Spatial and temporal patterns in the distribution of ... - CSIRO Publishing

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Marine and Freshwater Research, 2015, 66, 41–49 http://dx.doi.org/10.1071/MF13209

Spatial and temporal patterns in the distribution of large bivalves in a permanently open, temperate estuary: implications for management Alan J. Kendrick A,D, Michael J. Rule A,C, Paul S. Lavery B and Glenn A. Hyndes B A

Marine Science Program, Department of Parks and Wildlife, Locked Bag 104, Bentley Delivery Centre, WA 6983, Australia. B Centre for Marine Ecosystems Research, School of Natural Sciences, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia. C Oceans Institute, University of Western Australia, Hackett Drive, Crawley, WA 6009, Australia. D Corresponding author. Email: [email protected]

Abstract. To inform management, baseline ecological studies in estuaries must be implemented at spatial scales that accommodate both environmental gradients and likely anthropogenic pressures. We describe fine-scale spatial patterns in the abundances and size structure of large infaunal bivalves inhabiting shallow sand habitats in the lower reaches of a relatively undisturbed, permanently open, temperate estuary. Sampling over 3 years at 19 sites during the autumn, when freshwater influence was minimal, revealed that Soletellina alba, Wallucina assimilis and Paphies elongata were consistently the most abundant of nine species present. Although most abundant near the ocean entrance, S. alba was distributed widely and shells of differing lengths, and presumably ages, were present at most sites, suggesting that this species recruited continuously throughout the study area when conditions were appropriate. In contrast, W. assimilis and P. elongata occurred only near or in the entrance channel of the Nornalup Inlet in areas where seagrass rhizomes may grow and where oceanic influences caused relatively turbulent conditions respectively. Sediment structure appeared to exert only a moderate and intermittent influence on the bivalve assemblage at some sites where particularly large grain sizes occurred. This study provides important baseline information on the distribution and abundance of large bivalves in this estuary. These species are likely to be important in the trophic ecology of this system and are potential indicators of disturbance and ecosystem health. Additional keywords: conservation, monitoring, soft-sediment. Received 5 August 2013, accepted 19 March 2014, published online 26 November 2014

Introduction Temperate estuaries are now commonly subject to high levels of direct and indirect human disturbance, such as fishing, shoreline modification and catchment degradation (Kennish 2002). Such effects have caused significant environmental changes in many temperate estuaries and major declines of some species have occurred (Beck et al. 2011). Effective management of such environments must include monitoring that provides a quantitative understanding of resource ‘condition’ and the significance of both natural variation and anthropogenic threats acting on that resource over time (Fancy et al. 2009). Macrobenthic invertebrate communities are often employed as indicators of estuarine ecosystem condition as they can be sensitive to human disturbance (Wildsmith et al. 2009, 2011). These communities can play an ecologically significant role in estuaries by removing nutrients from the water column, creating habitat for other organisms and providing an important prey resource for other animals (Potter and Hyndes 1999; Gutie´rrez et al. 2003; Beck et al. 2011; Kellogg et al. 2013). Gradients in Journal compilation Ó CSIRO 2015

physical factors, such as salinity and sediment structure, have been shown to influence the spatial distribution and composition of estuarine invertebrate assemblages (e.g. Kennish et al. 2004; Gime´nez et al. 2014). The ability of bivalves to burrow into sediments of differing grain sizes, for example, can affect their distribution (Alexander et al. 1993). Furthermore, faunal distributions also vary temporally as salinity fluctuates with hydrological cycles (Attrill and Thomas 1996). Marine species, for example, may enter the lower reaches of estuaries as adults, juveniles or early life stages when marine-like conditions exist and persist there until events like sand-bar closure or winter flooding make environmental conditions unsuitable (Matthews and Constable 2004). Understanding the spatial distributions of estuarine fauna and how those distributions vary over time, both naturally and in response to human influences, is an important requirement of management and a precursor to effective long-term resource monitoring. However, such baseline studies of temperate estuarine invertebrate assemblages are often undertaken at a www.publish.csiro.au/journals/mfr

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Marine and Freshwater Research

