Species-Specific Biomass Allocation Shapes Seagrass Biomass Carbon in an Indonesian Coastal National Park: Evidence from West Bali National Park

The ocean floor holds a quiet, powerful secret in the form of seagrass meadows. These are not the grasses that grow on land, but flowering plants that have adapted to live entirely underwater, forming dense, green carpets along coastlines. Like their terrestrial cousins, they capture carbon dioxide from the air and water, storing it in their leaves, stems, and roots. This stored carbon is often called "blue carbon" because it is found in the ocean. Scientists have long known that these underwater gardens are vital for slowing climate change, but calculating exactly how much carbon they hold has been tricky. A common approach has been to count how many individual plants are growing in a patch, assuming that more plants automatically means more stored carbon. However, this method overlooks a crucial detail: not all seagrass plants are built the same way. Some grow tall and heavy with thick roots, while others stay small and thin. To understand the true carbon value of a meadow, researchers must look past simple counts and examine the specific type of plant and how it distributes its weight between the parts visible above the sand and the hidden parts buried beneath it.

In the waters of West Bali National Park in Indonesia, a team of researchers set out to test whether the identity of the seagrass species matters more than the sheer number of plants when it comes to carbon storage. They focused on three distinct locations within the park: Labuan Lalang, Teluk Terima, and Karang Sewu. Here, the seagrass meadows are a mix of three main species: the large, robust Enhalus acoroides, the medium-sized Thalassia hemprichii, and the small, delicate Halophila ovalis. The scientists collected samples from sixty-three different spots, carefully separating the plants into their above-ground leaves and their below-ground roots and stems. They measured how many shoots were in each spot, the size of the leaves, and the dry weight of the plant material. Crucially, they also burned small samples of the dried plant matter to determine exactly how much of it was made of carbon. This process allowed them to calculate the total amount of carbon stored in the meadows based on the actual weight of the plants, rather than just guessing based on how crowded the plants looked.

The results revealed a story that contradicts the simple idea that more plants equal more carbon. The researchers found that the large Enhalus acoroides was the heavyweight champion of carbon storage. Even though it made up only about seventeen percent of the total number of plant shoots in the meadows, this single species contributed more than half of the total carbon stored in the community. In contrast, the tiny Halophila ovalis was the most numerous, accounting for nearly fifty-six percent of all the shoots, yet it contributed less than twenty percent of the total carbon. When the researchers looked at the numbers for the large species, they saw it held an average of 483.98 grams of dry plant weight per square meter, storing 154.61 grams of carbon in that same space. The small species, despite being everywhere, held only 50.39 grams of carbon per square meter. This disparity showed that counting shoots was a misleading way to estimate carbon; the type of plant and how it built its body were far more important.

The study also uncovered a confusing pattern that only made sense when the plants were looked at individually. When the researchers mixed all the data together, they saw a negative link between the number of shoots and the amount of carbon: areas with more shoots seemed to have less carbon. This happened because the areas with the most shoots were dominated by the small, low-carbon species. Once the scientists separated the data by species, this confusing pattern vanished. The relationship between plant density and carbon storage was not a simple rule that applied to the whole meadow; it was an illusion created by mixing different types of plants. Furthermore, the team noticed that the small Halophila ovalis stored significantly less carbon in its roots and stems than in its leaves, a trait linked to its lack of dense, heavy underground tissues. This difference in how the plants allocated their biomass—deciding where to put their weight—was a key factor in how much carbon they could hold.

Across the entire meadow, the average carbon stock was 2.85 megagrams per hectare, and this amount did not vary significantly from one site to another. The consistency of this finding across different locations reinforced the idea that the species composition of the meadow is the primary driver of carbon storage, not the specific location or the density of the plants. The work in West Bali National Park demonstrates that to accurately assess the climate benefits of these underwater forests, we must move beyond simple counts. The identity of the seagrass and the way each species builds its structure are the true determinants of how much carbon is locked away in the sand. For coastal protected areas, this means that conservation efforts and carbon calculations need to be sensitive to the specific mix of species present, recognizing that a meadow full of small plants may store far less carbon than a sparser meadow dominated by the large, heavy growers.