**1. Introduction**

Over the last few years, the frequency of mass mortalities in marine ecosystems linked to Global Climate Change (GCC) has increased greatly, and several authors have described the dramatic effects of such anomalous events on benthic communities and populations [1–4] among others. However, few research studies address the delayed effects on, and the long-term growth trends of the affected populations, whose complex dynamics cannot be adequately expressed through the narrow terms of "survival" or "extinction".

In marine ecosystems, temperature is one of the main factors controlling species distribution, and unusual deviations from typical seasonal patterns may represent the greater impacts of global change on such systems [5]. The effects of abnormal warming of the water column are clearly and easily observable not only at shallow depths as environmental degradation and the direct responses of the organisms (e.g., lower population growth rates), but also in the deep sea, where modifications of the water's physicochemical characteristics are impacting biodiversity [6].

Habitat-forming species such as sea grasses, kelps, corals, and oysters are among the main organisms profoundly influenced by thermal stressors [7], although neither all species nor all communities are equally affected or respond in the same way. For example, among tropical hexacorals, differential susceptibility and resistance to bleaching and mass

**Citation:** Ruffaldi Santori, S.; Benedetti, M.C.; Cocito, S.; Peirano, A.; Cupido, R.; Erra, F.; Santangelo, G. After the Fall: The Demographic Destiny of a Gorgonian Population Stricken by Catastrophic Mortality. *Oceans* **2021**, *2*, 337–350. https:// doi.org/10.3390/oceans2020020

Academic Editor: Rupert Ormond

Received: 11 January 2021 Accepted: 9 April 2021 Published: 19 April 2021

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mortalities have been observed due to species peculiarity, the environmental variability between reefs, and even genetic differences between colonies of the same species on the same reef [8–10].

In recent decades, several worldwide populations of octocorals have been affected by stressors linked to GCC, making it essential that a demographic study of these populations is carried out in order to formulate reliable survivorship predictions [11–19]. It has been observed that the frequency of negative events may be a main factor determining the survival of such populations [17]. Even if they do endure, these populations could dramatically change their demographic structure and, depending upon their peculiar life-history traits, respond to anomalous mortality events in unexpected ways [13,20–23]. Octocorals are among the most common suspension feeders dwelling within the so-called animal marine forest *sensu* Rossi [24], where they often act as *habitat engineers*. Thus, the dynamics of their populations may have profound effects on the whole species assemblage. In the Mediterranean Sea, some populations of the temperate red gorgonian *Paramuricea clavata* (Risso 1826) have been affected by mass mortality events following unusual thermal stress [20,25–27] among others. Widely distributed along boulders and cliffs between 12 and 120 m depth in highly hydrodynamic areas [28,29], *P. clavata* is one of the main components of the coralligenous species assemblages [30] and determines the composition of the entire epibenthic community [31]. *P. clavata* is a large sea fan gorgonian (up to 1 m tall) that can form dense patches (up to 59 colonies per m2, [32]) and is characterized by high reproductive output and long life cycle [21]. This species has recently been classified as a "bioindicator" [33] and "vulnerable" by the IUCN (International Union for Conservation of Nature). A recent genetic study has highlighted the strategic role of the *P. clavata* populations in the eastern Ligurian Sea (northwestern Mediterranean, Italy) for the persistence of the species at a regional level [34].

At summer's end of both 1999 and 2003, the benthic suspension feeders living along the coastal shores of the Ligurian Sea were affected by two mass mortality events cooccurring with exceptional warming of the water column [35,36]. Exposure to abnormal temperature increases was likely the cause of an ensuing outbreak of opportunistic pathogens such as bacteria, protozoans, and fungi on the *P. clavata* colonies, leading to their total or partial death [35,37]. The local population of this gorgonian living at the western edge of the Spezia Gulf (Figure 1) was stricken, and 78% of the colonies died or were heavily damaged [32]. This population was monitored from 1998 (pre-mortality) to 2010 [13,21]. In the years following the catastrophic events both adult and recruit density fell drastically, and the population size/age structure was altered due to the mortality disproportionally affecting the larger colonies, thereby shifting it towards the smaller ones [32]. Since 2007, a significant increase in adult and recruit density has occurred [13,20].

This paper analyzes the population size/age structure, the adult and annual recruit densities, and the percentage cover of *P. clavata* and the main organisms living in the area in 2013. The results have been compared with the previous findings to identify long-term variations affecting both the *P. clavata* population and the whole community after the catastrophic mortality events.

**Figure 1.** Sampling area, Tinetto islet: stars represent the sampling sites (Eastern Liguria Sea, NW Mediterranean, Italy).
