Monitoring of Posidonia meadows under the EC Habitats Directive: vehicular videography can estimate trends in coverage at low cost and high precision

Regular monitoring of Posidonia oceanica (L.) (Posidoniaceae) is required under EC directives, and its purpose is to inform management agencies when and where actions are necessary to protect and restore ecological health and water quality. The Habitats Directive requires each EU member nation to report specifically on temporal trend in areal extent of Posidonia habitat. The success of the HD hinges on the power of the monitoring protocol to reveal a significant negative trend, wherever that trend occurs in nature. Statistical power is determined by the size of the sampling error of the protocol, which includes measurement uncertainty and natural spatial variation in the descriptor. We review several descriptors of Posidonia status, and consider published estimates of their typical sampling errors, to compare their ability to demonstrate a 10% temporal trend. We find that the protocols with potentially high statistical power are fixed-plot methods such as SeagrassNet and balisage, high resolution aerial photography, and remote underwater videography. The lowest statistical power is found for random plot protocols using descriptors from the interior meadow such as shoot density measured by SCUBA in 40 x 40 cm quadrats. The main advantage of videography over direct SCUBA methods is the ability to sample 20-30 times the area sampled by the equivalent SCUBA labour, but with none of the fatigue or discomfort. The main advantage of videography over aerial photography or acoustic imaging is its high resolution and near-perfect habitat classification accuracy for live seagrass, dead matte, and non-seagrass benthic habitats. We recommend remote underwater videography as a low-cost method of uncovering trends in Posidonia cover with high statistical power.