Sources of uncertainty in estimation of eelgrass depth limits

In coastal areas seagrasses have considerable ecological importance and respond to eutrophication pressures. Seagrasses have, therefore, become an important parameter for assessing ecological status of marine water bodies. In this study we analyzed the sources of uncertainty associated with the monitoring of the maximum depth limit of eelgrass (Zostera marina). Based on a long-term marine monitoring of eelgrass depth limit in Danish coastal waters we estimated the uncertainty contribution of years, divers, transects, and replicates in water bodies and their sub-areas. For all these components the absolute uncertainty increased with the maximum depth limit. We used either a Spheric or a Gaussian function to describe the relationship between uncertainty and the maximum depth limit for each variable. This parameterization of the depth-specific uncertainty allowed estimation of the total variance associated with monitoring, which can be used to evaluate monitoring designs. The variance components were included in a time budget for optimizing eelgrass monitoring. With a maximum time limit of 100 or 200 h allocated for monitoring, the design that resulted in the lowest variance of the estimated maximum depth limit used 2 or 3 divers, respectively, and involved 2 or 3 years of monitoring and 4-8 transects.