The effects of El Nino-Southern Oscillation events on intertidal seagrass beds over a long-term timescale

El Nino-Southern Oscillation (ENSO) events can cause dramatic changes in marine communities. However, we know little as to how ENSO events affect tropical seagrass beds over decadal timescales. Therefore, a diverse array of seagrass (Thalassia hemprichii) habitat types were surveyed once every 3months for 16years (January 2001 to February 2017) in a tropical intertidal zone that is regularly affected by both ENSO events and anthropogenic nutrient enrichment. La Nina and El Nino events had distinct effects on the biomass and growth of T.hemprichii. During La Nina years, higher (a) precipitation levels and (b) seawater nitrogen concentrations led to increases in seagrass leaf productivity, canopy height, and biomass. However, the latter simultaneously stimulated the growth of periphyton on seagrass leaves; this led to decreases in seagrass cover and shoot density. More frequent La Nina events could, then, eventually lead to either a decline in intertidal seagrass beds or a shift to another, less drought-resistant seagrass species in those regions already characterized by eutrophication due to local anthropogenic activity.