Variation of fucoid community metabolism during the tidal cycle: Insights from in situ measurements of seasonal carbon fluxes during emersion and immersion

Intertidal communities dominated by canopy-forming brown seaweed are typically highly productive systems. However, their metabolism can vary significantly over time, due to fluctuations in abiotic parameters. If tidal and/or seasonal cycles play an important role in the regulation of metabolism in these communities, they may therefore strongly influence community functioning and dynamics. Here, we investigated the low mid-intertidal Fucus serratus community, measuring in situ carbon fluxes of its primary production and respiration during different seasons. To perform direct comparisons of its underwater and aerial metabolism, these measurements were carried out during immersion and emersion, analyzing the changes in dissolved inorganic carbon and in CO2 concentrations under closed benthic chambers. Our results showed that during both emersion and immersion periods, primary production and respiration varied seasonally with minimum values in winter and maximum values in summer. These values were, on average, 5 and 3.5 times higher when the community was exposed to air than when immersed, due to the large changes that occur in abiotic environmental conditions during the tidal cycle. Primary production fluctuated greatly during immersion and was generally limited by underwater light availability. In contrast, primary production remained high during emersion, partly because canopies limit the water loss of their understory layers. During both tidal periods, community respiration was mainly driven by temperature. Our results suggest that the relatively high metabolism during emersion contributes substantially to the total energy budget of fucoid mid-intertidal communities, even those that are low on the shore and spend most of their time underwater.