Seagrass-Watch Magazine Issue 47 - March 2013

Australia dredging effects on food webs Article & photography by Kathryn McMahon A third of the world's population lives in coastal areas. As a consequence there is significant development and infrastructure here. Ports with Western Australia associated access channels and facilities are a key part of the transport system supporting the coastal infrastructure. For example, in Australia 20% of the gross domestic product is from the export industry, which relies on ports. Dredging is carried out to maintain existing ports and for expansion to allow access for larger ships, and for the construction of new ports, as well as other activities such as land reclamation, beach nourishment and laying of pipelines and cables. Seagrasses, which live in coastal waters, are vulnerable to dredging activities. They can be impacted directly through physical removal at the dredge site, or smothering by sediments at the dredge disposal site. Indirect impacts also occur through the generation of turbid plumes by sediment particles, which are suspended in the water column and reduce light reaching the meadows. Seagrasses are incredibly sensitive to reductions in light as they have a high light requirement. Dredging-related seagrass losses have occurred at scales of km and it has been estimated that up to 21,000 hectares of seagrass meadow has been lost world-wide in the past 50 years, most likely an underestimate. Dredging does not always result in impacts to seagrass, but in some cases it can cause the complete loss of seagrass meadows and therefore the ecological services that these meadows provide is also lost. In other cases, meadows are not lost but are impacted through reductions in the amount and structure of seagrass present. Research in Western Australia found that reductions in the amount and structure of the seagrass present, including the associated algal epiphytes resulted in about a 40% decline in small invertebrates living in the seagrasses, gastropods were most sensitive. This has significant consequences for the associated food webs that the seagrass meadows support. Modelling studies suggested that although the macroinvertbrates could recover once the seagrass meadow recovered, within approximately one year, there were longer term implications for larger fauna such as fish, which could take from 2-10 years to recover. 16 WWW.SEAGRASSWA TCH.ORG

Seagrass-Watch Magazine Issue 47 - March 2013 | Page 16