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The influence of geomorphology and sedimentary processes on shallow-water benthic habitat distribution: Esperance Bay, Western Australia
Estuarine, Coastal and Shelf Science 72 (2007) 379e386 www.elsevier.com/locate/ecss
The influence of geomorphology and sedimentary processes on shallowwater benthic habitat distribution: Esperance Bay, Western Australia David A. Ryan a,*, Brendan P. Brooke a, Lindsay B. Collins b, Gary A. Kendrick c, Katrina J. Baxter c, Andy N. Bickers c, Paulus J.W. Siwabessy d, Charitha B. Pattiaratchi e a
Petroleum and Marine Division, Geoscience Australia, GPO Box 378, Canberra ACT, New South Wales 2601, Australia b Department of Applied Geology, Curtin University of Technology, Perth, Western Australia 6102, Australia c School of Plant Biology, University of Western Australia, 35 Stirling Highway, Perth, Western Australia 6009, Australia d Centre for Marine Science and Technology, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia e Centre for Water Research, University of Western Australia, 35 Stirling Highway, Perth, Western Australia 6009, Australia Received 15 August 2005; accepted 9 October 2006 Available online 20 November 2006
Abstract The mapping of seabed environments is fundamental to successful fisheries management and environmental monitoring, however, there is an emerging need to better characterise habitats based upon appropriate physical parameters. In this study, relationships between seabed geomorphology and the distribution of benthic habitats were examined using multibeam sonar, underwater video, predicted wave energy, and sediment data for Esperance Bay, part of the Recherche Archipelago. This shallow (<50 m), high energy, biogenic sediment dominated environment is located in temperate southwestern Australia. Exposure to wave energy appears to determine the distribution of unconsolidated substrate, and is the most useful regional scale predictor of rhodolith and seagrass habitats. Although they are intermittently smothered by mobile sediments, limestone reefs provide habitat for a wide range of sessile organisms, even in very high wave exposure environments. The distribution of rhodolith beds is related to poorly sorted sediments that contain high gravel, mud, and CaCO3 percentages. Our results reveal that in the Recherche Archipelago, wave abrasion coupled with localised sediment transport and accumulation play a major role in increasing the diversity of inner shelf benthic habitats. This highlights the value of assessing geomorphic processes in order to better understand the distribution and structure of benthic habitats. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: benthic habitat mapping; shelf sediments; cool water carbonates; rhodoliths; Recherche Archipelago; Great Australian Bight
1. Introduction The delineation of seabed benthic environments such as seagrass beds and rocky reefs is a basic requirement for fisheries management and environmental monitoring (Diaz et al., 2004). The development of new sonar systems and data analysis techniques that can rapidly assess benthic features has greatly enhanced our ability to identify these environments, and has led to a global increase in seabed mapping activities
* Corresponding author. E-mail address: [email protected] (D.A. Ryan). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.10.008
(Freeman and Rogers, 2003; Kenny et al., 2003; Roff et al., 2003; Lathrop et al., 2006). However, relatively few existing benthic habitat maps include adequate descriptors based on fundamental environmental processes and parameters (Mumby and Harborne, 1999). Internationally, there is an emerging need to better characterise these mapped habitats using geological and oceanographic information, in order to better predict habitat distribution in areas that lack ecological data (Freeman and Rogers, 2003). Although sediment grainsize alone is a poor determinant of species distribution, appreciation of process related factors such as sediment mobility, currents and wave exposure can provide useful insights into habitat distribution (Hemer,
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2006; Post et al., 2006). Physical attributes can therefore become surrogates for predicting the potential spatial distribution of benthic organisms, in the absence of detailed ecological data. Geomorphology also determines the long-term stability of the substrate, which represents a major control on biological diversity (Freeman and Rogers, 2003). This study examines the inner shelf of the Recherche Archipelago, an example of a storm dominated, temperate marine environment in southern Australia. Recently, the distribution of ecological habitats in the archipelago were mapped (Kendrick et al., 2005), providing an opportunity to relate physical datasets to mapped ecological habitats. This paper aims to: (1) provide insights into how geomorphic processes influence the formation and stability of shallow benthic habitats; and (2) investigate quantitative relationships between physical parameters (bathymetry, sediments, geomorphology and wave energy) and the distribution of mapped habitats. In particular, high resolution multibeam sonar is used to characterise the distinctive benthic morphological features. Our
approach provides insights that are relevant to the characterisation and predictive mapping of similar shallow-water environments elsewhere. 2. Regional setting 2.1. Physical environment The Recherche Archipelago is located in southern Western Australia (Fig. 1), on the western margin of the Great Australian Bight (GAB), the world’s largest cool water carbonate environment (James et al., 2001). Esperance Bay is a southwest facing embayment in the archipelago (Fig. 1), bordered by sand barriers and rocky headlands (Sanderson et al., 2000). The embayment is dominated by the Woody and Remark Island groups, which comprise outcrops of Middle Proterozoic granites, gneisses and migmatites (Fig. 1). These protrude through mainly flat lying Cenozoic limestones (Cann and Clarke, 1993). Limestone outcrops dominate in <100 m water depth
Fig. 1. Location of Esperance Bay, Western Australia, showing bathymetry, sample sites (indicated by squares), multibeam coverage, and habitat classes (Kendrick et al., 2005). Still images are derived from underwater video, and show: (A) Rhodoliths; (B) Seagrasses; (C) Bare sands; (D) Subaqueous dunes; (E) Low profile reef; and F) High profile reef.
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due to powerful wave erosion, and are in places overlain by a thin (<2 m), discontinuous veneer of mainly biogenic sediments (James et al., 2001). 2.2. Climate and oceanography The semi-arid Mediterranean climate that dominates southwestern Australia is characterised by moderate temperatures (averaging from 8 to 26 C seasonally), winter rainfall, and very low fluvial discharge to the ocean. The dominant wind direction in summer is from the southeast, whereas in winter southwest storms prevail (Sanderson et al., 2000). In summer, the eastward flowing Leeuwin Current elevates sea temperatures to over 20 C (Cann and Clarke, 1993). The low turbidity, mostly oligotrophic seawater has a salinity that reaches 36 ppt, and the spring tidal range is 1.1 m (Li et al., 1999). The narrow southwest continental shelf is exposed to the most extreme wave energy of the entire Australian coastline (Hemer, 2006).
