Seagrass productivity peak linked to sediment microbial changes and organic matter peak near stormwater drains

Estuaries support important foundation species, such as seagrasses, which promote biodiversity and contribute to ecosystem functioning. However, growing urbanisation has increased stormwater inputs into estuaries, which can cause physical scour, reduced salinity, increased sedimentation and the introduction of microbial communities and metal contaminants. The impacts are often greatest in sediments surrounding stormwater drains. These factors may affect seagrass performance directly and/or via disruptions of surface-associated and below-ground microbes that influence seagrass performance. This study investigated how seagrass ( Zostera muelleri) productivity, sediment characteristics and seagrass associated above- and below-ground microbial communities vary with distance from stormwater drains in Lake Macquarie, Australia. We hypothesised that (i) sites closest to stormwater drains scour mark (-1m (inside the scour), 0m (edge of scour), 1m, and 5 m(within seagrass meadow)) would have lower organic matter in sediments, higher metal contamination in sediments and lower seagrass productivity than control sites (200m from stormwater drain scour); and (ii) bacterial and fungal communities on seagrass leaves, roots and associated sediments would differ between sites closest to the drains compared to control sites. We found that sediment nearest to stormwater drains had the highest metal concentrations while seagrass productivity was lowest. Sediment organic matter and seagrass productivity both peaked at intermediate distances (5m). These patterns suggest stressful conditions for the seagrass close to drains, while the organic matter increase 5m away from the drains may help to explain the peak in seagrass productivity. Sedimentary bacterial community structure differed between control sites and all four distances close to stormwater drains with putative nutrient cycling and organic decomposing taxa being less abundant near the drains where organic matter was also lower. This suggests that both sedimentary microbes and sediment organic matter are important in the functioning of seagrass plants however, further investigation is required to disentangle the specific effects of microbial activity from those of organic matter and the mechanisms by which the processes occur.