Temporal Lags in the Dynamics of Carbon Storage Following the Collapse of Seagrass Ecosystems Depend on Type of Stressor

Abstract

Seagrass meadows, like the Mediterranean coastal meadows dominated by Poseidonia spp., are key blue carbon ecosystems, but they are increasingly threatened by global change stressors. This implies that seagrass carbon storage capacity may be compromised turning seagrass meadows from carbon sinks to carbon sources. However, the large difference in the timescales between seagrass growth dynamics and soil carbon dynamics can cause long temporal lags that could create the impression that seagrass degradation has limited cascading effect on stored soil carbon. Here, using a mechanistic stoichiometric model parameterised for Mediterranean seagrass meadows of Posidonia oceanica, we investigated the transient dynamics of soil carbon under three stressors applied at varying rates of change: mechanical damage, eutrophication, and warming. Our findings reveal marked differences between equilibrium and transient dynamics, including the emergence of rate-dependent lags, mismatches in the response between seagrass biomass and carbon stored, as well as unexpected transient patterns such as increases in seagrass biomass and carbon stored triggered by eutrophication stress before seagrass collapse. These results highlight the importance of considering transient dynamics and stressor rates in understanding seagrass responses to stress with potential implications for their management as carbon sinks.