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Esperance Bay (Fig. 2A), developed by Kendrick et al. (2005). Wave height index values were low (0e1 m), moderate (1e2 m), and high (2e3 m). Sediment grainsize was measured using standard sieves to obtain the ratio of mud:sand:gravel, and with a Malvern Mastersizer laser grainsize analyser to provide detailed volumetric particle size information for the <2 mm fraction. CaCO3 proportions were determined using the ‘bomb’ gas evolution method (Muller and Gastner, 1971). A principal component analysis (PCA) was used to investigate patterns in and explore the relationships between all sediment samples (Fig. 1). The input variables for the PCA were mean grainsize, sorting, skewness and kurtosis; percentages of gravel, mud and CaCO3; and water depth. Sediment samples were analysed under a reflected
2.3. Benthic habitats The seabed habitats of the Recherche Archipelago were mapped by Kendrick et al. (2005) using sidescan sonar, underwater video, and satellite imagery. A visual decision tree classification was employed to describe the dominant community cover, and occurrence of various biological assemblages (Kendrick et al., 2005). The map units (Fig. 1) include high and low profile reef, rhodoliths and associated biota, seagrass beds of varying density and species, bare sands, and sand waves, with further descriptors detailing dominant ecological communities (Kendrick et al., 2005). The study provided a comprehensive qualitative assessment of habitat distributions in the archipelago, however it lacked a detailed quantitative assessment of substrate composition based on physical sampling. In this paper we adopt the terminology of Kendrick et al. (2005) for the broad habitat classes (Fig. 1). 3. Methods 3.1. Field program Sediment grab samples (0.5 litre) were collected in a transect across Esperance Bay, and as grids around island groups (Fig. 1). Video imagery of the seabed was acquired using procedures reported in Kendrick et al. (2005), allowing the selection of representative sampling sites (Fig. 1AeF). Bathymetric data for the various habitats was acquired using a 455 kHz ResonTM 8125 multibeam sonar (Fig. 1). The system resolves returning signals into 240 beams in an arc of 120 across track, and 1 along track. Depth soundings were decimated into 0.5 m grid cells. 3.2. Analyses Sample locations were indexed for their degree of exposure to wave energy based on modelled mean wave height for
Fig. 2. (A) Map of mean (RMS) wave height (m) for the Esperance region (Kendrick et al., 2005). The rose diagram indicates the frequency of wave direction; and (B) Principal components analysis plot for all samples (Fig. 1), showing the first two principal components indexed by habitat class.
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light microscope to identify detrital constituents. The medium to coarse sand fraction of each sample was split using a sample splitter, from which 300 grains were classified (Lewis and McConchie, 1994).
waves, or subaqueous dunes (Ashley, 1990) occur in small patches in outer Esperance Bay (Fig. 1D). Rhodolith beds (Fig. 1A) occur in both the inner and outer bay, in water depths ranging from 25e60 m, and are associated with moderate wave energy environments (Fig. 2A).
4. Results 4.2. Sediment texture 4.1. Geomorphology and benthic habitats The seabed of Esperance Bay gradually slopes towards the southwest, and shoals in areas adjacent to islands and bedrock reefs (Figs. 1 and 3A). Shallow-water areas typically occur on the northeastern flanks of islands, and experience lower wave energy conditions (Fig. 2A). Extensive seagrass beds lie within 5 km of the shoreline and in patches adjacent to the northeastern margins of islands (Fig. 1B, Table 1). High profile reef comprises granitic outcrops (Fig. 1F), whereas low profile reefs comprise northwest to southeast trending limestone outcrops, and both are heavily colonised by sessile organisms (Fig. 1E). Bare sand areas are mostly planar, occur throughout the inner bay and on the lee side of islands (Fig. 1C). Sand
Sediments in Esperance Bay typically comprise very poorly sorted gravelly sands. Grainsize trends in the shore normal transect (Fig. 3A,B) show that medium to fine sand occurs inshore, grading to more variable fine to coarse sands offshore. Broad relationships occur between the mapped habitat classes (Fig. 1) and sediment properties (Table 1). Seagrass beds and bare sand habitats are dominated by medium sands, with little mud or gravel, whereas low profile reefs and subaqueous dune habitats comprise gravelly sands. Rhodolith beds are typically gravel rich, are the only habitat with a significant proportion of mud (up to 28%), and occur where there is low to moderate wave exposure. The PCA shows sedimentological relationships for all samples (Fig. 2B). The dominant principal component (factor 1)
Fig. 3. Line plots for a transect across Esperance Bay (Fig. 1) showing: (A) depth and percentage mud; (B) percentage gravel and CaCO3; and (C) the relative abundance of bioclastic, lithic (quartz þ rock) and relict components. Fossil composition of (D) low profile reefs; (E) rhodolith beds; (F) offshore seagrass beds; (G) inshore seagrass beds; and (H) bare sand.
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Table 1 Mean sedimentary characteristics for the broad scale benthic habitats of Kendrick et al. (2005). Seabed morphology was determined from multibeam bathymetry and underwater video Habitat
% CaCO3
% Gravel
% Sand
% Mud
Swell Exp.
