RECLESS: Baltic 2026

In large parts of the world’s oceans, nitrogen is one of the key elements controlling how much plankton and other organisms can grow. Nitrogen therefore also influences how much carbon dioxide the ocean can take up from the atmosphere. However, in oxygen-depleted marine areas, the nitrogen cycle changes. Microorganisms can convert biologically available nitrogen into nitrogen gas and nitrous oxide — a powerful greenhouse gas.

Oxygen minimum zones expected to become more common

Oxygen-depleted areas, known as oxygen minimum zones, occur in several parts of the world’s oceans and are expected to become more common as the climate changes. The Baltic Sea is a particularly important area to study, as parts of it regularly experience very low oxygen levels. The Gotland Deep is one of the best-known deep basins in the Baltic Sea and provides a natural setting for investigating how microorganisms control the movement of nitrogen through the ecosystem.

During the expedition to the Gotland Deep in summer 2026, the research team will collect water samples and carry out advanced measurements of oxygen, nitrogen and microbial processes. Using new technology, including highly sensitive oxygen sensors and analyses of microorganisms at single-cell level, the researchers aim to understand what is actually happening in these oxygen-depleted water masses.

Seeking new knowledge about the role of nitrogen

The results from the Baltic Sea will be compared with data from other oxygen-depleted marine areas around the world. The aim is to improve understanding of how nitrogen is transformed in the ocean, how much nitrogen is lost, and how these processes may affect ocean productivity, ecosystems and climate in the future.

A laboratory on the seafloor – description of in situ incubator 

Many measurements of microbial activity in the ocean are made by bringing water from depth onboard a ship and incubating it in the lab. This can bias results because temperature and pressure change during retrieval, and in low‑oxygen waters, the samples can be contaminated with oxygen. Such shifts can stimulate some processes and suppress others relative to the microbes’ natural environment. 

No exposure to oxygen

To avoid these biases, we built an autonomous in situ incubator that collects and incubates water directly at depth, preserving the original temperature, pressure, and oxygen conditions. The instrument, which is largely built of glass and titanium to minimize oxygen contamination, fills its incubation chambers once at depth and then injects stable‑isotope tracers to quantify specific microbial processes.

Over the incubation period, it automatically collects a series of small samples at pre-set times and chemically fixes them, creating a time series, all without exposure to oxygen. 

Predict future climate feedbacks

Only after the experiment is complete are the fixed samples brought back to the ship for analysis. Comparisons reveal clear differences between in situ and shipboard measurements. Our goal is to more accurately quantify microbial processes in low‑oxygen marine waters and use this improved understanding to strengthen global ecosystem models used to predict climate feedbacks.