The authors have declared that no competing interests exist.
Conceived and designed the experiments: JT LK SS. Performed the experiments: JT. Analyzed the data: JT. Contributed reagents/materials/analysis tools: JT LK. Wrote the paper: JT LK SS.
Intertidal rocky reefs are complex and rich ecosystems that are vulnerable to even the smallest fluctuations in sea level. We modelled habitat loss associated with sea level rise for intertidal rocky reefs using GIS, high-resolution digital imagery, and LIDAR technology at fine-scale resolution (0.1 m per pixel). We used projected sea levels of +0.3 m, +0.5 m and +1.0 m above current Mean Low Tide Level (0.4 m). Habitat loss and changes were analysed for each scenario for five headlands in the Solitary Islands Marine Park (SIMP), Australia. The results indicate that changes to habitat extent will be variable across different shores and will not necessarily result in net loss of area for some habitats. In addition, habitat modification will not follow a regular pattern over the projected sea levels. Two of the headlands included in the study currently have the maximum level of protection within the SIMP. However, these headlands are likely to lose much of the habitat known to support biodiverse assemblages and may not continue to be suitable sanctuaries into the future. The fine-scale approach taken in this study thus provides a protocol not only for modelling habitat modification but also for future proofing conservation measures under a scenario of changing sea levels.
There is strong consensus that sea levels will rise by as much as 1 m by 2100 in response to a warming climate
While the magnitude of SLR is clearly difficult to predict with certainty, there is little doubt that intertidal habitats are likely to be the first to experience sea level rise-related effects
Predicting the magnitude of changes to intertidal rocky reefs, however, relies not only on accurate predictions of SLR, but also on the development of technologies to map and quantify habitats and environmental processes at appropriate spatial scales, in an efficient and cost-effective way
Following the development and assessment of a combined approach using LIDAR, high-resolution digital imagery and GIS
The SIMP is located on the mid-north-coast of NSW, Australia. The five headlands chosen for this research were: Arrawarra Headland (153°12′07′′E-30°3′33′′S); Oceanview Headland (153°12′14′′E-30°04′01′S); Mullaway Headland (153°12′15′′E-30°4′33′′S); Woolgoolga Headland (153°12′17′′E-30°06′30′′S); and Flat Top Point (153°12′26′′E-30°07′48′′S) (
Arrawarra Headland; Ocean View Headland; Mullaway Headland; Woolgoolga Headland and Flat Top Point.
Three-dimensional digital habitat maps were compiled for each of the five intertidal rocky reefs. The maps were obtained by coupling topographic LIDAR data (wavelength = 1550 µ; Leica ASL60 system) and high-resolution imagery in ArcGIS software
Habitat Classification | Habitat Description (Mean Sea Level (MSL) = 0.9-m) |
Upper Boulder Field | ≥2 boulders (>0.25-m) per m2 and located above MSL |
Lower Boulder Field | ≥2 boulders (>0.25-m) per m2 and located below MSL |
Upper Shallow Pool | Rock pools no deeper than 0.5-m, with bottom composed |
of sand, cobble, boulder or a mix of these, and located | |
above MSL | |
Lower Shallow Pool | Rock pools no deeper than 0.5-m, with bottom composed |
of sand, cobble, boulder or a mix of these, usually with | |
high algal cover and located below MSL | |
Deep Pool | Rock pools deeper than 0.5-m, with bottom composed of |
sand, cobble, boulder or a mix of these (this habitat type | |
only occurred at the seaward edge of the lower shore at | |
the five study locations). | |
Upper Platform | Emerged bedrock located above MSL |
Lower Platform | Emerged bedrock located below MSL |
Flooding maps were created as a layer over the five original habitat maps using ArcMap
The results from the SLR projections show a dramatic change in the rocky shore landscape under a 1-m SLR, with an overall decline in habitat area for all habitat types with rare exceptions (
The four projected sea levels were 0+0.3 m,+0.5 m and 1 m and the five headlands AR-Arrawarra Headland; OV-Ocean View Headland; MU-Mullaway Headland; WO-Woolgoolga Headland; FT-Flat Top Point.
