Journal of Environmental Management 261 (2020) 110203

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Journal of Environmental Management

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Research article Predicting the impact of sea-level rise on intertidal rocky shores with remote sensing

Nina Schaefer a,*, Mariana Mayer-Pinto a,b, Kingsley J. Griffin a, Emma L. Johnston a, William Glamore c, Katherine A. Dafforn a,b,d a Centre for Marine Science & Innovation and Evolution & Ecology Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, 2052, Australia b Sydney Institute of Marine Science, Mosman, NSW, 2088, Australia c Water Research Laboratory, School of Civil and Environmental Engineering, UNSW, Sydney, NSW, Australia d Department of Environmental Sciences, Macquarie University, North Ryde, NSW, 2109, Australia

ARTICLE INFO ABSTRACT

Keywords: Sea-level rise is an inevitable consequence of climate change and threatens coastal ecosystems, particularly Sea level rise intertidal habitats that are constrained by landward development. Intertidal habitats support significant biodi­ Climate change versity, but also provide natural buffers from climate-threats such as increased storm events. Predicting the LiDAR effects of climate scenarios on coastal ecosystems is important for understanding both the degree of habitat loss IUCN red list of ecosystems for associated ecological communities and the risk of the loss of coastal buffer zones. We take a novel approach Conservation management by combining remote sensing with the IUCN Red List of Ecosystem criteria to assess this impact. We quantified the extent of horizontal intertidal rocky shores along ~200 km of coastline in Eastern Australia using GIS and remote-sensing (LiDAR) and used this information to predict changes in extent under four different climate change driven sea-level rise scenarios. We then applied the IUCN Red List of Ecosystems Criterion C2 (habitat degradation over the next 50 years based on change in an abiotic variable) to estimate the status of this ecosystem using the Hawkesbury Shelf Marine Bioregion as a test coastline. We also used four individual rocky shores as case studies to investigate the role of local topography in determining the severity of sea-level rise impacts. We found that, if the habitat loss within the study area is representative of the entire bioregion, the IUCN status of this ecosystem is ‘near threatened’, assuming that an assessment of the other criteria would return lower categories of risk. There was, however, high spatial variability in this effect. Rocky shores with gentle slopes had the highest projected losses of area whereas rocky shores expanding above the current intertidal range were less affected. Among the sites surveyed in detail, the ecosystem status ranged from ‘least concern’ to ‘vulnerable’, but reached ‘endangered’ under upper estimates of the most severe scenario. Our results have important implications for conservation management, highlighting a new link between remote sensing and the IUCN Red List of Ecosystem criteria that can be applied worldwide to assess ecosystem risk to sea-level rise.

1. Introduction predictions for the level of this rise vary depending on the amount of anthropogenic contributions (in the form of emissions and land-use Climate change threatens marine ecosystems at a global scale change) to radiative forcing. Radiative forcing is a measure of change through changes in temperature, ocean acidification and sea-level rise in the balance between incoming solar radiation and outgoing infrared (Brierley and Kingsford, 2009; Doney et al., 2012; Hoegh-Guldberg and radiation due to a forcing agent (IPCC, 2013). Four global scenarios Bruno, 2010). Sea-level rise is a consequence of thermal expansion of the developed by the International Panel for Climate Change (IPCC) are ocean and the melting of water stored in glaciers and ice-caps (Church used to represent the effect of radiative forcing in 2100, relative to et al., 2011; IPCC, 2013). Under climate change, sea-level rise has been preindustrial levels: RCP2.6, RCP4.5, RCP6.0, RCP8.5 (IPCC, 2013). projected to exceed previously observed rates (IPCC, 2013), but Under these scenarios, global sea-level rise is expected to increase at a

