The constraint of ignoring the subtidal water climatology in evaluating the changes of coralligenous reefs due to heating events

Abstract

Predicting community-level responses to seawater warming is a pressing goal of global change ecologists. How far such predictions can be derived from a fine gradient of thermal environments needs to be explored, even if ignoring water climatology does not allow estimating subtidal marine heat waves. In this study insights about the influence of the thermal environment on the coralligenous community structure were gained by considering sites (Sardinia, Italy) at different temperature conditions. Heating events were measured (by loggers at 18 m, 23 m, 28 m, 33 m and 38 m deep) and proxies for their duration (the maximum duration of events warmer than the 90th percentile temperature), intensity (the median temperature) and variability (the number of daily ΔT larger than the mean daily ΔT, and the number of heating events larger in ΔT than the 90th percentile ΔT) were selected by GAM models. Reliable predictions of decrease in coralligenous richness of taxa/morphological groups, with relevant increment in turfs and encrusting coralline algae abundance at the expenses of bryozoans were made. Associations to the different types of heating descriptor have highlighted the aspect (intensity, duration or variability) of the heating events and the threshold for each of them responsible for the trajectories of change.

The Mediterranean bioconstructor Lithophyllum stictiforme shows adaptability to future warming

“…In coralligenous reefs, CCA constitute a secondary substrate which hosts highly diverse assemblages (Ballesteros, 2006). Spatial analyses of assemblages associated to CCA have evidenced biogeographical patterns (Piazzi et al, 2021), supporting the hypothesis that temperature is a key driver affecting the horizontal and vertical distribution of these communities (Ceccherelli et al, 2020;Pinna et al, 2021). On the contrary, little is known about the influence of temperature on CCA presence and distribution in the Mediterranean Sea, although several diseases referable to thermal effects have been reported (Hereu and Kersting, 2016) and a time-integrated thermogeographic model has been proposed by Adey and Steneck (2001) to demonstrate conditions under which CCA assemblages evolve biogeographic patterns in their distribution and abundance.…”

“…Both sites are far from any anthropogenic sources of disturbance and are characterized by a high oligotrophy (Barisiello et al, 2002;Regione Autonoma della Sardegna, 2016). However, large differences in sea water temperature were evidenced between sites in the Sea Surface Temperature (SST) climatology, water stratification during the hot season in terms of duration, temperature intensity and variability, and MHWs occurrence (Ceccherelli et al, 2020). Probably due also to such differences, the coralligenous assemblages of the two sites have different geomorphological structures (Piazzi et al, 2021): in particular, CP, which is on average colder than CC, is characterized by massive bioconstructions from 15 m of depth, while in CC the same type of constructions is found only deeper than 30 m (Pinna et al, 2021).…”

“…23 and 34 m). Based on temperature variations measured by Ceccherelli et al (2020), CC has considerably warmer water than CP, and the rationale of the BSE relies on the fact that, at the same depth, the algae of the two sites can experience different temperatures for several months, at least from May to November (Ceccherelli et al, 2020). At each combination of site and depth, eight L. stictiforme thalli were fixed on an exposed horizontal substratum next to the coralligenous cliff: four transplanted from the other site (same depth) and four translocated (same site and same depth, sensu Chapman, 1986).…”

“…Nowadays, there is a growing need of field experiments which may serve as a useful bridge between laboratory results and natural environmental processes (Chave, 2013). Taking advantage of natural differences in temperature conditions between coastal areas in Sardinia (Italy, Western Mediterranean, Pansini et al, 2021) and along depth gradients (Ceccherelli et al, 2020), we performed crossed transplants of L. stictiforme to examine the temperature effects in terms of mortality, growth, and reproductive structures for the first time in the field. Two experiments were conducted: in the first (between sites experiment), to evaluate the adaptability of the alga to a different thermal regime, we transplanted algae from a cold to a warmer site at two depths; in the second (between depths experiment), transplants were done within the same site from a deeper to a shallower depth under different light irradiance manipulations to evaluate the influence of the thermocline, which could be increasingly deeper and persistent due to global climate change effects (Cerrano et al, 2000;Collins et al, 2010).…”

Effects of Current and Future Summer Marine Heat Waves on Posidonia oceanica: Plant Origin Matters?

“…The temperature change projected in the Mediterranean Sea ranges between 0.81 and 3.71 • C in the upper layer (0-150 m) by the end of the 21st century, depending on the greenhouse gas emissions scenario (Soto-Navarro et al, 2020). The rising SST has accelerated MHWs occurrence and effects on Mediterranean benthic communities have already been described (Garrabou et al, 2009;Marbà and Duarte, 2010;Rubio-Portillo et al, 2016), even though their effects on subtidal water conditions are anything but obvious: SST is undoubtedly a useful proxy for very shallow water temperature, but using loggers remains recommended to have more accurate and precise temperature estimates as deep water temperature can be scarcely predicted from SST (Ceccherelli et al, 2020). The most severe impacts of MHWs on benthic communities are expected in summer when the intensity of these extreme events can easily exceed the maximum tolerance limit of the species (e.g., those in 2003(e.g., those in , 2012(e.g., those in , 2015(e.g., those in , Darmaraki et al, 2019b.…”