Acute effects of ammonia and methanol on dominant zooplankton species in the northern Baltic Sea.

To reduce greenhouse gas emissions, an increasing number of ships are expected in future to operate on ammonia and methanol. However, the effects of these chemicals on zooplankton in case of a spill are not yet fully understood. We conducted shipboard experiments in the northern Baltic Sea on seven species of copepods and cladocerans, differing in their size and feeding mode. Zooplankton were individually exposed to chemical concentrations representing realistic spill scenarios for 24-48 h, and their mortality, faecal pellet production and hatching success (only Eurytemora affinis) were measured. Moreover, we assessed whether species traits and environmental parameters influenced zooplankton responses. Mortality was species- and population-specific. For ammonia, LC50 ranged from 0.5 to 1.2 mg L-1 NH3-N, with the cladoceran Bosmina longispina and the copepod Temora longicornis as the most and least tolerant species, respectively. Ambient oxygen concentration was positively correlated with LC50, whereas ambient ammonium and total nitrogen were negatively correlated with LC50 (p < 0.05). For methanol, Limnocalanus macrurus experienced elevated mortality (80%) at 25 mL L-1, whereas mortality of all other species remained below 50%. Faecal pellet production increased at low concentrations (up to 0.5 mg L-1 NH3-N and 2.5 mL L-1 methanol) and decreased at higher concentrations for feeding-current feeding copepods, but was largely unaffected for ambush feeders. Hatching success was negatively impacted by ammonia. The results indicate that zooplankton response to ammonia and methanol varies between species and areas underscoring the importance of testing toxic effects on natural populations to determine risks under environmentally relevant conditions.