* Corresponding author. E-mail address: [email protected] (N. Schaefer). https://doi.org/10.1016/j.jenvman.2020.110203 Received 31 January 2019; Received in revised form 16 January 2020; Accepted 25 January 2020 Available online 2 March 2020 0301-4797/© 2020 Elsevier Ltd. All rights reserved. N. Schaefer et al. Journal of Environmental Management 261 (2020) 110203

rate of 4.4, 6.1, 7.4 and 11.2 mm/year (values represent median values), rocky shorelines (a triangular irregular network of contour data) found respectively (IPCC, 2013). that a rise in sea-level between 0.3 and 1.9 m will, in some areas, result Sea-level rise will have the greatest ecological impact along low lying in a loss of 10%–50% of rocky shore extent (Jackson and McIlvenny, coastlines through increasing inundation of the intertidal zone, which 2011). Furthermore, under a 1.9 m sea-level rise scenario, slopes are supports important ecological assemblages such as mangroves, sea­ predicted to steepen in these areas, with at least 50% of rocky shores � grasses, saltmarshes and rocky shores (FitzGerald et al., 2008; Nicholls becoming vertical (�45 ) (Jackson and McIlvenny, 2011). The transi­ and Cazenave, 2010; Nicholls et al., 1999). Apart from providing habitat tion to a steeper relief from a flatrocky shore may force organisms into for intertidal biodiversity, these habitats provide a buffer from greater densities and increase pressure from competition, particularly in destructive ocean forces, reducing the impact of storm events and areas where static vertical barriers such as seawalls prevent a landward mitigating erosion (Gedan et al., 2011; Shepard et al., 2011; Spalding migration (Pontee, 2013). Yet little is known about the effect of sea-level et al., 2014). It is therefore important to quantify the risks to important rise on intertidal rocky shores in other areas of the world or in the coastal habitats from sea-level rise. In this study we used the Hawkes­ context of multiple climate change scenarios. Sea-level rise is an inevi­ bury Shelf Marine Bioregion as a test region and applied remote sensing table consequence of climate change, and understanding the possible to investigate the threat of sea-level rise to intertidal rocky shores negative consequences is essential to inform conservation and mitigate (Fig. 1). impacts. We take a novel approach to understand this change and its Intertidal rocky shores are the most common coastal habitat world­ potential impacts by combining remote sensing data (LiDAR) and the wide and are ecologically valuable (Thompson et al., 2002). They sup­ IUCN Red List of Ecosystems criteria. Remote sensing provides solutions port a diverse array of species, which is attributed to the high structural to rapidly collect geospatial data over large spatial scales, whereas the complexity of rocky shores (Blanchard and Bourget, 1999; Chapman, IUCN Red List of Ecosystem criteria provide a consistent framework for 2003; Sebens, 1991). Intertidal rocky shores and the communities living ecosystem risk assessments that can be applied worldwide. on them provide numerous ecosystem functions and services. Here, by following the framework of the IUCN Red List of Ecosystems Filter-feeders such as oysters improve water quality and further promote criteria (Keith et al., 2013), we assessed the current status of ~200 km of biodiversity by creating additional habitat for other intertidal organisms coastline within the Hawkesbury Shelf Marine Bioregion in order to (Coen et al., 2007; Grabowski et al., 2012). Intertidal rocky shore also estimate the status of intertidal rocky shores of the entire bioregion and act as important nursery and feeding ground for fish during high tide discuss potential effects of sea-level rise on associated biota. Under the and shorebirds during low tide (Burrows et al., 1999; Cantin et al., 1974; IUCN system, the status of an ecosystem is assessed against fivecriteria, Rangeley and Kramer, 1995). Yet rocky shores are also amongst the most with the final ecosystem status determined based on the highest risk vulnerable marine systems, facing a variety of anthropogenically returned for any one category. We focus on criterion C2, which involves induced threats (Halpern et al., 2007; Thompson et al., 2002). an assessment of the extent and relative severity of habitat degradation Since alternating periods of emersion and submersion is the key in the next 50 years. We used a high-resolution LiDAR (light detection physical driver on intertidal rocky shores (Menge and Branch, 2001), and ranging) survey of coastal elevation to estimate the net loss/gain of rapid changes in sea-level can have particularly severe consequences for intertidal rocky shores under sea-level rise scenarios. the availability of habitat. In Scotland, a study used existing maps of

