Environmental predictors of Seagrass Phenotype Variability in a Temperate Lagoon Ecosystem.

Abstract

Abstract Seagrass ecosystems provide essential services but have declined worldwide due to human and natural influences. Thermal stress from diurnal and seasonal fluctuations, coupled with climatic factors, impact seagrass productivity and distribution. To survive in dynamic environments, plants must adapt or acclimate. Following concerning declines, environmental factors responsible for spatial and temporal variability were investigated in a dwarf eelgrass (Zostera capensis) habitat within a temperate lagoon. Significant differences in shoot densities, leaf size, and biomass of Zostera populations near the mouth and at the end of the lagoon were observed, with distinct seasonal responses. Severe diebacks were observed in summer with subsequent recovery under favourable conditions. Generalized additive mixed modelling revealed seagrass densities to be primarily (> 80%) predicted by water temperature, turbidity, and exposure. Those exposed longer during low tide exhibited a small-leaved morphotype in higher densities. Conversely, deeper intertidal stands supported a sparser large-leaved morphotype. These traits represent a phenotypic response enabling populations to acclimate to the prevailing environmental conditions, altering their characteristics and interactions. Large-leaved populations supported higher epiphyte loads and faunal diversity compared to small-leaved populations. High gene flow suggested that morphotypic variations are predominantly phenotypically based rather than genetically driven. Trends in Zostera cover within the lagoon reveal greater declines closer to the mouth, implying a concurrent decline in associated macro-epifaunal communities dependent on large-leaved populations. With climate change-induced warming, further decreases in Zostera and ensuing loss of large-leaved populations are expected, with likely negative repercussions on associated epifaunal communities and other trophic levels within the system.