Long-term variability of the siphonophores Muggiaea atlantica and M. kochi in the Western English Channel

- Muggiaea atlantica and Muggiaea kochi were analysed in the Western English Channel.
- M. kochi was a transient low abundance member of the plankton.
- M. atlantica established a resident population with increased density in the 1960s.
- Specific environmental conditions determined M. atlantica population growth in situ.
- Establishment of M. atlantica was attributed to structural ecosystem changes.

We investigated long-term variability of the calycophoran siphonophores Muggiaea atlantica and Muggiaea kochi in the Western English Channel (WEC) between 1930 and 2011. Our aims were to describe long-term changes in abundance and temporal distribution in relation to local environmental dynamics. In order to better understand mechanisms that regulate the species' populations, we identified periods that were characteristic of in situ population growth and the environmental optima associated with these events. Our results show that between 1930 and the 1960s both M. atlantica and M. kochi were transient components of the WEC ecosystem. In the late 1960s M. atlantica, successfully established a resident population in the WEC, while the occurrence of M. kochi became increasingly sporadic. Once established as a resident species, the seasonal abundance and distribution of M. atlantica increased. Analysis of environmental conditions associated with in situ population growth revealed that temperature and prey were key determinants of the seasonal distribution and abundance of M. atlantica. Salinity was shown to have an indirect effect, likely representing a proxy for water circulation in the WEC. Anomalies in the seasonal cycle of salinity, indicating deviation from the usual circulation pattern in the WEC, were negatively associated with in situ growth, suggesting dispersal of the locally developing M. atlantica population. However, our findings identified complexity in the relationship between characteristics of the environment and M. atlantica variability. The transition from a period of transiency (1930-1968) to residency (1969-2011) was tentatively attributed to structural changes in the WEC ecosystem that occurred under the forcing of wider-scale hydroclimatic changes.