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Detecting an environmental impact of dredging on seagrass beds with a BACIR sampling design
Aquatic Botany 53 ( 19%)235-243
Technical communication
Detecting an environmental impact of dredging on seagrass beds with a BACIR sampling design B.G. Long * , D.M. Dennis, T.D. Skewes, I.R. Poiner CSIRO Marine Laboratories, PO Box 120, Cleveland, Qld. 4163 Australia
Accepted 24 November 1995
Abstract The impact of maintenance dredging an access channel to a canal estate in Deception Bay, Australia, on the nearby seagrasses was monitored over 18 months with a Before/After, Control/Impact, Repeated measures (BACIR) sampling design. Three seagrasses were collected in the study area; Zostera capricorni Aschers., Halophifa ovalis (R.Br.) Hook. f. and Halophila spinulosa (R.Br.) Aschers. All seagrasses were found less than 700 m offshore. The biomass of Z. capricorni, the numerically dominant seagrass, was significantly lower in the access channel border compared with the control area before dredging, which was attributed to direct or indirect effects associated with the channel. There was no significant effect of maintenance dredging statistically detected for Z. capricorni biomass in the access channel border even though seagrass was absent in the access channel 14 months after dredging. This was due to the high background variability of seagrass biomass in the control area. In contrast the biomass of H. oualis declined at a significantly higher rate in the control area than in the access channel border but had also disappeared from the access channel border 14 months after dredging. Without a control we may have concluded that the disappearance of seagrass from the access channel border was due to the effects of dredging, whereas with a BACIR sampling program there remained a possibility that the decline in seagrass was due to larger scale changes in the bay. Keywords: Seagrass; BACIR; Environmental impact; Dredging; Australia
1. Introduction Seagrasses are important for the management of coastal shallow inshore waters because they stabilise sediments and provide nursery grounds, habitat and food for
* Corresponding author. Tel: (07) 286 8222 [International +6 [International + 61 + 7 286 25821; e-mail: [email protected].
+7 286 82221; fax: (07) 28 2582
0304-3770/%/$15.00 8 1996 Elsevier Science B.V. All rights reserved SSDI 0304-3770(95)01006-8
236
B.C. Long et d/Aquatic
Botany 53 (1996) 235-243
commercially important prawns and fish (Young, 1978; Bell and Pollard, 19891, wading birds and dugongs (Preen et al., 1992). Seagrasses can be destroyed by natural events (e.g. cyclones) and man-made impacts (e.g. dredging). Dredging removes seagrass and also affects surrounding seagrass beds by smothering (Hatcher et al., 1989; Zieman, 19751, changes in current patterns (Ogden and Gladfelter, 1983) and increased turbidity (Giesen et al., 1990). Positive effects of dredging have also been shown by an increase in the growth of seagrass, which was attributed to the addition of nutrients from the dredge spoil (Odum, 1963). In all of the above studies, however, the assessment of the effects of dredging was descriptive only. This is unsatisfactory in many instances because a descriptive approach is often unable to distinguish between the effects due to the impact and the natural background variation. A more formal method of impact assessment is to use analysis of variance techniques in an experimental design with appropriate controls (Underwood, 1993). Before/After, Control/Impact, Repeated measures (BACIR) experimental designs have been developed to detect impact effects (Green, 1993), but these techniques have apparently not yet been used on seagrass. ‘Ihe aim of this study was to use a BACIR experimental design to assess the impact of maintenance dredging in an access channel into a canal estate, on the seagrasses next to the channel, in southern Queensland, Australia. The seagrasses in the area have previously been described by Young and Kirkman (19751, Young (1978) and Hyland et al. (1989) provided anecdotal historical information on seagrass distribution in the study area.