Seabed morphology
Seagrass Beds (inshore & offshore)
63.4 22.7 (inshore w20, offshore w80) n/a
9.7 16.4
89.4 16.3
0.94 1.6
Low
Surface variation of 30 cm over 2e3 m laterally, depending on seagrass density
n/a
n/a
n/a
Varied
High relief (>10 m) weathered granitic geomorphology Relief w2 m, typically dense macroalgae, sponges, & ascidians Mostly planar; relief less than the resolution of the multibeam system Smooth, featureless surface Large asymmetric subaqueous dunes with amplitude w1 m and wavelength 5e130 m
High profile reef (granite) Low profile reef (limestone) Rhodoliths
69.8 20.1
20.7 23.2
79.0 23.3
0.31 0.2
High
83.6 5.9
40.1 30.1
56.3 30.3
3.6 6.6
Mod
Bare Sand Subaqueous Dunes
69.7 16.3 86.3 3.1
5.9 12.4 22.0 31.8
93.2 13.2 77.8 32.0
0.92 1.5 0.25 0.3
Low High
accounts for 41% of dataset variance, and is largely determined by sorting and percentage mud. Major scores for factor 2 (20.4% variance) are percentage CaCO3, and depth. The third principal component (15.2% variance) is dominated by kurtosis and skewness. Samples from rhodolith habitat (Fig. 2B) vary with factor 1, mainly comprising poorly sorted, muddy, high CaCO3 sediments. A large degree of overlap is apparent between most samples from low profile reefs and bare sands, suggesting little sedimentological difference between these environments. Several seagrass sites are distinctive, comprising well sorted sediments from inshore, whereas offshore seagrass sites are similar to the bare sand samples. Samples in the lee of islands comprise moderately sorted, low CaCO3 material. 4.3. Sediment composition Compositional trends can be ascertained from the aggregated percentage abundance of bioclastic (mainly fragmented coralline algae, bryozoans, foraminifera, and molluscs), lithic (granite or quartz) and relict (weathered CaCO3 and weakly cemented calcarenite fragments) material. Transect data indicate that inshore sediments are dominated by lithic (quartz) material, whereas the proportion of relict grains increases offshore, with greater wave exposure (Fig. 3C). Moving offshore, CaCO3 greatly increases, from <5% inshore to 75% in the outer bay (Fig. 3B). Terrigenous minerals occurred in greater quantities close to the granitic islands. Sediment samples from low profile reef and bare sand habitats (Fig. 3D,H) have a variable composition, with large proportions of lithic, relict, and algal components. Rhodolith habitats (Fig. 3E) mainly comprise calcareous algae and bryozoans. Inshore seagrass beds (Fig. 3G) are dominated by lithic (quartz) grains, whereas offshore beds mainly comprise coarse CaCO3 (Fig. 3F). However, all seagrass beds occur in areas of moderate to low wave exposure (Fig. 2A). 4.4. Multibeam bathymetry A representative multibeam sonar coverage was acquired for all habitat types. High profile reef (Fig. 4A) has vertical relief up to 10 m formed by rounded granite outcrops and
boulders. In contrast, extensive areas of low profile reef (Fig. 4B) typically have <2 m relief and often display northwest to southeast linear or arcuate structure. Seagrass beds comprise irregular banks with a vertical relief of <30 cm (Fig. 4C). Sediment bedforms, with up to 1 m vertical relief and wavelengths of 60e130 m, occur between Remark and Long Islands, and in an exposed area northeast of Long Island (Fig. 4D). The orientation and geometry of these subaqueous dunes suggests shoreward sediment transport (Ashley, 1990). Smaller scale (<2 m wavelength) dunes of similar orientation also occur in the outer bay (Fig. 1). Bathymetry data indicates the presence of mounded sediment aprons on the northeast margin of several islands. At Canard Island the deposit is 500 m wide and rises 5 m above the surrounding flat limestone seabed (Fig. 4E). Similarly, bathymetric data around Remark Island reveals a steep and uneven windward margin, contrasting with the smooth low gradient leeward margin where sediment appears to have accumulated (Fig. 4F). 5. Discussion 5.1. Seabed geomorphology Analysis of the distribution of seabed habitats (Fig. 1), sediment texture, and fossil composition (Table 1), suggests that three characteristic sedimentary environments occur in Esperance Bay. These include near shore quartz sands, biocalcarenites, and relict limestones with associated sediment lags. The depth range, distribution, texture, composition, and wave exposure characteristics of these units provide conceptual insights into the geomorphic processes that operate in the Recherche Archipelago (Fig. 5). Extensive quartz sand environments occur within 4 km of the shoreline. This sediment is likely derived from reworked coastal deposits and transported to the east by the seasonal longshore current (Sanderson et al., 2000; Fig. 5). Numerous topographic highs in the Recherche Archipelago form a large, morphologically complex littoral zone (Fig. 5). Consequently, in comparison to the majority of the GAB, there is a large nearshore area available for colonisation by shallow-water
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CaCO3 producing organisms, including rhodoliths (Foster, 2001). This, coupled with warm current flows and the presence of subtropical biota (Cann and Clarke, 1993; Li et al., 1999), creates a potentially productive shallow-water CaCO3 ‘factory’, which effectively dilutes proportions of relict sediment grains (James et al., 2001). Bathymetric data show that much of this biogenic sediment accumulates on the lower energy, leeward sides of bedrock islands (Fig. 4E,F). Terrigenous sediment derived from the weathering of the granitic outcrops also contributes significantly to the leeward accumulations (Figs. 3F and 5). Low profile reefs correspond to the ‘shaved shelf’ environments described on other parts of the GAB, and probably experience negligible sediment accumulation (James et al., 2001). Linear structures in the limestone (Fig. 4B) may represent the eroded remnants of subaerial or shoreline calcarenite that formed during periods of lower sea level (Fig. 5). The presence of outer bay asymmetric sediment dunes indicates that CaCO3 material from the middle shelf is being transported inshore, and may be supplementing the inner shelf sediment budget. Overall, the distribution of physical environments on the Recherche inner shelf appears to be primarily controlled by wave energy. As a consequence, sediment accumulation is restricted to the littoral zone and in the lee of islands, and there are large tracts of seabed with exposed limestone hardgrounds. 5.2. Depositional environments and habitat characteristics
Fig. 4. Selected examples of multibeam bathymetry within Esperance Bay (Fig. 1). (A) High profile reef, south of Long Island; (B) Low profile reef, north of Remark Island; (C) Seagrasses, southeast of Woody Island, with a bathymetric profile; (D) Subaqueous dunes, south of Long Island, and bathymetric profile; (E) Sediment apron leeward of Canard Island, and bathymetric profile; and (F) Bathymetry surrounding Remark Island, with bathymetric profiles. A to D are shown in greyscale with a 45 sun angle; E and F are shown on a coloured scale.
