Growing Deep – How to succeed as a deepwater seagrass

What are the major drivers of deepwater (>10m) seagrasses and how best can this resource be managed to curb coastal development impacts such as dredge plumes? To address this question, we constructed a multidimensional... [ view full abstract ]

What are the major drivers of deepwater (>10m) seagrasses and how best can this resource be managed to curb coastal development impacts such as dredge plumes? To address this question, we constructed a multidimensional research program to detail seasonal dynamics and growth strategies of Halophila spp. in deep water at three locations along the Great Barrier Reef lagoon. We mapped seasonal abundance, reproductive effort, seed bank status, productivity and environmental parameters over three years. While all Halophila spp. thrived under relatively low light, the extent of the growing period and whether plants were present or absent depended on the species and latitudinal position. The light climate and growing phases at field sites guided customised light and temperature treatments for H. decipiens and H. spinulosa during laboratory experiments into tolerances and thresholds to maintain seagrass condition. Overall, H. decipiens and H. spinulosa maintained their condition in aquaria at light levels as low as 3.2 mol photons m-2 d-1, in line with observed seasonal germination and production at field sites. A 66% reduction in light from ambient led to decreased shoot density for H. decipiens and H. spinulosa after two and four weeks respectively. Remarkably, temperature did not measurably affect shoot density or respiration of either species. Differences in sexual reproductive effort between the two species indicate disparate life history strategies to cope with impacts. H. decipiens relies heavily on fruit and seed dispersal into local sediments, whereas H. spinulosa preserves a standing crop from which it can proliferate when conditions improve. Overall, small reductions in light over relatively short timeframes can quickly exhaust a deepwater seagrass population. Ensuring an annual seed bank is established should be a management priority prior to impacts to the light climate, as caused by dredging, to ensure subsequent annual germination and recruitment. Designing a light threshold for managing deepwater seagrasses must therefore collectively address their monocarpic life history strategies to be an effective management tool in mitigating light-driven loss.