Seagrass shoots interact with hydrodynamic forces and thereby a positively or negatively influence the survival of associated species. The modification of these forces indirectly alters the physical transport and flux of edible particles within seagrass meadows, which will influence the growth and survivorship of associated filter-feeding organisms. The present work contributes to gaining insight into the mechanisms controlling the availability of resources for filter feeders inhabiting seagrass canopies, both from physical (influenced by seagrass density and patchiness) and biological (regulated by filter feeder density) perspectives. A factorial experiment was conducted in a large racetrack flume, which combined changes in hydrodynamic conditions, chlorophyll a concentration in the water and food intake rate (FIR) in a model active filter-feeding organism (the cockle). Results showed that seagrass density and patchiness modified both hydrodynamic forces and availability of resources for filter feeders. Chlorophyll a water content decreased to 50% of the initial value when densities of both seagrass shoots and cockles were high. Also, filter feeder density controlled resource availability within seagrass patches, depending on its spatial position within the racetrack flume. Under high density of filter-feeding organisms, chlorophyll a levels were lower between patches. This suggests that the pumping activity of cockles (i.e. biomixing) is an emergent key factor affecting both resource availability and FIR for filter feeders in dense canopies. Applying our results to natural conditions, we suggest the existence of a direct correlation between habitat complexity (i.e. shoot density and degree of patchiness) and filter feeders density. Fragmented and low-density patches seem to offer both greater protection from hydrodynamic forces and higher resource availability. In denser patches, however, resources are allocated mostly within the canopy, which would benefit filter feeders if they occurred at low densities, but would be limiting when filter feeder were at high densities.

Seagrass beds act as blue carbon sinks globally as they enhance the trapping of recalcitrant (i.e., low biodegradability) organic carbon in their sediments. Recent studies also show that the recalcitrant fraction of the dissolved organic carbon (DOC) pool in seawater has an important role as long-term carbon sequestration in oceans. Although seagrasses are known for the large amount of DOC they export, little attention has been given to its biodegradability, which ultimately determinates its fate in the coastal carbon cycle. In turn, invasive algae are a major global concern in seagrass ecosystems since they can deeply modify their structure and functions, which may affect carbon metabolism and DOC release. This work assesses how the presence of Halimeda incrassata, an invasive tropical calcareous macroalga, modifies carbon metabolism and DOC fluxes in invaded areas dominated by the seagrass Cymodocea nodosa. Our results show that stands with the presence of this seagrass (i.e., both monospecific and mixed meadow) had the highest production values, acting as high DOC producers in both winter (mainly of labile DOC; DOC_L) and summer (mainly as recalcitrant DOC; DOC_R). In contrast, monospecific H. incrassata beds exhibited low production values, and the presence of this macroalga (either as monospecific beds or mixed with C. nodosa) triggered the shift from a net DOC-producing-system in summer (mainly DOC_L) to a net DOC-consuming-system in winter. This work thus suggests that C. nodosa meadows have the potential to export a significant fraction of both labile and recalcitrant DOC, and that the spread of this invasive alga might decrease the C export capacity of seagrass meadows. Such shift would imply the reduction of a quick and efficient transfer of carbon and energy to higher trophic levels, and might reduce the blue carbon potential of seagrasses as dissolved form in the water column.

Dissolved organic carbon (DOC) plays an essential role in the global marine carbon cycle, with coastal vegetated communities as important DOC producers. However, the ultimate fate of this DOC remains still largely unknown due to the lack of knowledge about its chemical composition and lability. Furthermore, global change could alter both DOC fluxes and its bioavailability, affecting the carbon sequestration capacity of coastal vegetated communities. This study explores, in two contrasting seasons (winter and summer), the effects of an in situ simulated marine heatwave on carbon metabolism and DOC fluxes produced by seagrass (Cymodocea nodosa) and macroalgae (Caulerpa prolifera) communities. In addition, the fluorescent characteristics and biodegradability of the dissolved organic matter released directly by the communities under such conditions are evaluated. Under marine heatwave conditions, a significant increase in net community production (NCP) in C. nodosa and a shift to negative DOC fluxes in C. prolifera were observed. In control treatments, the seagrass‐dominated community produced a substantial amount of labile (between 44% and 58%) and recalcitrant DOC (between 42% and 56%), while C. prolifera community produced mainly recalcitrant DOC (between 64% and 87%). Therefore, this research revealed that temperature is an important factor determining the NCP in benthic communities and the chemical structure and bioavailability of DOC produced by these communities, since both communities tended to produce more humic‐like and less bioavailable DOC with increasing temperature.