2. Materials and methods 2.1. Study area The access channel border was defined as a 100 m wide corridor bordering each side of a 1300 m X 60 m access channel into the Newport Waters Canal Estate, Deception Bay, Queensland (Fig. 1). A control area (100 m alongshore by 1300 m offshore), was located 3.5 km to the west of the canal entrance (Fig. 1). The 1300 m long access channel and part of the internal canal system was dredged to approximately 2.0 m below low tide mark between December 1991 and January 1992, which entailed the removal of about 0.5 m of sediment from the access channel. The dredge spoil was pumped ashore to a holding basin west of the canal and allowed to settle. The excess water was returned to Deception Bay, via a small unnamed creek opening into the bay approximately 400 m west of the canal entrance. The canal estate and coastline were digitised from a 1:84000 nautical chart of Moreton Bay (Moreton Bay: Caloundra to Dunwich; Queensland Department of Harbours and Marine) into a Geographic Information System (GIS) (SPANS”, Tydac, 1990). A grid of 78 sites in the access channel border, and 39 sites in the control area, was constructed using the GIS. Sites were separated by 50 m alongshore and 100 m offshore in both the access channel border and control area (Fig. 2).
B.G. Long et al. /Aquatic
Botany 53 (19%) 235-243
237
, 153’ 05’ E
Deception
. . .ControlArea ...
Bay Access
.
channel
Fig. 1. Study area showing the control area, access channel border and 2 m depth contour in the Newport Canal Estate study area.
2.2. Field sampling and sample processing The sites were located in the field using a Global Positioning System (GPS). At all sites a sample of the bottom was taken with a 0.085m’ seagrass grab (Long et al., 1994). The sample was sieved over the side of the dinghy in a 4 mm mesh lug-basket to remove the sediment from the seagrass rhizomes. The seagrass was transferred to plastic
canbol area
Access
ChaMd
a-. ... ...
border
-0. 0..
0.. . . . . . .
0.. . . . . . .
_--
I/
..a --
,.. .** IT!& 0.0 ..* ,..
.*.
:::
0.0 ,..
0’.
.*
.*a . ..
rot:: ...*a=
0.. . . .
/---
r. ..
OH
-_
5OOm
-.
\
0..
5OOm
Fig. 2. Sites sampled for seagrass in the control area and access channel border in the Newport Canal Estate study area, 0: sites used in the repeated measures ANOVA.
238
B.G. Lang et al./Aquatic
Botany 53 (19%) 235-243
bags and stored on ice. The seagrass samples were returned to the laboratory for processing. Seagrasses were separated into species and the above-ground shoots and stems were separated from the below-ground rhizomes. The above- and below-ground components were dried to a constant weight at 60°C for 48 h and weighed to the nearest 0.1 g. The sediment texture at each site was qualitatively classified as mud, sandy mud, muddy sand or sand by squeezing a small handful taken from the grab. Sampling was carried out in September 1991, 2 months before dredging, one month after dredging (March, 19921, and 14 months after dredging (May, 1993). 2.3. Data analysis A repeated measures analysis of variance was used to test whether maintenance dredging had an effect on seagrass biomass in the access channel border (Green, 1993). Seagrass biomass was log ,Otransformed to reduce the correlation between the mean and the variance, and to normal& the data. There were one pre-dredge and two post-dredge sampling occasions in both the control and access channel border.
3. Results 3.1. Seagrass distribution Zostera capricorni Aschers. and Halophila oualis (R.Br.1 Hook. f. were found in the control area and the access channel border. Halophila spinulosa (R.Br.) Aschers. was
Table 1 Repeated measures analyses of variance of the effect of maintenance Newnort Canal Estate access channel border for (a) Zosrera caoricorni
dredging on seagrass biomass and (b) Haloahila oualis
df
ss
MS
F-ratio
1.23 18.80 1.79 0.60 24.49 52.92
7.23 0.59 0.90 0.30 0.38
12.25 ’ * 1.55 2.37 0.79
Total
1 32 2 2 64 101
(b) Halophila oualis Location Sites (location) Time Location X time Error Total
1 32 2 2 64 101
0.729 0.07 1 0.580 0.465 0.046
10.26 1.54 12.61 * * 10.11 * *
Source of variation
(a) Zostera capricorni Location Sites (location) Time Location X time Error
0.729 2.265 1.159 0.93 1 2.964 8.048
Degrees of freedom (df); sums of squares (SS); mean squares (MS); * * P < 0.01.
