Does seasonal proliferation of Enteromorpha sp. affect the annual benthic metabolism of a small macrotidal estuary? (Roscoff Aber Bay, France)

Proliferation of fast growing ephemeral macroalgae may occur in intertidal environments and potentially affect the annual gross (GPPa) and net (NPPa) primary production of the system. Seasonal macroalgal proliferation was studied in situ in the Roscoff Aber bay and its influence on the annual benthic metabolism was investigated during low tide. CO2 exchanges at the air-sediment interface were regularly measured within a benthic chamber in order to calculate in situ net (NPP), gross (GPP) primary production and benthic community respiration (BCR). These measurements were assessed on bare sediments at 3 representative sampling sites and in an additional sampling site regularly affected by macroalgae deposit. On bare sediments, under optimal irradiance daily GPP and NPP exhibited fortnightly fluctuations. Annual net primary production (NPPa) was net autotrophic at the coarsest sites (up to 16 gC m 2 y 1) and net heterotrophic at the muddiest station (up to 22 gC m 2 y 1). On macroalgal mats GPP and BCR increased respectively from 3 to 10 and 3 to 20 times compared to bare sediments. This indicated that, at an hourly scale, the impact of macroalgae deposit on benthic metabolism was very significant. However, at the annual scale, macroalgae deposit and sediment microalgae respectively accounted for 12% and 88% of the annual net benthic primary production indicating that macroalgae proliferation poorly affected the annual carbon budget.