Abstract
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 >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.
Highlights
Coastal marine ecosystems have experienced dramatic changes during the last century, often driven by urbanisation and exemplified by the proliferation of man-made structures such as seawalls, breakwaters, and groynes (Heery et al 2017, Todd et al 2019)
Findings from our bilateral 1 yr study indicate that lowering the pH of concrete did not significantly increase the abundance and species richness of intertidal benthic organisms on retro-fitted enhancement tiles, and did not significantly alter the community composition they supported
Concrete is generally considered harmful to the environment, yet it remains one of the most commonly used materials in the world and is prevalent in the construction of marine and coastal infrastructure (Bulleri & Chapman 2010, Waters & Zalasiewicz 2018), including marine biodiversity enhancement units
Summary
Coastal marine ecosystems have experienced dramatic changes during the last century, often driven by urbanisation and exemplified by the proliferation of man-made structures such as seawalls, breakwaters, and groynes (Heery et al 2017, Todd et al 2019). It has been suggested that concrete has a negative effect on the recruitment of marine biota due to its high surface alkalinity (pH ~13) (Lukens & Selberg 2004, PerkolFinkel & Sella 2014), reducing initial rates of species colonisation (Nandakumar et al 2003) and favouring alkotolerant taxa such as barnacles and serpulids over algae (Hatcher 1998, Dooley et al 1999) This high surface alkalinity potentially compounds the known negative effects of hard coastal defences on intertidal organisms such as the loss of habitat area (Lai et al 2015), compression of the intertidal zone due to steep gradients (Firth et al 2014, Loke et al 2019b), low structural complexity (Chapman & Bulleri 2003, Moreira et al 2007), and higher risk of desiccation (Tan et al 2018, Zhao et al 2019) and temperature stress (Aguilera et al 2019). We tested the following hypotheses: (1) carbonated tiles will support higher macrofaunal abundance and species richness than standard non-carbonated tiles, and (2) carbonated tiles will support different biological communities from standard non-carbonated tiles, and these differences will be consistent across time and sites with different community assemblages
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