?? ??
in the
B.G. Long er al/Aquatic
Botany 53 (1996) 235-243
Access channel border
Control
D
a40 E 20 %
ET 0 2
%
cz
239
LJLI
1 month postdredging
1,.
,_
.
_
14 months post-dredging
20 0 40 :1
500
1000
560
1060
Distanceoffshore(m) Fig. 3. Stacked bar chart showing the distribution of Zosrera capricorni, ??, and Halophila ooalis, • I , with distance from shore in the control area and access channel border for the pre-dredge (September, 1991). one-month post-dredging (March, 1992) and 14 months post-dredging (May, 1993) sampling occasion.
found at one near-shore site in the access channel border pre-dredge samples and was absent from all post-dredge samples. Seagrass was not found further than 600 m offshore in the control area and 300 m offshore in the access channel border over the 3 year study period. Because of this, only those sites less than 600 m from shore in the control area and less than 300 m from shore in the access channel border were used in the repeated measures analysis of variance. The distribution of Zostera capricorni and Halophila oualis in the control area contracted over the study period from 200 to 600 m offshore in the pre-dredge samples to 300 to 600 m one month after dredging, and 400 to 600 m, 14 months after dredging (Fig. 3). The distribution range of Zostera capricorni and Halophila ooalis in the access channel border also contracted over the study period, from 300 m offshore or less, to absent, 14 months after dredging. 3.2. Repeated measures analysis of variance
Above and below ground biomass were significantly correlated for Zostera caprzcorm (r = 0.576; P < 0.05; n = 28) and Halophila oualis (r = 0.912; P < 0.01; n = 22), therefore, above- and below-ground biomass were pooled and used as the response variable in the repeated measures analysis of variance.
240
B.G. Long et al. /Aquatic
Botany 53 (19%) 235-243
Sampling occasion Fig. 4. Seagrass biomass (g m-*) and standard e rror in the control area, ?? , and access channel border, 0, for the pre-dredging (September, 1991). one-month post-dredging (March, 1992), and 14 months post-dredging (May, 1993) sampling occasions for (a) Zostera capricorni and (b) Halophila ooalis.
There was no significant interaction between location (control versus access channel border) and time (pre- and post-dredging) for Zosferu capricorni, and hence no impact effect of maintenance dredging was detected for Z. capricorni (Table 1). Seagrass biomass was higher in the control area compared with the access channel border over
Distance offshore (m) Fig. 5. Sediment type profiles with distance offshore in the control area and access channel border for the pre-dredging (September, 1991) and one-month post-dredging (March, 1992) sampling period. Note: the 14 month post-dredging (May, 1993) sediment profiles were the same as the one-month post-dredging (March, 1992) profiles.
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Botany 53 (19%) 235-243
241
the study period (Fig. 4). In contrast, there was a significant interaction between location and time for Hulophilu ovalis (Table I). This was due to the relatively large decline in H. ovalis biomass in the control area between the pre- and post-dredging sampling, compared with a relatively smaller decline in the impact area (Fig. 4). 3.3. Sediment analysis Before dredging, the sediments in the access channel border were predominantly mud, except for some sandier sediments close to shore. In the control area, the sediments were mainly muds, inshore ( < 300 m), becoming sandier offshore (700 m) and reverting back to muds further offshore. After dredging the sediments in the access channel border became coarser grained whereas the sediments in the control area essentially remained unchanged (Fig. 5).
