Season and sediment nutrient additions affect root architecture in the temperate seagrasses Posidonia australis and P. sinuosa

We examined the root systems of 2 temperate seagrasses, Posidonia australis and P. sinuosa, testing the effects of nutrients and season on root architecture. Transplants of each species were grown in pots containing a standardised sand medium and a localised supply of nitrogen (N), phosphorus (P), or N and P combined. P. australis and P. sinuosa showed similar root architecture (primary roots with numerous second-order laterals and few higher-order laterals), but P. australis produced a larger root system with longer laterals making up the bulk of root length. Seagrass with combined N and P produced a higher lateral root density (330 +/- 18 [SE] branches m(-1) root compared to controls with 250 +/- 22), and a lower topological index (TI: 0.72 +/- 0.03 compared to controls with 0.84 +/- 0.02). Despite this response in root architecture, there was no significant change in traits associated with absorptive capacity. Separate nutrient additions had little effect on roots. Root architecture was more complex in summer for both species, with more higher-order laterals (TI: 0.73 +/- 0.02) resulting in a more dichotomous pattern compared to winter (TI: 0.86 +/- 0.01). P. australis had twice the root length and higher specific root length in summer than winter, mostly due to increases in lateral root number and length. This study demonstrated that P. australis and P. sinuosa display architectural and morphological plasticity with season and to a lesser extent nutrient addition. The complex root architecture in the slow-growing temperate Posidonia species may reflect hydrodynamic exposure and the importance for anchorage in wave-swept environments.