Fig. 1. Map of the coastline (~210 km) that has been assessed for rocky shores (highlighted in blue) on the coast of NSW, Australia (projection: GCS_GDA_1994). Approximate extent of the Hawkesbury Shelf Marine Bioregion is indicated. Rocky shores that were additionally assessed are highlighted in green (BH: Bradleys Head, CB: Cape Banks, D: Delwood, F: Freshwater). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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2. Materials and methods Raffaelli and Hawkins, 2012; Underwood, 1980). Intertidal rocky shores can be broadly categorized into three main zones: the low, mid- and high 2.1. IUCN red list of ecosystems intertidal, which vary in their heights above sea-level, representing areas of different exposure (Druehl and Green, 1982). In New South The IUCN Red List of Ecosystems was established to fillthe demand Wales, littorinids (e.g. Austrolittorina unifasciata) are abundant in the for biodiversity assessments that address levels of biodiversity above upper sections of rocky shores, followed by areas often occupied by those of single species, such as those of ecosystems (Rodríguez et al., barnacles and the polychaete Galeolaria sp., whereas the lower intertidal 2011). It assesses the risk of an ecosystem collapse, which occurs when is characterised by foliose algae and tunicates (Dakin et al., 1966; Un­ an ecosystem loses its defining biotic or abiotic features, and the char­ derwood, 1981). Common mobile species in the middle part of the rocky acteristic native biota and ecosystem processes are lost (Keith et al., shores include various gastropods and chitons (e.g. Cellana tramoserica, 2013). The framework of the IUCN Red List of Ecosystems to assess the Bembicium nanum, Nerita atramentosa Austrocochlea spp., Tenguella status of an ecosystem comprises five criteria, which assess different marginalba, Sypharochiton pelliserpentis) (Dakin et al., 1966; Underwood, “symptoms” of ecosystem collapse, with the finalecosystem status being 1981). determined based on the highest risk returned for any one category (Keith et al., 2013). 2.4. Habitat mapping Criterion C involves an assessment of environmental degradation based on change in an abiotic variable, which can be determined using To define the extent of intertidal rocky shores, a shapefile was digi­ either present (C1), future (C2) or historic change (C3). For criterion C2, tised from publicly available aerial imagery (DFSI) (1 pixel ¼ 0.07 an assessment of degradation over a 50-year period is required. The m–156543.03 m, depending on zoom level) using the editor tool in status of the ecosystem is then determined based on percentage of extent ArcMap (version 10.4.1). This was done to differentiate rocky shore (% extent) and severity (%severity) (Table S1) (Keith et al., 2013). As habitat from other shore types (e.g. beach). Upper (landward) limits of net loss/gain of intertidal rocky shores was determined, the relative rocky shores were determined based on visible rock outcropping, while severity of the loss of available area was assumed to be 100%. lower limits were extended well past the minimum low-tide area and refined later in the analysis based on tidal and elevation data. The in­ 2.2. Study location clusion of all visible rocky shore habitat allowed us to account for po­ tential gain of intertidal area with sea-level rise. Vertical intertidal The Hawkesbury Marine Shelf Bioregion includes the estuaries, surfaces such as tall cliffs and intertidal boulder fields were avoided coastline and marine waters from Newcastle to Wollongong, NSW, where possible. When shading or vegetation prevented a proper view of Australia (Fig. 1). The assessment of intertidal rocky shores in this study the intertidal areas, other sources of imagery were reviewed to obtain was limited to the coastal area between the northern side of Port outlines (e.g. Google Earth). Areas were not incorporated in the shape­ � 0 0 � 0 0 Hacking (34 03 55 S, 151 08 05 E) in the south and the south end of file if they could not be visually