It’s not all black and white – the effects of substrate brightness on the intertidal communities of seawalls
Data files
Mar 03, 2026 version files 388.85 KB
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mobile_otu_table.xlsx
72.95 KB
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mobile_tax_table.xlsx
11.89 KB
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README.md
9.57 KB
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sessile_otu_table.xlsx
110.19 KB
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sessile_tax_table.csv
3.33 KB
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Temp_Monthly_summary.csv
165.88 KB
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Tile_colour.xlsx
15.04 KB
Abstract
Substrate brightness can influence colonisation of marine surfaces, but is typically overlooked in the design of marine constructions for ecological co-benefits. Many micro-organisms, invertebrate larvae and algal spores are negatively phototactic, preferentially settling on dark surfaces. Brightness may also influence post-settlement processes in the intertidal by influencing passive warming of substrates, and by influencing the crypsis of organisms susceptible to visually feeding predators.
To inform eco-engineering designs, this study assessed how the brightness of settlement panels influenced biofilm establishment, passive warming of the substrate and microbial and macrobenthic community development on three seawalls in Sydney Harbour, Australia.
Brightness was manipulated using grey-scale customised concrete panels, spanning black, white and three intermediate brightness values matching seawalls constructed of local quarried sandstone and concrete, as well as Hawkesbury sandstone rocky shores – the most similar natural habitat analogue. Biofilms were sampled from one site after 6 weeks, and the temperature and macro-community development was tracked across brightness treatments at all three sites quarterly for two years.
Maximum temperatures were generally greater on darker (blacker) than brighter (whiter) tiles, with this pattern strengthening with tidal elevation and sun-exposure. White tiles hosted distinct microbial and macrobiotic communities, which were generally more taxonomically rich and contained greater organism abundances/covers than darker tiles. Effects of brightness were greater in summer than winter, and at sunny than shaded sites, diminishing through time as tiles became fouled.
Synthesis and applications. Our results suggest that brightness should be considered when designing marine structures for ecological co-benefits. Particularly in environments exposed to high solar irradiance, bright surfaces can reduce thermal stress to organisms, resulting in more biodiverse communities. Where construction using dark materials at sun-exposed orientations is unavoidable, timing the construction to occur in cooler, winter periods, may allow establishment of communities that ameliorate thermal effects before peak summer temperatures occur.
Dataset DOI: 10.5061/dryad.dz08kps8f
Description of the data and file structure
The study was conducted in Sydney Harbour, New South Wales, Australia, between November 2018 and October 2020. Effects of brightness on benthic community development were investigated using customised concrete panels affixed to three vertical intertidal seawalls, ~9 km from the mouth of the Harbour at: Kurraba Point (33.844564 °S, 151.222579 °E); North Sydney (33.84503 °S, 151.2186 °E); and McMahons Point 33.84899 °S, 151.20553 °E). At all sites, tides were semi-diurnal and of ~1.8 m amplitude, and salinities were close to 35 ppt.
Brightness was manipulated across a greyscale by altering the amount of black oxide in tiles fabricated from white Portland cement. Five brightness levels were established, each described by a mean brightness (B2) value that was calculated as the total reflectance divided by the number of wavelengths measured across the spectrum. The five levels were: white (B2=59.5; hereafter D1), black (B2=2.96, hereafter D5), and three intermediate greyscale levels matching the brightness of the two most common types of seawall in Sydney Harbour -- sandstone (B2=5.26, hereafter D4) and concrete (B2=18.8, hereafter D2) -- as well as an intermediate brightness level that matched natural Hawkesbury sandstone rocky shores (B2 =12.24, hereafter D3). All tiles were flat, 25 x 25 cm in area, 2-3 cm thick and contained white Portland cement (250 ml) to which builders’ sand (750 ml), water 250 ml and alkaline resistant glass fibres (75 ml) had been added. Black oxide was added to all but the white (D1) treatment to achieve the required brightness (Table S1). To remove the effect of any chemical differences among tiles, all were coated with two layers of non-toxic pond resin (Crommelin waterproofing).
A total of 30 tiles per brightness treatment were produced which, when evenly divided across the two tidal elevations at each of the three sites, gave n=5 for each treatment by elevation by site combination. In November 2018, tiles were affixed to seawalls on plastic frames that were bolted to their surface. Each frame held one tile of each of the five brightness treatments, deployed in random order and each separated by ~10 cm. At each of the three sites, five frames were deployed at each tidal elevation. Tiles remained in place for 24 months, until October 2020. Loss of some tiles from the McMahons and North Sydney sites reduced the final number of tiles retrieved. To test hypotheses about effects of substrate brightness on early colonisation and subsequent community development, and to assess the extent to which this might be driven by thermal effects, tile temperatures, ecological communities were sampled.
A single waterproofed iButton logger (DS1921G, Thermodata Pty Ltd, Warnambool Australia, accuracy 0.5°C), programmed to record temperature every 60 min, was attached to each tile with epoxy (Fisher FIS EM Plus 390 S epoxy mortar). iButtons were replaced every three months, providing near continuous monitoring of the surface temperature of tiles over the 24-month experiment. The monthly mean, maximum and minimum temperatures were calculated for each logger.
