediment
granulometry and infaunal assemblages differed between sites with and without dredge spoit but not between spoil affected
sites differing in exposure to wake. Epifaunal assemblages differed between sites with and without wake irrespective of sediment
granulometry. The effect of wake on epifauna was primarily due to lesser abundances of the gastropodBittiolum varium and the slipper limpet,Crepidula fornicata, at wake exposed sites. These results suggest that because of their opposing effects on sediment granulometry, boat-wake
and sediment disposal may have compensatory effects on infaunal assemblages. The detection of an effect of wake on epifauna
despite the absence of a sedimentological effect of the disturbance shows that ecological impacts do not necessarily mirror
physical effects and should be considered separately when adopting strategies of management. |
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