A. J. Kendrick et al.

N 0 215 430

860

1290

1720

Metres

1 2 10 9 3

Nornalup Inlet 4 8 5

11 7

6

14

12

17 15 18 16

13

19 Western Australia

Fig. 1. Map of 19 shallow sand sampling sites in the Walpole and Nornalup Inlets Marine Park.

relatively coarse spatial scale (e.g. Platell and Potter 1996; Dye and Barros 2005), despite the significant environmental gradients and fluxes that typically occur in these systems. Anthropogenic impacts on estuaries may also occur at widely differing spatial scales, ranging from broad urban or catchment influences to localised point-source inputs (Kuk-Dzul et al. 2012). For this reason, baseline ecological studies that inform the development of long-term monitoring in estuaries must be implemented at spatial scales that accommodate both environmental gradients and likely anthropogenic pressures. In this study, we undertook a 3-year baseline study of finescale spatial patterns in the abundances, and in some instances size structure, of large (.10 mm) infaunal bivalves inhabiting shallow subtidal sand-flat habitats in a relatively undisturbed, permanently open, temperate estuary. We also examined the extent to which sediment grain structure could explain the distribution of the bivalve assemblage and discuss the study outcomes in relation to the long-term management of this and other estuaries. Methods This study was undertaken in the Walpole and Nornalup Inlets Marine Park (35.018S, 116.738E), a 1442 ha wave-dominated

estuary on Western Australia’s southern coast (Fig. 1). Comprising two connected basin inlets and the tidal reaches of three rivers, the inlets are permanently open to the sea because of a relatively high local rainfall (,1300 mm annually) and the presence of a rocky headland that prevents a wave-deposited sand-bar forming at the estuary mouth (Hodgkin and Hesp 1998). Marine-like conditions persist within the lower reaches of the estuary except during periods of typically winter freshwater discharge (Hodgkin and Clark 1999). Sampling was undertaken in the austral autumn following the typically dry summer to minimise the chances that bivalves in the inlets had recently been influenced by freshwater discharge. Annual rainfall in the region was approximately average during the study period and monthly river flow data indicated that no significant unseasonal flows occurred (BoM 2014; DoW 2014). In each April of 2011, 2012 and 2013, large (.10 mm) bivalves were sampled at 19 sites located on the large areas of shallow (,0.5 m) subtidal sand-flat that are present around the periphery of the Nornalup Inlet and along the sides and centre of the entrance channel (Fig. 1). Sampling was not conducted in the smaller Walpole Inlet as it had little equivalent sand-flat habitat. The shoreline length of the sampling sites varied from ,250 to 1000 m and depended on the extent of appropriate shallow sand and the

Fine scale estuarine bivalve distributions


43

Table 1. The total number and percentage contribution (in parentheses) of each bivalve species in the total catch from shallow sand habitats of the Walpole and Nornalup Inlets Marine Park in 2011, 2012, 2013 and in all 3 years combined

Soletellina alba Wallucina assimilis Paphies elongata Katelysia scalarina Spisula trigonella Soletellina sp. Macomona deltoidalis Venerupis crenata Laternula creccina

2011

2012

2013

Total

1754 (69.60) 603 (23.93) 105 (4.17) 24 (0.95) 18 (0.71) 0 13 (0.52) 1 (0.04) 2 (0.08)

2502 (80.04) 454 (14.52) 123 (3.93) 17 (0.54) 0 22 (0.70) 4 (0.13) 4 (0.13) 0

1873 (74.98) 521 (20.86) 48 (1.92) 29 (1.16) 15 (0.60) 10 (0.40) 2 (0.08) 0 0

6129 (75.26) 1578 (19.38) 276 (3.39) 70 (0.86) 33 (0.41) 32 (0.39) 19 (0.23) 5 (0.06) 2 (0.02)