4. Discussion There was no significant impact of maintenance dredging detected on the distribution of Zosteru capricorni biomass in the access channel border, despite the disappearance of seagrass from the access channel border 14 months after dredging. This was because of the high background variation in the biomass of Z. capricorni in the control area which was due to the patchy distribution of seagrass. In a similar study to monitor the effects, with a BACIR sampling design, of cutting a large channel (60 m wide (3158 m long (5.5 m deep) through thick seagrass beds at McArthur River in the Gulf of Carpentaria, northern Australia, no significant impacts due to dredging were detected on the nearby seagrasses (Kenyon et al., 1995). In contrast there was a significant interaction between location and time for H. ovalis which was due to a relatively large decline in biomass in the control area compared with a relatively smaller decline in the access channel border. The decline in Hulophilu ovalis biomass in the control area was not attributed to the effects of maintenance dredging because the control area, 3.5 km to the west, was outside any obvious influences of the dredging operations. Moreover, the sediment type was the same in the control area before and after dredging. The offshore distribution range for Z. cupricorni and H. ovalis contracted over the study period. Changes in the overall distribution of seagrass in the study area may be part of larger scale changes that occur there based on previous reports of seagrass diebacks occurring in Deception Bay (Young and Kirkman, 1975) and subsequent recoveries (Hyland et al., 1989). Moreover, effects due to seasonal changes of seagrass biomass need to be considered in the sampling design. The biomass of seagrass was significantly higher in the control area than the access channel border. The channel was originally constructed in the 197Os, over 20 years ago, and it would appear unlikely that the observed differences now were due to factors associated with the construction of the charmel. A more plausible explanation may be that factors correlated with changes in the local hydrology near the access channel, or direct effects such as speedboats passing over the access channel borders and physically
242
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Botany 53 (1996) 235-243
disturbing the seabed, may be responsible for the significantly lower biomass of seagrass in the access channel border than the control area. This study illustrates well the importance of having a control when monitoring environmental impacts. By comparing changes in seagrass biomass in the control area with changes in the impacted area, the natural variation in seagrass biomass was statistically taken into account and provided a powerful reference when deciding whether an impact had occurred or not. Without the control we may have concluded that the disappearance of Zosreru capricorni and Halophila oualis in the access channel border was due to the effects of dredging. The full extent of these changes, however, can not be determined from a single control. Therefore, we recommend the use of multiple controls sampled over a number of occasions before, and after, dredging where possible, as advocated by Underwood (19931, to ensure that the control areas are a representative sample of the natural background variation. In reality, compromises must be made which are based on limited resources and funding.
Acknowledgements We wish to thank C. Peterken for her diligence and help in tracking down the relevant literature on seagrass impact studies and N. Loneragan for his useful suggestions on ways to improve the manuscript. The assistance of the Redcliffe City Council, by providing funds for the impact assessment, is greatly appreciated.
References Bell, J.D. and Pollard, D.A., 1989. Ecology of fish assemblages and fisheries associated with seagrasses. In: A.W.D. Larkum, A.J. McComb and S.A. Shepard (Editors), The Biology of Seagrasses - An Australian Perspective. Elsevier, Amsterdam, pp. 565-609. Giesen, W., van Katwijk, M.M. and den Hartog, C., 1990. Eelgrass condition and turbidity in the Dutch Wadden Sea. Aquat. Bot., 37: 71-85. Green, R.H., 1993. Application of repeated measures designs in environmental impact and monitoring studies. Aust. J. Ecol., 18: 81-98. Hatcher, B., Johamres, R. and Robertson, A., 1989. Review of research relevant to the conservation of shallow tropical marine ecosystems. Oceanogr. Mar. Biol. Ann. Rev., 27: 337-414. Hyland, S.J., Courtney, A.J. and Butler, C.T., 1989. Distribution of seagrass in the Moreton Region from Coolangatta to Noosa. Queensland Department of Primary Industries, Information Series QJ89010, 42 pp. Kenyon. R., Poiner, 1. and Burridge, C., 1995. Impact of the McArthur river project mine transhipment facility on the marine environment: post-construction survey. CSIRO Division of Fisheries Report, April 1995, 82 PPOdum, H.T., 1963. Productivity measurements in Texas turtle grass and the effects of dredging in an intercoastal channel. Publ. Instit. Mar. Sci. Univ. Texas, 9: 48-58. Ogden, J.C. and Gladfelter. E.H., 1983. Coral reefs, seagrass beds and mangroves: their interaction in the coastal zones of the Caribbean. UNESCO Rep. Mar. Sci., 23, 133 pp. Long, B.G., Skewes, T.D. and Poiner, I.R., 1994. An efficient method for estimating seagrass biomass. Aquat. Bot., 47: 277-291. Preen, A., Thompson, J. and Corkeron, P., 1992. Wildlife and management: dugongs, waders and dolphins. In: O.N. Crimp (Editor), Moreton Bay in the Balance. Australian Littoral Society Inc., Brisbane, pp. 59-68.