assessed from available imagery. � 0 0 � 0 0 Fishermans Beach (33 44 12 S, 151 18 22 E) in the north (Fig. 1). The assessment also included the outer Harbour to the Harbour Bridge and 2.5. Elevation data Spit Bridge and the Harbour Bridge and in Botany Bay the outer Harbour to the Captain Cook Bridge (Fig. 1). This represents approximately 210 Elevation data of the intertidal rocky shores was acquired using km length of coastline within the greater bioregion, with rocky shores LiDAR, which measures reflected laser pulses (Bachman, 1979). LiDAR comprising approximately 50 km of the assessed area. data were collected between the 10–24.04.2013 (LPI NSW 2013). The LiDAR data used in the following analysis were compiled from two 2.3. Ecosystem description separate datasets (Sydney North and Sydney South) with an average point density of 1.57 and 1.56 points per square meter, respectively. The 2.3.1. Abiotic features horizontal spatial accuracy was 0.8 m and the vertical spatial accuracy The rocky shores in this study and in the Sydney region in general are 0.3 m. Accuracy specifications (95% CI) meet the Intercontinental composed of sandstone and mainly horizontal or gently-sloped Committee on Surveying and Mapping guidelines for digital elevation (Chapman, 2003; Chapman and Bulleri, 2003), with the landward data (ICSM, 2009). Due to the marginal differences between the data­ edge backed by cliffs. The tidal range is ~1.9 m (MHL 2016). sets, they were combined without any correction. Elevation values refer to the zero level on the Fort Denison Tide Gauge (Zero Fort Denison ¼ 2.3.2. Biota ZFD), being approximately the level of the Lowest Astronomical Tide In Sydney Harbour alone, approximately 162 taxa can be found on (LAT). We therefore take ZFD ¼ LAT. To quantify the current extent of natural intertidal rocky shores (Mayer-Pinto et al., 2018). There is, rocky intertidal shores, rocky shores were defined as the area between however, large spatial variability in the distribution of species, which is 0 and 1.9 m above LAT. This classification was based on the minimum common along New South Wales (Schaefer et al., 2019; Underwood and (0.069 m above Zero Camp Cove (¼ZFD) and maximum (1.920 m above Chapman, 1998). Mobile fauna in rock pools in the outer zone of Sydney Zero Camp Cove (¼ZFD)) annual averages of Port Jackson (Manly Hy­ Harbour are dominated by Bembicium nanum, Austrocochlea spp. and draulics Laboratory, 2016). Nerita atramentosa, whereas the inner zone of Sydney Harbour is char­ acterised by greater densities of B. auratum, Patelloida spp. and 2.6. Data cleaning and spatial analyses Siphonaria spp. (Schaefer et al., 2019). Similarly, sessile assemblage composition within rock pools differs between inner and outer Harbour, Data was cleaned based on the tidal height and unsupervised clas­ with the brown encrusting algae Ralfsia spp., the bryozoan Watersipora sification of LiDAR returns before analysis. To avoid including the spp. and the polychaete Galeolaria caespitosa being more common in the elevation of water covering the intertidal surface, elevation data that outer zone, and Corallina spp. and oysters being more abundant in rock was below the tidal height at the time of collection was excluded based � 0 0 � 0 0 pools in the inner zone (Schaefer et al., 2019). on the local (Fort Denison) tide gauge (33 51 16.8 S, 151 13 32.8 E) Besides this variability in composition over larger spatial scales, (OEH NSW2015). Tide data was interpolated linearly from an hourly intertidal assemblages also show distribution patterns on smaller spatial measure to a per minute estimate to match the LiDAR timecodes. scales, particularly across the vertical gradient, which is driven by Excluding data points from below the waterline eliminates potential abiotic factors such as temperature and wave exposure, as well as biotic errors from signal noise from the water. As an additional measure, factors such as predation and competition (Connell, 1961, 1972; LiDAR points underwent an unsupervised classification at the time of

