Effects of tidal elevation and substrate type on settlement and postsettlement mortality of the Sydney rock oyster, Saccostrea glomerata, in a mangrove forest and on a rocky shore.
KEY WORDS: conspecific, habitat, oyster, settlement, supply-side ecology, zonation, Saccostrea glomerata
A fundamental concern of ecology is the identification of factors that determine the distribution and abundance of organisms. For sessile organisms, such as many marine invertebrates, movement as an adult is not possible and patterns of distribution can only be determined by the settlement of propagules (i.e., larvae) or postsettlement survival of individuals through to adult stages (Keough & Downes 1982, Connell 1985). Settlement is dependent on the availability of competent propagules, propagule dispersion by water and wind, and, for animals, active site selection (reviewed by Keough and Downes (1982)). Postsettlement mortality is dictated by abiotic conditions as well as inter- and intraspecific interactions, such as competition, predation, and facilitation (Keough & Downes 1982). The net effect of successful settlement and survival is recruitment (sensu Keough & Downes 1982) to larger age or size classes.
On intertidal shores, many species of algae and invertebrates display clearly defined vertical distributions (e.g., Connell 1972, Lubchenco 1980, Peterson 1991) that decades of studies have sought to explain (e.g., Knight-Jones 1953, Connell 1961, Lubchenco 1980). Some have focused on the role of settlement in shaping distributions, explaining distributions in terms of the tidal inundation gradient of time available for water-borne larvae to settle (e.g., Pineda 1984), and the distribution of adult populations that release settlement cues (Knight-Jones 1953, Wethey 1984). Others have explained distributions in terms of postsettlement processes, suggesting that the upper distributional limits of many intertidal species are set by their tolerance of heat and desiccation (Connell (1972), but see Underwood (1980) for an example where this is not the case), and the lower limits by biological interactions such as competition and predation (Connell 1961, Connell 1972, Raffaelli & Hawkins 1996).
The Sydney rock oyster (Saccostrea glomerata) is a common intertidal species of eastern Australia. In estuaries of New South Wales, S. glomerata is the dominant species on rocky intertidal shores, forming patches of 90-100% cover, and is also abundant in mangrove forests, where it attaches to the pneumatophores (peg roots) and trunks of mangrove trees, forming dense aggregations (Chapman & Underwood 1995, Bishop et al. 2010). Like many other sessile marine invertebrates, S. glomerata displays a pattern of decreasing abundance with increasing tidal elevation, on rocky shores and in mangroves (Bishop et al. 2010).
The vertical distribution of Saccostrea glomerata may, like that of many other species of bivalve and barnacle, reflect greater larval settlement at low tidal elevations compared with mid or high tidal elevations (e.g., Grosberg 1982, Minchinton & Scheibling 1991, Raimondi 1991, Roegner & Mann 1995). Supply-side ecology predicts that greater tidal inundation increases the time available for larvae to settle, and this pattern may be reinforced where adult populations low on the shore produce settlement cues that lead to gregarious settlement (Hidu 1969, Hidu et al. 1978, Turner et al. 1994) or that enhance the surface area of substrate available for attachment (Summerhayes et al. 2009). Alternatively, differential rates of S. glomerata mortality among tidal elevations and substrate types may play a more important role. Tidal emersion reduces the time available for suspension-feeding invertebrates to feed (e.g., Hatton 1938, Barnes & Powell 1953, Suchanek 1978) and, in the absence of refugia, can enhance physiological stress (Raffaelli & Hawkins 1996). Furthermore, the complex habitat provided by adult conspecifics low on the shore may reinforce patterns set at settlement by providing refugia from predation, and moderation of physical stress (Gutierrez et al. 2003).
