Selection of multiple seagrass indicators for environmental biomonitoring

The need to monitor the environmental condition of ecosystems worldwide has resulted in a large number of potential bioindicators being proposed in the scientific literature. However, only a few have been validated at an adequate scale to monitor environmental problems and to solve management questions. Here we compiled a list of candidate seagrass indicators (n = 59) obtained from the literature. We empirically validated them on a temperate seagrass ecosystem (Posidonia oceanica) across a wide anthropogenic gradient ranging from undisturbed to severely disturbed sites. We discarded about 75% of the candidate indicators because of their lack of sensitivity at the relevant spatial scale for biomonitoring (i.e. 10s of km against 10s of m) or across the environmental quality gradient. This illustrates the need for a careful validation of indicators prior to their use in monitoring programmes. Bathymetric variability strongly influenced indicator responses to the quality gradient. Deep meadows responded more clearly to differences in environmental quality, whereas shallow meadows were more influenced by natural sources of variability such as herbivory and physical disturbances. The 16 indicators unequivocally related to the environmental status gradient were representative of physiological, biochemical, individual, and population levels of biotic organisation. Their combination was necessary to cover the entire environmental gradient and to reflect the multiple anthropogenic disturbances causing the gradient. The selection process of indicators described here is an important step that needs to take place before the integration of these indicators to extract ecologically relevant information useful for policy and management goals.