Headland | 0m | +0.3m | +0.5m | +1m |
Arrawarra Headland | 0.5 | 0.7 | 0.9 | 0.6 |
Ocean View Headland | 2.5 | 3.7 | 3.5 | 3.4 |
Mullaway Headland | 1.3 | 0.8 | 0.6 | 0.4 |
Woolgoolga Headland | 2.7 | 2 | 1.4 | 0.7 |
Flat Top Point | 1.4 | 1.1 | 1.3 | 0.4 |
Our modelling indicated that there will be an overall decline in the total area of upper shallow pool habitat at each of the five headlands under 1-m SLR. A reduction by >50% of habitat area will occur under 0.3-m SLR at all headlands except Ocean View Headland. At Mullaway Headland and Flat Top Point, the upper shallow pool habitat will completely disappear under 1-m SLR, whilst at Arrawarra Headland and Woolgoolga Headland, only 2% of the habitat will remain. Habitat loss will be lowest at Ocean View Headland which will maintain nearly half of its upper shallow pool habitat area under 1-m SLR. However, the habitat quality tends to change with SLR, with a decrease in the number of pools for all headlands. The mean size of pools (m2) will be reduced at Arrawarra Headland, Mullaway Headland and Flat Top Point whereas Ocean View Headland and Woolgoolga Headland will show a variable change in pool size as sea level rises (
Headland | Sea Level | Upper Shallow Pool | Lower Shallow Pool | ||
n | Mean ± SE | n | Mean ± SE | ||
Arrawarra Headland | 0m | 32 | 5.57±0.79 | 60 | 25.82±5.24 |
+0.3m | 21 | 3.96±0.66 | 32 | 20.14±5.73 | |
+0.5m | 18 | 3.82±0.7 | 14 | 7.81±1.36 | |
+1m | 1 | 4.34±4.34 | 17 | 3.79±0.74 | |
Ocean View Headland | 0m | 17 | 10.21±3.34 | 19 | 10.73±3.45 |
+0.3m | 13 | 9.59±3.88 | 6 | 9.98±4.5 | |
+0.5m | 9 | 9.31±5.36 | 8 | 11.20±3.71 | |
+1m | 6 | 12.28±7.75 | 3 | 3.38±0.8 | |
Mullaway Headland | 0m | 48 | 16.21±5.11 | 44 | 8.86±1.09 |
+0.3m | 13 | 11.63±5.73 | 46 | 16.77±5.12 | |
+0.5m | 11 | 5.85±1.29 | 36 | 17.15±6.36 | |
+1m | − | − | 11 | 5.85±1.29 | |
Woolgoolga Headland | 0m | 106 | 12.42±2.21 | 61 | 11.75±1.79 |
+0.3m | 57 | 11.18±2.37 | 69 | 12.68±2.88 | |
+0.5m | 29 | 9.66±2.54 | 72 | 13.81±3.07 | |
+1m | 2 | 11.9±5.41 | 27 | 9.5±2.69 | |
Flat Top Point | 0m | 24 | 9.85±2.09 | 20 | 27.73±5.38 |
+0.3m | 19 | 6.12±1.03 | 19 | 25.32±5.12 | |
+0.5m | 10 | 5.44±1.13 | 15 | 12.13±3.14 | |
+1m | − | − | 10 | 5.44±1.13 |
Lower shallow pool habitat shows an irregular pattern of modification under 1-m SLR. At all headlands, with the exception of Woolgoolga Headland, habitat extent will be reduced by >80% under 1-m SLR. At Mullaway and Woolgoolga headlands, SLR will initially lead to an increase in lower shallow pool habitat (under 0.5-m SLR), with a subsequent decline. Ocean View Headland shows a dramatic reduction, to nearly 29% of the current extent, under 0.3-m SLR, followed by an increase to 44% under 0.5-m SLR, and a further decline to 5% under 1-m SLR. Flat Top Point shows the greatest reduction of habitat (50%) between 0.3-m and 0.5-m SLR. The number of pools will be reduced at all headlands; however, the mean pool size will tend to a steady decrease only in Arrawarra Headland, Ocean View Headland and Flat Top Point. Mullaway Headland and Woolgoolga Headland will initially show an increase in mean pool size (m2) as the sea level rises 0.3 m and 0.5 m, and a subsequent decrease under 1-m SLR.
