Little evidence that lowering the pH of concrete supports greater biodiversity on tropical and temperate seawalls

Abstract

<jats:p>Concrete is one of the most commonly used materials in the construction of coastal and marine infrastructure despite the well known environmental impacts which include a high carbon footprint and high alkalinity (~pH 13). There is an ongoing discussion regarding the potential positive effects of lowered concrete pH on benthic biodiversity, but this has not been investigated rigorously. Here, we designed a manipulative field experiment to test whether carbonated (lowered pH) concrete substrates support greater species richness and abundance, and/or alter community composition, in both temperate and tropical intertidal habitats. We constructed 192 experimental concrete tiles, half of which were carbonated to a lower surface pH of 7-8 (vs. control pH of &gt;9), and affixed them to seawalls in the United Kingdom and Singapore. There were 2 sites per country, and 6 replicate tiles of each treatment were collected at 4 time points over a year. Overall, we found no significant effect of lowered pH on the abundance, richness, or community assemblage in both countries. Separate site- and month-specific generalised linear models (GLMs) showed only sporadic effects: i.e. lowered pH tiles had a small positive effect on early benthic colonisation in the tropics but this was later succeeded by similar species assemblages regardless of treatment. Thus, while it is worth considering the modification of concrete from an environmental/emissions standpoint, lowered pH may not be a suitable technique for enhancing biodiversity in the marine built environment.</jats:p>

Greening of grey infrastructure should not be used as a Trojan horse to facilitate coastal development

“…These experiments should be revisited, repeated and expanded on (e.g. Hsiung et al., 2020) to improve the evidence base for policy development. The rise of the global research network (e.g.…”

Influence of the Intrinsic Characteristics of Cementitious Materials on Biofouling in the Marine Environment

“…The pH was also verified using phenolphthalein indicators, which gave a pink coloration after contact with the surface of the specimens, indicating a pH higher than 9. Therefore, the surface pH measured was lower than expected (pH~13) [101,102]. This might be due to the 30 days of contact between air and mortar specimens before the start of the immersion test.…”

“…Cementitious materials have a very high surface alkalinity (pH~13) [101,102]. Due to this very basic pH, their constituent materials have been cited as inhibitors of the recruitment of marine biota [36].…”

“…These results and conclusions are in keeping with those from the study performed by Hsiung et al, which showed that the decrease in pH did not affect the long-term bioreceptivity of cementitious materials in the marine environment. At the end of their study, Hsiung et al suggested that manipulation of the physical structure (surface roughness) of the concrete was a more effective eco-engineering approach to enhancing ecological value and species diversity in the marine environment [102].…”

Concrete Seawalls: Load Considerations, Ecological Performance, Durability, and Recent Innovations

“…The higher diversity in these alternative mixtures has tentatively been suggested to be caused by lower pH [88,99,101]. On the other hand, a recent study concluded that carbonated (lowered pH) concrete substrates did not support greater species richness and abundance, and/or alter community composition, in both temperate and tropical intertidal habitats [102]. It is possible that the use of 'natural' materials in the concrete mixture design could help increase biodiversity.…”