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

Theodor Kindeberg,May-Linn Paulsen,Nicholas R Bates,Fred T Mackenzie,Alyssa Griffin,Tyler Cyronak,Travis A Courtney,Andreas J Andersson

doi:10.1007/s10498-020-09378-8

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.

Highlights

Coastal ecosystems play an important role in the global carbon cycle, largely due to the lateral transport of carbon and nutrients from rivers, terrestrial runoff and groundwater, intense benthic and pelagic metabolism and carbon transformation pathways in biomes such as seagrass beds, coral reefs, kelp forests, wetlands and saltmarshes (Duarte et al 2005; Bauer et al 2013)
Our goal was to examine the following questions: (i) how do changes in porewater carbonate chemistry correlate with changes in the overlying water column?; (ii) is there a difference in vertical porewater profiles of dissolved inorganic carbon (DIC), total alkalinity (TA) and pH between the different study sites, including differences between the vegetated and unvegetated sediment in Mission Bay?; and, (iii) what is the diel variability of these parameters in the different sediments? These questions were addressed by conducting a 24-h study at each site measuring an array of physical and chemical parameters with temporal resolution ranging from minutes to hours
The highest DIC and TA values were observed at morning low tide and the lowest at evening high tide. pHT and ΩAr exhibited the opposite trend with minimum values observed at morning low tide and maximum values observed at evening high tide (Fig. 4)

Summary

Introduction

Coastal ecosystems play an important role in the global carbon cycle, largely due to the lateral transport of carbon and nutrients from rivers, terrestrial runoff and groundwater, intense benthic and pelagic metabolism and carbon transformation pathways in biomes such as seagrass beds, coral reefs, kelp forests, wetlands and saltmarshes (Duarte et al 2005; Bauer et al 2013). Burdige et al (2002, 2008) showed that seagrass enhances carbonate sediment dissolution by fueling high rates of organic matter (OM) remineralization in the sediments by pumping oxygen via their roots and rhizomes that subsequently leads to elevated CO2, lower carbonate saturation state (Ω) and elevated rates of carbonate mineral dissolution These authors proposed that the alkalinity generated from carbonate sediment dissolution in seagrass beds could constitute a negative feedback mechanism to increasing atmospheric CO2 (Burdige and Zimmerman 2002; Burdige et al 2008). Regardless of whether this is the case or not, the mechanism of transporting photosynthetically derived oxygen downward from shoots to roots (Smith et al 1984; Caffrey and Kemp 1991; Borum et al 2007) can have a significant influence on both porewater and water column chemistry. No measurement of diel variability in porewater DIC was carried out and it is unknown to what extent this tentative process is modifying porewater carbonate chemistry (Delgard et al 2016)

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Conclusion