Changes in primary productivity and carbon balance of intertidal macroalgal assemblages

Marine macroalgae remain largely overlooked in global assessments of carbon budgets, despite being dominant and highly productive primary producers in coastal zones. Such assessments require studies at the level of whole assemblages as well as a better understanding of how human-induced shifts affect algal forest metabolism. This study aimed to quantify primary productivity and the carbon balance and stocks of different intertidal macroalgal assemblages in the Ría de Vigo (Galicia, NW Spain), in order to evaluate how structural shifts from canopy-forming to turf-forming species influence ecosystem functioning.

We examined assemblages dominated by large canopy-forming brown algae (Fucus vesiculosus, Ascophyllum nodosum) and by opportunistic and/or turf-forming species. Using incubation chambers in an outdoor mesocosm system, we measured net primary productivity (NPP) and respiration of whole assemblages, quantifying the contribution of macroalgae and of the associated fauna to respiration rates. Estimates of gross primary productivity (GPP), total gross productivity (GP) and net (NP) productivity (i.e. trophic state) were made for each assemblage. Photosynthetic (PQ) and respiration (RQ) quotients were calculated from changes in dissolved inorganic carbon (DIC) and O2, and the values were used to calculate the diel carbon balance. The carbon and nitrogen contents of the dominant macroalgae were also quantified to assess the storage capacity.

The study findings show positive diel carbon balances (autotrophic metabolism) during summer in both intertidal fucoid-dominated and turf-dominated assemblages, with a non-significant trend for greater values in the fucoid-dominated site. The observed inverse trends in the diel balances of net O2 fluxes may be linked to low PQ values of fucoid assemblages and higher RQ values of the turf-dominated community. Assemblages dominated by A. nodosum exhibited the highest biomass and specific carbon and nitrogen contents.

Projected shifts from large canopy-forming species toward smaller, turf-forming macroalgae maintain a positive carbon balance but are likely to reduce the carbon gains and overall productivity in intertidal habitats.