presence of rock outcrops, headlands and shoreline structures. Five randomly selected samples were collected from each of the 19 sites by digging the substratum from within a 1.0-m2 quadrat to a depth of ,0.3 m and sieving the contents to 5 mm, which equated to the approximate height of a 10 mm long Soletelina alba. Pilot sampling had shown that additional bivalves were rarely collected by digging deeper than 0.3 m and that the adult forms of the species encountered would be retained by 5-mm mesh. The retained bivalves were identified in the laboratory and the maximum length of each undamaged shell was measured to 0.1 mm. Abundance data from each site were expressed as the mean number of bivalves per square metre and, for the most abundant species, box plots were compiled from the length data showing the 25th/75th, 10th/90th and 5th/95th percentiles at different sites when sample sizes permitted. Differences in the bivalve assemblages were analysed using PRIMER v6 with PERMANOVAþ (Clarke and Warwick 2001; Anderson et al. 2008). Abundance data were transformed (see Table 2) and a Bray–Curtis similarity matrix was generated. Patterns in the overall bivalve assemblage were examined using non metric multi-dimensional scaling (nMDS) and cluster analysis. Bivalve data were also analysed with PERMANOVA with tests based on 999 permutations and using year (3 levels) and site (19 levels) as random factors. Differences in the abundance of individual major species at the sites at which they occurred were also tested with the same PERMANOVA model, but with matrices based on Euclidean distances (Anderson et al. 2008). In all instances, data were tested for heterogeneity of dispersion using PERMDISP, which is equivalent to Levene’s test for homogeneity of variances. Significance was accepted at P , 0.05 except in cases when transformation failed to produce homogeneity of dispersion and significance was accepted at P , 0.01. Data collected before this study, in 2010, were used to examine broad-scale sediment characteristics across the study area. Six replicate 5 cm diameter and 20 cm deep core samples were taken haphazardly across the extent of shallow sand flats in each of five broad regions that comprised the entrance channel and the south-west, south-east, north-west and north-east aspects of the Nornalup Inlet (n ¼ 30 samples in total). These broad regions overlie our bivalve sampling sites such that the entrance channel encompassed sites 12–19, the south-west region covered sites 6–7, the south-east region covered sites 3–11, the north-west region covered sites 8–10 and the north-

east region covered sites 1–2. Sediment samples were dried at 608C until their weights stabilised and then sieved for 20 min into 4-mm, 2-mm, 1-mm, 500-mm, 250-mm, 125-mm, 63-mm, 32-mm and ,32-mm fractions using a mechanical shaker. Mean grain size and sorting characteristics, based on the percent of each sediment sample retained in each sieve, were calculated using the Gradistat v7 program (Blott and Pye 2001) and plotted for each of the five broad regions. In eight instances, the above broad-scale sediment samples collected in 2010 were at the same sites where bivalves were subsequently sampled in 2011–13 (i.e. sites 1, 2, 4, 5, 6, 9, 10 and 12), providing an opportunity to determine if a relationship existed between the bivalve assemblage and sediment composition. This comparison assumes that the sediment structure documented in 2010 remained the same during 2011–13 when the bivalves were collected. Our observations suggest that the sediment structure of this estuary is stable and this comparison is valid, particularly as no significant disturbance event, such as flooding, occurred during the study period. The RELATE function in PRIMER v6 was used to compare Bray–Curtis similarity matrices derived from the bivalve assemblages collected at these eight sites in each year with the Euclidean distance similarity matrix derived from the percentage composition of the different sediment fractions at the same sites. This procedure tested the hypothesis that no relationship existed between the multivariate pattern from the bivalve assemblages and sediment composition and the matrices were considered to be significantly correlated if P , 0.05. The degree of correlation between the data can be gauged by the Spearman coefficient (r) which ranges from 1 (i.e. complete opposition) to 1 (i.e. complete agreement), with a value of zero indicating no correspondence (Clarke and Warwick 2001). In instances where matrices were significantly correlated, the BEST procedure was used to determine which sediment variable(s) best grouped the data with patterns in the bivalve assemblage. To illustrate which sites had sediment characteristics that contributed to the differences in the bivalve assemblage, the percentage contributions of these sediment variables were overlaid as circles of proportionate size on nMDS plots produced from the bivalve data. Results In total, 8144 bivalves were collected over the 3 years of this study, among which Soletellina alba (n ¼ 6129; 75.3%),

44



150

Table 2. Results of multivariate and univariate PERMANOVA analysis to test between year and site Significance was accepted at P , 0.05 except for the bivalve assemblage and Wallucina assimilis where data were not transformed and significance was accepted at P , 0.01. Data for Soletellina alba and Paphies elongata were fourth root and square root transformed respectively

