Review of techniques for monitoring impacts of dredging on seagrass: a case study from north Queensland

Measuring impacts to seagrass in tropical ecosystems is confounded by high short and long-term variation in the distribution and composition of these communities: few monitoring programs run for long enough to adequately... [ view full abstract ]

Measuring impacts to seagrass in tropical ecosystems is confounded by high short and long-term variation in the distribution and composition of these communities: few monitoring programs run for long enough to adequately account for this, or to establish the methodologies that are most suitable. While BACI designs can assist, it is often difficult to find suitable control locations, and the scale of ‘natural’ temporal change is commonly underestimated.

This paper reviews techniques used for seagrass monitoring and references a relatively long-term monitoring program in north Queensland.

Construction of the Port of Airlie involved the excavation and handling of approximately 1,000,000 m3 of material within an enclosed area of Boathaven Bay at Airlie Beach, north Queensland, and dredging of approximately 120,000 m3 to construct a navigation channel.

Four different techniques were used to assess potential impacts:
• mapping the distribution and community composition of seagrass
• monitoring changes in the depth range of the dominant species, Zostera muelleri
• monitoring changes in biomass along predicted impact gradients, and
• videoing seagrass communities.

Seagrass was monitored for two years prior to construction (2004 & 2005), during construction (2007 & 2008), and after dredging was completed (2012 & 2014).

Fortuitously, seagrass depth range in Boathaven and nearby bays had also been monitored for approximately 10 years prior to construction.

With the exception of the video transects, all of the techniques provide useful information to measure change, and assess likely impact. Mapping seagrass communities and depth range transects have indicted substantial variation in seagrass distribution, with the greatest depth range recorded 6 years post-construction. Impact gradient assessment indicates that near-field impacts can be less than previously predicted. While mapping seagrass distribution provides valuable insights into the means of impact, where in-water visibility is poor, it can be very time consuming. In contrast, depth range and impact gradient transects are a relatively rapid form of assessment.
We compare the assessment techniques used at Port of Airlie to a range of other commonly used techniques.