Biochemistry

Ocean biochemistry links physical climate to marine life. Measures of changing ocean chemistry include changes in surface‐ocean acidity (pH), phytoplankton abundance, phytoplankton size structure, and subsurface oxygen concentrations. Surface ocean pH is a measure of ocean acidification. As the amount of CO2 in the atmosphere has increased, a substantial portion (as much as 26%) has been absorbed by the ocean (IPCC, 2021). This has caused ocean pH to decline (the ocean has become more acidic). This makes it harder for corals and plankton to form calcium carbonate, the main mineral used to build their hard skeletons and shells. This adversely impacts coral reefs and shellfish, and threatens other marine ecosystems including pelagic fisheries.

Phytoplankton are microscopic marine algae. They form the foundation of the marine food web. Chlorophyll-a concentration derived from satellite ocean color is used as a proxy for phytoplankton abundance (IPCC, 2021). Phytoplankton abundance and size affect food availability for all marine organisms, ranging from zooplankton to apex predators and provide insights into ecosystem productivity. Decreasing phytoplankton abundance and size reduces habitat quality and has the potential to negatively impact ocean and coastal fisheries.

Oxygen concentrations in the ocean subsurface are a critical indicator of marine ecosystem health and functioning (IPCC, 2021). When oxygen levels decline below the surface, marine habitats can shrink, causing shifts in species distribution. This affects marine biodiversity and fisheries. Subsurface oxygen levels are closely tied to the development of hypoxic “dead zones”, where oxygen concentrations are too low to sustain most marine life.

These biochemical indicators are observed and analyzed with complementary tools. Including satellite ocean color and SST, in-situ water-column profiles, and physics–biogeochemistry reanalyses that fuse observations with models. Values for these biochemical indicators vary with space and time. With respect to ENSO, in the ocean around Palau El Niño typically suppresses nutrients and can shift pH and oxygen through changes in upwelling and air–sea fluxes (Behrenfeld et al., 2006; NOAA GFDL, 2022). In contrast, La Niña often favors higher chlorophyll and larger median cell size in parts of the western Pacific through enhanced mixing or upwelling.

In this section we look at four indicators of change in ocean biochemisty. We will create plots and maps that show changes in: ocean pH; Chlorophyll-a concentration estimated from satellite remotely sensed observations of ocean color; estimated median phytoplankton size as derived from satellite remotely sensed sea surface temperature and ocean color data; and subsurface oxygen concentration.