Bivalve assemblage

Soletellina alba

Wallucina assimilis

Paphies elongata

Year Site Year Site Residual Year Site Year Site Residual Year Site year Site Residual Year Site Year Site Residual

d.f. 2 18 36 206 2 18 35 193 2 12 20 58 2 4 6 15

MS 7783.8 10408 3056.1 1714.8 1357.6 930.73 243.2 161.94 1025.5 5582.7 1640.2 1626.6 1.0021 8.0599 0.40534 1.6894

Pseudo-F

P(perm)

2.6013 3.4316 1.7822

0.007 0.001 0.001

5.7436 3.8548 1.5018

0.003 0.001 0.038

0.6261 3.4058 1.0083

0.679 0.004 0.491

1.5876 14.189 0.23993

0.306 0.011 0.953

100 50 0 150

Mean density

Source

2011

2012

100 50 0 150

2013

100 50 0 1

Wallucina assimilis (n ¼ 1578; 19.4%) and Paphies elongata (n ¼ 276; 3.4%) were the most abundant species overall, collectively accounting for 98.1% of the total catch (Table 1). The numbers of S. alba contributing to the catch of a single year ranged from 1754 (69.6%) in 2011 to 2502 (80.0%) in 2012, while numbers of W. assimilis and P. elongata across the 3 years ranged from 454 (14.2%) to 603 (23.9%) and from 48 (1.9%) to 123 (3.9%) respectively. All other species contributed ,1.0% to the total catch or to the catch of a single year, with the exception of Katelysia scalarina which amounted to 1.2% of the 2013 catch. A relatively uncommon species identified as Soletellina sp. appeared to be juveniles and have been tentatively identified as S. biradiata (Willan 1993). PERMANOVA showed that the bivalve assemblage varied significantly by site and by year, and that a site/year interaction was also significant (Table 2). However, nMDS and cluster analysis did not show clear patterns in the separation of bivalve assemblage samples by site and these plots are not included. Univariate analysis showed that there was significant variability of S. alba by year and by site and that an interaction between these factors was also present (Table 2). While S. alba was collected at all sites sampled in this study, the mean abundance (1 s.e.) of this species ranged from zero at site 1 in 2011 to 99.0 21.3 m2 at site 14 in 2012, and it tended to be most abundant on sand-bars in the entrance channel (i.e. sites 14 and 15; Figs 1, 2). The sizes of S. alba collected during this study rarely exceeded 35 mm in length, while the lack of small shells (i.e. ,10 mm in length) was most likely because they were ,5 mm high and would not have been consistently retained in the sieves used in the study (Fig. 3). Despite collecting only larger individuals of S. alba, a relatively wide size range of ,20 mm was frequently evident for this species at most sites during the sampling period when sample sizes were not small (Fig. 3). PERMANOVA also showed that there was significant variability in the densities of W. assimilis and P. elongata by site, but

2

3

4

5

6

7

8 10 9 11 12 13 14 15 16 17 18 19

Site Fig. 2. Mean densities per square metre (þ1 s.e.) of Soletellina alba collected from 19 shallow sand sites in the Walpole and Nornalup Inlets Marine Park during April of 2011, 2012 and 2013 (n ¼ 5 in all instances).

