Seagrass population dynamics and biodiversity assemblages indicate negative effects of short-term nutrient enrichment in tropical island ecosystem

This study assessed the influence of anthropogenic short-term nutrient enrichment (hereafter enriched) effects on seagrass population dynamics (recruitment, growth rate and mortality), morphometric traits, productivity, and leaf biodiversity assemblages in the islands of Andaman and Nicobar (ANI) of India and contrasted these findings with away from these enriched areas (hereafter pristine). Seagrass (Thalassia hemprichii and Cymodocea rotundata), and sediment samples were collected in the dry season (October-May) of ANI. Reconstruction techniques, an indirect measurement of plant growth was used to derive leaf plastochrone interval (PI), i.e., number of days required to produce one leaf by the seagrass. Sediment, organic matter (OM) and carbon (C) were quantified using, loss on ignition method and CHNS elemental analyser. The total N in leaves of T. hemprichii and C. rotundata increased 3.3-fold and 2.4-fold than pristine conditions. Increased N accumulation resulted in higher shoot densities, below ground biomass, and productivity for both seagrasses. T. hemprichii and C. rotundata took 26.07 and 19.76 days respectively to produce new seagrass leaf under enriched conditions. Low apex densities resulted in lower meadow migration and increased meadow fragmentation under enriched conditions. The above ground-biomass and leaf length of T. hemprichii and C. rotundata decreased under enriched conditions leading to lower leaf meiofauna abundance. The long-term average recruitment for both T. hemprichii and C. rotundata increased under enriched conditions resulting in 3.5-fold and 11-fold higher current population growth rates resulting in increased younger plants. Contrastingly, these younger plants did not survive longer under enriched conditions, reducing the long-term seagrass population longevity to 4 years, compared to 6-7 years longevity under pristine conditions. This study highlights that nutrient enrichment in tropical islands benefits seagrass in short-term but reduces seagrass meadow migration, population longevity and biodiversity assemblages, thus reducing seagrass ecosystem service provisions, which calls for urgent monitoring and conservation of seagrass ecosystems of ANI, India.