Porewater Carbonate Chemistry Dynamics in a Temperate and a Subtropical Seagrass System

Abstract

Seagrass systems are integral components of both local and global carbon cycles and can substantially modify seawater biogeochemistry, which has ecological ramifications. However, the influence of seagrass on porewater biogeochemistry has not been fully described, and the exact role of this marine macrophyte and associated microbial communities in the modification of porewater chemistry remains equivocal. In the present study, carbonate chemistry in the water column and porewater was investigated over diel timescales in contrasting, tidally influenced seagrass systems in Southern California and Bermuda, including vegetated (Zostera marina) and unvegetated biomes (0–16 cm) in Mission Bay, San Diego, USA and a vegetated system (Thallasia testudinium) in Mangrove Bay, Ferry Reach, Bermuda. In Mission Bay, dissolved inorganic carbon (DIC) and total alkalinity (TA) exhibited strong increasing gradients with sediment depth. Vertical porewater profiles differed between the sites, with almost twice as high concentrations of DIC and TA observed in the vegetated compared to the unvegetated sediments. In Mangrove Bay, both the range and vertical profiles of porewater carbonate parameters such as DIC and TA were much lower and, in contrast to Mission Bay where no distinct temporal signal was observed, biogeochemical parameters followed the semi-diurnal tidal signal in the water column. The observed differences between the study sites most likely reflect a differential influence of biological (biomass, detritus and infauna) and physical processes (e.g., sediment permeability, residence time and mixing) on porewater carbonate chemistry in the different settings.

Overstated Potential for Seagrass Meadows to Mitigate Coastal Ocean Acidification

“…Net alkalinity production in the sediments, when bottom waters are oxygenated, can occur through denitrification or through the burial of reduced products of sulfate and iron reduction (FeS x ). While these and other alkalinity sources are likely to affect seagrass ecosystem carbon chemistry (Van Dam et al, 2019;Akhand et al, 2020), even in the same field sites occupied in R21 ("MB"; Kindeberg et al, 2020), such sediment-water alkalinity fluxes are only just now explicitly being incorporated into "blue carbon" budgets (Reithmaier et al, 2021). Still, this leaves only 509, or ∼20%, of positive [H + ] measurements as potentially related to a seagrass photosynthesis effect (Figure 1C, left teal bars), far below the 65% claimed in R21.…”

Investigating the Relationship between Growth Rate, Shell Morphology, and Trace Element Composition of the Pacific Littleneck Clam (Leukoma staminea): Implications for Paleoclimate Reconstructions

“…Coastal waters around Bodega Bay experience seasonal upwelling during the spring-early summer months associated with strong alongshore winds and Ekman transport, with temperatures from ~10 to 12 • C, followed by autumn relaxation periods with temperatures around ~13 to 15 • C, and wet, moderate winters with seawater temperatures ~11 • C [53,54]. Average seawater pH varies widely from ~7.6 to ~8.0 throughout the region due to seasonally influenced physical processes, including wind stress and upwelling, which lower pH, as well as diel cycles of photosynthesis and respiration [54][55][56][57].…”

On the delimitation of the carbonate burial realm

“…Generally, the vertical distribution of redox-sensitive species in shallow pore water profiles has been well studied and can be explained by the depth sequence of redox reactions based on the bacterially mediated oxidation of organic matter (Lückge et al, 1999). The literature dealing with pore water dynamics and carbonate diagenesis in the upper portion of the sediment column is substantial and reflects the complexity of the system (Boudreau & Canfield, 1993;Ge et al, 2020aGe et al, , 2020bGeske et al, 2015b;Hillgärtner et al, 2001;Huettel & Webster, 2001;Kindeberg et al, 2020;Reimers et al, 1996;Riding, 2000). These concepts are well-documented for many shallow, well-oxygenated marine environments and particularly refer to grainy sediments experiencing thermobaric circulation or pore water advection.…”