Abstract
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.
Highlights
Blooms of jellyfish are a characteristic feature of planktonic ecosystems (Boero et al, 2008; Condon et al, 2012) and have been for millennia (e.g. Hagadorn et al, 2002)
Our study shows that between 1930 and the late 1960s Muggiaea atlantica and Muggiaea kochi were transient components of the Western English Channel (WEC) ecosystem
Their sporadic occurrence and infrequent in situ population development at this time indicated the influence of advection, probably from source populations to the south (Corbin, 1947; Southward, 1962)
Summary
Blooms of jellyfish (pelagic Cnidaria and Ctenophora) are a characteristic feature of planktonic ecosystems (Boero et al, 2008; Condon et al, 2012) and have been for millennia (e.g. Hagadorn et al, 2002). Jellyfish are important predators (Mills, 1995), with complex ecosystem-level effects (Pitt et al, 2009) and a range of socio-economic impacts (Purcell et al, 2007; Lucas et al, 2014). Developing our understanding of factors affecting their populations is of high importance (Condon et al, 2012). Both biological and physical factors influence jellyfish populations. Environmental factors, including temperature, food availability and salinity, have been shown to directly affect jellyfish populations (Purcell, 2005); localised environmental variability may regulate populations.
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