Above- and belowground biomass production in intertidal vegetated ecosystems of Cadiz Bay (Spain): implications for resilience to sea-level rise

Saltmarshes and seagrass meadows are highly productive coastal ecosystems that provide essential ecosystem services, such as carbon cycle regulation, sediment stabilization, and protection against extreme events. Unfortunately, these valuable systems are increasingly threatened by the effects of climate change, particularly due to the accelerating rise in sea level. Their resilience largely depends on their capacity to sustain positive substrate accretion through particulate matter retention and biomass production, especially within belowground compartments, thereby enabling compensation for sea-level rise. However, plant production remains poorly constrained, due to methodological challenges associated with its quantification and the heterogeneous environmental conditions that characterize them.

To help bridge this knowledge gap, this study estimated annual above- and belowground biomass production in two intertidal saltmarsh areas representative of Cadiz bay (Spain): Puerto Real (PT) and Santibañez (ST). Sampling locations were selected in homogeneous vegetation patches, with 14 sites established in PT and 11 in ST to encompass existing spatial variability. Aboveground production was assessed using circular exclusion structures of 25 cm in diameter, from which the initial aboveground vegetation was removed and biomass regrowth was quantified after 12 months. Belowground production was quantified using a modified ingrowth core method, which involved inserting partially open, mesh-wrapped cylinders, filled with root-free sediment. The cores were retrieved after 12 months under natural conditions to quantify root colonization. In the case of seagrass meadows, above- and belowground production was estimated exclusively from plant crowns, considered as the functional structural unit.

Results revealed clear differences between the studied vegetation types. In seagrass meadows, annual production averaged approximately 25 gPS·m⁻²·yr⁻¹ for aboveground biomass and 42 gPS·m⁻²·yr⁻¹ for belowground biomass. In contrast, saltmarsh communities showed markedly higher values, reaching 310 gDW·m⁻²·yr⁻¹ and 475 gDW·m⁻²·yr⁻¹, respectively. These findings highlight the predominant role of belowground compartments in the production balance of both ecosystems, where roots and rhizomes directly contribute to sediment stabilization. The spatial variability observed among sampling points suggests the influence of environmental and biological factors, such as dominant species or relative elevation, whose assessment will allow for a better understanding of the mechanisms driving resilience to sea-level rise.

Overall, the combined methodological approach provides a robust and transferable framework for quantifying productivity in intertidal ecosystems and constitutes a solid basis for upscaling biomass production from local measurements to larger spatial scales. By integrating field-derived production rates with spatial information on vegetation distribution, this approach enables ecosystem-scale assessments of productivity, carbon accumulation and sediment dynamics. The dominance of belowground production underscores its fundamental role in maintaining surface elevation and enhancing resilience to sea-level rise, offering key insights to support conservation and management strategies under climate change.