Provision of refugia and seeding with native bivalves can enhance biodiversity on vertical seawalls

Published source details Bradford T.E., Astudillo J.C., Lau E.T.C., Perkins M.J., Lo C.C., Li T.C.H., Lam C.S., Ng T.P.T., Strain E.M.A., Steinberg P.D. & Leung K.M.Y. (2020) Provision of refugia and seeding with native bivalves can enhance biodiversity on vertical seawalls. Marine Pollution Bulletin, 160, 111578.
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This study is summarised as evidence for the following.

	Action	Category
	Create small ridges or ledges (1–50 mm) on intertidal artificial structures Action Link
	Transplant or seed organisms onto intertidal artificial structures Action Link
	Create groove habitats (1–50 mm) on intertidal artificial structures Action Link
	*Create grooves and* small protrusions, ridges or ledges (1–50 mm) on intertidal artificial structures** Action Link

Create small ridges or ledges (1–50 mm) on intertidal artificial structures

A replicated, randomized, paired sites study in 2016–2017 on two intertidal seawalls in the Pearl River estuary, Hong Kong (Bradford et al. 2020) found that small ridges created on the seawalls supported lower macroalgae and invertebrate species richness than groove habitats created in between them, while species diversity varied depending on the ridge height. After 12 months, macroalgae and invertebrate species richness on settlement plates was similar on tall and short ridges (both 3–4 species/plate), and lower on both than in the grooves in between (8–9/plate). The same was true for species diversity, except that tall ridges supported similar diversity to grooves (data reported as Shannon index). Concrete settlement plates (250 × 250 mm) were moulded with five tall (height: 50 mm) or short (25 mm) vertical small ridges (length: 250 mm; width: 17–65 mm) between four grooves (length: 250 mm; width: 15–50 mm; depth: 50 or 25 mm). Five of each were randomly arranged at midshore on each of two vertical concrete seawalls in November 2016 (month/year: M. Perkins pers. comms.). Plates had textured surfaces. Macroalgae and invertebrates on ridges and in grooves were counted in the laboratory after 12 months. One plate with tall ridges was missing and no longer provided habitat.

(Summarised by: Ally Evans)
Transplant or seed organisms onto intertidal artificial structures

A replicated, randomized, controlled study in 2016–2017 on two intertidal seawalls in the Pearl River estuary, Hong Kong (Bradford et al. 2020) found that 36% of oysters Saccostrea cuccullata transplanted onto settlement plates survived, and that transplanting oysters increased non-mobile invertebrate and oyster abundance on plates, decreased barnacle abundance, and had mixed effects on mobile invertebrate abundance and macroalgae and invertebrate species richness, depending on the presence of grooves and small ridges on plates and the site. After 12 months, 36% of transplanted oysters survived. At one of two sites, settlement plates with transplanted oysters supported higher macroalgae and invertebrate species richness (14–17 species/plate) and mobile invertebrate abundance (51–106 individuals/plate) than plates without oysters (9–12 species/plate, 15–81 individuals/plate). At the second site, the same was true for flat plates (with oysters: 16 species/plate, 65 individuals/plate; without: 9 species/plate, 19 individuals/plate) but no significant differences were found for plates with grooves and ridges (12–13 species/plate with and without oysters, 43–49 vs 31–45 individuals/plate). At both sites, plates with transplanted oysters supported higher abundance of non-mobile invertebrates (48–58% cover) and new oyster recruits (1–4 g/plate) but fewer barnacles (Cirripedia) (0–4 g/plate) than plates without (non-mobile invertebrates: 13–42%; oyster recruits: 0–2 g/plate; barnacles: 1–13 g/plate). Three mobile invertebrate species recorded on plates with transplanted oysters were absent from those without. Oysters collected locally were attached to concrete settlement plates (250 × 250 mm) using epoxy glue and transplanted onto vertical concrete seawalls. Plates had oyster patches covering 225 cm²/plate or no oysters, and textured surfaces with or without deep/tall (50 mm) or shallow/short (25 mm) grooves and small ridges. Five of each transplant-grooves/ridges combination were randomly arranged at midshore on each of two seawalls in November 2016 (month/year: M. Perkins pers. comms.). Macroalgae and invertebrates on plates were counted from photographs and in the laboratory, and barnacle and oyster recruit biomass (dry weight) was measured after 12 months. One plate was missing and no longer provided habitat.

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

A replicated, randomized, paired sites study in 2016–2017 on two intertidal seawalls in the Pearl River estuary, Hong Kong (Bradford et al. 2020) found that groove habitats created on the seawalls supported higher macroalgae and invertebrate species richness than small ridges created in between them, while species diversity varied depending on the groove depth. After 12 months, macroalgae and invertebrate species richness on settlement plates was similar in deep (8–9 species/plate) and shallow (9/plate) grooves, and higher in both than on the ridges in between (both 3–4/plate). The same was true for species diversity, except that deep grooves supported similar diversity to ridges (data reported as Shannon index). Concrete settlement plates (250 × 250 mm) were moulded with four deep (depth: 50 mm) or shallow (25 mm) vertical groove habitats (length: 250 mm; width: 15–50 mm) between five small ridges (length: 250 mm; width: 17–65 mm; height: 50 or 25 mm). Five of each were randomly arranged at midshore on each of two vertical concrete seawalls in November 2016 (month/year: M. Perkins pers. comms.). Plates had textured surfaces. Macroalgae and invertebrates in grooves and on ridges were counted in the laboratory after 12 months. One plate with deep grooves was missing and no longer provided habitat.

(Summarised by: Ally Evans)
Create grooves and small protrusions, ridges or ledges (1–50 mm) on intertidal artificial structures

A replicated, randomized, controlled study in 2016–2017 on two intertidal seawalls in the Pearl River estuary, Hong Kong (Bradford et al. 2020) found that creating groove habitats and small ridges on the seawalls had mixed effects on macroalgae and invertebrate species richness, and invertebrate abundances and biomass, depending on the depth/height of grooves and ridges and the site. After 12 months, in two of four comparisons, settlement plates with grooves and ridges supported higher macroalgae and invertebrate species richness (12–13 species/plate) and non-mobile invertebrate abundance (38–42% cover) than plates without (9 species/plate; 17% cover). In three of four comparisons, plates with grooves and ridges supported higher mobile invertebrate abundance (45–81 individuals/plate) and barnacle (Cirripedia) and oyster (Saccostrea cuccullata) recruit biomass (5–14 g/plate) than plates without (15–19 individuals/plate; 1 g/plate). In all other comparisons, plates with and without grooves and ridges were similar (richness: 11–12 vs 9 species/plate; non-mobiles: 14–27 vs 13% cover; mobiles: 31 vs 19 individuals/plate; barnacles/oysters: 2 vs 1 g/plate). Two mobile invertebrate species recorded on plates with grooves and ridges were absent from those without. Concrete settlement plates (250 × 250 mm) were moulded with and without groove habitats and small ridges. Plates with grooves and ridges had four vertical grooves (length: 250 mm; width: 15–50 mm) between five ridges (length: 250 mm; width: 17–65 mm). Grooves and ridges were either deep/tall (depth/height: 50 mm) or shallow/short (25 mm). Five of each were randomly arranged at midshore on each of two vertical concrete seawalls in November 2016 (month/year: M. Perkins pers. comms.). Plates had textured surfaces. Macroalgae and invertebrates on plates were counted from photographs and in the laboratory, and barnacle and oyster recruit biomass (dry weight) was measured after 12 months. One plate with deep/tall grooves and ridges was missing and no longer provided habitat.

(Summarised by: Ally Evans)