not by year and an interaction between these factors was not present (Table 2). Owing to the lack of annual or interactive effects, data for each of these two species were pooled across the 3 years and presented for sites only (Fig. 4). Wallucina assimilis occurred at 13 of the 19 sites, but was most abundant at sites towards the seaward end of the Nornalup Inlet (i.e. sites 6, 11, 12 and 17) (c.f. Figs 1, 4). The mean abundance (1 s.e.) of this species when it was present ranged from 0.2 0.1 m2 at site 16 to 35.1 7.4 m2 at site 12. Although present at five sites, P. elongata was abundant only close to the ocean entrance (i.e. sites 15 and 16) (c.f. Figs 1, 4). The mean abundance (1 s.e.) of this species when it was present ranged from 0.3 0.2 m2 at sites 13 and 18 to 14.6 2.9 m2 at site 16. Sediment samples from shallow habitats at the entrance channel and south-west, south-east, north-west and north-east reaches of the Nornalup Inlet predominantly comprised fine to coarse sands (i.e. 125 mm , grain size , 1 mm) (Blott and Pye 2001) and grain sizes of ,125 mm or .1 mm rarely contributed .1% to the sediment structure (Fig. 5). While the modal grain size class was 125 mm at the entrance channel and the south-east and north-east regions of the Nornalup Inlet, it increased to 250 mm at the south-west and north-west regions. Sediments across these five broad-scale regions ranged from moderately well sorted to moderately sorted (Blott and Pye 2001), indicating that sediment particles were neither highly uniform nor highly variable in size across the study area (Fig. 5). The mean sorting values differed significantly (ANOVA; F4,25 ¼ 33.11, P , 0.001) and Scheffe’s test indicated that the mean sorting values (1 s.e.) of the entrance channel (1.55 0.03) and the south-west (1.53 0.02) and south-east (1.55 0.02) regions were not significantly different, but that these differed from both northern regions of the Nornalup Inlet. The mean sorting values of the north-east (1.97 0.04) and north-west (1.85 0.06) regions were not significantly different from each other, but



45

2011 5

40

27

30

120

18

59

11

92

36

49

17 20

92 126

64 445 360 104 8

7

10

2012 Shell length (mm)

32 40

43

22

30

189 119

20

96

130 104 161

20

486 175

243 189

50

56 108

98

92

10

2013 46 42

40 26

67 113

30

47

35

191

75

30

258 193 32 154 147 152

16

108

20

2

10 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

Site Fig. 3. Box plots of the length of Soletellina alba collected from 19 shallow sand sites in the Walpole and Nornalup Inlets Marine Park during April of 2011, 2012 and 2013 (boxes ¼ 25th/75th percentile with median, whiskers ¼ 25th/75th percentile and ¼ 5th/95th percentile. Sample sizes are above each plot). 50

in 2013 (r ¼ 0.44) indicated that this relationship was not strong. For the 2013 bivalve assemblage data, BEST analysis showed that the maximum correlation of r ¼ 0.63 occurred in relation to the 1- and 2-mm sediment fractions. Bubble plots on the bivalve assemblage data from 2013 showed that the 1- and 2-mm sediment fractions occurred on the right side of the plot, primarily at sites 1, 2, 9 and 10 and at 1 and 10 respectively (Fig. 6).

Wallucina assimilis

Mean density

25

0 20

Paphies elongata

10

0 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19

Site Fig. 4. Mean densities per square metre (þ1 s.e.) of Wallucina assimilis and Paphies elongata collected from 19 shallow sand sites in the Walpole and Nornalup Inlets Marine Park. Data have been pooled across years (n ¼ 15 in all instances).

differed from all other regions. These findings indicate that shallow sand habitats of the northern part of the Nornalup Inlet had a greater spread of sediment grain sizes around the average than did sediments from the other three regions. When yearly bivalve assemblage data were compared to the composition of sediment samples taken from the same sites, there was no significant relationship in 2011 (P ¼ 0.41, r ¼ 0.03) or 2012 (P ¼ 0.42, r ¼ 0.01), but there was in 2013 (P ¼ 0.03). The correlation between bivalve and sediment data

Discussion Distribution and size of abundant species By intensive spatial sampling over 3 years, this study has demonstrated that the dominant large infaunal bivalve species inhabiting shallow sand-flats in the lower reaches of this permanently open estuary exhibited markedly different patterns of distribution. The three most abundant species were found predominantly or in greatest abundance at sites at the entrance channel and the southern reaches of the Nornalup Inlet, highlighting the importance of these shallow sand areas as bivalve habitat relative to similar sand habitats further inside the estuary basin. The dominance of S. alba, W. assimilis and P. elongata in this bivalve assemblage is notable as these species are not exclusively estuarine and also occur in inshore marine waters across southern Australia (Willan 1993; Barnes and Hickman 1999; Edgar 2008). Their presence emphasises the typically strong marine influence in the lower reaches of this and other permanently open estuaries in this region (Chalmer et al. 1976; Hodgkin 1978) compared with those that are seasonally open, (Tweedley et al. 2012).