In this study, we assessed the relative importance of larval settlement and postsettlement mortality in setting the vertical distribution of Saccostrea glomerata on a rocky shore and in a mangrove forest. We also assessed how substrate type (bare, with oyster shells, or with live oysters) modulates the establishment of the vertical distribution of S. glomerata. We hypothesized that (1) new S. glomerata recruits to mangrove forests and rocky shores would display a spatial pattern of decrease with increasing tidal elevation regardless of substrate type, but would also display a pattern of greater abundance among live oyster cover than on bare substrate or among dead oysters; (2) the abundance of recruiting oysters would diminish over time as a consequence of cumulative mortality; and (3) rates of mortality would be greater on the high shore than the low shore, decreasing with tidal elevation, and would be greater on bare substrate than on live or dead oyster shells. Given the similar vertical distributions displayed by S. glomerata in mangrove forests and on rocky shores, we expected that patterns in oyster settlement and postsettlement mortality would not differ between the 2 habitat types despite the environmental differences between them.
MATERIALS AND METHODS
Experimental Design and Sampling
We assessed patterns of Saccostrea glomerata recruitment and postsettlement mortality in Port Stephens, New South Wales, Australia. Port Stephens, a large drowned river valley (125 [km.sup.2]), supports extensive populations of S. glomerata in mangrove forests and on intertidal rocky shores (Bishop et al. 2010). Experiments were conducted at Corlette Point (rocky shore; (32[degrees]43' S, 152007' E) and the Karuah River (mangrove; 32[degrees]39' S, 151 [degrees]58' E). The 2 sites selected for the settlement and recruitment experiments were adjacent to commercial spat catching areas, with a good supply of S. glomerata larvae. Experiments were initiated in December (early summer) 1993 to coincide with the beginning of the peak settlement season of S. glomerata, which can extend from late November to early May (McOrrie 1990, McOrrie 1995).
In each of the 2 habitats (rocky shore and mangrove) we manipulated substrate availability at 3 tidal elevations: low, mid, and high. The 3 tidal elevations spanned most of the intertidal range of Saccostrea glomerata in Port Stephens, which extended from the subtidal up to 1.4 m above Indian Spring Low Water (Bishop et al. 2010). At each tidal elevation of each habitat, we established 3 experimental treatments: bare (rock or pneumatophores without additional structure), shell (rock or pneumatophores with dead shell), and oyster (rock or pneumatophores with live oysters attached).
On the rocky shore, we manipulated substrate availability within 12 randomly selected 0.5 x 0.5-m plots per elevation (n = 36 plots total), each containing > 90% cover of oysters. Each plot was assigned randomly to 1 of the 3 substrate treatments, giving 4 replicate plots of each treatment at each elevation. For plots assigned to the bare treatment, all oysters were cleared using an oyster knife and hammer. In the plots assigned to the shell treatment, we left the lower valve of oysters attached to the rock substrate. In the oyster treatment, plots were left untouched. In general, the dead oyster treatment provided the greatest surface area; the live oyster treatment, an intermediate surface area; and the bare substrate, the least. Pilot studies indicated that the disturbance associated with removal of oysters did not influence recruitment, with similar recruitment recorded between cleared and naturally bare plots.
In the mangrove forest, we manipulated substrate using pneumatophore mimics constructed of 200-ram lengths of 7-mm-diameter maple wood dowel, inserted 100 mm into the sediment. Mimics assigned to the bare treatment had no further structure attached. The mimics assigned to the shell treatment, had a single upper valve (shell height, 65 80 mm) of a 3-y-old cultivated Saccostrea glomerata shell attached, whereas those assigned to the oyster treatment held 1 live adult S. glomerata (shell height, 65-80 mm). This density of oysters was chosen based on a pilot study that indicated there was, on average, 1 [+ or -] 1 (1 SE) oysters attached to each pneumatophore at our study site. Measurements using foil cutouts of the surface area for settlement indicated that it was greatest in the live oyster treatment (0.0048 [m.sup.2]), followed closely by the shell treatment, which included only the flat left valve (0.0042 [m.sup.2]), with the bare treatment offering substantially less (0.0011 [m.sup.2]). Shells and live oysters were attached to relevant pneumatophore mimics using a neutral cure Silastic (Dow Coming). The glue was allowed to cure for 1 day prior to the dowel being inserted 0.1 m into the mud substrate. In pilot studies, the presence of the glue did not influence settlement patterns. We deployed 75 pneumatophore mimics within each of three 0.5 x 0.5-m plots per tidal elevation (n = 9 total), each of which had been cleared of all pneumatophores. The plots were subdivided into twenty-five 0.1 x 0.1-m quadrats, each of which received 1 U of each of the 3 experimental treatments (bare, shell, oyster).