The PCA (Fig. 2B) shows that the relationship between sediment parameters and benthic habitats is complex, with sediment sorting and mud content showing the strongest correlation to benthic habitats. As suggested in previous studies, sediment grainsize alone is not sufficient as a surrogate for species distribution (Post et al., 2006). Although the majority of seagrass in Esperance Bay occurs on the nearshore quartz sand environment (Fig. 3G), seagrass areas also occur in the lee of islands, on a substrate of coarse CaCO3 sand (Fig. 1 and 3F). Therefore, seagrass distribution in Esperance Bay is primarily related to wave exposure rather than sediment type. All seagrass sites have a low proportion of mud, demonstrating that even dense seagrass beds in Esperance Bay are unable to trap fine sediment. Rhodolith habitat is characterised by exposure to moderate wave energy. This is consistent with previous studies which suggest that in low energy environments rhodoliths are buried, whereas in higher energy conditions they tend to be removed (Marrack, 1999; Foster, 2001). Unsurprisingly, the rhodolith substrates comprise CaCO3 gravel, which comprises broken rhodolith thalli. However, the <2 mm fraction of rhodolith samples tends to be poorly sorted and contains significant quantities of mud (Table 1, Fig. 2B). The high rugosity of these beds appears to trap fine sediment, and provides three dimensional microhabitat for numerous other organisms (Fig. 3E; Foster, 2001). Limestone and granitic reef habitats occur in various depths and wave energy settings and form habitat for a wide range of
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Fig. 5. Conceptual model of the geomorphic processes in Esperance Bay, including plan view and cross section (A-A0 ).
sessile organisms (Kendrick et al., 2005; Figs. 1 and 3). High rates of sediment production and transport has resulted in the partial burial of low profile reef, particularly inshore (Fig. 5). Mobile bedforms on sections of this reef show that these areas represent unstable habitat that is subject to intermittent burial. In lower energy settings it has been shown that bioclastic material may accumulate and eventually permanently bury sessile biota (James et al., 2001). 6. Summary and conclusion In Esperance Bay, exposure to wave energy appears to be the most useful regional scale predictor of the distribution of rhodolith and seagrass habitats, and also controls the distribution of unconsolidated substrate. Limestone reefs provide habitat for a wide range of sessile organisms, even in very high wave exposure environments. Although these habitats are intermittently inundated by sediment, periodic erosion may prevent sessile biota from being permanently smothered by their own sediment. Rhodolith beds comprise poorly sorted sediment that contains large proportions of gravel, mud, and CaCO3. Although fine sediments are typically rare in ‘shaved shelf’
environments (James et al., 2001), high rugosity rhodolith beds, rather than seagrass beds, provide the only environment in which mud can be trapped. The lower energy environments and extensive sediment accumulations in Esperance Bay contrast strongly with the heavily storm influenced and nondepositional inner shelf of the GAB (James et al., 2001). This study demonstrates that a process based conceptual understanding provides useful insights into the relationships between physical processes and habitat distribution. Our results reveal that in the Recherche Archipelago, wave abrasion coupled with localised accumulation of CaCO3 sediment, and a complex sediment transport system, play a major role in increasing the diversity of inner shelf benthic habitats. This work highlights the value of assessing geomorphic processes in order to better understand the distribution and structure of benthic habitats. Acknowledgements Many thanks to: David Johnson for wave modelling; Tony Watson and Alex McLachlan for sedimentology assistance; Jonathon Clarke, Alix Post, Vicky Passlow, Yvonne Bone, Lynda Radke, and Alan Orpin for useful comments; and Paul
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Rose, Simon Grove, and Marcus Gray for field assistance. This study was supported by the Cooperative Research Centre for Coastal Zone, Estuary and Waterway Management. Parts of this study were funded by the Fisheries Research and Development Corporation (FRDC) project 2001/060. Published with permission of the Executive Director, Geoscience Australia. References Ashley, G.M., 1990. Classification of large-scale subaqueous bedforms: a new look at an old problem. Journal of Sedimentary Petrology 60, 160e172. Cann, J.H., Clarke, J.D.A., 1993. The significance of Marginopora vertebralis (Foraminifera) in surficial sediments at Esperance, Western Australia, and in last interglacial sediments in northern Spencer Gulf, South Australia. Marine Geology 111, 171e187. Diaz, R.J., Solan, M., Valente, R.M., 2004. A review of approaches for classifying benthic habitats and evaluating habitat quality. Journal of Environmental Management 73, 165e181. Foster, M.S., 2001. Rhodoliths: Between rocks and soft places. Journal of Phycology 37, 659e667. Freeman, S.M., Rogers, S.I., 2003. A new analytical approach to the characterisation of macro-epibenthic habitats: linking species to the environment. Estuarine, Coastal and Shelf Science 56, 749e764. Hemer, M.A., 2006. The magnitude and frequency of combined flow bed shear stress as a measure of exposure on the Australian continental shelf. Continental Shelf Research 26, 1258e1280. James, N.P., Bone, Y., Collins, L.B., Kyser, T.K., 2001. Surficial sediments of the Great Australian Bight: facies dynamics and oceanography on a vast cool-water carbonate shelf. Journal of Sedimentary Research 71, 549e567. Kendrick, G.A., Harvey, E., McDonald, J., Pattiaratchi, C., Cappo, M., Fromont, J., Shortis, M., Grove, S., Bickers, A., Baxter, K., Goldberg, N., Kletczkowski, M., Butler, J., 2005. Characterising the fish habitats of the
Recherche Archipelago. Final Report, Fisheries Research Development Corporation Project 2001/060, 582 pp. (http://www.marine.uwa.edu.au/recherche/). Kenny, A.J., Cato, I., Desprez, M., Fader, G., Schuttenhelm, R.T.E., Side, J., 2003. An overview of seabed-mapping technologies in the context of marine habitat classification. ICES Journal of Marine Science 60, 411e418. Lathrop, R.G., Cole, M., Senyk, N., Butman, B., 2006. Seafloor habitat mapping of the New York Bight incorporating sidescan sonar data. Estuarine, Coastal and Shelf Science 68, 221e230. Lewis, D.W., McConchie, D., 1994. Analytical Sedimentology. Chapman and Hall, New York, 197 pp. Li, Q., James, N.P., Bone, Y., McGowran, B., 1999. Palaeoceanographic significance of recent foraminiferal biofacies on the southern shelf of Western Australia: a preliminary study. Palaeogeography, Palaeoclimatology, Palaeoecology 147, 101e120. Marrack, E.C., 1999. The relationship between water motion and living rhodolith beds in the southwestern Gulf of California, Mexico. Palaios 14, 159e 171. Muller, G., Gastner, M., 1971. The ‘‘karbonate bombe’’ a simple device for the determination of the carbonate content in sediments, soils and other materials. Neues Jahrbuch fur Mineralogie Monatshefte 10, 466e469. Mumby, P.J., Harborne, A.R., 1999. Development of a systematic classification scheme of marine habitats to facilitate regional management and mapping of Caribbean coral reefs. Biological Conservation 88, 155e163. Post, A.L., Wassenberg, T.J., Passlow, V., 2006. Physical surrogates for macrofaunal distributions and abundance in a tropical gulf. Marine and Freshwater Research 57, 469e483. Roff, J.C., Taylor, M.E., Laughren, J., 2003. Geophysical approaches to the classification, delineation and monitoring of marine habitats and their communities. Aquatic Conservation: Marine and Freshwater Ecosystems 13, 77e90. Sanderson, P.G., Eliot, I., Hegge, B., Maxwell, S., 2000. Regional variation of coastal morphology in southwestern Australia: a synthesis. Geomorphology 34, 73e88.