3 N. Schaefer et al. Journal of Environmental Management 261 (2020) 110203 measurement, and any points classified as ‘water’ were excluded. projected under the RCP 8.5 scenario (Table S2). Elevation data within the rocky shore was clipped to the polygons and At the four sites selected for detailed investigation, the area of rocky was then standardised to a 1 m resolution point-cloud for analysis of the shore, topography, and relative severity of sea-level rise effects varied. total area. Because the LiDAR data was acquired at varying tide heights, Cape Banks was the largest rocky platform assessed in detail, with a total the number of points available for assessment of the entire intertidal current intertidal area of 18,304 m2, followed by Bradleys Head and zone (0–1.9 m above LAT) varied. Freshwater (5864 m2 and 5295 m2, respectively) and Delwood (2352 m2). Among these four sites the extent of loss of intertidal rocky shore area 2.7. Predicted sea-level rise scenarios varied greatly, with the greatest losses projected to occur at Delwood (Fig. 3a, Fig. 4a, c, e) – ranging from ~14% (RCP2.6) to ~ 37% (RCP8.5) Four scenarios (RCP2.6, RCP4.5, RCP6 and RCP8.5) of projected (Table S2). At Bradleys Head, the intertidal area is projected to remain global mean sea-level rise were taken from the 5th Assessment Report of almost constant under all predictions, except the RCP8.5 scenario where the Intergovernmental Panel on Climate Change (Church et al., 2013). more than 30% of area is likely to be lost (Figs. 3b and Fig .4b, d, f) Current sea-level was set at 0 m ZFD/LAT in 2013. (Table S2). Under the upper ranges of sea-level rise for scenario RCP8.5, We calculated sea-level rise for the year 2063 (50 years from when both Delwood and Bradleys Head rocky shores are reduced by more than the data was collected) using the predicted linear rate of global mean half of their current extent (Table S2). Rocky shores at Freshwater sea-level rise under all scenarios (Table 1) and added it to the tide data (Figs. 3c and 5a, c, e) and Cape Banks (Figs. 3d and 5b, d, f) were less for each LiDAR point. This method is commonly known as “bathtub affected than Delwood or Bradleys Head. Losses at Freshwater ranged approach”, which classifies flooded areas based on elevation only between ~3 and 19% (RCP 2.6, RCP 8.5), and the platform at Cape (Seenath et al., 2016). Therefore, this study does not account for other Banks decreased ~ 9% under scenario RCP2.6, but was not constrained shoreline dynamics such as wind and wave action. Global values were in available landwards rock, with smaller losses under the more severe chosen due to limited data availability of local sea-level rise for all scenarios (Table S2). scenarios. However, comparisons of global values with available pre­ dictions for Sydney (Webb and Hennessy, 2015) suggest that they fall 3.2. Classification with IUCN Red List of ecosystems within the range of global predictions. We then compared the predicted intertidal area to its current extent. This was done so we could apply the Using our estimates of available habitat under sea-level rise scenarios criteria used in the IUCN Red List of Ecosystems. We calculated the net and assuming that sea-level rise is the most serious threat to this loss of rocky shore along 210 km of coastline within the Hawkesbury ecosystem, the overall threat of sea-level rise to intertidal rocky shores in Shelf Marine Bioregion, which accounts for potential gain of intertidal the Hawkesbury Shelf Marine Bioregion against criterion C2 of the IUCN rocky shore area above the current intertidal zone. In addition, we criteria under the upper predictions of RCP8.5 is ‘vulnerable’. For the selected four intertidal rocky shores from the dataset to predict the other, less extreme, scenarios, the degradation of rocky shores in this loss/gain of intertidal area in relation to different topographies and region does not exceed the threshold for ‘vulnerable’ classification. spatial extent. Two of the rocky shores (Bradleys Head and Delwood) Overall, the ecosystem status of rocky shores should therefore be clas­ were gently sloping and backed up by beach, cliffs or bushland, while sified as ‘near threatened’. However, at a site-specific level, these pre­ the other two shores were large areas expanding above the current dictions are elevated to ‘endangered’ under the upper projections of the intertidal range (Freshwater and Cape Banks) (personal observation). RCP8.5 scenario at Delwood and Bradleys Head. Although losses at Freshwater do not reach the status of ‘vulnerable’ under any of the RCP 3. Results scenarios, this site fell into the category ‘near threatened’ due to high losses under RCP8.5. Cape Banks should be classified as ‘least concern’ 3.1. Net loss of intertidal rocky shore extent because losses at this site are limited, even under upper predictions of scenario RCP8.5. The current area of intertidal rocky shores is estimated at 374,689 m2 along approximately 50 km of rocky shore within the assessed 210 km of 4. Discussion the NSW coastline. However, the area is likely to be underestimated due to the lack of reliable elevation data available in the low and mid- Coastal ecosystems are changing worldwide as a result of climate intertidal zones (see Methods: Spatial analysis). change, including sea-level rise. Here we quantified the sea-level rise Model predictions based on median sea-level rise rates suggest that threat to rocky intertidal communities along ~200 km of coastline in SE the available habitat for intertidal organisms will be reduced over the Australia using the criteria applied in the IUCN Red List of Ecosystems. next 50 years at an overall rate per year of ~0.11%, ~0.17%, ~0.24% Under the IUCN Criterion C2, which considers the degradation of an and ~0.43% under scenarios RCP2.6, RCP4.5, RCP6.0 and RCP8.5, abiotic variable over the next 50 years, we found that intertidal rocky respectively. These rates are however not linear and accelerate with shores in this bioregion should be classified as ‘near threatened’ due to time (Fig. 2a, Fig. S1). In 50 years, the effect of sea-level rise is predicted the predicted losses caused by sea-level rise on these habitats. The threat to reduce the area of current intertidal rocky shores by at least ~5.85% of sea-level rise for rocky shores is, however, predicted to vary spatially under the most benign scenario (RCP 2.6) and a maximum of ~21.75% and is linked to local topography and the landward availability of rocky under the most extreme scenario (RCP 8.5) (Fig. 2b, Table S2). Losses surfaces. Higher predicted losses of intertidal rocky shores occurred at may however exceed 30%, based on the upper range of sea-level rise locations where gently sloping shores (personal observation) are con­ strained by sandy shoreline (beach) and bushland, whereas shores with Table 1 extended rocky slopes allowed for migration of the intertidal zones. To Median values and likely ranges for projections of rate of global mean sea-level protect coastal biodiversity and preserve buffers between ocean forces rise (GMSLR) (mm/year) (Church et al., 2013) as well as projections for sea-level and coastal infrastructure, similar efforts are required to identify suit­ rise (SLR) in 50 years (mm) for the four RCP scenarios used for analyses. able sites for the long-term preservation of intertidal habitats. Examples RCP2.6 RCP4.5 RCP6.0 RCP8.5 include the establishment of no-construction zones in areas of coastal Rate of 4.4 [2.0–6.8] 6.1 [3.5–8.8] 7.4 11.2 development, and proactive surface preparation to allow for species GMSLR [4.7–10.3] [7.5–15.7] migration. The results of this study highlight the importance of fore­ SLR in 50 220 305 370 560 casting sea-level rise effects using detailed spatial data, without which years [100–340] [175–440] [235–515] [375–785] we may fail to notice the small-scale losses of coastal habitat that are

