Seagrasses





Seagrasses are true plants underwater, different than algae or seaweeds. Seagrass meadows are one of the most productive and multifunctional ecosystems on Earth, yet they are often overlooked or ignored by snorkelers and divers. Seagrasses are found in coastal environments around the world in a range of climates, including Guam. Locally, seagrasses are commonly found around the coastline from Agana Bay to Agat, Cocos Lagoon, and Pago Bay. In fact, Tape grass (Enhalus acoroides) is the most common seagrass found on Guam and it is also the largest species of seagrass in the world. Seagrasses provide many benefits:

  • Seagrasses provide habitat for valuable commercial and recreational fisheries, as well as a wide variety of other marine life beneficiary to recreational use. Seagrass meadows are home to many species of fish, sharks, turtles, marine mammals, octopus, squid, cuttlefish, snails, bivalves, sponges, shrimp, crabs, worms, sea urchins, sea anemones, and others (Smithsonian Institute 2020).
  • Seagrasses generate oxygen in the water through photosynthesis, they are known as the “lungs of the sea”.
  • Seagrasses cycle nutrients from the soil and sediment back into the ecosystem.
  • Seagrasses improve water quality by filtering pollutants in runoff from the land.
  • Seagrasses stabilize sand and sediment in place and reduce coastal erosion.
  • Seagrass meadows moderate the impact of storms and waves in conjunction with coral reef counterparts.

 
Mangroves are small trees and shrubs that live in the intertidal zone, or the area where the land meets the sea between high tide and low tide. Mangroves are especially important for commercial and recreational fisheries. It is estimated 1/3 of the fish caught in Southeast Asia are supported by mangroves (UNEP 2014). On Guam, mangroves are found primarily in Apra Harbor, which is the largest and most established mangrove forest in the Marianas (Burdick et al 2008). There are two smaller areas of mangroves in Merizo and Inarajan. Mangrove forests provide many benefits:


  • Mangroves reduce wave energy by more than 70% and prevent erosion from storm surges, currents, waves, and tides by stabilizing sediments (Guannel et al 2016).
  • Mangroves contribute to biodiversity by serving as nesting and breeding habitat.
  • Mangroves provide fish and other marine life food, shelter from predators, and nursery habitat for young.
  • Mangroves improve water quality and clarity by filtering sediment and pollutant run off from the land.
  • Mangroves reduce climate change by storing and sequestering carbon.


Loss of Seagrasses and Mangroves over time
 
A 2008 meta-analysis found seagrasses around the world are declining, and the rate of loss is increasing. There was an estimated 29% global decline in seagrass areal extent from 1879-2006 (Waycott et al 2009). Specifically, on Guam, there was an average 22% reduction from 2004-2015 of Tape Grass. However, some areas have experienced significantly more loss such as Cocos Lagoon (94%) and East Agana Bay (82%) (LaRoche et al 2018). The biggest drivers of seagrass meadow loss are coastal development and poor water quality.
 
Global mangrove loss is attributed to human activity, primarily coastal development and land use changes for aquaculture and agriculture. Between 20%-35% of mangroves were lost globally since approximately 1980 (Polidoro et al 2010). Although the trend on Guam is not clear, historically significant losses of wetlands have occurred. For example, 500 hectares of wetlands including mangrove communities was filled in the late 1940s by the US Navy to construct the port facilities in Apra Harbor (Wiles and Ritter 1993).
 
As you can see, the degradation of coastal environments is a concern both globally and locally. However, local-level management efforts and community cooperation can increase the resilience of these habitats and the valuable ecosystem services they provide for future generations. In the next module, we will explore deeper what is driving the decline of coastal environments and what we can do to protect them.