Biology and ecology of the siphonophore Muggiaea atlantica in the northeast Atlantic

Abstract

In many regions of the world the abundance and distribution of jellyfish is increasing. These changes have important consequences for marine ecosystems and the services that they provide to humans. It is a fundamental goal of marine ecologists to develop understanding of how jellyfish populations respond to environmental change. Two key processes are involved in the regulation of jellyfish populations: local demographics (i.e. production and mortality) and dispersal (i.e. immigration and emigration). A failure to discriminate between the contributions of these two fundamental processes can obscure the specific environmental factors that drive changes in jellyfish populations, impeding understanding.
This thesis aims (1) to assess recent changes in the abundance and distribution of Muggiaea atlantica in the coastal waters of the United Kingdom; and to (2) investigate the influence of environmental variability on both the biological (i.e. demography) and physical (i.e. dispersal) processes that drive these changes. This study utilised data collected from a network of coastal monitoring stations, together with data on a range of local and regional hydroclimatic environmental factors.
I show that since the late 1960s, there has been a progressive northward extension of this species’ distribution from its centre of population in the Bay of Biscay and Iberian Coast region. These changes involved the establishment of a resident population in the Western English Channel (WEC) and the subsequent development of transient seasonal populations on the east and west coasts of Scotland. In the WEC the M. atlantica population was capable of overwintering, sustaining a resident population irrespective of immigration. This population was modulated primarily by the availability of suitable local environmental conditions that influenced demography (temperature and food availability) and secondarily by changes to water circulation patterns that influenced its dispersal. In contrast, in Scotland M. atlantica was incapable of overwintering and its population was primarily modulated by immigration and secondarily by the availability of suitable local environmental conditions. On the west coast of Scotland, the European Slope Current (ESC) was identified as the source of immigration, whereas on the east coast the mechanism of immigration was not identified. Despite the importance of the ESC as a transport mechanism from the WEC region, a direct source-sink relationship between the WEC and Scottish coastal waters was not clearly demonstrated, suggesting the importance of a network of sources. Key environmental requirements for local M. atlantica production were consistent between the localities, these were: a critical limit of 10 °C for reproduction, with an optimum of 13–15°C, together with the availability of small copepod prey. Changes in the timing of this critical temperate modulated trophic phasing with copepod prey, influencing the phenology of the blooms of M. atlantica. This thesis provides the basis for developing a mechanistic understanding of the factors that modulate distribution in a species of jellyfish. The principles and methods used could be applied to better understand changes in the abundance and distribution of M. atlantica in other areas, and also for different species of jellyfish.

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ORCID for Catherine Lucas: orcid.org/0000-0002-5929-7481

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