The influence of geomorphology and sedimentary processes on shallowwater benthic habitat distribution: Esperance Bay, Western Australia David A. Ryan a,*, Brendan P. Brooke a, Lindsay B. Collins b, Gary A. Kendrick c, Katrina J. Baxter c, Andy N. Bickers c, Paulus J.W. Siwabessy d, Charitha B. Pattiaratchi e a
Petroleum and Marine Division, Geoscience Australia, GPO Box 378, Canberra ACT, New South Wales 2601, Australia b Department of Applied Geology, Curtin University of Technology, Perth, Western Australia 6102, Australia c School of Plant Biology, University of Western Australia, 35 Stirling Highway, Perth, Western Australia 6009, Australia d Centre for Marine Science and Technology, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia e Centre for Water Research, University of Western Australia, 35 Stirling Highway, Perth, Western Australia 6009, Australia Received 15 August 2005; accepted 9 October 2006 Available online 20 November 2006
Abstract The mapping of seabed environments is fundamental to successful fisheries management and environmental monitoring, however, there is an emerging need to better characterise habitats based upon appropriate physical parameters. In this study, relationships between seabed geomorphology and the distribution of benthic habitats were examined using multibeam sonar, underwater video, predicted wave energy, and sediment data for Esperance Bay, part of the Recherche Archipelago. This shallow (<50 m), high energy, biogenic sediment dominated environment is located in temperate southwestern Australia. Exposure to wave energy appears to determine the distribution of unconsolidated substrate, and is the most useful regional scale predictor of rhodolith and seagrass habitats. Although they are intermittently smothered by mobile sediments, limestone reefs provide habitat for a wide range of sessile organisms, even in very high wave exposure environments. The distribution of rhodolith beds is related to poorly sorted sediments that contain high gravel, mud, and CaCO3 percentages. Our results reveal that in the Recherche Archipelago, wave abrasion coupled with localised sediment transport and accumulation play a major role in increasing the diversity of inner shelf benthic habitats. This highlights the value of assessing geomorphic processes in order to better understand the distribution and structure of benthic habitats. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: benthic habitat mapping; shelf sediments; cool water carbonates; rhodoliths; Recherche Archipelago; Great Australian Bight
1. Introduction The delineation of seabed benthic environments such as seagrass beds and rocky reefs is a basic requirement for fisheries management and environmental monitoring (Diaz et al., 2004). The development of new sonar systems and data analysis techniques that can rapidly assess benthic features has greatly enhanced our ability to identify these environments, and has led to a global increase in seabed mapping activities
* Corresponding author. E-mail address: [email protected] (D.A. Ryan). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.10.008
(Freeman and Rogers, 2003; Kenny et al., 2003; Roff et al., 2003; Lathrop et al., 2006). However, relatively few existing benthic habitat maps include adequate descriptors based on fundamental environmental processes and parameters (Mumby and Harborne, 1999). Internationally, there is an emerging need to better characterise these mapped habitats using geological and oceanographic information, in order to better predict habitat distribution in areas that lack ecological data (Freeman and Rogers, 2003). Although sediment grainsize alone is a poor determinant of species distribution, appreciation of process related factors such as sediment mobility, currents and wave exposure can provide useful insights into habitat distribution (Hemer,
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2006; Post et al., 2006). Physical attributes can therefore become surrogates for predicting the potential spatial distribution of benthic organisms, in the absence of detailed ecological data. Geomorphology also determines the long-term stability of the substrate, which represents a major control on biological diversity (Freeman and Rogers, 2003). This study examines the inner shelf of the Recherche Archipelago, an example of a storm dominated, temperate marine environment in southern Australia. Recently, the distribution of ecological habitats in the archipelago were mapped (Kendrick et al., 2005), providing an opportunity to relate physical datasets to mapped ecological habitats. This paper aims to: (1) provide insights into how geomorphic processes influence the formation and stability of shallow benthic habitats; and (2) investigate quantitative relationships between physical parameters (bathymetry, sediments, geomorphology and wave energy) and the distribution of mapped habitats. In particular, high resolution multibeam sonar is used to characterise the distinctive benthic morphological features. Our
approach provides insights that are relevant to the characterisation and predictive mapping of similar shallow-water environments elsewhere. 2. Regional setting 2.1. Physical environment The Recherche Archipelago is located in southern Western Australia (Fig. 1), on the western margin of the Great Australian Bight (GAB), the world’s largest cool water carbonate environment (James et al., 2001). Esperance Bay is a southwest facing embayment in the archipelago (Fig. 1), bordered by sand barriers and rocky headlands (Sanderson et al., 2000). The embayment is dominated by the Woody and Remark Island groups, which comprise outcrops of Middle Proterozoic granites, gneisses and migmatites (Fig. 1). These protrude through mainly flat lying Cenozoic limestones (Cann and Clarke, 1993). Limestone outcrops dominate in <100 m water depth
Fig. 1. Location of Esperance Bay, Western Australia, showing bathymetry, sample sites (indicated by squares), multibeam coverage, and habitat classes (Kendrick et al., 2005). Still images are derived from underwater video, and show: (A) Rhodoliths; (B) Seagrasses; (C) Bare sands; (D) Subaqueous dunes; (E) Low profile reef; and F) High profile reef.
