Effects of ammonia on North and Baltic Sea key copepod species.

The shipping industry, in line with the International Maritime Organisation’s strategy to reduce greenhouse gas emissions, is introducing carbon-free engines powered by ammonia. Commercial deployment of such vessels is expected to begin globally from mid-2026. With a growing ammonia-fuelled fleet, the probability of accidental releases during operations, maintenance, or maritime accidents will increase. Thus, it is critical to assess the effects of ammonia spills in marine environments, especially in ecosystems that are already under heavy anthropogenic stress, such as the Baltic Sea and the North Sea.
Copepods, which constitute a key link in marine food webs, are particularly relevant for risk assessment. Known impacts of ammonia exposure include cellular osmotic stress, as well as reduced growth, reproduction, and survival. However, existing studies have largely focused on species and conditions that do not apply to complex natural systems.
We present the results of laboratory and ship-board experiments exposing different Baltic and North Sea copepod species to a range of ammonia concentrations and measuring mortality (LC₅₀), feeding, egestion and juvenile development. Our results demonstrate that the environment and organismal condition play a role in copepod responses, like the LC₅₀ of E. affinis being 0.64 and 1.06 mg L^-1 NH₃-N in the Finnish Gulf and Bothnian Sea, respectively. In addition, we observed significant sub-lethal effects of ammonia on feeding activity and fecal pellet production both in Baltic and North Sea key copepod species above concentrations of 0.5 mg L-¹ NH₃-N, although the response differed between feeding strategies.
These findings highlight the vulnerability of Baltic copepods to ammonia exposure and underline the importance of incorporating ecological complexity into spill impact assessments as ammonia emerges as a marine fuel. Our findings provide critical evidence to support risk assessments, regulatory frameworks, and spill-response strategies related to ammonia use in shipping.