Consequences of the marine climate and ecosystem shift of the 1980-90s on the Ligurian Sea biodiversity (NW Mediterranean)

Abstract

A rapid temperature increase in the 1980-90s has been accompanied by dramatic and unprecedented changes in the biota and communities of the Ligurian Sea. This review uses existing historical series (a few of which have been purposely updated) to assess extent and consequences of such changes. A number of warm-water species, previously absent or occasional in the comparatively cold Ligurian Sea, has recently established thanks to warmer winters. Occurrence among them of invasive alien species is causing concern because of their capacity of outcompeting autochthonous species. Summer heatwaves, on the other hand, caused mass mortalities in marine organisms, some of which found refuge at depth. New marine diseases appeared, as well as other dysfunctions such as the formation of mucilage aggregates that suffocated and entangled benthic organisms. Human pressures have combined with climate change to cause phase shifts (i.e., abrupt variations in species composition and community structure) in different habitats, such as the pelagic environment, seagrass meadows, rocky reefs, and marine caves. These phase shifts implied biotic homogenization, reduction of diversity, and dominance by invasive aliens, and may be detrimental to the resilience of Ligurian Sea ecosystems. Another phase of rapid warming has possibly started in the 2010s and there are clues pointing to a further series of biological changes, but data are too scarce to date for proper assessment. Only well addressed long-term studies will help understanding the future dynamics of Ligurian Sea ecosystems and their possibilities of recovery.

Abrupt Change in a Subtidal Rocky Reef Community Coincided with a Rapid Acceleration of Sea Water Warming

“…There is growing evidence that a major shift in Ligurian Sea climate and ecosystem regime occurred around the 1980-90s [28,29], in coincidence with similar changes observed in other European seas and perhaps globally. Such changes were caused by complex ocean-atmosphere interactions and involved plankton, fish, and top predators [30][31][32].…”

“…Theory predicts that regime shifts (i.e., changes in the factors that govern the dynamics of the ecosystem) cause corresponding phase shifts in the biotic communities [55], and this is apparently what happened at Mesco Reef, suggesting that the previous community as known in 1961 and 1990-and possibly little modified from that existing in the 1930s [33]-has not been capable to resist climate change. The community passed from a complex configuration with a dense canopy of large gorgonians (Paramuricea clavata), an understory dominated by long-lived calcified bryozoans (Cellaria fistulosa), and a basal layer of encrusting corallines (Lithophyllum stictiforme), to a simpler configuration with a reduced P. clavata's canopy (due to the mass mortality induced by the heat waves of the 1990s [28]), an understory with soft corals (Parazoanthus axinellae) and a basal layer of filamentous algae (dominated by the alien species Womersleyella setacea and Caulerpa cylindracea). Reduced canopy and decreased abundance of calcified organisms implied loss of topographic complexity, with likely consequences on the whole associate community and ecosystem services [56,57].…”

“…The novel community that emerged from the climate shift of the 1990s, with many once characteristic species lost, reduced complexity, biotic homogenization, greater diversity, and domination by alien species, may be able to persist [70], and there is little opportunity to go back for ecosystems under altered regime [16]. Recent analyses of Ligurian Sea temperature suggest that another phase of rapid warming is occurring, 2014 to 2018 having been the warmest years on records [28,46], thus conforming to the expected further rapid warming over the whole Mediterranean Sea [71,72]. Only continued monitoring will help envisaging the possibility for a reversal of the present phase shift or for further directional transformations driven by sea water warming and other components of global change.…”

“…A proper evaluation of ecosystem change requires the availability of long-time series [25,26], which are unfortunately rare in the Mediterranean Sea as well as worldwide [6,27]. The Ligurian Sea (NW Mediterranean) represents a partial exception, as historical data are available for a number of taxa and habitats [28].…”

Impacts of Marine and Lagoon Aquaculture on Macrophytes in Mediterranean Benthic Ecosystems

“…A wasting disease (90% to near 100% mortality) occurred in Spain in 2016 (Darriba, 2017;Vázquez-Luis et al, 2017). Subsequently, this mortality spread to the whole of the Mediterranean (González-Wangüemert et al, 2018;Pergent, 2018;Bianchi et al, 2019;Cabanellas-Reboredo et al, 2019). Mortality is caused by a unicellular parasite, Haplosporidium pinnae, a species previously undescribed (Catanese et al, 2018).…”