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Fig. 2. Remaining area (%) of all intertidal rocky shores combined (a) in each year for the next 50 years and (b) in 50 years using median projection values. Shaded areas (a) and error bars (b) represent remaining area (%) using upper and lower likely ranges of projections.

Fig. 3. Remaining area (%) in 50 years at (a) Delwood, (b) Bradleys Head, (c) Freshwater and (d) Cape Banks using median projection values. Error bars represent remaining area (%) using upper and lower likely ranges of projections. likely to act as precursors to larger, regional-scale losses of ecosystem Sea-level rise threatens rocky shore communities by forcing organ­ function and biodiversity. isms into a coastal squeeze in two ways: 1) loss of upper tidal zones The loss of intertidal rocky shores can have potential negative con­ where barriers (e.g. cliffs, bushland) prevent a shift of communities sequences for adjoining habitats such as beaches and bushland. Rocky further up shore; and 2) a decrease in the overall area within each zone. shores can dissipate wave action and thus prevent water from reaching Rocky shore communities display vertical distribution patterns, which areas higher on shore (Denny et al., 1992; Leatherman, 1990). The loss are a result of physiological limitations as well as competition and of the mitigating function of rocky shores may therefore lead not only to predation (Connell, 1961, 1972; Raffaelli and Hawkins, 2012; Under­ floodingof adjacent habitats, such as bushland, but also to greater influx wood, 1980). In the first case, which would be the case for the rocky of sediment or organic matter into the water when next to bushland or shores at Delwood and Bradleys Head, it is likely that only species found beaches, which can in turn alter adjoining subtidal ecosystems (Blanchet in the impacted zone will be affected, due to reduced habitat and et al., 2005). replacement by organisms migrating up the shore from lower elevations.

