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

Published source details Hsiung A.R., Tan W.T., Loke L.H.L., Firth L.B., Heery E.C., Ducker J., Clark V., Pek Y.S., Birch W.R., Ang A.C.F., Hartanto R.S., Chai T.M.F. & Todd P.A. (2020) Little evidence that lowering the pH of concrete supports greater biodiversity on tropical and temperate seawalls. Marine Ecology Progress Series, 656, 193-205.
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This study is summarised as evidence for the following.

	Action	Category
	Use environmentally-sensitive material on intertidal artificial structures Action Link
	Create pit habitats (1–50 mm) on intertidal artificial structures Action Link

Use environmentally-sensitive material on intertidal artificial structures

A replicated, controlled study in 2018–2019 on four intertidal seawalls on island coastlines in the Singapore Strait, Singapore, and in the Plym and Tamar estuaries, UK (Hsiung et al. 2020) found that reducing the pH of concrete settlement plates did not alter the macroalgae and invertebrate community composition or increase their species richness or abundance on plates. Over 12 months, reduced-pH-concrete settlement plates supported 59 invertebrate species in total (Singapore: 46; UK: 13), while standard-concrete plates supported 57 (Singapore: 48; UK: 9) (data not statistically tested). Ten invertebrate species (8 mobile, 2 non-mobile) recorded on reduced-pH plates were absent from standard-concrete plates. After 12 months, macroalgae and invertebrate community composition (data reported as statistical model results) and species richness was similar on reduced-pH plates (3–21 species/plate) and standard-concrete plates (3–20/plate). The same was true for invertebrate abundance (6–187 vs 11–216 individuals/plate) and cover of limpets (Patellidae, Fissurellidae, Siphonariidae, Lottioidea) (both 1–5% cover), barnacles (Cirripedia) (18–24 vs 18–25%), ephemeral green macroalgae (4–5 vs 5–8%) and encrusting macroalgae (35 vs 29%). Concrete settlement plates (200 × 200 mm) were moulded with reduced pH (pH 7–10) and standard pH (pH 12–13). Twenty-four of each were attached at a 60° angle at midshore on each of two seawalls in both Singapore and the UK during February–March 2018. Plates had water-retaining pits created on them. Macroalgae on plates were counted from photographs and invertebrates in the laboratory over 12 months. Eight plates were missing and no longer provided habitat.

(Summarised by: Ally Evans)
Create pit habitats (1–50 mm) on intertidal artificial structures

A replicated study in 2018–2019 on four intertidal seawalls on island coastlines in the Singapore Strait, Singapore, and in the Plym and Tamar estuaries, UK (Hsiung et al. 2020) reported that settlement plates with pit habitats supported macroalgae and invertebrates. Over 12 months, settlement plates with pits supported 67 invertebrate species in total (Singapore: 54; UK: 13). After 12 months, there were 3–21 species/plate and 6–216 individuals/plate. Plates supported 1–5% cover of limpets (Patellidae, Fissurellidae, Siphonariidae, Lottioidea), 18–25% cover of barnacles (Cirripedia), 4–8% cover of ephemeral green macroalgae, and 29–35% cover of encrusting macroalgae. Concrete settlement plates (200 × 200 mm) were moulded with 15 water-retaining round pit habitats (diameter: 6–28 mm; depth not reported) over half their surfaces. Plates had either reduced pH (environmentally-sensitive material) or standard pH. Twenty-four of each were attached at a 60° angle at midshore on each of two seawalls in both Singapore and the UK during February–March 2018. Macroalgae on plates were counted from photographs and invertebrates in the laboratory over 12 months. Eight plates were missing and no longer provided habitat.

(Summarised by: Ally Evans)