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Tydac International, 1990. Nepeau, Qnt., Canada. Underwood, A.J., 1993. The mechanics of spatially replicated sampling programmes to detect environmental impacts in a variable world. Aust. J. Ecol., 18: 99-l 16. Young, P.C. and Kirkman, H., 1975. The seagrass communities of Moreton Bay, Queensland. Aquat. Bot., 1: 191-202. Young, P.C., 1978. Moreton Bay, Queensland: a nursery area for juvenile penaeid prawns. Aust. J. Mar. Freshw. Res., 28: 745-773. Zieman, J.C., 197.5. Tropical seagrass ecosystems and pollution. In: F. Wood and R. Johannes (Editors), Tropical Marine Pollution. Elsevier, Amsterdam, pp. 63-74.
Technical communication
Detecting an environmental impact of dredging on seagrass beds with a BACIR sampling design B.G. Long * , D.M. Dennis, T.D. Skewes, I.R. Poiner CSIRO Marine Laboratories, PO Box 120, Cleveland, Qld. 4163 Australia
Accepted 24 November 1995
Abstract The impact of maintenance dredging an access channel to a canal estate in Deception Bay, Australia, on the nearby seagrasses was monitored over 18 months with a Before/After, Control/Impact, Repeated measures (BACIR) sampling design. Three seagrasses were collected in the study area; Zostera capricorni Aschers., Halophifa ovalis (R.Br.) Hook. f. and Halophila spinulosa (R.Br.) Aschers. All seagrasses were found less than 700 m offshore. The biomass of Z. capricorni, the numerically dominant seagrass, was significantly lower in the access channel border compared with the control area before dredging, which was attributed to direct or indirect effects associated with the channel. There was no significant effect of maintenance dredging statistically detected for Z. capricorni biomass in the access channel border even though seagrass was absent in the access channel 14 months after dredging. This was due to the high background variability of seagrass biomass in the control area. In contrast the biomass of H. oualis declined at a significantly higher rate in the control area than in the access channel border but had also disappeared from the access channel border 14 months after dredging. Without a control we may have concluded that the disappearance of seagrass from the access channel border was due to the effects of dredging, whereas with a BACIR sampling program there remained a possibility that the decline in seagrass was due to larger scale changes in the bay. Keywords: Seagrass; BACIR; Environmental impact; Dredging; Australia
1. Introduction Seagrasses are important for the management of coastal shallow inshore waters because they stabilise sediments and provide nursery grounds, habitat and food for
* Corresponding author. Tel: (07) 286 8222 [International +6 [International + 61 + 7 286 25821; e-mail: [email protected].
+7 286 82221; fax: (07) 28 2582
0304-3770/%/$15.00 8 1996 Elsevier Science B.V. All rights reserved SSDI 0304-3770(95)01006-8
236
B.C. Long et d/Aquatic
Botany 53 (1996) 235-243
commercially important prawns and fish (Young, 1978; Bell and Pollard, 19891, wading birds and dugongs (Preen et al., 1992). Seagrasses can be destroyed by natural events (e.g. cyclones) and man-made impacts (e.g. dredging). Dredging removes seagrass and also affects surrounding seagrass beds by smothering (Hatcher et al., 1989; Zieman, 19751, changes in current patterns (Ogden and Gladfelter, 1983) and increased turbidity (Giesen et al., 1990). Positive effects of dredging have also been shown by an increase in the growth of seagrass, which was attributed to the addition of nutrients from the dredge spoil (Odum, 1963). In all of the above studies, however, the assessment of the effects of dredging was descriptive only. This is unsatisfactory in many instances because a descriptive approach is often unable to distinguish between the effects due to the impact and the natural background variation. A more formal method of impact assessment is to use analysis of variance techniques in an experimental design with appropriate controls (Underwood, 1993). Before/After, Control/Impact, Repeated measures (BACIR) experimental designs have been developed to detect impact effects (Green, 1993), but these techniques have apparently not yet been used on seagrass. ‘Ihe aim of this study was to use a BACIR experimental design to assess the impact of maintenance dredging in an access channel into a canal estate, on the seagrasses next to the channel, in southern Queensland, Australia. The seagrasses in the area have previously been described by Young and Kirkman (19751, Young (1978) and Hyland et al. (1989) provided anecdotal historical information on seagrass distribution in the study area.
