Kep's Microscopic Guardians: Saving Cambodia's Marine Ecosystems

Dive with us into the turquoise waters of Cambodia's Kep archipelago. Just a few meters below the surface, microscopic creatures—smaller than a grain of sand—play a crucial role in saving marine ecosystems devastated by illegal fishing.

Posidonia seagrass meadow on the seabed of the Kep archipelago

A groundbreaking study reveals these Cambodian "meiofauna" for the first time, offering tangible hope against the destruction of seabeds.

Kep: A Fragile Paradise Under Siege

The Kep archipelago hosts an exceptional underwater world, with lush mangroves, undulating Posidonia seagrass meadows (herbaceous marine plants), and vibrant coral reefs teeming with life, all at an average depth of just 50 meters.

These treasures of southern Cambodia, rich in oyster and bivalve beds (shellfish like mussels or clams), have endured decades of damage from illegal trawling and climate warming. The result is clear: bare seabeds, unstable sediments, and plummeting biodiversity.

School of sergeant majors swimming around pillars of artificial reefs underwater

Since 2017, Marine Fisheries Management Areas have protected the Kep region. In these zones, artificial reefs—small concrete pyramids called "fishery production structures" (FPS)—repel illegal trawlers and invite marine life to return. The question remains: do these structures really work? The answer lies at the heart of the sand.

Meiofauna: Sentinels of the Seabed

Forget big fish or colorful corals: meiofauna are tiny animals (from 38 microns to 1 mm), such as nematodes (microscopic thread-like worms), copepods (planktonic crustaceans as small as water fleas), or foraminifera (protozoans with calcareous shells).

These organisms are your ideal spies, living exclusively in sediments and reacting first to pollution, oxygen depletion, or violent disturbances.

They signal ocean health like a reliable thermometer: abundant diversity indicates a stable ecosystem, while impoverishment signals a red alert. Until now ignored in Cambodia, these meiofauna fill a major scientific gap, following studies in neighboring Thailand.

Dive Mission: Three Worlds Underwater

From January to February 2023, around Koh Ach Seh island (at an average depth of 4.5 meters), a cosmopolitan team—led by Torrey Gorra (Belgium), Amick Haissoune (Marine Conservation Cambodia), and local experts—explored three distinct habitat types:

• Bivalve beds (BB): true forests of oysters like Magallana belcheri (native Cambodian oyster with a rough shell) or Saccostrea cucullata (common rock oyster in Southeast Asia), associated with mussels;

• FPS: concrete artificial reefs designed to counter trawlers;

• Impacted sites (IS): ravaged, completely bare sands.

The team used a simple, effective method: collecting 28 sediment cores (3.6 cm diameter and 5 cm deep). Analyses, conducted in Hanoi, identified 24 taxa (focusing on 17 permanent groups, like ostracods—tiny crustaceans with shells—or tardigrades, nicknamed "water bears" for their extreme resilience). Measurements included temperature (27-34 °C), salinity (30-34 PSU), light, granulometry (sand grain size), and oxic depth (aerated sediment zone).

The results are telling: bivalve beds (BB) and FPS show fine, stable sediments with deep oxic layers (up to 3.7 cm). In contrast, impacted sites (IS) have coarse grains and immediate anoxia at the surface. Marine life faithfully follows these conditions.

Surprising Discoveries: Numbers That Speak for Themselves

In total, researchers counted 39,200 meiofaunal individuals. Nematodes dominate everywhere (60%), followed by copepods and calcareous foraminifera. Yet each habitat tells its unique story:

Habitat	Taxonomic richness (number of taxa)	Diversity (Shannon index)	Local stars	Relative density
BB	14-18	2.6-2.7	Nematodes (651/10 cm²), Copepods	46% diversified
FPS	11-18	2.3	Testate amoebae (1,371!), Ostracods	47% variable
IS	15-16	2.4-2.5	Calcareous foraminifera (342)	9% opportunistic

Meiofaunal assemblages differ significantly between habitats (p<0.01): BB and FPS host stable communities, while IS shelter stressed, opportunistic species. Tardigrades and oyster larvae emerge as key discriminators between sites. The biodiversity peak occurs at BB02, where seagrass and oysters create a microbial paradise.

Major Lesson: Bivalves, Undisputed Ecological Heroes

Oysters and mussels act as true miracle engineers: they filter water, stabilize sand, oxygenate sediments through bioturbation (sediment stirring by their activities), and feed the entire food chain.

The FPS, meanwhile, serve as express refuges: they boost microbes (like those pink amoebae!) and promote sessile recruitment (organism attachment to structures).

However, vigilance is needed: excess organic matter risks hypoxia (oxygen depletion). Optimizing the design and placement of these structures is therefore essential.

Concrete Hope for Cambodia and the World

This pioneering study in Cambodia now guides restoration efforts: it urges protection of natural bivalve beds and expansion of FPS through projects like the Asian Development Bank's (ADB). Facing illegal fishing and devastating algal blooms (like those of 2016-2017), these discoveries hold the key to revitalizing fisheries and developing sustainable tourism. Although the sample focuses on a limited area, this solid baseline paves the way for more precise future monitoring (seasonal tracking, direct oxygen measurements).

In these threatened waters, invisible meiofauna prove a simple truth: restoring the microscopic world saves entire oceans. The Kep archipelago is reborn with hope for its seabeds.