46


80


Stress: 0.03

Entrance channel 6

40

1

0 80

South-east 12

40

2 10

% contribution

0

9

80

North-east

40

4

5

0 80

Stress: 0.03

South-west 6

40

1

0 80

North-west 12

40

2 10

0

9

⬍32 µm 32 µm 63 µm 125 µm 250 µm 500 µm 1 mm

2 mm

4 mm

Sediment grain size 4

2.0 1.5 1.0

t

t

or N

ut So

th

h-

-w

w

es

es

as th or N

So

ut

h-

-e

ea

ne an ch e

Fig. 6. Two-dimensional nMDS plots created from the bivalve assemblage at eight sites in the Walpole and Nornalup Inlets Marine Park during April of 2013 overlaid with circles representing the relative proportion of 1 mm (top) and 2 mm (bottom) sediment fractions at those sites.

En

tra

nc

t

st

0.5

l

Mean sorting

5 2.5

Region Fig. 5. The mean composition (þ1 s.e.) of sediments, expressed as percentages of ,32-mm, 32-mm, 63-mm, 125-mm, 250-mm, 500-mm, 1-mm, 2-mm and 4-mm grain-size fractions (top) and mean sorting characteristics (þ1 s.e.) (bottom) of shallow sand habitats at the entrance channel and south-east, north-east, south-west and north-west regions of the Nornalup Inlet (n ¼ 6 for all means).

Soletellina alba ranging in length from ,10–30 mm occurred at most of the sites during the 3 years of this study, suggesting that shells of varying ages were typically present at each site at this time of year. This suggests that recruitment of S. alba may occur often or continuously across these shallow sand habitats when mature sized shells are present and conditions are appropriate. In the present study, such conditions may have occurred for ,6 months before we sampled in the austral autumn (i.e. April), as the region has mainly winter rainfall and the Nornalup Inlet typically has relatively stable, marine-like conditions in other seasons (Hodgkin and Clark 1999). This finding concurs with Matthews and Fairweather (2003), who suggested that this thin-shelled and fast growing species can attain sexual maturity at a length of ,16 mm in ,4 months under optimal conditions, enabling them to grow and reproduce

in estuaries before they are affected by winter freshwater discharge. Sediment structure and bivalve distributions Soletellina alba was the only bivalve species consistently present at sites on the northern shore of the Nornalup Inlet (i.e. sites 1, 2, 9 and 10) where a moderate relationship between the bivalve assemblage and sediment composition was present in 2013, but not in the previous 2 years. Indeed, the number of bivalve species at these sites was typically low and during 2013 S. alba was the only species present at sites 1 and 9, which differed from all others in that coarse sediment fractions (i.e. .1 mm) comprised up to ,7% of the sediment structure. It may be that the relatively coarse sediments at these northern sites to some extent inhibit the settlement or recruitment of bivalves, although Matthews and Fairweather (2006) found no evidence that S. alba recruitment varied with regard to sediment grain size in a seasonally open, temperate estuary over a period of ten days. However, sediment grain size can affect the rate at which bivalves can burrow to avoid predators (Alexander et al. 1993) and it may be that predation on bivalves is greater in these coarser sediments in the study area. If so, however, this effect must not have been sufficiently strong to be apparent in each year. Alternatively, the distributions of adult bivalves across


