Seagrass and macroalgae are NOT the same!

Seagrass and macroalgae have very different life cycles and ecological roles within the Saipan lagoon environment and should not be considered the same. The most obvious difference is in the way that they look. Seagrasses look like underwater fields of grass, and have roots that penetrate into the sandy bottom, while macroalgae are large forms of algae, and can have a variety of different shapes. Most macroalgae don’t have roots and attach to the bottom (if at all) by sticking to hard surfaces.

Seagrasses and macroalgae also grow differently. Majority of macroalgae in the lagoon, especially those along the shore, are fast growing and have short life cycles. When nutrient rich stormwater drains into the lagoon tiny, microscopic algae called “phytoplankton” can undergo a population bloom. The numbers of phytoplankton can increase dramatically within a few short hours! As storms pass and drainages stop flowing, nutrients become depleted and the millions of phytoplankton die and settle to the bottom of the lagoon. The nutrients released by decaying phytoplankton are required for both macroalgae and seagrass growth; however macroalgae can assimilate them much faster than seagrass. Once the nutrients are gone, much of the macroalgae will die, until the next storm provides more nutrients. During non-storm events, when low levels of nutrients are available in the water, seagrasses can use their roots to access nutrients trapped below the sediment layer. This continuous food source is why seagrass is dominant in the Saipan Lagoon.

Seagrasses provide several benefits to the lagoon ecosystem that macroalgae don’t. Because seagrasses have roots, they hold and stabilize the sandy substrate, preventing erosion and shoreline shifting. Seagrasses also provide habitat for many fish and invertebrates among the blades of grass, and seagrass beds can also filter stormwater runoff before it reaches the coral reefs. If we had no nutrients running off the land, nearshore lagoon communities would consist solely of clean sand. As the runoff and nutrients start to increase, seagrass communities take over. And, when there is continuous runoff and too many nutrients, macroalgae dominate the ecosystem. We can see these changes by comparing historical aerial photographs with the present distribution of seagrass and macroalgae in the lagoon. From this comparison, we learned that macroalgae communities have increased, and thus we can infer that more and more nutrients are entering our waters. It is imperative for the health of the lagoon that we act now to clean our watersheds and polluted stormwater runoff. Nobody likes the sight and smell of macroalgae washing up onto our shores from southwest oceanic swells (passing typhoons). Over the past three years, our lagoon monitoring program has developed a baseline from which the success or failure of future management can be judged. Log onto www.deq.gov.mp (click onto “marine monitoring”) to learn more about this. (Peter Houk)

(Peter Houk is a Biologist at the Division of Environmental Quality.)