This habitat type is the most at risk at all headlands, with 100% loss under 1-m SLR, at all headlands except Arrawarra Headland. The rate of habitat loss will differ between headlands, with Ocean View, Mullaway and Woolgoolga losing this habitat at 0.5-m SLR and Flat Top Point only at 1-m SLR.
With the exception of Woolgoolga Headland, all headlands show a steady decline in the extent of upper boulder fields under 1-m SLR. At Woolgoolga Headland, habitat extent remains the same under 0.5-m SLR and then suffers a decrease of ∼50% under 1-m SLR. Flat Top Point will lose its entire upper boulder field habitat under 1-m SLR. While there is an overall decline at all headlands, this habitat type will suffer the second lowest overall loss (after lower platform habitat).
Lower boulder field habitat shows an overall decline under 1-m SLR. At all headlands, nearly 50% of the habitat area is predicted to be lost under 0.3-m SLR. Flat Top Point shows the slowest decline in habitat area with around 30% remaining under 1-m SLR, whereas the other headlands will lose >85% of this habitat. Woolgoolga Headland will lose this entire habitat under 0.5-m SLR but will then recover 2% under 1-m SLR.
There will be a decrease in the area of upper platform habitat at all headlands over the next century (1-m SLR). Mullaway Headland will experience by far the greatest loss with 54% remaining under 0.3-m SLR, and 6% remaining under 1-m SLR. All other headlands show greater declines under 0.5-m SLR, with the exception of Ocean View Headland, which will retain 63% of upper platform habitat at 1-m SLR.
Lower platform habitat shows an irregular pattern of change over the next century. Arrawarra Headland will suffer a steady decline with <30% of habitat left under 1-m SLR. Flat Top Point will have a reduction of 50% under 0.5-m SLR, but very little subsequent change up to 1-m SLR. Mullaway Headland and Woolgoolga Headland will have an initial increase in habitat area at 0.5-m SLR, however, Mullaway Headland subsequently lose 18% (relative to the current area), whereas Woolgoolga Headland show a net 15% increase, under 1-m SLR. Ocean View will have the slowest rate of change with 76% of habitat area remaining under 1-m SLR.
It is clear from the results of this study that we can make few generalizations about the rate and extent of intertidal habitat loss resulting from SLR. While the extent of most habitats is ultimately reduced in the worst-case model (1-m SLR), the rate-of-change and final outcome are dependent on the habitat type and specific locality. In other words, the effects of SLR on intertidal rocky reefs will be variable at small spatial scales.
To date, most of the research conducted to evaluate the impact of SLR on intertidal ecosystems refers to mangroves
An important extension of this is that, depending on the relative loss of critical habitat, headlands currently gazetted as sanctuary zones (no take) may not be effective locations to meet conservation targets in the future. If the objective is to protect representative habitats that are likely to persist, there is little point targeting headlands that will show high rates of habitat loss and ultimately support little habitat. It is instructive to apply this concept to the current conservation status of the five headlands. Both Flat Top Point and Arrawarra Headland are currently afforded the highest level of protection under the zoning plan for the Solitary Islands Marine Park (Special Purpose and Sanctuary, respectively)
Eighty percent of the world's oceanic coastlines comprise of rock platforms backed by steep cliffs
Present conservation planning is based on current sea levels. This leads to two questions: i) Will present sanctuary zones be effective for ongoing conservation in a changing environment?; ii) Does loss of habitat area equate to a commensurate loss of biodiversity for intertidal rocky reef ecosystems? Clearly, there is a need to determine if targets that foster prioritization of sites where habitat loss will be lower will actually translate into similar representation for the biota. This also needs to be considered in terms of other factors determining ecosystem functioning (e.g. various processes that can vary over a range of spatial scales)
Coffs Harbour City Council provided the remote sensing data and we had the support of the National Marine Science Centre, Southern Cross University.