Hidden World of Biodiversity 🌱

The Living Lattice: Underwater Worlds of Mangrove Prop Roots

Where land and sea embrace, mangroves rise with roots that seem to defy gravity. These prop roots, arching gracefully into the brackish waters, are more than just pillars of support for mangrove trees. they are living bridges between ecosystems. Beneath their tangled lattice lies a bustling world of organisms, each finding shelter, food, and survival within this unique habitat. Barnacles cluster like stubborn tenants, oysters cement themselves in layered colonies, sponges weave soft carpets, and colorful crabs scuttle through shaded crevices. Fish dart between the roots, while algae and microbes cloak the submerged wood, forming the first steps of an intricate food web.

Pambala–Chillaw Lagoon Complex

Pambala–Chillaw Lagoon Complex, one of Sri Lanka’s most biologically rich mangrove ecosystems, offers a fascinating glimpse into this hidden world. This study was carried out with the support of the Lanka Mangrove Museum. The museum provided access to mangrove sites and research facilities, enabling the collection and investigation of mangrove root-associated organisms across different habitats. The lagoon’s prop roots are not merely tree structures; they are underwater cities where sessile and fouling organisms thrive, influencing nutrient cycling, water quality, and even the survival of larger species. Studying these hidden communities reveals how foundation species like mangroves extend their ecological influence far beyond their own survival, shaping the biodiversity of the entire lagoon.

There are both natural and anthropogenic threats to mangroves, where aquaculture being the greatest threat to mangroves, especially in developing countries. This decline through to results from a combination of several factors, such as habitat destruction, natural disturbances such as chill shock, fungal infections affecting roots and leaves, crab predation on seedlings, and fouling by barnacles.

Mangrove prop roots serve as a critical ecological foundation, hosting a diverse and complex ecosystem that provides immense value to the marine environment. In the intertidal zone, where the roots are alternately submerged and exposed, they create a unique habitat that supports a wide range of sessile and mobile organisms. The intricate, submerged surfaces are colonized by an extensive community of filter-feeding epibionts, including oysters, mussels, and barnacles, all of which play a vital role in water purification. This hard-surface substrate, rare in the surrounding soft-sediment and muddy environment, also attracts an array of mobile invertebrates, such as crabs, snails, and shrimp, which utilize the roots for foraging, shelter, and breeding. Furthermore, the dense network of roots acts as a crucial nursery for juvenile fish. The ecosystem’s complexity extends beyond the roots, where decomposers like bacteria and fungi break down leaf litter, sustaining the entire food web and illustrating the intricate interdependencies that define this essential coastal habitat.

Mangrove Prop Roots and Their Role in Blue Carbon

Beyond their visible biodiversity, mangrove prop roots are silent custodians of the planet’s climate balance. Acting as living carbon vaults, mangroves capture and store atmospheric carbon dioxide through photosynthesis, locking it away in their woody tissues, prop roots, and the sediment below. This process, widely known as blue carbon sequestration, makes mangrove ecosystems some of the most effective natural solutions to climate change.

The prop roots play a unique part in this process. Their dense network slows down water currents, causing fine sediments rich in organic matter to settle and accumulate. Over time, these sediments bury leaf litter, detritus, and waste products of root-associated organisms such as oysters, barnacles, and sponges. By trapping and stabilizing this organic material, the roots prevent carbon from being released back into the atmosphere. Even the biofouling communities that colonize prop roots contribute indirectly: oysters and barnacles filter suspended particles from the water, enhancing water clarity and promoting the growth of benthic vegetation, which further boosts carbon capture.

However, several root-associated organisms have the potential to influence the growth and productivity of mangrove prop roots. For example, extensive barnacle colonization can obstruct gas exchange and aeration of roots or induce mechanical stress, thereby reducing root stability and physiological performance. Beyond such direct impacts, predation and other biotic interactions can indirectly affect mangrove growth by shaping the distribution, abundance, and composition of root-associated fauna. These processes carry broader ecological consequences, since the health of prop root systems directly determines a mangrove’s ability to stabilize sediments, buffer against hydrodynamic forces, and provide habitat for diverse aquatic organisms. Human activities add further pressure: in many mangrove forests worldwide, oyster harvesting by local communities frequently leads to the accidental breakage of aerial roots, compromising both tree stability and growth.

Three Unique Study Sites

The mangrove root samples were collected from three distinct sites within the Pambala–Chilaw Lagoon system, Sri Lanka. Each site is characterized by differences in hydrology, salinity, and mangrove composition, which influence the assemblages of root-associated organisms.

Site 1 – Open Lagoon Area

This site is located near the Pambala port, approximately 6.5 km from the Thoduwawa sea mouth and 11.5 km from the Chilaw sea mouth. The relatively long distance from both sea inlets results in moderate tidal influence and a comparatively lower flushing rate than areas closer to the sea mouths. The water flow is slower, allowing fine sediments and organic matter to accumulate, which provides a suitable substrate for root-associated fouling organisms. The salinity at this site was recorded as 24 ppt, indicating a brackish water environment characterized by a mix of freshwater inflow and limited seawater exchange.

Site 2 – Bata Canal/ බට ඇල

The second site is located in the Bata Canal, which connects to the lagoon and is lined with Rhizophora mucronata trees planted in 1995. It lies approximately 6 km from the Thoduwawa sea mouth and 11.5 km from the Chilaw sea mouth. Due to its canal environment, this site experiences a more channelized water flow compared to the open lagoon, enhancing water circulation around the prop roots. The salinity of 26 ppt indicates slightly higher marine influence than the open lagoon site, favoring the colonization of a diverse range of fouling communities. The established R. mucronata plantation provides a structurally complex root system that serves as an important microhabitat for sessile and mobile organisms.

Site 3 – Dutch Canal / ඕලන්ද ඇල

The third site is situated in the Dutch Canal, where Rhizophora mucronata trees planted in 1996 dominate the banks. It is positioned 3 km from the Thoduwawa sea mouth and 13 km from the Chilaw sea mouth. Being closer to the Thoduwawa sea mouth, this site experiences stronger tidal exchange and relatively higher water movement compared to the other two sites. Consequently, the environment is more marine-influenced, as indicated by the higher salinity of 30 ppt. The stronger flushing effect reduces sediment deposition on the roots while enhancing nutrient and larval supply, which can promote distinct patterns of organism settlement and growth compared to the more stagnant lagoonal areas.

These findings from ongoing research,

conducted by E. M. Amandhi Nirukshila Ekanayake (Undergraduate B.Sc. (Hons) in Fisheries and Marine Science, Faculty of Fisheries and Ocean Sciences | Ocean University of Sri Lanka.)

Supervised by Dr M.P. Kumara and Mr. Duglas Thesera (Director of Lanka Mangrove Museum). Collaborated with Lanka Mangrove Museum.