Experimental effects of warming and epiphyte grazing on the ecophysiology of two seagrass morphotypes

Seagrasses form important ecosystems providing essential provisioning and regulating services. They exhibit acclimative properties to contend with environmental change while intrinsic responses allow seagrasses to persist in sub- and supra-optimal temperatures, and are critical to understanding their ability to cope with a changing climate. An indoor mesocosm approach was used to investigate the effects of warming, i.e. mean summer (24 °C) and extreme (30 °C) temperatures, and epiphyte grazing on large-leaved and small-leaved morphotypes of a dwarf eelgrass (Zostera capensis) from a temperate lagoon habitat. Both morphotypes displayed similar responses under extreme temperatures with greater losses observed in shoot densities and total biomass compared to summer mean temperatures. However, higher photosynthetic rates and accumulations of carbon, nitrogen and phenolics were observed in small- compared to large-leaved plants. This suggests that small-leaved populations possess a larger propensity to store resources - a potential for resilience. Large-leaved populations appeared to have channelled resources towards growth suggesting a greater susceptibility to stress from e.g. fouling. Epiphyte biomass increased under extreme temperatures and, even though grazing had a positive effect on seagrass biomass under summer mean conditions, grazers did not regulate algal growth under extreme temperatures which may have been too rapid for grazer control. These findings indicate that seagrasses will be negatively affected by extreme warming with ensuing reductions in biomass, increasing their susceptibility to stressors. However, acclimation strategies including carbon and nitrogen stores provide high resilience potential and attest to a system's ability to cope with stress.