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Fig. 4. Height (m) above LAT of each LiDAR point (aþb) at each site with gentle-sloped rocky shores, (c þ d) an exemplary area of rocky shore within each site and (e þ f) gain/loss of intertidal rocky shore under the predicted climate change (using median values) scenarios in 50 years (”-” indicating loss, “þ” indicating gain). Delwood: a, c, e; Bradleys Head: b, d, f. Untransformed datasets with greater point densities were used for plotting and points higher than 60 m above LAT were excluded. Exemplary areas of rocky shore were restricted to points within the shapefile.

Species adapted to upper elevations have no room to move, and there­ may provide a surface for species dispersal and migration, greater fore could experience greater predation and competition pressure physical forces (e.g. wave action) on vertical walls increases the risk of (Connell, 1961) or even local extinction (Rybicki and Hanski, 2013). dislodgement (Denny et al., 1985) and may therefore prevent the This would likely affect gastropods of the family Littorinidae and many dispersal of organisms with little adhesive strength. Furthermore, in barnacles, since they mainly occupy areas higher on shore (Dakin et al., areas where planktonic dispersal of propagules from rocky shore biota is 1966). In the second case, a decrease in extent in habitat area can in­ limited due to slow water flow (e.g. inner estuarine embayments (Das crease biotic interactions, which can alter relative abundances of species et al., 2000)), the distance between suitable rocky substrata may become (Klein et al., 2011; Underwood, 1978), and result in an overall decrease insurmountable if some shores are lost. in the number of species (Fahrig, 1997). This can be particularly detri­ Besides the loss in the extent of intertidal area, sea-level rise may mental if habitat-forming species such as oysters and algae are affected, further result in reduced habitat heterogeneity and complexity through as this can have cascading effects for associated communities (Cole et al., the disappearance of important intertidal microhabitats like rock pools, 2017). Important ecosystem services such as the improvement of water which are only present on horizontal rocky shores. A study assessing the quality by filter-feedingoysters may also be impacted (Coen et al., 2007; impacts of sea-level rise on intertidal rocky reefs in a marine park on the Grabowski et al., 2012). north coast of NSW found that there will be a large loss of shallow pools Loss of horizontal rocky platforms can also result in the fragmenta­ in the lower part of the intertidal zone despite small losses of the lower tion of rocky shore habitat, resulting in a decrease in diversity along the platform habitat (Thorner et al., 2014). Shallow pools and deep pools in coastline (Goodsell et al., 2007). Greater distances among rocky shore the upper part of the shore also declined in abundance (Thorner et al., communities may further prevent migration and gene flow among 2014). Rock pools are the main habitat for many species on natural communities (Saunders et al., 1991). Although vertical rocky shores rocky shores, such as sea urchins, large whelks and sea stars (Chapman,

