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.
Kelp communities are experiencing exacerbated heat‐related impacts from more intense, frequent, and deeper marine heatwaves (MHWs), imperiling the long‐term survival of kelp forests in the climate ...change scenario. The occurrence of deep thermal anomalies is of critical importance, as elevated temperatures can impact kelp populations across their entire bathymetric range. This study evaluates the impact of MHWs on mature sporophytes of Pterygophora californica (walking kelp) from the bathymetric extremes (8–10 vs. 25–27 m) of a population situated in Baja California (Mexico). The location is near the southernmost point of the species's broad distribution (from Alaska to Mexico). The study investigated the ecophysiological responses (e.g., photobiology, nitrate uptake, oxidative stress) and growth of adult sporophytes through a two‐phase experiment: warming simulating a MHW and a post‐MHW phase without warming. Generally, the effects of warming differed depending on the bathymetric origin of the sporophytes. The MHW facilitated essential metabolic functions of deep‐water sporophytes, including photosynthesis, and promoted their growth. In contrast, shallow‐water sporophytes displayed metabolic stress, reduced growth, and oxidative damage. Upon the cessation of warming, certain responses, such as a decline in nitrate uptake and net productivity, became evident in shallow‐water sporophytes, implying a delay in heat‐stress response. This indicates that variation in temperatures can result in more prominent effects than warming alone. The greater heat tolerance of sporophytes in deeper waters shows convincing evidence that deep portions of P. californica populations have the potential to serve as refuges from the harmful impacts of MHWs on shallow reefs.
The surfgrass Phyllospadix scouleri grows in highly productive meadows along the Pacific coast of North America. This region has experienced increasingly severe marine heatwaves (MHWs) in recent ...years. Our study evaluated the impact of consecutive MHWs, simulated in mesocosms, on essential ecophysiological features of P. scouleri. Overall, our findings show that the plants' overall physiological status has been progressively declining. Interestingly, the indicators of physiological stress in photosynthesis only showed up once the initial heat exposure stopped (i.e., during the recovery period). The warming caused increased oxidative damage and a decrease in nitrate uptake rates. However, the levels of non-structural carbohydrates and relative growth rates were not affected. Our findings emphasize the significance of incorporating recovery periods in this type of study as they expose delayed stress responses. Furthermore, experiencing consecutive intense MHWs can harm surfgrasses over time, compromising the health of their meadows and the services they offer to the ecosystem.
•Surfgrasses are potentially threatened by more frequent marine heatwaves.•Responses to simulated consecutive heatwaves examined in Phyllospadix scouleri.•Elevated oxidative damage and reduced nitrate uptake evidenced thermal stress.•Decline in photosynthetic performance was more pronounced during recovery periods.•Repeated marine heatwaves cause progressive physiological weakening in P. scouleri.
Marine heatwaves (MHWs) can have detrimental effects on seagrasses, but knowledge about the impacts on their ecosystem services remains scarce. This work evaluated Phyllospadix scouleri (surgrass) as ...a biofilter for wastewater discharges, and how warming associated with MHW may affect this ecological function. The nitrogen uptake kinetics and assimilation abilities for ammonium, nitrate, and urea were examined under two different warming scenarios (single and repeated events) simulated in a mesocosm. N-uptake kinetics were related to urban sewage discharges close to surfgrass meadows. Our results revealed that surfgrasses can serve as effective biofilters because of their high nitrogen uptake rates and above-average canopy biomass. Nonetheless, exposure to both experimental warmings resulted in a significant decline in their ability to incorporate and assimilate nitrogen. Consequently, MHWs may reduce the capacity of surfgrasses to function as nitrogen sinks and green filters for sewage waters, jeopardizing their role as Blue Nitrogen systems.
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•Surfgrass acts as a biofilter for wastewater discharges.•Marine heatwaves reduce the biofiltering capacity of seagrasses.•Surfgrasses can be considered as blue nitrogen systems.•Surfgrass conservation is a nature-based solution to coastal eutrophication.