Each of the sites was visited every 2 wk to assess patterns of Saccostrea glomerata settlement and subsequent recruitment. On the rocky shore, nondestructive sampling was possible, and counts and lengths of recruits were quantified during each visit. In the mangrove forest, destructive sampling was necessary to quantify recruits accurately, which tended to be covered with mud, and quantitative sampling was conducted on 4 occasions (4, 12, 24, and 36 wk after the deployment of pneumatophore mimics). During the intervening periods, we assessed qualitatively whether new recruitment events had occurred in the mangrove forest by visual inspection. Nondestructive sampling of the rocky shore was done by subsampling 3 randomly selected 0.1 x 0.1-m quadrats within each 0.25-[m.sup.2] plot. We used a hand lens with 10x magnification to assist with measurements and counts. In the mangrove forest, 5 pneumatophore mimics of each of the 3 experimental treatments were sampled randomly from each plot at each sampling time. The model pneumatophores were transported to the laboratory for inspection under a dissecting microscope. Oyster larvae were enumerated and measured using a micrometer eyepiece.
We identified major recruitment episodes on the rocky shore and in the mangrove forest by producing size-frequency histograms for each sampling time, and identifying periods during which there was arrival of small recruits (shell height, <1.0 mm). For each of these recruitment episodes identified for each habitat, we ran separate 3-factor mixed-model ANOVAs to test for effects of substrate and tidal elevation on the abundance of new (shell height, <1 mm) Saccostrea glomerata recruits. In each analysis, the factors were treatment (3 levels: bare, shell, oyster), tidal elevation (3 levels: low, mid, high), and plot (3 levels in the mangrove forest, 4 on the rocky shore; random). For each site, treatment and tidal elevation were fully orthogonal. On the rocky shore, plots were nested within both tidal elevation and treatment, and there were 3 replicate quadrats per plot. Within the mangrove forest, the factor plot was nested within tidal elevation only, with 5 replicate pneumatophores of each treatment per plot. The rocky shore data were analyzed without standardizing by the surface area for settlement because measurements of these data were not available. Mangrove data were analyzed twice---once using unstandardized data and a second time using counts expressed per unit surface area.
Early postsettlement mortality of oysters was examined for the cohort in each habitat that contributed most to longer term recruitment. On the rocky shore, this was the late March settlement event (recruitment episode 3, Fig. 1G). In the mangrove forest, this was the early March settlement event (also the third recruitment episode for that habitat, Fig. 2B). On the rocky shore, we calculated mortality between March 13 and March 26, the subsequent sampling time, based on the number of oysters < 1.0 mm in shell height on March 13 that had survived to the 1.1-2.0 size class on March 26 (there was no addition settlement during this period). Previous sampling at this site indicated that new recruits could be expected to grow ~1.0 mm in 2 wk. Mortality in the mangrove forest was considered between March 26 and the subsequent sampling time of May 13, a period of longer duration. Based on field assessments of growth rates, mortality was calculated as the percentage of oysters that were 1.0-1.1 mm on March 26 that had recruited to the 2.7-2.9-mm size class by May 13. Percent mortality was calculated by pooling oyster counts for a newly settled cohort across quadrats/pneumatophores within a plot and determining the percentage reduction in oysters of this cohort between sampling intervals as a percentage of the starting number.
