Resilient consumers accelerate the plant decomposition in a naturally acidified seagrass ecosystem

Anthropogenic stressors are predicted to alter biodiversity and ecosystem functioning worldwide. However, scaling up from species to ecosystem responses poses a challenge, as species and functional groups can exhibit different capacities to adapt, acclimate, and compensate under changing environments. We used a naturally acidified seagrass ecosystem (the endemic Mediterranean Posidonia oceanica) as a model system to examine how ocean acidification (OA) modifies the community structure and functioning of plant detritivores, which play vital roles in the coastal nutrient cycling and food web dynamics. In seagrass beds associated with volcanic CO₂ vents (Ischia, Italy), we quantified the effects of OA on seagrass decomposition by deploying litterbags in three distinct pH zones (i.e., ambient, low, extreme low pH), which differed in the mean and variability of seawater pH. We replicated the study in two discrete vents for 117 days (litterbags sampled on day 5, 10, 28, 55, and 117). Acidification reduced seagrass detritivore richness and diversity through the loss of less abundant, pH-sensitive species but increased the abundance of the dominant detritivore (amphipod Gammarella fucicola). Such compensatory shifts in species abundance caused more than a threefold increase in the total detritivore abundance in lower pH zones. These community changes were associated with increased consumption (52%-112%) and decay of seagrass detritus (up to 67% faster decomposition rate for the slow-decaying, refractory detrital pool) under acidification. Seagrass detritus deployed in acidified zones showed increased N content and decreased C:N ratio, indicating that altered microbial activities under OA may have affected the decay process. The findings suggest that OA could restructure consumer assemblages and modify plant decomposition in blue carbon ecosystems, which may have important implications for carbon sequestration, nutrient recycling, and trophic transfer. Our study highlights the importance of within-community response variability and compensatory processes in modulating ecosystem functions under extreme global change scenarios.

The influence of light and temperature on detritus degradation rates for kelp species with contrasting thermal affinities

Kelp detritus fuels coastal food webs and may play an important role as a source of organic matter for natural carbon sequestration. Here, we conducted ex situ and in situ manipulations to evaluate the role of temperature and light availability in the breakdown of detrital material. We examined degradation rates of two North Atlantic species with contrasting thermal affinities: the 'warm water' kelp Laminaria ochroleuca and the 'cool water' Laminaria hyperborea. Detrital fragments were exposed to different temperatures in controlled conditions and across an in situ gradient of depth, corresponding to light availability. Overall, degradation rates (i.e. changes in Fv/Fm and biomass) were faster under lower light conditions and at higher temperatures, although responses were highly variable between plants and fragments. Crucially, as L. ochroleuca degraded faster than L. hyperborea under some conditions, a climate-driven substitution of the 'cool' for the 'warm' kelp, which has been observed at some locations, will likely increase detritus turnover rates and alter detrital pathways in certain environments. More importantly, ocean warming combined with decreased coastal water quality will likely accelerate kelp detritus decomposition, with potential implications for coastal food webs and carbon cycles.