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The capacity of macroalgae to absorb, hold, and release the relative amounts of carbon (C) and nitrogen (N) in its environment varies. C and N stable isotopes can be used to gain a better understanding of the ecosystem services of nutrient cycling by quantifying its usage in macroalgae. In this study, we conducted an in situ δ13C and δ15N tracer study of the macroalgae Ulva reticulata and Jania adhaerens in the seagrass meadows of Sungai Pulai estuary, Johor. Bulk tissues were labelled with enriched sodium bicarbonate (NaH13CO 3) and ammonium chloride (15NH 4 Cl). Within 1 to 18 days post-enrichment, the macroalgae were measured for C N contents and stable isotope signatures. U. reticulata showed more bulk losses for excess 13C and 15N compared to J. adhaerens. J. adhaerens tended to retain greater %C and %N incorporated in its tissues. There was a clear distinction between the amount of C and N retained and lost in the tissues of each species. The amount of nutrients, especially C, that remained at the end of the experiment demonstrated that both macroalgae species are capable of short-term carbon sequestration, with implications for meadow-scale nutrient subsidies. Higher N uptake and retention in the biomass of the macroalgae, especially in U. reticulata , suggest their relatively high growth capacity. Understanding the C and N dynamics and loss rate of the whole plant provides an estimation of the nutrient uptake and retention capacity of the macroalgae. Such insights are important as a means to formulating steps in preserving the ecological balance of marine ecosystems. [Display omitted] • C and N isotope signatures of Jania adhaerens and Ulva reticulata are reported. • After pulsing with 13C and 15N, these macroalgae were placed in a seagrass meadow. • Variations of δ13C and δ15N in their tissues were measured over 18 days. • J. adhaerens retained more of the initial 13C and 15N tracers than U. reticulata. • C and N changes allow insights into ecological roles of nutrient cycling. [ABSTRACT FROM AUTHOR]