Analyses of variance examined effects of treatment and tidal elevation on (1) the percent mortality of these cohorts contributing most to oyster recruitment between their first detection and subsequent sampling (2 wk later on the rocky shore, 2 mo later in the mangrove forest) and (2) the total number of oysters (across all cohorts) remaining in experimental plots at the conclusion of the experiment. Abundance data were [square root of (x + 1)] transformed and percent data arc-sine transformed prior to analysis to satisfy assumptions of ANOVA of homogeneity of variances (see Underwood 1997). Post hoc Student-Newman-Keuls tests identified differences in treatment means leading to significance of factors.
During the 12 mo of the experiment, 3 distinct recruitment episodes of Saccostrea glomerata were documented in the experimental plots on the Corlette Point rocky shore--early December (episode 1), late January (episode 2), and mid March (episode 3; Fig. 1). After both the December and January recruitment episodes, the abundance of oysters declined rapidly. Of the recruits first detected on December 11 (Fig. 1A), only 12% remained 1 wk later (Fig. 1B), and by January 8 (Fig. 1C), their abundance was 3% of December 11 (Fig. 1). After the second recruitment event in late January, we counted 161 oysters of 1.1-2 mm in shell height on February 12 (Fig. 1E), but only 59 (or 27% of those counted on February 12) less than 2 wk later, on February 25 (Fig. 1F). By March 13 (Fig. 1G), there were only 43 oysters that were of sufficient size (>2.0 mm) to be from recruitment episode 2; by March 26 (Fig. 1H), there were only 14 oysters (>2.0 mm) remaining from this cohort--less than 9% the abundance of February 12. Decline in abundance after the third recruitment pulse was less marked. Nevertheless, although 165 oysters < 1.0 mm were detected on March 13 (Fig. 1G), by March 26 (Fig. 1H), only 81 oysters of any size remained (49% of the starting number), and by April 23 (Fig. 1I), there were 67 remaining (41% of the number on March 13).
In the Karuah mangrove forest, 4 recruitment episodes were observed qualitatively: early January (episode 1), early February (episode 2), early March (episode 3), and late April (episode 4). These episodes were not captured fully by our destructive sampling, which detected new recruits < 1.0 mm on January 29, 1994, and on March 26 1994 (Fig. 2). Consequently, cohorts were difficult to track based on size-frequency distributions. Nevertheless, the first appearance of oysters > 5.0 mm in May, > 3 mo after the initial recruitment episodes, suggested (based on growth rates on the rocky shore) that oysters from the first 2 pulses did not survive through to this size class.
On the rocky shore and in the mangrove forest, abundances of new oyster recruits (shell height, <1.0 mm), after each of the recruitment episodes, were determined by both tidal elevation and substrate type. At each of the sites, their abundance of new recruits was generally greater on the low and mid shores than on the high shore (Table 1, Figs. 3 and 4). On the rocky shore, there were more new oyster recruits in plots with shell than with live oysters or with bare rock only (Table 1, Fig. 3). In the mangrove forest, by contrast, the abundance of new oyster recruits was generally similar between pneumatophore mimics with shell or live oysters, each of which supported more oysters than the bare pneumatophore mimics (Table 2, Fig. 4). In the mangrove forest, differences among treatments were less distinct in the high intertidal, where few oysters were recorded than lower on the shore. In this habitat, the pattern of more Saccostrea glomerata recruits on pneumatophore mimics with live or dead oysters in most instances disappeared when surface area was taken into consideration (Table 3, Fig. 4).The exception was on the high shore after recruitment episode 1, when bare pneumatophore mimics supported fewer oysters per unit area than pneumatophores with a shell or oyster.
On the rocky shore, mortality of episode 3 recruits was 60% within the 2-wk time interval considered (March 13-26). In the mangrove forest, 46% mortality of episode 3 recruits was evident between their first detection on March 26 and the subsequent sampling 1.5 mo later on May 13. In each of the habitats, initial postsettlement mortality of these cohorts was influenced by height on the shore but not treatment (Table 3). At each, percent mortality was greater on the high shore than at other tidal elevations, with 100% mortality of oysters experienced frequently in plots at this highest tidal elevation (Table 3).