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Fig. 5. Height (m) above LAT of each LiDAR point (aþb) at each site with rocky shore expanding above the current tidal range, (c þ d) an exemplary area of rocky shore within each site and (e þ f) gain/loss of intertidal rocky shore under the predicted climate change (using median values) scenarios in 50 years (”-” indicating loss, “þ” indicating gain). Freshwater: a, c, e; Cape Banks: b, d, f. Untransformed datasets with greater point densities were used for plotting and points higher than 60 m above LAT were excluded. Exemplary areas of rocky shore were restricted to points within the shapefile.

2003), which may become less abundant or absent with the loss of this included in the dataset. Additionally, the slope of intertidal areas was microhabitat. not accounted for in the estimates. The area between two adjoining The increasing rate of sea-level rise may also prevent the creation of points can be greater when this area is characterised by steep topog­ rock pools in places where rocky substrata are available higher on the raphy in contrast to a horizontal plane. Although most rocky shores were shore. Rock pools are formed by erosion and abrasion of rock, which gently-sloped, which should have limited the effect of slope, the pres­ happens slowly over time. Reduced habitat heterogeneity and ence of some platforms with steeper slopes may have contributed to a complexity may therefore lead to a decrease in diversity, at least in the slight underestimation of the current intertidal rocky shores. In addition, short-term, within remaining intertidal rocky shores. The impact of the creation of new rocky shores through erosion of sediment from global climate change on the extent of rocky shores is likely to be more beach habitat was not included in estimations due to uncertainties in the significant than just the effect of sea-level rise. Global tidal ranges are timing and extent of this process. Furthermore, calculations were based expected to change in the future, and will amplify or mitigate the effect on a point-cloud (one point per square meter) and not on an interpolated of sea-level rise (Pickering et al., 2017). Additionally, ocean warming digital surface model. The spatial (vertical and horizontal) accuracy may will likely result in greater wave forces (Reguero et al., 2019). Greater have further led to slight under- or overestimations of loss/gain of rocky wave forces can result in increased splash, which can reduce the extent shore extent. Higher resolution and improved spatial accuracy would of intertidal areas adjacent to cliffs or could promote the range extension enhance the accuracy of the predictions and would therefore be bene­ of the intertidal zone in rocky shore areas of low relief. These environ­ ficial in future analyses. Nevertheless, this study shows that remote mental drivers may further affect losses and gains, but more detailed sensing techniques can be a useful tool to assess potential effects of sea- data in the study area is needed to incorporate them in the predictions. level rise on rocky shore habitats and their communities. The present study estimated the loss of intertidal rocky shores in the Sydney region and found that rocky shores are threatened by sea-level 5. Conclusion rise. However, estimates of the current intertidal area do not account for some elevations that were covered by water and therefore were not This study highlighted the high degree of risk to intertidal rocky

7 N. Schaefer et al. Journal of Environmental Management 261 (2020) 110203 shore environments from sea-level rise. We found that the IUCN Connell, J.H., 1972. Community interactions on marine rocky intertidal shores. Annu. – threshold for ‘vulnerable’ classification is not exceeded for median es­ Rev. Ecol. Systemat. 3, 169 192. Dakin, W.J., Bennett, I., Pope, E., 1966. Australian Seashores: a Guide for the Beach- timates of sea-level rise in the next 50 years, but it is certain that rising Lover, the Naturalist, the Shore Fisherman, and the Student. Angus and Robertson. sea-levels will increase the inundation of rocky shores into the future. Das, P., Marchesiello, P., Middleton, J.H., 2000. Numerical modelling of tide-induced – The overall status of ‘near threatened’ highlights the need for ongoing residual circulation in Sydney harbour. Mar. Freshw. Res. 51, 97 112. Denny, M., Dairiki, J., Distefano, S., 1992. 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