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

Abstract

a b s t r a c tWe 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 to residency was tentatively attributed to structural changes in the WEC ecosystem that occurred under the forcing of wider-scale hydroclimatic changes.

Population ecology of Muggiaea atlantica (Cnidaria, Siphonophora) in the Western English Channel

“…Our results support this hypothesis and suggest a critical basal limit of 10°C for asexual reproductive activity of M. atlantica in the Western English Channel. This thermal limit is in agreement with the results of Blackett et al (2014) who showed that local development of the M. atlan ti ca polygastric population at an open-shelf station in the Western English Channel only occurred when the sea surface temperature was above 9°C.…”

“…However, recurrent reproductive activity in winter would appear unlikely given that the winter temperatures typically experienced are below the thermal tolerances of M. atlantica discussed above. The M. atlantica population in the Western English Channel likely represents the northern component of a spatially extensive Northeast Atlantic metapopulation (Blackett et al 2014). Pulses of winter abundance observed in the present study could thus indicate immigration from a sub-population located to the south, probably within the Bay of Biscay (reviewed in Mackie et al 1987).…”

“…Recent observations suggest that M. atlantica has expanded its geographical distribution, establishing new populations in the Western English Channel (Blackett et al 2014) and the Mediterranean and Adriatic (Licandro et al 2012, Batistić et al 2013, probably in re sponse to hydroclimatic changes. Large blooms of M. atlantica have caused dramatic ecological (Greve 1994, Kršinić & Njire 2001 and economic (Fosså et al 2003, Baxter et al 2011 ramifications, particularly in novel habitats.…”

High spatial heterogeneity of two planktonic cnidarian species related to the variability of a shelf-slope front at short time scales

“…In particular, when the front occurred over the shelf in early June, salinity was the variable most strongly related to the spatial distributions of both species (Tables 3 and 4). Salinity has previously been reported as a determinant factor for the distributions and abundance of gelatinous zooplankton at various locations in the NW Mediterranean (Gili et al 1988, Licandro et al 2012) and the North Atlantic (Blackett et al 2014, Greer et al 2015.…”

“…The gelatinous zooplankton followed a similar trend, varying in their spatial distributions at short time scales. The two cnidarian species studied are epipelagic, mainly occurring in the surface layer between 0 and 50 m (Gili et al 1987a, b), and holoplanktonic, making them particularly susceptible to surface-water dynamics (Mackie et al 1987, Blackett et al 2014.…”