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due to powerful wave erosion, and are in places overlain by a thin (<2 m), discontinuous veneer of mainly biogenic sediments (James et al., 2001). 2.2. Climate and oceanography The semi-arid Mediterranean climate that dominates southwestern Australia is characterised by moderate temperatures (averaging from 8 to 26 C seasonally), winter rainfall, and very low fluvial discharge to the ocean. The dominant wind direction in summer is from the southeast, whereas in winter southwest storms prevail (Sanderson et al., 2000). In summer, the eastward flowing Leeuwin Current elevates sea temperatures to over 20 C (Cann and Clarke, 1993). The low turbidity, mostly oligotrophic seawater has a salinity that reaches 36 ppt, and the spring tidal range is 1.1 m (Li et al., 1999). The narrow southwest continental shelf is exposed to the most extreme wave energy of the entire Australian coastline (Hemer, 2006).
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Esperance Bay (Fig. 2A), developed by Kendrick et al. (2005). Wave height index values were low (0e1 m), moderate (1e2 m), and high (2e3 m). Sediment grainsize was measured using standard sieves to obtain the ratio of mud:sand:gravel, and with a Malvern Mastersizer laser grainsize analyser to provide detailed volumetric particle size information for the <2 mm fraction. CaCO3 proportions were determined using the ‘bomb’ gas evolution method (Muller and Gastner, 1971). A principal component analysis (PCA) was used to investigate patterns in and explore the relationships between all sediment samples (Fig. 1). The input variables for the PCA were mean grainsize, sorting, skewness and kurtosis; percentages of gravel, mud and CaCO3; and water depth. Sediment samples were analysed under a reflected
2.3. Benthic habitats The seabed habitats of the Recherche Archipelago were mapped by Kendrick et al. (2005) using sidescan sonar, underwater video, and satellite imagery. A visual decision tree classification was employed to describe the dominant community cover, and occurrence of various biological assemblages (Kendrick et al., 2005). The map units (Fig. 1) include high and low profile reef, rhodoliths and associated biota, seagrass beds of varying density and species, bare sands, and sand waves, with further descriptors detailing dominant ecological communities (Kendrick et al., 2005). The study provided a comprehensive qualitative assessment of habitat distributions in the archipelago, however it lacked a detailed quantitative assessment of substrate composition based on physical sampling. In this paper we adopt the terminology of Kendrick et al. (2005) for the broad habitat classes (Fig. 1). 3. Methods 3.1. Field program Sediment grab samples (0.5 litre) were collected in a transect across Esperance Bay, and as grids around island groups (Fig. 1). Video imagery of the seabed was acquired using procedures reported in Kendrick et al. (2005), allowing the selection of representative sampling sites (Fig. 1AeF). Bathymetric data for the various habitats was acquired using a 455 kHz ResonTM 8125 multibeam sonar (Fig. 1). The system resolves returning signals into 240 beams in an arc of 120 across track, and 1 along track. Depth soundings were decimated into 0.5 m grid cells. 3.2. Analyses Sample locations were indexed for their degree of exposure to wave energy based on modelled mean wave height for
Fig. 2. (A) Map of mean (RMS) wave height (m) for the Esperance region (Kendrick et al., 2005). The rose diagram indicates the frequency of wave direction; and (B) Principal components analysis plot for all samples (Fig. 1), showing the first two principal components indexed by habitat class.
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light microscope to identify detrital constituents. The medium to coarse sand fraction of each sample was split using a sample splitter, from which 300 grains were classified (Lewis and McConchie, 1994).
waves, or subaqueous dunes (Ashley, 1990) occur in small patches in outer Esperance Bay (Fig. 1D). Rhodolith beds (Fig. 1A) occur in both the inner and outer bay, in water depths ranging from 25e60 m, and are associated with moderate wave energy environments (Fig. 2A).
4. Results 4.2. Sediment texture 4.1. Geomorphology and benthic habitats The seabed of Esperance Bay gradually slopes towards the southwest, and shoals in areas adjacent to islands and bedrock reefs (Figs. 1 and 3A). Shallow-water areas typically occur on the northeastern flanks of islands, and experience lower wave energy conditions (Fig. 2A). Extensive seagrass beds lie within 5 km of the shoreline and in patches adjacent to the northeastern margins of islands (Fig. 1B, Table 1). High profile reef comprises granitic outcrops (Fig. 1F), whereas low profile reefs comprise northwest to southeast trending limestone outcrops, and both are heavily colonised by sessile organisms (Fig. 1E). Bare sand areas are mostly planar, occur throughout the inner bay and on the lee side of islands (Fig. 1C). Sand
Sediments in Esperance Bay typically comprise very poorly sorted gravelly sands. Grainsize trends in the shore normal transect (Fig. 3A,B) show that medium to fine sand occurs inshore, grading to more variable fine to coarse sands offshore. Broad relationships occur between the mapped habitat classes (Fig. 1) and sediment properties (Table 1). Seagrass beds and bare sand habitats are dominated by medium sands, with little mud or gravel, whereas low profile reefs and subaqueous dune habitats comprise gravelly sands. Rhodolith beds are typically gravel rich, are the only habitat with a significant proportion of mud (up to 28%), and occur where there is low to moderate wave exposure. The PCA shows sedimentological relationships for all samples (Fig. 2B). The dominant principal component (factor 1)
Fig. 3. Line plots for a transect across Esperance Bay (Fig. 1) showing: (A) depth and percentage mud; (B) percentage gravel and CaCO3; and (C) the relative abundance of bioclastic, lithic (quartz þ rock) and relict components. Fossil composition of (D) low profile reefs; (E) rhodolith beds; (F) offshore seagrass beds; (G) inshore seagrass beds; and (H) bare sand.
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Table 1 Mean sedimentary characteristics for the broad scale benthic habitats of Kendrick et al. (2005). Seabed morphology was determined from multibeam bathymetry and underwater video Habitat
% CaCO3
% Gravel
% Sand
% Mud
Swell Exp.
