Conference Agenda

The ocean absorbs ~25% of anthropogenic CO₂ emissions annually, mediated by physio-chemical and biological processes. While physical drivers of oceanic CO₂ uptake are relatively well characterized, biological contributions remain poorly constrained. Advancing our understanding of biological controls is essential for monitoring and predicting climate-driven changes in the ocean carbon cycle. Therefore, we first propose a new global ocean ecological biome delineation based on ocean colour remote sensing. Next, we examine CO₂ dynamics in a temperate shelf sea at high spatial and temporal resolution.

Ecological biomes, i.e. regions of coherent biological and biogeochemical structure, have proven essential for carbon cycle studies. Yet, existing classifications rely heavily on physical variables (e.g. sea surface temperature, SST) with limited biological representation. We present a biologically-informed segmentation at 0.25° resolution based on 26 years of satellite ocean color data (ESA Ocean Colour Climate Change Initiative, OC-CCI), spanning the open and coastal ocean. Our biomes capture key surface ocean ecosystem features, including primary productivity, particulate organic carbon, and phytoplankton community structure. Their relevance for carbon cycle research is further demonstrated through biome-scale estimates of biological modulation of the seawater partial pressure of CO₂ (pCO₂).

Secondly, we examine pCO₂ dynamics within selected biome regions in the North Sea over the past decade at unprecedented (1km daily) resolution using in-situ pCO₂ observations (Surface Ocean CO₂ Atlas, SOCAT) and satellite observations of i.a. ocean colour (OC-CCI) and SST. By applying regionally-optimized retrieval algorithms, we estimate key biogeochemical drivers of pCO₂, including chlorophyll-a, suspended particulate matter and particulate organic carbon. We identify distinct biogeochemical regions shaped by primary productivity, riverine inputs, and sediment dynamics, with varying impacts on pCO₂ dynamics, from locally enhancing the CO₂ uptake to degassing CO₂. This study provides new insights into coastal carbon dynamics applicable to coastal regions globally.

Coastal regions are highly dynamic environments, where physical, chemical, and biological interactions regulate carbon exchange between the ocean and the atmosphere. Extreme warming events, such as marine heatwaves (MHWs), can strongly disrupt these fluxes, yet their fine-scale, short-term impacts on the full carbonate system remain poorly understood. The Belgian Part of the North Sea (BPNS), with its extensive in-situ observations of key carbonate system variables, provides an ideal setting to study these effects.

In this study, we combine in-situ carbonate system observations from the Surface Ocean CO₂ Atlas (SOCAT) and the Integrated Carbon Observation System (ICOS) with machine learning to reconstruct daily, 1 km-resolution maps of sea surface partial pressure of CO₂ (pCO₂, 2000–2024) and dissolved inorganic carbon (DIC, 2017–2024). Using CO2SYS, we derive pH and total alkalinity, completing the full carbonate system and enabling high-resolution assessment of air–sea CO₂ fluxes (FCO₂) during MHWs.

From 2000–2024, over 100 MHW events were detected in the BPNS using Hobday et al.’s (2016) criteria, with a 90% threshold, minimum five-day duration, up to two-day gaps, and the 1983–2012 SST climatology (daily ESA SST CCI and C3S data at 0.05°, interpolated to 1 km). On average, MHWs lasted two weeks, reached ~0.29 °C above the 90th percentile, and affected ~19% of the region. FCO₂ anomalies during MHWs, expressed as a percentage relative to the climatological FCO₂, represent deviations in CO₂ flux: positive anomalies indicate increased outgassing or reduced uptake, negative anomalies enhanced uptake or reduced outgassing. Preliminary analyses show average flux anomalies of 15 ± 13% (5%-trimmed mean), with short (<14 days) and long (≥14 days) events producing similar effects (~20% vs. ~16%). FCO₂ anomalies are primarily driven by sea surface pCO₂ changes, with limited influence from wind variability, highlighting the need for sustained CO₂ monitoring to understand coastal carbon responses under climate change.

Marine heatwaves are periods of anomalous sea surface temperatures (SST) sustained for long periods. These heatwaves have wide ranging impacts on marine ecosystems and biodiversity but can also alter the marine carbonate system and air-sea CO₂ exchange. The regional responses of the carbonate system and air-sea CO₂ exchange are likely to be different and vary during and after the marine heatwave. Within this work, we used a satellite observation–based approach to detect heatwaves in the SST records, alongside a well-characterised observational carbonate system dataset (OceanSODA ETHZ), to examine changes in the marine carbonate system before, during, and after five documented marine heatwaves: (1) Southern Ocean (2016), (2) Northeast Pacific (“The Blob”; 2015), (3) Western Australia (2011), (4) South Pacific (2016) and (5) Equatorial Indian Ocean (2016).

In all heatwaves, a significant reduction of dissolved inorganic carbon (DIC) was observed during the event with DIC anomalies increasing in magnitude beforehand and declining afterwards. The magnitude of these anomalies differed among the four heatwaves. Variations in DIC and SST appeared to drive anomalies in the fugacity of CO₂ (fCO_{2 (sw)}) and pH, due to their influence on carbonate chemistry. The impact of these heatwaves on the air-sea exchange of CO₂ varied during the heatwaves lifetime and the region and was driven by a combination of the carbonate system state, thermodynamics, and meteorological condition. We identified that the strongest anomalies in the carbonate system, and air-sea CO₂ exchange did not always coincide with the heatwave period but could occur prior to or after the heatwave. These results therefore support a need to consider the temporal sequence of any compounding events and their feedbacks.