At the end of sampling, the total abundance of oysters (integrating across all cohorts) was generally greater on the low to mid shores than on the high shore, but did not differ among treatments (Table 4, Fig. 5).
Despite the very different biotic and abiotic conditions on rocky shores and in mangrove forests, we found that patterns of oyster settlement and postsettlement mortality were similar between each. Regardless of habitat type, larval settlement (as estimated from the abundance of new recruits; shell height, <1.0 mm) was influenced by the substrate provided (bare, oyster shell, or live oysters), and was greater on the low or mid shore than on the high shore. Postsettlement mortality, by contrast, was determined only by tidal elevation, with few oysters surviving high on the shore. The net effect was that the number of oysters recruiting to larger size classes was determined primarily by tidal elevation.
Diminished settlement into the high intertidal zone has been reported previously for other species of oyster and barnacle (Michener & Kenny 1991, Roegner & Mann 1995), and is consistent with predictions of supply-side ecology--that increasing emergence time should decrease time available for settlement. Nevertheless, emergence time alone could not predict differences in the number of new recruits among elevations. In each of the habitats, the density of new recruits was often, although not always, indistinguishable between low and mid tidal elevations (Tables 1 and 2), despite the greater emergence of the mid tidal elevation. Similarly, the 450-760% difference in new oyster recruits between the mid and high intertidal elevations (Figs. 3 and 4) could not be explained by the 30% difference in inundation. The Sydney rock oyster Saecostrea glomerata may be displaying selectivity in settlement, responding to cues from conspecifics or other species that are similarly more abundant on the low and mid shores than the high shore. Alternatively, during the period between settlement and our first detection of new recruits, differential postsettlement mortality among tidal elevations may already be acting on oyster abundance.
Effects of biogenic habitat on the abundance of new Saccostrea glomerata recruits were generally independent of tidal elevation. The recruitment of S. glomerata was affected positively by the presence of conspecifics, and shell remnants in the mangrove forest, but shell remnants only on the rocky shore. Although S. glomerata can respond to substrata in response to chemical cues (Anderson 1996), we suspect that this was not the primary mechanism for greater settlement on biogenic substrate here. Instead, differences in settlement among substrates appeared to be proportional to the surface area provided by the substrate. In the mangrove forest, where the surface areas provided by each of the 3 substrates could be calculated, standardization by surface area eliminated differences in recruitment among substrates. On the rocky shore, the dead oyster substrate, which was determined qualitatively to provide the greatest surface area, supported the most recruits. In a previous study also manipulating substrate availability, the greater recruitment of invertebrates to half shells of S. glomerata than whole live or dead shells was attributed to the great surface area and interstices that the half shells provide (Summerhayes et al. 2009).
In each of the 2 habitats, oysters settled during multiple recruitment events that occurred throughout the summer period. In each of the habitats, however, few of the oysters settling in December or January (summer) survived more than a couple of weeks. Recruitment events in March or April (early autumn) contributed the bulk of recruits that survived through to the following summer in rocky shore or mangrove habitats. In a similar study, the American oyster Crasssostrea virginica was found to display a similar pattern of most successful recruitment to its native habitat in fall (Roegner & Mann 1995). High temperatures, or seasonally abundant predators or competitors might be prohibitive of successful oyster recruitment after summer settlement events. Temperatures exceeding 40[degrees]C can be lethal to Saccostrea glomerata, especially when exposure is prolonged (Potter & Hill 1982, Krassoi 2001).
In summary, our study represents one of the first to compare the contribution of settlement and postsettlement mortality with the spatial distribution of a sessile marine invertebrate between sites of contrasting habitat. We have documented that on a rocky shore and in a mangrove forest, settlement and early postsettlement mortality varied similarly across tidal elevation gradients and substrate types to determine the distribution of Saccostrea glomerata. Although small-scale variation in substrate type influenced settlement patterns, in each habitat, postsettlement mortality drove patterns of distribution, and tidal elevation was ultimately more important in determining the distribution of adult S. glomerata oysters than small-scale habitat structure.