Seabed morphology
Seagrass Beds (inshore & offshore)
63.4 22.7 (inshore w20, offshore w80) n/a
9.7 16.4
89.4 16.3
0.94 1.6
Low
Surface variation of 30 cm over 2e3 m laterally, depending on seagrass density
n/a
n/a
n/a
Varied
High relief (>10 m) weathered granitic geomorphology Relief w2 m, typically dense macroalgae, sponges, & ascidians Mostly planar; relief less than the resolution of the multibeam system Smooth, featureless surface Large asymmetric subaqueous dunes with amplitude w1 m and wavelength 5e130 m
High profile reef (granite) Low profile reef (limestone) Rhodoliths
69.8 20.1
20.7 23.2
79.0 23.3
0.31 0.2
High
83.6 5.9
40.1 30.1
56.3 30.3
3.6 6.6
Mod
Bare Sand Subaqueous Dunes
69.7 16.3 86.3 3.1
5.9 12.4 22.0 31.8
93.2 13.2 77.8 32.0
0.92 1.5 0.25 0.3
Low High
accounts for 41% of dataset variance, and is largely determined by sorting and percentage mud. Major scores for factor 2 (20.4% variance) are percentage CaCO3, and depth. The third principal component (15.2% variance) is dominated by kurtosis and skewness. Samples from rhodolith habitat (Fig. 2B) vary with factor 1, mainly comprising poorly sorted, muddy, high CaCO3 sediments. A large degree of overlap is apparent between most samples from low profile reefs and bare sands, suggesting little sedimentological difference between these environments. Several seagrass sites are distinctive, comprising well sorted sediments from inshore, whereas offshore seagrass sites are similar to the bare sand samples. Samples in the lee of islands comprise moderately sorted, low CaCO3 material. 4.3. Sediment composition Compositional trends can be ascertained from the aggregated percentage abundance of bioclastic (mainly fragmented coralline algae, bryozoans, foraminifera, and molluscs), lithic (granite or quartz) and relict (weathered CaCO3 and weakly cemented calcarenite fragments) material. Transect data indicate that inshore sediments are dominated by lithic (quartz) material, whereas the proportion of relict grains increases offshore, with greater wave exposure (Fig. 3C). Moving offshore, CaCO3 greatly increases, from <5% inshore to 75% in the outer bay (Fig. 3B). Terrigenous minerals occurred in greater quantities close to the granitic islands. Sediment samples from low profile reef and bare sand habitats (Fig. 3D,H) have a variable composition, with large proportions of lithic, relict, and algal components. Rhodolith habitats (Fig. 3E) mainly comprise calcareous algae and bryozoans. Inshore seagrass beds (Fig. 3G) are dominated by lithic (quartz) grains, whereas offshore beds mainly comprise coarse CaCO3 (Fig. 3F). However, all seagrass beds occur in areas of moderate to low wave exposure (Fig. 2A). 4.4. Multibeam bathymetry A representative multibeam sonar coverage was acquired for all habitat types. High profile reef (Fig. 4A) has vertical relief up to 10 m formed by rounded granite outcrops and
boulders. In contrast, extensive areas of low profile reef (Fig. 4B) typically have <2 m relief and often display northwest to southeast linear or arcuate structure. Seagrass beds comprise irregular banks with a vertical relief of <30 cm (Fig. 4C). Sediment bedforms, with up to 1 m vertical relief and wavelengths of 60e130 m, occur between Remark and Long Islands, and in an exposed area northeast of Long Island (Fig. 4D). The orientation and geometry of these subaqueous dunes suggests shoreward sediment transport (Ashley, 1990). Smaller scale (<2 m wavelength) dunes of similar orientation also occur in the outer bay (Fig. 1). Bathymetry data indicates the presence of mounded sediment aprons on the northeast margin of several islands. At Canard Island the deposit is 500 m wide and rises 5 m above the surrounding flat limestone seabed (Fig. 4E). Similarly, bathymetric data around Remark Island reveals a steep and uneven windward margin, contrasting with the smooth low gradient leeward margin where sediment appears to have accumulated (Fig. 4F). 5. Discussion 5.1. Seabed geomorphology Analysis of the distribution of seabed habitats (Fig. 1), sediment texture, and fossil composition (Table 1), suggests that three characteristic sedimentary environments occur in Esperance Bay. These include near shore quartz sands, biocalcarenites, and relict limestones with associated sediment lags. The depth range, distribution, texture, composition, and wave exposure characteristics of these units provide conceptual insights into the geomorphic processes that operate in the Recherche Archipelago (Fig. 5). Extensive quartz sand environments occur within 4 km of the shoreline. This sediment is likely derived from reworked coastal deposits and transported to the east by the seasonal longshore current (Sanderson et al., 2000; Fig. 5). Numerous topographic highs in the Recherche Archipelago form a large, morphologically complex littoral zone (Fig. 5). Consequently, in comparison to the majority of the GAB, there is a large nearshore area available for colonisation by shallow-water
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CaCO3 producing organisms, including rhodoliths (Foster, 2001). This, coupled with warm current flows and the presence of subtropical biota (Cann and Clarke, 1993; Li et al., 1999), creates a potentially productive shallow-water CaCO3 ‘factory’, which effectively dilutes proportions of relict sediment grains (James et al., 2001). Bathymetric data show that much of this biogenic sediment accumulates on the lower energy, leeward sides of bedrock islands (Fig. 4E,F). Terrigenous sediment derived from the weathering of the granitic outcrops also contributes significantly to the leeward accumulations (Figs. 3F and 5). Low profile reefs correspond to the ‘shaved shelf’ environments described on other parts of the GAB, and probably experience negligible sediment accumulation (James et al., 2001). Linear structures in the limestone (Fig. 4B) may represent the eroded remnants of subaerial or shoreline calcarenite that formed during periods of lower sea level (Fig. 5). The presence of outer bay asymmetric sediment dunes indicates that CaCO3 material from the middle shelf is being transported inshore, and may be supplementing the inner shelf sediment budget. Overall, the distribution of physical environments on the Recherche inner shelf appears to be primarily controlled by wave energy. As a consequence, sediment accumulation is restricted to the littoral zone and in the lee of islands, and there are large tracts of seabed with exposed limestone hardgrounds. 5.2. Depositional environments and habitat characteristics
Fig. 4. Selected examples of multibeam bathymetry within Esperance Bay (Fig. 1). (A) High profile reef, south of Long Island; (B) Low profile reef, north of Remark Island; (C) Seagrasses, southeast of Woody Island, with a bathymetric profile; (D) Subaqueous dunes, south of Long Island, and bathymetric profile; (E) Sediment apron leeward of Canard Island, and bathymetric profile; and (F) Bathymetry surrounding Remark Island, with bathymetric profiles. A to D are shown in greyscale with a 45 sun angle; E and F are shown on a coloured scale.
