Identifying robust bioindicators of light stress in seagrasses: a meta-analysis

Bioindicators are used to monitor responses to environmental pressures. They should reflect important ecological values, be scientifically defensible, respond in a predictable manner and be easy to measure and interpret. Seagrasses are significant marine habitat, which globally are under threat and are considered "sentinels" of coastal degradation. Light reduction via (for example) eutrophication, dredging and turbid terrestrial run-off is a key anthropogenic pressure impacting seagrasses. Consequently, seagrasses are regularly included in monitoring programs, both to protect them and for their value as indicators of change in light availability. This paper assessed published literature on seagrass responses to light reduction to identify which seagrass characteristics provide the most robust bioindicators of light reduction. ISI Web of Science was searched in July 2011 to retrieve refereed publications that documented the response of seagrasses to light reduction. Only studies with a control were included, giving confidence that the response was due to light reduction and not other, unexplained factors. This yielded a dataset of 58 published studies, covering eight of 11 seagrass genera and 18 species, with a wide geographic range. In each study, the response of each variable to light reduction was categorised into no effect, reduce or increase. Where studies tested the intensity and durations of light reduction, the consistency of responses at these different levels was also assessed. A set of consistent and robust bioindicators is proposed that respond to the pressure of light reduction and can indicate different timescales and levels of pressure. These include: those that respond early and reflect sub-lethal changes at the scale of the plant, such as rhizome sugars, shoot C:N, leaf growth and the number of leaves per shoot; and those that respond later, reflecting changes at the meadow-scale, such as shoot density or above-ground biomass. We recommend these variables for monitoring programs with the goal of detecting significant light reduction and indicating the severity and duration of impact