We thank J. Nell, S. Hunter, and W. O'Connor of NSW Fisheries for logistical support and K. Brown for useful discussion. Helpful comments from 2 anonymous reviewers and S. Shumway improved the quality of this article. This research was funded by an Australian Research Council Linkage Grant (to M. B. and W. O.).
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KA-MAN LEE, (1) ([dagger]) FREDERICK R. KRASSOI (2) AND MELANIE J. BISHOP (1) *
(1) Department of Biological Sciences, Macquarie University, NSW 2109 Australia; (2) Ecotox Services Australasia Pry Ltd, 27/2 Chaplin Drive, Lane Cove, NSW 2066 Australia
* Corresponding author. E-mail: email@example.com
([dagger]) Current address: Department of Biology, Hong Kong Baptist University, 224 Waterloo Road, Hong Kong.
TABLE 1. Summary of mixed-model ANOVAs testing for effects of substrate treatment (bare rock, rock + shell, and rock + oysters) and tidal elevation (low, mid, and high) on the abundance of new (shell height, <1.0 mm) Saccostrea glomerata recruits on the rocky shore after 3 major settlement events. Episode 2: January Episode 1: December 11 29 df MS F P value MS F P value Tr 2 0.44 12.9 <0.001# 0.86 5.8 0.008# Ti 2 0.11 3.16 0.058 1.17 7.9 0.002# Pl (Tr x Ti) 27 0.03 0.86 0.664 0.15 1.0 0.537 Tr x Ti 4 0.08 2.40 0.075 0.22 1.5 0.243 Residual 72 0.04 0.16 SNK Tr: (R = O) < S Tr: (R = O) < S Ti: (L = M) > H Episode 3: March 13 MS F P value Tr 3.64 8.71 0.001# Ti 1.51 3.62 0.040# PI (Tr x Ti) 0.42 1.63 0.052 Tr x Ti 0.56 1.33 0.282 Residual 0.26 SNK Tr: (R = O) < S Ti: L > (H = M) P values in bold type indicate significance at [alpha] = 0.05. Tr, substrate treatment (3 levels: R, rock; O, rock + oysters; S, rock + shell); Ti, tidal elevation (3 levels: L, low; M, mid; H, high); Pl, plots (4 levels, random) with n = 3 quadrats within each; SNK, Student-Newman-Keuls tests. Note: P values significance at [alpha] = 0.05 are indicated with #. TABLE 2. Summary of mixed-model ANOVAs testing for effects of substrate treatment (pneumatophore, pneumatophore + shell, and pneumatophore + oyster) and tidal elevation (low, mid, and high) on the abundance of new (shell height, <1.0 mm) Saccostrea glomerata recruits in the mangrove forest. Episode 1: January 29 Raw Standardized df MS F P value MS F P value Tr 2 4.91 341.7 <0.001# 324 15.6 <0.001# Ti 2 8.68 107.9 <0.001# 7,337 89.6 <0.001# Pl(Ti) 6 0.08 0.6 0.735 82 0.6 0.755 Tr X Ti 4 0.60 41.6 <0.001# 86 4.1 0.025# Tr X Pl(Ti) 12 0.01 0.1 0.999 21 0.1 1.000 Residual 108 0.14 144 SNK Tr X Ti: Tr X Ti: Tr(Ti) Tr(Ti) L:P < S < O L,M:P = S = OH:P < (S = 0) M, H: P < (S = 0) Ti(Tr) Ti(Tr) P: L > M > H P:L > M > H S, O: (L = M) > H S,O:(L = M) > H Episode 3: March 26 Raw Standardized MS F P value MS F P value Tr 2.87 21.6 <0.001# 199 1.0 0.415 Ti 9.67 45.2 <0.001# 5,975 16.8 0.004# PI(Ti) 0.21 1.7 0.124 356 2.5 0.028# Tr X Ti 1.48 11.2 <0.001# 438 2.1 0.146 Tr X PI(Ti) 0.13 1.1 0.397 210 1.5 0.148 Residual 0.12 144 Tr X Ti: Ti: (L = M) > H Tr(Ti) L,M:P < (S = 0) H: P = S = O Ti(Tr) P: L = M = H