Influence of elevated seawater temperature on amphisteginid Foraminifera in Maltese waters

Abstract

Sea warming due to climate change might favour the spread of invasive species by expanding their distribution into new areas. The Mediterranean Sea warming and its particular biogeographic history makes it a hotspot for marine invasions, and a case of a successful thermophilic invader in this sea is the marine benthic foraminifer Amphistegina lobifera, considered to be of Red Sea origin. The range expansion of A. lobifera is likely to trigger changes in ecosystem functions, including the displacement of local foraminiferal species, homogenization of foraminiferal assemblages and drastic alterations in coastal habitats. In the Maltese archipelago, the presence of this alien invasive species could bring about changes to the granulometric characteristics of marine sediments present in bottom habitats, changes which in turn may impact certain benthic species and communities. Elevated temperature has also been identified as causing morphological variations in the shells of benthic Foraminifera, including A. lobifera. The aim of the present study was to use the thermal effluent from the Delimara Power Station, Malta, as a proxy for sea warming in order to assess climate change impacts on the distribution and morphology of the invasive foraminiferan A. lobifera. This was made by comparing populations of A. lobifera at the impacted site (an inlet receiving thermal effluent) with two geographically similar inlets without thermal effluent (reference sites). No significant differences in the thickness/diameter ratio, lateral asymmetry and coiling direction of A. lobifera were noted between the impacted and reference sites. Overall, most of the specimens in the present study were sinistrally coiled, and tests were significantly larger and with a higher incidence of irregular keels at the impacted than at the two reference sites. Both absolute and relative abundances of A. lobifera at the impacted site were significantly different from values of the same attribute recorded from the reference sites, and significant differences were also found in the impact of amphisteginid invasions on the mean sediment grain size. However, no significant differences in sediment sorting values were noted between the impacted and reference sites. These findings match the described westward invasion of A. lobifera in the Mediterranean, and indicate that the colonisation of the alien in the studied sites is temperature-driven and possibly displacing local foraminiferal species. Amphisteginid invasions in Malta will probably intensify as the sea warms up and cause changes in the invaded micro-ecosystem, which is likely to also pose long-term impacts in local meso-ecosystems with significant ecological consequences for presently-occurring marine benthic communities.