Interacting effects of habitat structure and seeding with oysters on the intertidal biodiversity of seawalls

Published source details Strain E.M.A., Cumbo V.R., Morris R.L., Steinberg P.D. & Bishop M.J. (2020) Interacting effects of habitat structure and seeding with oysters on the intertidal biodiversity of seawalls. PLoS ONE, 15, e0230807.
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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 study in 2015–2016 of two intertidal seawalls in Sydney Harbour estuary, Australia (Strain et al. 2020) found that small ledges created on the seawalls supported lower macroalgae and invertebrate species richness and lower mobile invertebrate and oyster Saccostrea glomerata abundances than groove habitats created in between them, but that macroalgae and other non-mobile invertebrate abundance, fish species richness and fish abundance were similar on ledges and in grooves. After 12 months, small ledges supported lower macroalgae and non-mobile invertebrate species richness (4 species/ledge) than the grooves in between (6/groove). The same was true for mobile invertebrates (4/ledge vs 6/groove), but not fishes (both 2/sample). Abundances were lower on ledges than in grooves for mobile invertebrates (1 individual/ledge vs 18/groove) and oysters (14 vs 56% cover), but were similar for macroalgae and other non-mobile invertebrates (36 vs 38% cover) and fishes (both 1 individual/sample). Concrete settlement plates (250 × 250 mm) were moulded with five horizontal small ledges (length: 250 mm; width: 17–65 mm; height: 50 mm) between four grooves (length: 250 mm; width: 15–50 mm; depth: 50 mm). Five plates were attached at midshore on each of two vertical sandstone seawalls in November 2015. Plates had textured surfaces. Macroalgae and invertebrates were counted on ledges and in grooves during low tide, from photographs and in the laboratory after 12 months. Fishes were counted from time-lapsed photographs during two high tides.

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

A replicated, randomized, controlled study in 2015–2016 on two intertidal seawalls in Sydney Harbour estuary, Australia (Strain et al. 2020) reported that 17–52% of oysters Saccostrea glomerata transplanted onto settlement plates survived, and found that oyster survival, macroalgae, invertebrate and fish species richness and abundances varied depending on the presence of grooves and small ledges on plates, the species group and site. Over 12 months, at one of two sites, transplanted oyster survival was higher on settlement plates with grooves and ledges (52%) than without (17%), and higher in grooves (92%) than on ledges (23%). At the second site, no significant differences were found (69% with and without grooves and ledges; grooves: 77%; ledges: 85%). Macroalgae and non-mobile invertebrate abundance was similar on plates with transplanted oysters (24–39% cover) and without (31–46%). Their species richness was higher on flat plates with oysters (5 species/plate) than those without (2/plate), but no significant difference was found when grooves and ledges were present on plates (6/plate with and without oysters). Mobile invertebrate species richness was higher on plates with oysters (10–12 species/plate) than without (4–7/plate). Their abundance was higher on plates with oysters (38 individuals/plate) than without (20/plate) when grooves and ledges were present, but did not differ on flat plates (16/plate with and without oysters). Fish species richness and abundance were similar on and around plates with and without oysters (3 vs 2 species/plate, both 1 individual/plate). Twenty-two species (3 macroalgae, 2 non-mobile invertebrates, 14 mobile invertebrates, 3 fishes) recorded on and around plates with transplanted oysters were absent from those without. Hatchery-reared juvenile oysters were attached to concrete settlement plates (250 × 250 mm) using epoxy glue and transplanted onto vertical sandstone seawalls. Plates had 52 oysters/plate in patches of 4–5 individuals or no oysters, and textured surfaces with or without grooves and small ledges (50 mm). Five of each transplant-grooves/ledges combination were randomly arranged at midshore on each of two seawalls in November 2015. Macroalgae and invertebrates were counted on plates during low tide, from photographs and in the laboratory over 12 months. Fishes were counted on and around plates from time-lapsed photographs during two high tides.

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

A replicated study in 2015–2016 on two intertidal seawalls in Sydney Harbour estuary, Australia (Strain et al. 2020) found that groove habitats created on the seawalls supported higher macroalgae and invertebrate species richness and higher mobile invertebrate and oyster Saccostrea glomerata abundances than small ledges created in between them, but that macroalgae and other non-mobile invertebrate abundance, fish species richness and fish abundance were similar in grooves and on ledges. After 12 months, grooves supported higher macroalgae and non-mobile invertebrate species richness (6 species/groove) than the ledges in between (4/ledge). The same was true for mobile invertebrates (6/groove vs 4/ledge), but not fishes (both 2/sample). Abundances were higher in grooves than on ledges for mobile invertebrates (18 individuals/groove vs 1/ledge) and oysters (56 vs 14% cover), but were similar for macroalgae and other non-mobile invertebrates (38 vs 36% cover) and fishes (both 1 individual/sample). Concrete settlement plates (250 × 250 mm) were moulded with four horizontal groove habitats (length: 250 mm; width: 15–50 mm; depth: 50 mm) between five small ledges (length: 250 mm; width: 17–65 mm; height: 50 mm). Five plates were attached at midshore on each of two vertical sandstone seawalls in November 2015. Plates had textured surfaces. Macroalgae and invertebrates were counted in grooves and on ledges during low tide, from photographs and in the laboratory after 12 months. Fishes were counted from time-lapsed photographs during two high tides.

(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 2015–2016 on two intertidal seawalls in Sydney Harbour estuary, Australia (Strain et al. 2020) found that creating groove habitats and small ledges on the seawalls increased the macroalgae and invertebrate species richness and oyster Saccostrea glomerata abundance on seawall surfaces, but did not increase abundances of macroalgae and other invertebrates, or the species richness and abundance of fishes. After 12 months, species richness was higher on settlement plates with grooves and ledges than without for macroalgae and non-mobile invertebrates (6 vs 2 species/plate) and mobile invertebrates (7 vs 4/plate), while there was no difference for fishes (both 2/plate). Oyster abundance was higher on plates with grooves and ledges (34% cover) than without (8%) but there were no significant differences in the abundances of macroalgae and other non-mobile invertebrates (46 vs 31% cover), mobile invertebrates (20 vs 16 individuals/plate) or fishes (both 1/plate). Eighteen species (5 macroalgae, 3 non-mobile invertebrates, 9 mobile invertebrates, 1 fish) recorded on plates with grooves and ledges were absent from those without. Concrete settlement plates (250 × 250 mm) were moulded with and without groove habitats and small ledges. Plates with grooves and ledges had four horizontal grooves (length: 250 mm; width: 15–50 mm; depth: 50 mm) between five ledges (length: 250 mm; width: 17–65 mm; height: 50 mm). Five plates with grooves and ledges and five without were randomly arranged at midshore on each of two vertical sandstone seawalls in November 2015. Plates had textured surfaces. Macroalgae and invertebrates were counted on plates during low tide, from photographs and in the laboratory after 12 months. Fishes were counted on and around plates from time-lapsed photographs during two high tides.

(Summarised by: Ally Evans)