Structural and functional organization of fish assemblages in a Mediterranean shallow CO2 vent

The “business-as-usual emission scenario” simulated by the IPCC (Intergovernmental Panel on Climate Change) suggests that atmospheric CO2 levels could approach 800 ppm by the end of the century. Corresponding biogeochemical models indicate that surface ocean water pH will drop from a pre-industrial value of about 8.2 to 7.8 within 2100 (Feely et al., 2010). This phenomenon known as “Ocean Acidification” (OA) is caused by the increasing CO2 emissions due to anthropic activities, with a current consequence decrease of about 0.1 unit of pH (Caldeira & Wickett 2003) that is having effects on seawater carbonate chemistry and on marine ecosystems. Many short-term laboratory experiments have shown the effects of OA on marine calcareous organisms (Doney et al., 2009), but also on not-calcifying ones. For instance, experiments on fish have revealed effects on physiological and behavioral aspects (Dixson et al., 2010; Munday et al., 2009), but many other aspects are still unknown (Ishimatsu et al., 2008). On the other hand, field experiments have been conducted in naturally acidified marine ecosystems, known as CO2 vents, which are currently investigated to study the long-term effects of OA on species, communities and ecological processes (Hall-Spencer et al. 2008).

Shallow CO₂ vents are widespread in Mediterranean (Dando et al., 1999) and represent a sort of natural mesocosms, where marked pH gradients are present at small spatial scales. The aim of this PhD project is to assess the effect of high pCO2/low pH on the structural and functional organization of fish assemblages in a Mediterranean shallow CO₂ vent (Aeolian Archipelago, NE Sicily). In particular, we compare the responses of a chronic exposed fish assemblage living near the primary vent (mean pH = 7.8; hereafter “Low pH”) with other two fish assemblages living at normal pH (mean pH = 8.2; hereafter “Control 1” and “Control 2”) in Vulcano and Lipari Islands. We hypothesized that the organization of fish assemblage at the low pH site is different from that in controls. To test our hypothesis we use several descriptors and different methodologies. First, we compared fish community structure by using Underwater Visual Census technique to assess species richness and abundance (frequency of occurrence). Then we carried out samplings to evaluate trophic organization of fish assemblages (we used stable isotopes of carbon and nitrogen to analyze food web and trophic levels), bioaccumulation and biomagnification of trace elements (concentration and bio-availability of several trace elements, also toxic ones, may increase due to direct input from the vent and to peculiar pH and Eh conditions), and the characteristics of carbonate structures like otoliths (to assess the effect of acidification on these structures by morphological analysis). Otoliths are also used as natural tags to study fish “site fidelity” of this particular site through microchemistry analysis of trace elements and isotopic composition.

This study provided a complete and exhaustive frame of fish assemblages structure and trophic organization at different pH levels. As scant data are available in the literature on this topic, the results of this research provide information about the ecological effects of long-term exposure to high CO2 levels on fish, a key biological component whose monitoring is relevant not only from the ecological side, but also for the economic one and for the implications on human health. Moreover, this study confirms the importance to use the naturally acidified environments to test ecological hypotheses on the effects of OA on communities and ecosystems.

Mirasole A., 2017. Structural and functional organization of fish assemblages in a Mediterranean shallow CO2 vent. PhD thesis, Università degli Studi di Palermo, 114 p. Thesis.


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