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 three-fold 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.