Intertidal seagrass species assemblage in the Andamans

Team members:  Rachana Rao, Elrika D’Souza, Teresa Alcoverro, Rajeswari B.T, Wenzel Pinto, Evan Nazareth

Project timeline: 2019 - Ongoing

Between the tides is a place of extremes. Twice a day, as the sea retreats and advances, it creates conditions so harsh, that only the toughest can survive. Yet, even this difficult seascape can be a battleground, as species jostle and shove for a comfortable spot.

Most community studies on the intertidal systems have focused on rocky shores and tide pools. Zonation patterns are often not quite as clear in soft-sediment communities where environmental gradients may not map as reliably between the high and low tide lines. As a result, soft-sediment assemblages often exist as patchy, sometimes ephemeral mosaics, giving way to more predictable and stable communities in calmer waters below the tide line.

Seagrass meadows, found in shallow sandy bottoms, have had a poor research representation in the tropics which have characteristic multi-species assemblage. While biotic interactions ought to affect multi-species assembly, species tolerance to environmental gradients might have a stronger influence in determining species distribution in intertidal seagrass meadows that experience several hours of aerial exposure each day that could also coincide with the hottest part of the day. Aerial exposure studies typically focus on photosynthetic performance, which is a sub-lethal stress response. Whether prolonged desiccation, characteristic of high tide areas can lead to differential shoot mortality and can help explain species distributions is less studied. Furthermore, species that share similar tolerance limits would compete for the same space. The understanding of whether one outcompetes the other, whether species adapt to different strategies in order to coexist or whether they perform better with another species than a conspecific individual, is lacking.

In this study, we explored the potential role of aerial exposure as an environmental filter explaining distributional patterns in a multi-species intertidal seagrass meadow in the Andaman Archipelago, Indian Ocean. We combined species distributional studies and field-based transplant experiments along gradients of desiccation to determine if differential tolerance to desiccation is linked to the distribution of seagrass species within the meadows. We further looked at two species that share similar tolerance to desiccation to determine the mechanism of their coexistence in the meadow.

Funders: MOEFCC; National Centre for Biological Sciences, Bangalore; Tata Institute of Fundamental Research, Mumbai; Centre for Wildlife Studies, Bangalore; Marine Biological Association of India, Kochi