Development of communities, comprising organisms >1 mm in diameter, was sampled quarterly, to give 8 sampling times across the 24-month experiment. Sampling was conducted in situ at low tide using a 25 x 25 cm quadrat strung with 5 x 5 evenly spaced strings to give 25 intersection points. The point intercept method was used to score the cover of sessile organisms by taxon, with both primary (attached directly to the substrate) and secondary (growing on top of the primary species) cover recorded separately under each intersection point. Total (primary + secondary) cover could, consequently, exceed 100%. Sessile biota that were present within the quadrat but did not fall beneath a point intercept were given a count value of 0.1 (0.4%) to note their presence. Mobile species within each quadrat were counted separately by taxon. All organisms were identified to species level, or where this was not possible, morphospecies.
Files and variables
File: Tile_colour.xlsx
Description: summary of the spatial arrangement of tiles of the varying treatments, and their brightness scores.
Variables
- Site: site of deployment, where KP = Kurraba Point; MM = McMahons Point, NS = North Sydney.
- Height: tidal height of tile placement, where H = high and Mid = mid intertidal.
- Structure: bracket to which tiles were attached (five tiles -- one of each brightness treatment -- attached to each of 10 brackets, numbered 1-10)
- Tile_position: the position (1-5) from left to right of tiles on the bracket
- Colour: tile brightness treatment, where brightness was matched on a grey scale to the following: BLCK = black, WHT = white, NATR = natural rock, SAND = sandstone, CONC = concrete
- B2_score: the brightness score recorded for an individual panel, reflecting minor tile-to-tile variation in the application of colour treatments.
- Colour_D: categoric brightness treatment, from D1 to D5 where D5 is darkest and D1 the brightest.
File: Temp_Monthly_summary.csv
Description: Summary of substrate temperatures, by month, recorded on each of the tiles by ibutton temperature loggers
Variables
- year_mon: the year (YY) and month of temperature measurement
- Site: site of deployment, where KP = Kurraba Point; MM = McMahons Point, NS = North Sydney.
- Colour: tile brightness treatment, where brightness was matched on a grey scale to the following: BLCK = black, WHT = white, NATR = natural rock, SAND = sandstone, CONC = concrete
- Colour_D: categoric brightness treatment, from D1 to D5 where D5 is darkest and D1 the brightest.
- Tidal_Ht: tidal height of tile placement, where H = high and Mid = mid intertidal.
- Tile_position: the position (1-5) from left to right of tiles on the bracket
- Structure: bracket to which tiles were attached (five tiles -- one of each brightness treatment -- attached to each of 10 brackets, numbered 1-10)
- t_mean: mean substrate temperature (in degrees Celsius) across all measurements taken by an individual logger placed on the tile surface over the corresponding month
- t_max: maximum substrate temperature (in degrees Celsius) recorded by an individual logger over the corresponding month
- t_min: minimum substrate temperature (in degrees Celsius) recorded by an individual logger over the corresponding month
- t_range: range in substrate temperature (in degrees Celsius) recorded by an individual logger over the corresponding month (i.e. t_max minus t_min)
- t_sd: standard deviation in substrate temperature (in degrees Celsius) recorded by an individual logger over the corresponding month
- t_n: number of temperature recordings made by an individual logger over the corresponding month
- t_SE: standard error in substrate temperature (in degrees Celsius) recorded by an individual logger over the corresponding month
File: mobile_tax_table.xlsx, sessile_tax_table.csv
Description: Summary of operational taxonomic units against which the abundance of mobile species or cover of sessile taxa were scored on tiles.
Variables
- Spp_name: the species/morphospecies names used in the data matrices, mobile_otu_table.csv and sessile_otu_table.csv
- Phylum, Class, Order, Genus, Species: provided where available; NA denotes ‘not available’ due to an inability to adequately taxonomically resolve.
- Broad_funct_grp: broad functional group; scored as ‘algae’, ‘sessile invert’[ebrate] or ‘mobile invert’[ebrate]
- Cat_grp = category group; scored as ‘sessile’ or ‘mobile’
File: mobile_otu_table.xlsx, sessile_otu_table.xlsx
Description: The cover of sessile or count of mobile taxa present on each of the tiles, on each of the sampling dates.
Variables
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Tile_month: a unique sample identifier in the format ‘1_1_KP_3’. The first two numbers denote the ‘Structure’ (bracket to which tiles are attached) at each site (numbered 1-10) and the ‘Tile position’ within the structure (from 1-5), respectively. The letters denote the site of deployment: KP = Kurraba Point; MM = McMahons Point, NS = North Sydney. The final number denotes the month of sampling: 3, 6, 9, 12, 15, 18, 21, or 24 months following deployment of panels. In the latter months some structures and tiles were missing so are not included in the data set. Tile_colour.csv provides full details of the brightness of each tile, by its Structure, Tile Position and Site.
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Column B and columns to the left include the cover (as a percentage) for sessile species, or count (per quadrat) for mobile species by taxon (=operational taxonomic unit, OTU. OTUS are listed in Row 1 with full details of these provided in the files sessile_otu_table.csv and mobile_otu_table.csv, explained above.
Both the primary [directly attached to tiles] and secondary [attached to primary] cover of sessile species were documented on tiles, so covers may sum to > 100%. Sessile species present but not falling underneath an intersection point were given a value of 0.4% cover to denote their presence.
Access information
Other publicly accessible locations of the data:
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Data was derived from the following sources:
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