Abstract
We applied the concept of source–sink dynamics to investigate a recent (1999–2013) increase in the occurrence of the siphonophore Muggiaea atlantica in Scottish coastal waters. Our aim was to determine whether this change represented the establishment of resident populations (i.e. “sources”), or transient populations reliant on immigration (i.e. “sinks”). First, we show that local production was not always sufficient to account for recruitment (a “source” prerequisite), suggesting reliance on immigration (a “sink” prerequisite). Using variation partitioning, we then discriminated between the exclusive effects of immigration [indexed by the European Slope Current (ESC)] and local production (indexed by local sea temperature and food availability). On the west coast (Loch Ewe), interannual variability in the species’ abundance was determined by, in order of increasing importance: (i) suitable local environmental conditions (13%); (ii) the role of the ESC in modulating these conditions (20%); and (iii) immigration via the ESC (29%). These results provided a strong indication that Loch Ewe represents a sink habitat for M. atlantica. However, on the east coast (Stonehaven) our results were less conclusive, probably due to the less direct influence of the ESC. For both locations, we suggest that low winter temperatures prevented overwintering, necessitating annual re-colonization via immigration.
Highlights
Jellyfish (Cnidaria and Ctenophora) represent an important functional group of secondary consumers in marine pelagic ecosystems (Condon et al, 2012; Lucas et al, 2014)
We previously demonstrated the transition of the Western English Channel (WEC) from a sink to a source habitat for M. atlantica in the late 1960s (Blackett et al, 2014)
Their inability for long-distance volitional swimming means that jellyfish populations are readily dispersed in ocean currents and may aggregate in areas that do not normally support a population, an “apparent” bloom (Graham et al, 2001). This classification represents opposite ends of a continuum (Graham et al, 2001); these biological and physical factors can often combine, resulting in the formation of opportunistic blooms when jellyfish are translocated to areas where suitable conditions exist temporarily (Costello et al, 2012). Considering these factors, we investigated the biogeographical expansion of M. atlantica in the context of source–sink dynamics (Pulliam, 2000)
Summary
Jellyfish (Cnidaria and Ctenophora) represent an important functional group of secondary consumers in marine pelagic ecosystems (Condon et al, 2012; Lucas et al, 2014). The ecological interactions of jellyfish can modify ecosystem structure and function (Mills, 1995; Pitt et al, 2009), with knock-on effects for a range of available online at academic.oup.com/plankt. A critical aspect of this understanding is our capacity to distinguish the effects of both biological (e.g. changes to demographic rates) and physical (e.g. changes to immigration and emigration rates) processes on jellyfish populations (Graham et al, 2001)
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