Assessing the Ecological Impacts of Bicarbonate-Enriched Seawater on Benthic Communities: A Mesocosm Experiment in the Gulf of La Spezia

Since the pace and scale of geological CO₂ storage deployment have fallen short of expectations, there is a growing interest in ocean-based CO₂ storage options, such as the storage in the form of bicarbonates ions in seawater. Limenet technology stores CO₂ in the form of bicarbonate ions (HCO₃^-) by seawater alkalinization through the addition of calcium hydroxide (Ca(OH)₂) at pH-equilibrated conditions, i.e., increasing seawater alkalinity without pH alteration.

Calcium hydroxide, generated from the calcination of calcium carbonate (CaCO₃), reacts with captured CO₂ and seawater in reactors to form stable bicarbonate ions that is then released in the sea, ensuring carbon sequestration and neutralizing ocean acidification.

This study investigated the ecological impact of bicarbonate-enriched seawater released from Limenet pilot plant on the recruitment of benthic communities in the Gulf of La Spezia (western Mediterranean Sea, northern Italy). A controlled mesocosm experiment, conducted from March to the end of May 2024, assessed the responses of benthic organisms to different concentrations of bicarbonate-enriched seawater.

Land mesocosms were housed within a purpose-built shelter, designed to simulate natural marine conditions while allowing for precise experimental control. Seawater was sourced directly from the gulf, ensuring natural baseline conditions. Important parameters such as pH, alkalinity, temperature, dissolved oxygen, and salinity were continuously monitored.

Five treatments were tested, representing different concentrations of bicarbonate-enriched seawater as a proportion of the total mesocosm volume (1,000 liters): control (no treated seawater), low (3.3% treated water), medium (6.7% treated water), high (13.3% treated water), very high (26.3% treated water). In this experiment, the “very high” treatment represented oversaturated seawater (Ω_aragonite7.1), specifically designed to investigate the potential adverse effects of operating above the saturation limit.

Six calcareous plates (10×10 cm) were placed in each mesocosm to serve as substrates for benthic organisms. Sampling was conducted at two intervals: after 50 days to assess early recruitment and after 85 days to evaluate survival indices and the long-term community structure.

Benthic communities were analyzed using a combination of stereomicroscopy, advanced imaging, and software tools.

Serpulid worms (Serpulidae) were the dominant group across all treatments, with no significant differences in their abundance between natural and treated water conditions, except, as expected, for the very high treatment. In this case, serpulids abundance was lower, likely due to high saturation levels and extensive precipitation of bicarbonates into calcium carbonate, which may have created less favorable conditions for recruitment and survival.

These findings represent an important first step in safely deploying Limenet and other seawater alkalinization solutions without causing alteration to benthic communities, when they are applied within established thresholds.