2. Materials and methods 2.1. Study area The access channel border was defined as a 100 m wide corridor bordering each side of a 1300 m X 60 m access channel into the Newport Waters Canal Estate, Deception Bay, Queensland (Fig. 1). A control area (100 m alongshore by 1300 m offshore), was located 3.5 km to the west of the canal entrance (Fig. 1). The 1300 m long access channel and part of the internal canal system was dredged to approximately 2.0 m below low tide mark between December 1991 and January 1992, which entailed the removal of about 0.5 m of sediment from the access channel. The dredge spoil was pumped ashore to a holding basin west of the canal and allowed to settle. The excess water was returned to Deception Bay, via a small unnamed creek opening into the bay approximately 400 m west of the canal entrance. The canal estate and coastline were digitised from a 1:84000 nautical chart of Moreton Bay (Moreton Bay: Caloundra to Dunwich; Queensland Department of Harbours and Marine) into a Geographic Information System (GIS) (SPANS”, Tydac, 1990). A grid of 78 sites in the access channel border, and 39 sites in the control area, was constructed using the GIS. Sites were separated by 50 m alongshore and 100 m offshore in both the access channel border and control area (Fig. 2).
B.G. Long et al. /Aquatic
Botany 53 (19%) 235-243
237
, 153’ 05’ E
Deception
. . .ControlArea ...
Bay Access
.
channel
Fig. 1. Study area showing the control area, access channel border and 2 m depth contour in the Newport Canal Estate study area.
2.2. Field sampling and sample processing The sites were located in the field using a Global Positioning System (GPS). At all sites a sample of the bottom was taken with a 0.085m’ seagrass grab (Long et al., 1994). The sample was sieved over the side of the dinghy in a 4 mm mesh lug-basket to remove the sediment from the seagrass rhizomes. The seagrass was transferred to plastic
canbol area
Access
ChaMd
a-. ... ...
border
-0. 0..
0.. . . . . . .
0.. . . . . . .
_--
I/
..a --
,.. .** IT!& 0.0 ..* ,..
.*.
:::
0.0 ,..
0’.
.*
.*a . ..
rot:: ...*a=
0.. . . .
/---
r. ..
OH
-_
5OOm
-.
\
0..
5OOm
Fig. 2. Sites sampled for seagrass in the control area and access channel border in the Newport Canal Estate study area, 0: sites used in the repeated measures ANOVA.
238
B.G. Lang et al./Aquatic
Botany 53 (19%) 235-243
bags and stored on ice. The seagrass samples were returned to the laboratory for processing. Seagrasses were separated into species and the above-ground shoots and stems were separated from the below-ground rhizomes. The above- and below-ground components were dried to a constant weight at 60°C for 48 h and weighed to the nearest 0.1 g. The sediment texture at each site was qualitatively classified as mud, sandy mud, muddy sand or sand by squeezing a small handful taken from the grab. Sampling was carried out in September 1991, 2 months before dredging, one month after dredging (March, 19921, and 14 months after dredging (May, 1993). 2.3. Data analysis A repeated measures analysis of variance was used to test whether maintenance dredging had an effect on seagrass biomass in the access channel border (Green, 1993). Seagrass biomass was log ,Otransformed to reduce the correlation between the mean and the variance, and to normal& the data. There were one pre-dredge and two post-dredge sampling occasions in both the control and access channel border.
