Decomposition of the tropical macroalga Caulerpa cupressoides (West) C. Agardh: Field and laboratory studies

Abstract

The decomposition of the marine tropical green macroalga Caulerpa cupressoides (West) C. Agardh was studied in order to assess its importance as a means of supplying nutrients to the sediments of seagrass beds which are in the early stages of colonization. Caulerpa is a colonizer species in the succession of seagrass beds. The dynamics of Caulerpa decomposition were also compared to that of other macroalgae and marine vascular plants. Caulerpa lost 50% of its original biomass in 7 days in litter bags positioned in the water column and sediments of Tague Bay lagoon, St. Croix, U.S. Virgin Islands. This rate is extremely rapid in comparison to that of vascular plants. Caulerpa was allowed to decompose in flasks for 141 days. During this time reducing conditions (Eh < − 100 mv) were established within 7 days and subsequently up to 1.8 mmol dissolved nitrogen was released from 25 g wet weight algae. The predominant form of nitrogen released was dissolved organics. Nitrate and nitrite were found only in trace concentrations after reducing conditions were established. The C N ratio in the particulate matter of decomposing Caulerpa decreased significantly over time in the flasks and increased significantly in the sediments. The low percent N and high C N ratios of decomposing Caulerpa suggest it would be poor quality food for detritivores. The amount of nitrogen released during the decomposition of Caulerpa was extrapolated to conditions in nature and was found sufficient to support a minimum of 10% of the nitrogen requirement of seagrasses as they colonize Tague Bay lagoon.