Community gross primary production and respiration in epilithic macroalgae and Posidonia oceanica macrophytodetritus accumulation in the Bay of Revellata (Corsica)

We report estimates of community gross primary production (GPP), community respiration (CR), and net community production (NCP) based on the change of dissolved O₂ during incubations over epilithic turf-forming macroalgae (Halopteris scoparia, Padina pavonica, and Dictyota dichotoma) on 7 occasions and in accumulations of Posidonia oceanica macrophytodetritus (i.e. litter) on 8 occasions in the Bay of Revellata (Corsica) from March 2009 to May 2011. In the epilithic macroalgae community, GPP ranged between 7.8 and 82.2 mmol O₂ m⁻² d⁻¹, CR ranged between −108.5 and −13.6 mmol O₂ m⁻² d⁻¹, and NCP ranged between −53.2 and −5.7 mmol O₂ m⁻² d⁻¹. In the P. oceanica macrophytodetritus accumulation, GPP ranged between 5.7 and 91.6 mmol O₂ m⁻² d⁻¹, CR ranged between −112.8 and −27.2 mmol O₂ m⁻² d⁻¹, and NCP ranged between −46.8 and −9.9 mmol O₂ m⁻² d⁻¹. GPP in both the epilithic macroalgae community and the P. oceanica macrophytodetritus accumulation peaked in summer and was lowest in fall, following the seasonal variation of incoming light. GPP correlated to macroalgal biomass but was unrelated to the biomass of living macroscopic plant material in the P. oceanica macrophytodetritus accumulation. The annual average of GPP was equivalent in the epilithic macroalgae and P. oceanica macrophytodetritus accumulation communities (17.6 and 19.4mol O₂ m⁻² yr⁻¹). Both the epilithic macroalgae community and the P. oceanica macrophytodetritus accumulation were net heterotrophic with an annual average NCP of −6.1 and −8.8mol O₂ m⁻² yr⁻¹, respectively. The NCP of the adjacent P. oceanica meadow at 10 m depth based on simultaneous measurements based on the open water O₂ mass balance from moored O₂ probes (optodes) was 28.9mol O₂ m⁻² yr⁻¹. The potential export of dissolved organic carbon from the P. oceanica meadow could quantitatively meet the carbon demand to sustain the net heterotrophy of the adjacent epilithic macroalgae community in the Bay of Revellata. We also show the limitation and possibly over-estimation of extrapolating decay rates based on litter bag experiments with small quantities of material to “real” macrophytodetritus biomass densities.