3. Results 3.1. Seagrass distribution Zostera capricorni Aschers. and Halophila oualis (R.Br.1 Hook. f. were found in the control area and the access channel border. Halophila spinulosa (R.Br.) Aschers. was
Table 1 Repeated measures analyses of variance of the effect of maintenance Newnort Canal Estate access channel border for (a) Zosrera caoricorni
dredging on seagrass biomass and (b) Haloahila oualis
df
ss
MS
F-ratio
1.23 18.80 1.79 0.60 24.49 52.92
7.23 0.59 0.90 0.30 0.38
12.25 ’ * 1.55 2.37 0.79
Total
1 32 2 2 64 101
(b) Halophila oualis Location Sites (location) Time Location X time Error Total
1 32 2 2 64 101
0.729 0.07 1 0.580 0.465 0.046
10.26 1.54 12.61 * * 10.11 * *
Source of variation
(a) Zostera capricorni Location Sites (location) Time Location X time Error
0.729 2.265 1.159 0.93 1 2.964 8.048
Degrees of freedom (df); sums of squares (SS); mean squares (MS); * * P < 0.01.
?? ??
in the
B.G. Long er al/Aquatic
Botany 53 (1996) 235-243
Access channel border
Control
D
a40 E 20 %
ET 0 2
%
cz
239
LJLI
1 month postdredging
1,.
,_
.
_
14 months post-dredging
20 0 40 :1
500
1000
560
1060
Distanceoffshore(m) Fig. 3. Stacked bar chart showing the distribution of Zosrera capricorni, ??, and Halophila ooalis, • I , with distance from shore in the control area and access channel border for the pre-dredge (September, 1991). one-month post-dredging (March, 1992) and 14 months post-dredging (May, 1993) sampling occasion.
found at one near-shore site in the access channel border pre-dredge samples and was absent from all post-dredge samples. Seagrass was not found further than 600 m offshore in the control area and 300 m offshore in the access channel border over the 3 year study period. Because of this, only those sites less than 600 m from shore in the control area and less than 300 m from shore in the access channel border were used in the repeated measures analysis of variance. The distribution of Zostera capricorni and Halophila oualis in the control area contracted over the study period from 200 to 600 m offshore in the pre-dredge samples to 300 to 600 m one month after dredging, and 400 to 600 m, 14 months after dredging (Fig. 3). The distribution range of Zostera capricorni and Halophila ooalis in the access channel border also contracted over the study period, from 300 m offshore or less, to absent, 14 months after dredging. 3.2. Repeated measures analysis of variance
Above and below ground biomass were significantly correlated for Zostera caprzcorm (r = 0.576; P < 0.05; n = 28) and Halophila oualis (r = 0.912; P < 0.01; n = 22), therefore, above- and below-ground biomass were pooled and used as the response variable in the repeated measures analysis of variance.
240
B.G. Long et al. /Aquatic
Botany 53 (19%) 235-243
Sampling occasion Fig. 4. Seagrass biomass (g m-*) and standard e rror in the control area, ?? , and access channel border, 0, for the pre-dredging (September, 1991). one-month post-dredging (March, 1992), and 14 months post-dredging (May, 1993) sampling occasions for (a) Zostera capricorni and (b) Halophila ooalis.
There was no significant interaction between location (control versus access channel border) and time (pre- and post-dredging) for Zosferu capricorni, and hence no impact effect of maintenance dredging was detected for Z. capricorni (Table 1). Seagrass biomass was higher in the control area compared with the access channel border over
Distance offshore (m) Fig. 5. Sediment type profiles with distance offshore in the control area and access channel border for the pre-dredging (September, 1991) and one-month post-dredging (March, 1992) sampling period. Note: the 14 month post-dredging (May, 1993) sediment profiles were the same as the one-month post-dredging (March, 1992) profiles.
