Rock‐protecting seaweed? Experimental evidence of bioprotection in the intertidal zone

Abstract

Biogeomorphological processes are an important component of dynamic intertidal systems. On rocky shores, the direct contribution of microorganisms, plants and animals to weathering and erosion is well known. There is also increasing evidence that organisms can alter rock breakdown indirectly, by moderating temperature and moisture regimes at the rock-air interface. These influences have been purported to represent mechanisms of bioprotection, by buffering microclimatic fluctuations associated with weathering processes such as wetting and drying and salt crystallization. However, virtually nothing has been done to test whether microclimatic buffering translates to differences in actual rock breakdown rates.Here we report a preliminary laboratory experiment to assess how an artificial canopy (chosen to represent seaweed) affects mechanical rock breakdown. Using a simplified and accelerated thermal regime based on field data from a rocky shore platform in southern England, UK, we find that breakdown (mineral debris release) of mudstone covered with a canopy is reduced by as much as 79% relative to bare rock after around 100 thermal cycles. Reduction in rock surface hardness (measured using an Equotip device) was also greater for bare rock (17%) compared to covered rock (10%) over this period. Measurements of salt crystal formation indicate that the mechanism driving these differences was a reduction in the frequency of crystallization events, via moisture retention and shading of the rock surface.

Cool barnacles: Do common biogenic structures enhance or retard rates of deterioration of intertidal rocks and concrete?

“…And seaweeds have been shown to buffer surface thermal extremes and limit the amplitude of temperature and humidity cycles over diurnal and shorter timescales (Coombes et al, 2013a). In doing so, some organic covers are thought to limit wetting-drying cycles, and damaging salt crystallisation and hydration events (e.g., Gowell et al, 2015). In contrast to microorganisms and plants, we are aware of only one published empirical study (Pappalardo et al, 2016) exploring the potential influence of common sedentary marine animals (such as barnacles, mussels and encrusting worms) on rock breakdown in the coastal zone.…”

“…Direct heating in this way better reflects field conditions than indirect (convective) heating alone (Warke and Smith, 1998). The lamp was set to switch on/off at 15 minute intervals to simulate short-term fluctuations in temperature as frequently occur on rocky shores due to interruptions in insolation by passing cloud and wind gusts (Coombes, 2011a;Gowell et al, 2015).…”

“…The characteristic saw-tooth pattern induced by the lamp switching on and off (e.g., Gowell et al, 2015;Warke and Smith, 1998) is superimposed on a general trend of increasing temperatures that reflect the cabinet programme. Block temperatures frequently rose above ambient air temperature, although there were notable differences between blocks depending on barnacle cover (discussed in Sections 3.1.1 and 3.1.2).…”

“…Weight change during the cabinet simulations was used as a measure of evaporative loss (of water retained both by barnacle tests and within rock pores) and implied evaporative cooling (e.g., Gowell et al, 2015). The relationship between barnacle cover and weight loss is shown in Figure 4c.…”

“…These influences can alter the efficacy of weathering processes (e.g., Gowell et al, 2015) and the potential suitability of a surface for subsequent colonisers (e.g., Coombes et al, 2015b;Pinn et al, 2008).…”