Rocks of different mineralogy show different temperature characteristics: implications for biodiversity on rocky seashores

Abstract

As some intertidal biota presently live near their upper tolerable thermal limits when emersed, predicted hotter temperatures and an increased frequency of extreme-heat events associated with global climate change may challenge the survival and persistence of such species. To predict the biological ramifications of climate change on rocky seashores, ecologists have collected baseline rock temperature data, which has shown substrate temperature is heterogenous in the rocky intertidal zone. A multitude of factors may affect rock temperature, although the potential roles of boulder surface (upper versus lower), lithology (rock type) and minerology have been largely neglected to date. Consequently, a common-garden experiment using intertidal boulders of six rock types tested whether temperature characteristics differed among rock types, boulder surfaces, and whether temperature characteristics were associated with rock mineralogy. The temperature of the upper and lower surfaces of all six rock types was heterogeneous at the millimetre to centimetre scale. Three qualitative patterns of temperature difference were identified on boulder surfaces: gradients; mosaics; and limited heterogeneity. The frequency of occurrence of these temperature patterns was heavily influenced by cloud cover. Upper surfaces were generally hotter than lower surfaces, plus purple siltstone and grey siltstone consistently had the hottest temperatures and white limestone and quartzite the coolest. Each rock type had unique mineralogy, with maximum temperatures correlated with the highest metallic oxide and trace metal content of rocks. These baseline data show that rock type, boulder surface and mineralogy all contribute to patterns of heterogenous substrate temperature, with the geological history of rocky seashores potentially influencing the future fate of species and populations under various climate change scenarios.

Where three snail species attach while emersed in relation to heterogenous substrate temperatures underneath intertidal boulders

“…While both seashores have similar oceanic and environmental conditions such as low wave exposure, a relatively flat shore slope, and are free from shading by cliffs, localised variations in wave splash and wind speed/direction may occur. Janetzki, Benkendorff & Fairweather (2021) showed that grey siltstone was consistently one of the hottest rock types sampled from this region, while quartzite was one of the coolest ( Appendix Fig. A1 ).…”

“…Small boulders (maximum dimension ≤ 30 cm) represent one type of habitat that gastropods may seek refuge under during low tide. Janetzki, Benkendorff & Fairweather (2021) found that substrate temperature can be highly variable underneath boulders, with differences of up to 15.5 C between the hottest and coolest areas on a lower boulder surface. Moreover, the thermal quality of under-boulder habitats was influenced by lithology, with rock types differing in both maximum temperature and the qualitative patterns of temperature that developed across boulder surfaces when emersed (Janetzki, Benkendorff & Fairweather, 2021).…”

“…Janetzki, Benkendorff & Fairweather (2021) found that substrate temperature can be highly variable underneath boulders, with differences of up to 15.5 C between the hottest and coolest areas on a lower boulder surface. Moreover, the thermal quality of under-boulder habitats was influenced by lithology, with rock types differing in both maximum temperature and the qualitative patterns of temperature that developed across boulder surfaces when emersed (Janetzki, Benkendorff & Fairweather, 2021). Consequently, gastropod thermoregulatory behaviour may extend beyond simply retreating under boulders, as organisms may also respond to the heterogenous temperatures underneath them.…”

“…To determine whether gastropod occupancy under boulders is associated with the patterns of heterogenous surface temperature identified by Janetzki, Benkendorff & Fairweather (2021) , a field study of their experimental results was completed. We first established whether the qualitative patterns of surface temperature (definitions and representative images of these qualitative patterns are supplied in the methods) and rock-type differences in maximum surface temperature identified under experimental conditions ( Janetzki, Benkendorff & Fairweather, 2021 ), also occur for boulders on the seashore. We then investigated how the specific locations occupied by three snail species underneath boulders were related to any patterns of heterogenous surface temperature.…”

“… Jones & Boulding, 1999 ; Harley, 2008 ; Gedan et al, 2011 ). Boulders of two rock types were sampled because surface temperatures can differ among rock types ( Janetzki, Benkendorff & Fairweather, 2021 ), and this in turn can influence ectotherm responses to substratum temperature ( Raimondi, 1988 ; Marshall, McQuaid & Williams, 2010 ; Judge, Botton & Hamilton, 2011 ). Untangling associations between intertidal gastropods and substratum temperatures is important given predictions of hotter air temperatures and an increased frequency of extreme heat events associated with global climate change ( IPCC, 2013 ).…”