Broad plasticity in the salinity tolerance of a marine copepod species, Acartia longiremis, in the Baltic Sea

We assessed feeding, reproduction, survival and respiration in the boreal-polar copepod Acartia longiremis over the salinity range of 3-16 in the Baltic Sea. Feeding and egg production were not affected at a salinity of 7-16, representing the range in which the species naturally occurs, but decreased significantly at a lower salinity. Survival experiments showed a broad physiological plasticity with no increase in mortality upon immediate exposure to salinities of 16-7. Acclimation of females to low salinity extended the survival range to a salinity of 5. While the response in vital rates was characteristic of a tolerant, brackish water species, unusually high respiration rates at a salinity of 7-16 indicated that the species experienced osmotic stress, and that the mechanism maintaining physiological integrity was energetically expensive. Divergent responses of an increase in respiration rate and a decrease in feeding rate at a salinity below 7 indicated a disruption of the energetic balance under which the osmotic stress could not be counteracted. Our results show that A. longiremis persists close to its physiological limit in the Baltic Sea, which makes the species vulnerable to small changes in future salinity.