Seagrass beds represent highly productive ecosystems that can sustain a variety of fisheries for use by Micronesian societies. However, as watershed development and urbanization grow, seagrass ecosystems can become overwhelmed by excessive pollution, and opportunistic seaweeds can become dominant (such as drifting and clumping macroalgae). The transition from seagrass to macroalgae is bad because macroalgae don’t have roots and can’t stabilize the sand substrate below, macroalgae offer less habitat structure for juvenile fishes, and macroalgae have faster growth-and-death cycles that leads to the accumulation of decaying plant matter and enhances nutrient availability. At the same time, fish and sea cucumbers are vital to the seagrass ecosystems because they feed upon seagrass, macroalgae, and decaying plant matter. The end result is that both good water quality and sea cucumber abundances are critical for healthy seagrass ecosystems.
These ideas formed the basis for a recent study in Yap that investigated nearshore seagrass habitats. Like many places in Micronesia, fishermen and community members from the villages of Weloy and Dalipebinaw, western coast of Yap Proper, have noticed changes to their nearshore seagrass habitats over time. Generally, the communities have noticed increased growth of macroalgae (mostly unattached seaweed), and in some areas, a reduced amount of seagrass. In order to better understand their situation, a local community organization, Kaday Community and Cultural Development Organization, teamed up with the Yap Environmental Protection Agency, and scientists associated with the Palau Coral Reef Research Center, Pacific Marine Resources Institute, and the University of Guam Marine Lab.
The study first examined two potential sources of watershed pollution, one from surface discharge following rain events, and another from groundwater seepage that percolates through the limestone bedrock, into the island aquifer, and out to the seagrass habitats. Dr. Peter Houk (project scientist) explained how the findings were somewhat unintuitive. Groundwater percolating through the limestone bedrock provided a larger and more consistent source of freshwater addition to seagrass beds compared with surface discharge during storm events. Groundwater discharge was observed when tidal heights fell below ~0.4 m, representing lower tides that were associated with new and full moon periods. In contrast, Dr. Yimnang Golbuu described that surface runoff, even following major rain events, remained in the top 0.5 m of the water column until mixing with offshore waters.
These findings were influential because groundwater discharge patterns were strongly associated with the distribution of seagrass and macroalgae. A team of scientists and community members together documented higher abundances of macroalgae and less seagrass in association with groundwater discharge. However, the team of observers also documented low sea cucumber densities with macroalgae, suggesting a reduced ability to process the decaying plant matter and keep the ecosystem clean. Interestingly, the density of sea cucumbers in the study area (less than 5 individuals per 100 m2) was much lower than comparable seagrass beds in Palau (over 50 individuals per 100 m2). Thus, it appears that both water quality discharge patterns and sea cucumbers are tied with the health of the seagrass habitats. The study concluded with recommendations to improving sea cucumber populations, while developing a follow-up study to better tease apart the exact roles of water quality and secondary consumers in maintaining healthy seagrass habitats.
A final concern for the Yap communities was the potential influence of excess decaying plant matter being exported to their locally-managed marine protected area. The protected area is adjacent to extensive macroalgal growth, and thus, there is a concern that nutrients being released with decaying plant matter may enrich the waters of the protected area. The present work was published in the journal Marine Pollution Bulletin and is available here. The team is grateful to the University of Guam Water and Energy Research Institute, the Micronesian Conservation Trust, and Dr. Bob Richmond from the University of Hawaii for funding and equipment.