Release of dissolved and particulate organic matter by marine macroalgae and its biogeochemical implications

Understanding the release of organic matter by macroalgae provides insight into carbon and nutrient cycling in ecosystems. However, there is little information on how the various fractions of dissolved and particulate organic matter released from macroalgae contribute to coastal biogeochemical cycles. To address this gap, we carried out short-term (5 d) laboratory incubation experiments with 11 species of typical macroalgae. During incubation, the average proportions of released dissolved organic carbon, nitrogen, and phosphorous as compared to total carbon, nitrogen, and phosphorus were 14.39%, 37.38%, and 31.32%, respectively. The average released proportions of particulate organic carbon, nitrogen, and phosphorous as compared to total carbon, nitrogen, and phosphorus were 9.02%, 12.67%, and 13.89%, respectively. This study showed that, during the vegetative growth stage, the macroalgae released both dissolved and particulate organic matter, and that more dissolved organic matter than particulate organic matter was released into the seawater. These results support the proposed importance of macroalgae in contributing dissolved and particulate organic matter that can be directly incorporated into the carbon and nutrient cycle in shallow ocean ecosystems. This research also found that the growth rate of the macroalgae had a linear correlation with algal tissue carbon, nitrogen, and phosphorus accumulation rates and the release rates of dissolved organic matter. The carbon:nitrogen and carbon:phosphorus ratio of both dissolved and particulate organic matter were substantially lower than that of algal tissue, which suggested that macroalgae tend to release organic nitrogen and phosphorus and enrich carbon.