Effect of marine heat waves on carbon metabolism, optical characterization, and bioavailability of dissolved organic carbon in coastal vegetated communities

Abstract

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.

Nitrogen availability regulates the effects of a simulated marine heatwave on carbon sequestration and phycosphere bacteria of a marine crop

“…Therefore, the effects of heatwave depend on their intensity and duration (Saha et al 2020). Temperature increases can benefit aquaculture seaweeds but only if within their thermal tolerance window (Egea et al 2022). For instance, a seawater temperature rise from 20 C to 24 C can enhance growth of G. lemaneiformis (Liu et al 2018) but an increase from 25 C to 30 C reduces its growth (Wang et al 2016).…”

“…Few studies have documented the impacts of marine heatwaves on DOC excretion by seaweeds and its use by phycosphere bacteria. Egea et al (2022) found that simulated marine heatwaves in summer did not affect net community productivity dominated by the macroalga Caulerpa prolifera but decreased DOC daily fluxes and increased the proportion of RDOC. However, the potential mechanisms remain unknown.…”

Long-term sediment organic carbon remineralization in different seagrass and macroalgae habitats: implication for blue carbon storage

“…The sediment organic carbon (OC) stock found in upper sediments was in the same range than those reported for Cadiz Bay habitats by de los Santos et al (2023), and similar to those reported by Bañolas et al (2020) for seagrasses and by Holmer et al (2004) for the macroalgae. This high sediment OC stock is explained by the high macrophyte density and fine grain sizes, which feature the donor sampling area (de los Santos et al, 2023;Egea et al, 2023b).…”

“…A significant fraction of this DOC is formed by bioavailable material (i.e., the labile fraction; DOC L ) being an important driver of secondary production, acting as a quick transfer of carbon across the food web (Egea et al, 2019;Moran et al, 2022). Otherwise, another fraction of this DOC is apparently non-accessible to rapid microbial degradation (i.e., recalcitrant fraction; DOC R ) and it can be exported and sequestered in the deep ocean, and therefore it is considered nowadays a cornerstone in the contribution of oceans to carbon sequestration (Krause-Jensen and Jimeńez-Ramos et al, 2022;Zhang et al, 2023;Egea et al, 2023b). On the other hand, seagrass and macroalgae are highly variable in terms of size, biomass, and meadow canopy complexity, which ultimately affect the properties of sediment deposits -main OM origin (autochthonous vs allochthonous), dry bulk density (DBD), OC and CaCO 3 contents, etc (Mazarrasa et al, 2018).…”

HOTFLOOR: A benthic chamber system to simulate warming on the seafloor

“…While conducting experimental work in situ, especially in the subtidal, is accompanied by a number of logistical difficulties, conducting heating experiments is particularly demanding since heating the marine environment requires an exceptional amount of energy due to the high thermal conductivity of water. The most recent and notable advance has been achieved by placing small rigid cylinders with gas‐permeable plastic bags and 100 W heaters connected to the battery of a boat for 110 h in a shallow seagrass meadow, resulting in a 3°C above ambient water temperature (Egea et al 2023). The gas‐permeable, flexible bags enabled an oxygen exchange to avoid unnatural concentrations, for example, oversaturation during light hours and anoxia during dark hours, and were claimed to prevent water stagnation through hydrodynamic forcing.…”