shallow sand habitats in the estuary may be influenced by patterns of water circulation and tidal flow which transport pelagic early life stages. Such processes have been shown to influence the distribution of pelagic fish larvae in this system (Neira and Potter 1994), and differences in water movement may partly explain, for example, why the widespread S. alba tended to be more abundant at sites on the entrance channel sandbars compared to those at the northern side of the estuary where coarser sediments were present. The more limited distributions of W. assimilis and P. elongata in the study area were clearly not related to sediment composition. In both instances, the densities of these species changed markedly over relatively small spatial scales (i.e. hundreds of metres) in parts of the inlet uniformly dominated by 125–250-mm grain sizes and where there was clearly no appreciable change in sediment structure. It is more likely that the particular prevalence of W. assimilis at sites near the seaward end of the Nornalup Inlet relates to the presence of rhizomes of the ephemeral seagrass Zostera polychlamys, which occurs in this part of the system (Huisman et al. 2011). Previous studies have noted an association between lucinid bivalves and marine angiosperms (Van der Heide et al. 2012), and W. assimilis has been recorded in very high densities in sediments of marine Posidonia and Amphibolis meadows (Poore and Rainer 1974, Barnes and Hickman 1999). Occurring mostly close to the oceanic end of the entrance channel (i.e. site 16), the distribution of P. elongata is consistent with the well documented affinity of Paphies for high-energy sandy beach surf zones where they rapidly re-burrow as sands are re-distributed by wave action (McLachlan et al. 1995). Unlike the more typical low-energy and relatively stable shallow sand habitats within the Nornalup Inlet, the oceanic end of the entrance channel is characterised by high water turbulence and sediments that are constantly re-worked due to the influences of oceanic wave action, tidal flows and estuarine discharge. Management implications Understanding the ‘condition’ of biodiversity resources over time in relation to the natural processes and anthropogenic pressures acting on the resource is paramount to informing and assessing the effectiveness of current management and development of new management strategies (Fancy et al. 2009). In this regard, we provide a quantitative baseline understanding of the diversity and abundance of large infaunal bivalves on shallow subtidal sand-flat habitats of the lower reaches of this largely unmodified (NLWRA 2002) estuary. The prevalence of bivalves in shallow sand habitats at the southern part of the Nornalup Inlet and the entrance channel compared to similar habitats in the inlet basin should be noted by managers as this part of the system is also subject to high recreational and tourism activity (DEC 2009). While most visitor activities other than collecting are unlikely to directly affect these typically buried species, managers should also consider possible indirect effect, such as the disturbance of seagrass rhizomes or animals that may gather to feed on bivalves in this relatively confined area. Numerous fishes feed on bivalves (Sarre et al. 2000, Platell et al. 2006), including elasmobranchs like the southern eagle ray Myliobatis australis (Sommerville et al. 2011), which are common in the Nornalup Inlet and make the system unique


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compared to other estuaries in the region (Potter and Hyndes 1994). The prominence of S. alba as the only large bivalve that is both abundant and widely distributed throughout shallow sand habitats in this system also highlights the limited functional redundancy that is common in estuaries (Elliott and Whitfield 2011). Managers should be aware that disturbances that affect this bivalve species may therefore have broader ecosystem implications for the estuary. This study also highlights a common problem faced by environmental managers who are expected to assess the ‘condition’ of natural attributes in managed conservation estate with limited or no historical data and insufficient knowledge of how natural processes and anthropogenic impacts influence the resource. For example, the collection of only 17 to 29 ridged cockle Katelysia scalarina in each year of this study contrasts markedly with local anecdotal evidence that this fished species was much more abundant in shallow sand habitats of this estuary until recent decades (DEC 2009). Its apparent prevalence in the Nornalup Inlet was noted in a 1925 newspaper: ‘Fine, fat cockles, which can literally be scooped up by the handful, and which make very fine eating, both for fish and hungry campers’ (Western Mail, 26 February 1925, p. 34). The cause of this apparent decline is unknown, but may have been influenced by sustained fishing pressure, a major flood event in 1982 or altered hydrology caused by region-wide drying climatic conditions (Hope et al. 2006). While baseline studies are important (Tweedley et al. 2012), a better understanding of natural processes and anthropogenic impacts are also needed to make informed management decisions. In this and other managed estuarine and marine managed areas, research should be guided by a structured analysis of knowledge gaps that will assist management. Acknowledgements For their assistance in the field, thanks are extended to Shannon Armstrong, Justin Ettridge, Shaun Ossinger, Kathryn McMahon, Christin Sawstrom and Edith Cowan University’s 2011–13 Coastal and Marine Management students. Shelby Noble and Federico Vitelli processed the samples, Jennifer Higbid assisted with figures and George Kendrick, Anne Brearley and Shaun Wilson provided taxonomic expertise and comments on the draft. This manuscript benefited from the comments of two anonymous referees.

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