B.C. Long et al./Aqua:ic
Botany 53 (19%) 235-243
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the study period (Fig. 4). In contrast, there was a significant interaction between location and time for Hulophilu ovalis (Table I). This was due to the relatively large decline in H. ovalis biomass in the control area between the pre- and post-dredging sampling, compared with a relatively smaller decline in the impact area (Fig. 4). 3.3. Sediment analysis Before dredging, the sediments in the access channel border were predominantly mud, except for some sandier sediments close to shore. In the control area, the sediments were mainly muds, inshore ( < 300 m), becoming sandier offshore (700 m) and reverting back to muds further offshore. After dredging the sediments in the access channel border became coarser grained whereas the sediments in the control area essentially remained unchanged (Fig. 5).
4. Discussion There was no significant impact of maintenance dredging detected on the distribution of Zosteru capricorni biomass in the access channel border, despite the disappearance of seagrass from the access channel border 14 months after dredging. This was because of the high background variation in the biomass of Z. capricorni in the control area which was due to the patchy distribution of seagrass. In a similar study to monitor the effects, with a BACIR sampling design, of cutting a large channel (60 m wide (3158 m long (5.5 m deep) through thick seagrass beds at McArthur River in the Gulf of Carpentaria, northern Australia, no significant impacts due to dredging were detected on the nearby seagrasses (Kenyon et al., 1995). In contrast there was a significant interaction between location and time for H. ovalis which was due to a relatively large decline in biomass in the control area compared with a relatively smaller decline in the access channel border. The decline in Hulophilu ovalis biomass in the control area was not attributed to the effects of maintenance dredging because the control area, 3.5 km to the west, was outside any obvious influences of the dredging operations. Moreover, the sediment type was the same in the control area before and after dredging. The offshore distribution range for Z. cupricorni and H. ovalis contracted over the study period. Changes in the overall distribution of seagrass in the study area may be part of larger scale changes that occur there based on previous reports of seagrass diebacks occurring in Deception Bay (Young and Kirkman, 1975) and subsequent recoveries (Hyland et al., 1989). Moreover, effects due to seasonal changes of seagrass biomass need to be considered in the sampling design. The biomass of seagrass was significantly higher in the control area than the access channel border. The channel was originally constructed in the 197Os, over 20 years ago, and it would appear unlikely that the observed differences now were due to factors associated with the construction of the charmel. A more plausible explanation may be that factors correlated with changes in the local hydrology near the access channel, or direct effects such as speedboats passing over the access channel borders and physically
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disturbing the seabed, may be responsible for the significantly lower biomass of seagrass in the access channel border than the control area. This study illustrates well the importance of having a control when monitoring environmental impacts. By comparing changes in seagrass biomass in the control area with changes in the impacted area, the natural variation in seagrass biomass was statistically taken into account and provided a powerful reference when deciding whether an impact had occurred or not. Without the control we may have concluded that the disappearance of Zosreru capricorni and Halophila oualis in the access channel border was due to the effects of dredging. The full extent of these changes, however, can not be determined from a single control. Therefore, we recommend the use of multiple controls sampled over a number of occasions before, and after, dredging where possible, as advocated by Underwood (19931, to ensure that the control areas are a representative sample of the natural background variation. In reality, compromises must be made which are based on limited resources and funding.
Acknowledgements We wish to thank C. Peterken for her diligence and help in tracking down the relevant literature on seagrass impact studies and N. Loneragan for his useful suggestions on ways to improve the manuscript. The assistance of the Redcliffe City Council, by providing funds for the impact assessment, is greatly appreciated.
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Tydac International, 1990. Nepeau, Qnt., Canada. Underwood, A.J., 1993. The mechanics of spatially replicated sampling programmes to detect environmental impacts in a variable world. Aust. J. Ecol., 18: 99-l 16. Young, P.C. and Kirkman, H., 1975. The seagrass communities of Moreton Bay, Queensland. Aquat. Bot., 1: 191-202. Young, P.C., 1978. Moreton Bay, Queensland: a nursery area for juvenile penaeid prawns. Aust. J. Mar. Freshw. Res., 28: 745-773. Zieman, J.C., 197.5. Tropical seagrass ecosystems and pollution. In: F. Wood and R. Johannes (Editors), Tropical Marine Pollution. Elsevier, Amsterdam, pp. 63-74.