Temporal and depth-associated changes in the structure, morphometry and production of near-pristine Zostera marina meadows in western Ireland

Abstract

The eelgrass Zostera marina is a dominant, subtidal meadow-forming seagrass in temperate regions in the northern hemisphere. Due to the numerous ecological services they provide, seagrass systems rank amongst the most valuable ecosystems worldwide but there is a lack of data regarding the structure and productivity of Irish seagrass populations. To address this gap, this investigation assessed temporal changes and depth acclimations of vegetative parameters, such as, shoot length and weight, shoot density, seagrass cover and below- and above-ground biomass, in three near-pristine subtidal Z. marina meadows in western Ireland. Most parameters revealed a marked temporal pattern, displaying clear unimodal responses, with peaks attained in July, intermediate values in April and November, and lowest values observed in January. Moreover, eelgrass cover and density decreased from 76.1 +/- 6.5% in shallow meadows to 34 +/- 4.5% at greater depths; inversely, other parameters such as shoot biomass or shoot length increased 2-3-fold with depth. Furthermore, annual leaf formation rates and above-ground biomass were estimated and compared to those of eelgrass populations from different latitudes (28 degrees-66 degrees) reported in the literature. Results suggest that the vegetative development of Irish populations corresponds to global data reported at different local and global scales of Z. marina populations inhabiting stable environments; our data serve as a first baseline for the assessment of potential future environmental impacts, including climate change and eutrophication.