Seagrass litter decomposition: an additional nutrient source to shallow coastal waters

Area of Science:

• Marine Ecology
• Biogeochemistry
• Coastal Ecosystems

Background:

• Seagrass ecosystems provide vital regulatory services but face degradation from human activities.
• Understanding seagrass decomposition is key to assessing nutrient cycling in coastal waters.

Purpose of the Study:

• To compare the decomposition of two tropical seagrass species, Cymodocea serrulata and Cymodocea rotundata.
• To quantify nutrient release (carbon, nitrogen, phosphorus) during decomposition under varying microbial conditions.
• To estimate the nutrient supply potential of seagrass litter in Palk Bay.

Main Methods:

• Incubation experiments were conducted on shore-washed seagrass from Palk Bay, India.
• Decomposition was studied under bacteria-active and bacteria-inhibited conditions over 25 days.
• Leaching rates of dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and total dissolved phosphorus (TDP) were measured.

Main Results:

• Bacteria-inhibited conditions showed nearly three times higher dissolved organic carbon (DOC) release compared to bacteria-active conditions.
• Maximum leaching rates for DOC, TDN, and TDP were 294, 65.1, and 11.2 μM/g dry wt/day, respectively.
• Microbial processes significantly increased the release of bio-available dissolved inorganic nitrogen (DIN), over 1.3 times higher in bacteria-active conditions.
• Initial decomposition (up to 8 days) was faster (0.014 day⁻¹) than later stages (0.005 day⁻¹), with a half-life of 41 days.
• Estimated seagrass litter in Palk Bay could supply ~0.9 Mg of nitrogen and ~0.3 Mg of phosphorus.

Conclusions:

• Seagrass decomposition is a significant source of nutrients for coastal waters.
• Microbial activity plays a critical role in transforming seagrass biomass into bio-available nutrients.
• The rapid initial decomposition phase highlights the importance of leaching processes in nutrient release.