The PCA (Fig. 2B) shows that the relationship between sediment parameters and benthic habitats is complex, with sediment sorting and mud content showing the strongest correlation to benthic habitats. As suggested in previous studies, sediment grainsize alone is not sufficient as a surrogate for species distribution (Post et al., 2006). Although the majority of seagrass in Esperance Bay occurs on the nearshore quartz sand environment (Fig. 3G), seagrass areas also occur in the lee of islands, on a substrate of coarse CaCO3 sand (Fig. 1 and 3F). Therefore, seagrass distribution in Esperance Bay is primarily related to wave exposure rather than sediment type. All seagrass sites have a low proportion of mud, demonstrating that even dense seagrass beds in Esperance Bay are unable to trap fine sediment. Rhodolith habitat is characterised by exposure to moderate wave energy. This is consistent with previous studies which suggest that in low energy environments rhodoliths are buried, whereas in higher energy conditions they tend to be removed (Marrack, 1999; Foster, 2001). Unsurprisingly, the rhodolith substrates comprise CaCO3 gravel, which comprises broken rhodolith thalli. However, the <2 mm fraction of rhodolith samples tends to be poorly sorted and contains significant quantities of mud (Table 1, Fig. 2B). The high rugosity of these beds appears to trap fine sediment, and provides three dimensional microhabitat for numerous other organisms (Fig. 3E; Foster, 2001). Limestone and granitic reef habitats occur in various depths and wave energy settings and form habitat for a wide range of
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Fig. 5. Conceptual model of the geomorphic processes in Esperance Bay, including plan view and cross section (A-A0 ).
sessile organisms (Kendrick et al., 2005; Figs. 1 and 3). High rates of sediment production and transport has resulted in the partial burial of low profile reef, particularly inshore (Fig. 5). Mobile bedforms on sections of this reef show that these areas represent unstable habitat that is subject to intermittent burial. In lower energy settings it has been shown that bioclastic material may accumulate and eventually permanently bury sessile biota (James et al., 2001). 6. Summary and conclusion In Esperance Bay, exposure to wave energy appears to be the most useful regional scale predictor of the distribution of rhodolith and seagrass habitats, and also controls the distribution of unconsolidated substrate. Limestone reefs provide habitat for a wide range of sessile organisms, even in very high wave exposure environments. Although these habitats are intermittently inundated by sediment, periodic erosion may prevent sessile biota from being permanently smothered by their own sediment. Rhodolith beds comprise poorly sorted sediment that contains large proportions of gravel, mud, and CaCO3. Although fine sediments are typically rare in ‘shaved shelf’
environments (James et al., 2001), high rugosity rhodolith beds, rather than seagrass beds, provide the only environment in which mud can be trapped. The lower energy environments and extensive sediment accumulations in Esperance Bay contrast strongly with the heavily storm influenced and nondepositional inner shelf of the GAB (James et al., 2001). This study demonstrates that a process based conceptual understanding provides useful insights into the relationships between physical processes and habitat distribution. Our results reveal that in the Recherche Archipelago, wave abrasion coupled with localised accumulation of CaCO3 sediment, and a complex sediment transport system, play a major role in increasing the diversity of inner shelf benthic habitats. This work highlights the value of assessing geomorphic processes in order to better understand the distribution and structure of benthic habitats. Acknowledgements Many thanks to: David Johnson for wave modelling; Tony Watson and Alex McLachlan for sedimentology assistance; Jonathon Clarke, Alix Post, Vicky Passlow, Yvonne Bone, Lynda Radke, and Alan Orpin for useful comments; and Paul
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Rose, Simon Grove, and Marcus Gray for field assistance. This study was supported by the Cooperative Research Centre for Coastal Zone, Estuary and Waterway Management. Parts of this study were funded by the Fisheries Research and Development Corporation (FRDC) project 2001/060. Published with permission of the Executive Director, Geoscience Australia. References Ashley, G.M., 1990. Classification of large-scale subaqueous bedforms: a new look at an old problem. Journal of Sedimentary Petrology 60, 160e172. Cann, J.H., Clarke, J.D.A., 1993. The significance of Marginopora vertebralis (Foraminifera) in surficial sediments at Esperance, Western Australia, and in last interglacial sediments in northern Spencer Gulf, South Australia. Marine Geology 111, 171e187. Diaz, R.J., Solan, M., Valente, R.M., 2004. A review of approaches for classifying benthic habitats and evaluating habitat quality. Journal of Environmental Management 73, 165e181. Foster, M.S., 2001. Rhodoliths: Between rocks and soft places. Journal of Phycology 37, 659e667. Freeman, S.M., Rogers, S.I., 2003. A new analytical approach to the characterisation of macro-epibenthic habitats: linking species to the environment. Estuarine, Coastal and Shelf Science 56, 749e764. Hemer, M.A., 2006. The magnitude and frequency of combined flow bed shear stress as a measure of exposure on the Australian continental shelf. Continental Shelf Research 26, 1258e1280. James, N.P., Bone, Y., Collins, L.B., Kyser, T.K., 2001. Surficial sediments of the Great Australian Bight: facies dynamics and oceanography on a vast cool-water carbonate shelf. Journal of Sedimentary Research 71, 549e567. Kendrick, G.A., Harvey, E., McDonald, J., Pattiaratchi, C., Cappo, M., Fromont, J., Shortis, M., Grove, S., Bickers, A., Baxter, K., Goldberg, N., Kletczkowski, M., Butler, J., 2005. Characterising the fish habitats of the
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