S:(L = M) > H O:L > M > H Analyses were run using raw counts (Raw) and counts standardized according to the surface area available for settlement on sampling units (Standardized). P values in bold type indicate significance at [alpha] = 0.05. Tr, substrate treatment (3 levels: P, pneumatophore; O, pneumatophore + oyster; S, pneumatophore + shell); Ti, tidal elevation (3 levels: L, low; M, mid; H, high); Pl, plots (3 levels, random) with n = 5 replicate pneumatophores within each; SNK, Student-Newman-Keuls tests. Note: P values significance at [alpha] = 0.05 are indicated with #. TABLE 3. Summary of 2-way ANOVAs testing for effects of substrate treatment (bare, shell, and oysters) and tidal elevation (low, mid, and high) on the percent mortality of Saccostrea glomerata on the rocky shore and in the mangrove forest immediately after major recruitment episodes (rocky shore: March 13-26; mangrove: March 26-May 13). Rocky Shore Mangrove df MS F P value df MS F P value Tr 2 243 0.2 0.854 2 170 0.4 0.696 Ti 2 12,395 8.1 0.002# 2 5,413 11.7 <0.001# Tr x Ti 4 1,000 0.7 0.632 4 889 1.9 0.493 Residual 27 1,537 18 461 SNK Ti: (L = M) < H Ti: L < M < H Percent mortality was calculated using abundances pooled across replicate samples (quadrats or pneumatophores) within a plot to give n = 4 on the rocky shore and n = 3 in the mangrove forest). P values in bold type indicate significance at [alpha] = 0.05. Tr, substrate treatment (3 levels: B, bare; O, oyster; S, shell); Ti, tidal elevation (3 levels: L, low; M, mid; H, high); SNK, Student-Newman-Keuls tests. Note: P values significance at [alpha] = 0.05 are indicated with #. TABLE 4. Summary of mixed-model ANOVAs testing for effects of substrate treatment (bare, shell, and oysters) and tidal elevation (low, mid, and high) on the total abundance of oysters (pooled across all cohorts) at the conclusion of the experiments (rocky shore: December 3; mangrove forest: July 23). Rocky Shore df MS F P -value Tr 2 0.20 1.6 0.219 Ti 2 0.86 6.8 0.004# Pl (Tr x Ti) 27 0.13 0.8 0.776 Tr x Ti 4 0.04 0.3 0.859 Residual 72 0.17 SNK Ti: (L = M) > H Mangrove df MS F P -value Tr 2 0.04 1.5 0.249 Ti 2 1.49 58.3 <0.001# Tr x Ti 18 0.50 19.5 <0.001# Residual 108 0.03 SNK Tr x Ti: Tr(Ti) L: (P = S) > O M: (P = S) < O) H:P = S = 0 Ti(Tr) P,S: (L = M) > H O: M > (H = L) Values in bold type indicate significance at [alpha] = 0.05. Tr, substrate treatment (3 levels: B, bare; O, oyster; S, shell); Ti, tidal elevation (3 levels: L, low; M, mid; H, high); Pl, plots (4 levels, random), with n = 3 on the rocky shore. Plot average (n = 3) were used as replicates in the analysis of mangrove data because of a large number of missing replicates. SNK, Student- Newman-Keuls tests. Note: Values indicate significance at [alpha] = 0.05 are indicated with #.
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|Author:||Lee, Ka-Man; Krassoi, Frederick R.; Bishop, Melanie J.|
|Publication:||Journal of Shellfish Research|
|Date:||Dec 1, 2012|
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