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Title: Effects of deployment of artificial reefs on the marine benthic environment, with special reference to sediment physico-chemical characteristics
Other Titles: Ren gong yu jiao de bu shu dui hai yang di qi huan jing you qi shi chen ji wu wu li hua xue xing zhi ying xiang zhi yan jiu
Authors: Wai, Ho Yin (衛浩賢)
Department: Department of Biology and Chemistry
Degree: Master of Philosophy
Issue Date: 2009
Publisher: City University of Hong Kong
Subjects: Marine ecology.
Artificial reefs -- Environmental aspects.
Notes: CityU Call Number: QH541.5.S3 W34 2009
xx, 194 leaves : ill. 30 cm.
Thesis (M.Phil.)--City University of Hong Kong, 2009.
Includes bibliographical references (leaves 144-191)
Type: thesis
Abstract: Marine resources have been declining all over the world since the 1950s. A solution to this problem is to maintain habitat heterogeneity particularly in coastal areas. This can be achieved by the deployment of artificial reefs (ARs). These are man-made structures that are placed on the seabed so as to simulate natural reefs. The typical objectives of deployment include the enhancement of fisheries production and mitigation so that a new habitat is created that can compensate for loss or damage to natural seabed due to anthropogenic activities. While ARs can enhance fisheries and serve as biofiltration units, there are concerns that these man-made structures might impact the surrounding benthic communities especially those living in the adjacent soft-bottom sediments. These impacts may be caused by changing the localized hydrographic regime such as water circulation, wave action and sedimentation rate. The general attitude is that the deployment of ARs may alter the soft-bottom assemblage by modifying the physical nature of the surrounding substratum. This study was an attempt to investigate impacts resulting from the presence of ARs on soft-bottom seabed in Hong Kong, through comparison of the physico-chemical characteristics of sediments collected from the areas with the presence of ARs and the adjacent areas nearby in Hong Kong. Particular attention was focused on water content, silt/clay fraction and nutrients. In addition, fatty acid profiles of the epifauna on the ARs, suspended particulates and sediments at the AR sites were analyzed to identify the effect of ARs on trophodynamics of the soft-bottom communities. The study area was Outer Port Shelter, which is located in eastern coastal waters of Hong Kong. Artificial reefs were deployed at the study site in 2002-03. Sediment samples were collected at AR deployment area and its adjacent area during the period March 2006 to February 2007. Subsequent laboratory analyses included total organic carbon (TOC), total Kjeldahl nitrogen (TKN), total phosphorus (TP), sediment enzymatically hydrolyzed protein, water content and silt/clay fraction. The results of the analysis of sediment parameters indicated that there was an alteration of sediment physico-chemical characteristics between the AR and control areas. For physical parameters, significantly lower water content was found in sediments in the AR area (mean 37.9%) than the control area (mean 41.3%). However, ANOVA results showed that there was no significant difference in silt/clay fraction between the AR and the control areas (mean: 75.1-79.9%). For chemical characteristics, TOC, TKN and protein content in sediments were significantly lower in the AR area (mean values of TOC: 0.66%, TKN: 921.7 mg/kg, protein: 234.9 mg/kg) when compared with the control area (mean values of TOC: 0.85%, TKN: 1,111.7 mg/kg, protein: 250.7 mg/kg). In contrast, TP content showed no significant spatial difference in sediments between the AR and control areas (mean: 418.3-437.2 mg/kg). TOC, TP and protein content in sediments also showed temporal differences during the sampling period, in which TOC level in May 2006 (mean: 0.59%) was significantly lower than that in April, July and October 2006 (mean: 0.83-0.85%). TP detected in sediments in July 2006 (mean: 670.8 mg/kg) was significantly higher than the most of the other sampling months (mean: 331.3-351.5 mg/kg). Protein content was also observed to be the lowest in November 2006 (mean: 190.4 mg/kg) and the highest in February 2007 (mean: 281.5 mg/kg). No significant temporal differences, however, were observed for TKN, water content and silt/clay fraction in the sediment. The present data suggested that the deployment of ARs on the seabed does modify the sediment physico-chemical characteristics in that area, leading to a potential variation in nutrient levels, and hence, food availability, to other benthic organisms. Such alteration may also cause impacts on surrounding physical and biological habitats, including the diversity and abundance of benthic communities. In order to evaluate the spatio-temporal changes of marine marcobenthic community caused by the AR deployment, marcobenthos (>500 μm) were collected using 0.1 m2 van Veen grab in April, July, October 2006 and January 2007. Animals in the sediment were hand-sorted, identified and enumerated. Inside the reef area, the mean species number (S) ranged from 26.7 to 30, individual number (N) from 55.8 to 68.2 (ind/0.5 m2), species diversity (H’) from 2.91-3.12 and species evenness (J) from 0.89-0.92, whereas outside the reef area, the mean S ranged from 20.2 to 26.5, N from 36.8 to 55.5 (ind/0.5 m2), H’ from 2.63-2.95 and J from 0.87-0.91, respectively. Results of two-way ANOVA showed that significant spatial differences were noted with higher values of S, N and H’ at the sampling sites inside than outside the reef area. However, there was no significant spatial difference for J. There were also no significant temporal differences for S, N, H’ and J among the sampling seasons. The species composition of the macrobenthos inside the artificial reef area differed from that outside the reefs. Within the reef area, species tended to be more diverse, with more dominant taxa than that outside the reefs. In particular, amphipods were abundant at the reef sampling sites, whereas outside the reef area the snapping shrimp Alpheus brevicristatus was commonly found. Over the study period, seasonal changes in macrobenthic community structure inside and outside the reef area were not evident and there were also no significant temporal changes in S, N, H’ and J of the samples. For analysis of fatty acid profiles, sediment samples were collected by divers using cores in the AR area in July-August 2007 and May-June 2008. In each sampling, sediment samples were taken at stations along a transect 1, 5, 20, 50 m from selected ARs. One transect was laid at the base of the AR upstream of the water current direction whereas another transect was laid downstream of the water current direction. Six sediment traps were also deployed at each of the selected ARs where transect core samples were collected. Three traps were set on top of the AR and three near the bottom of the AR. The traps were deployed for 90 days prior to retrieval by divers. In addition to collection of sediment samples, total particulate matter in the seawater column and epifaunal organisms on the AR surface were collected. Laboratory analyses of the collected samples included nutrient concentrations (TOC, TKN and TP), silt/clay fraction, water content and fatty acid profiles. Based on the sediments collected in the sediment traps at the ARs, the calculated mean sedimentation rates (644.3 g/m2/day) for the lower sediment traps were significantly higher than that measured in the upper traps (399.9 g/m2/day). No temporal difference was detected between the sedimentation rates measured in July-August 2007 and May-June 2008. The higher sedimentation rate at the bottom of the ARs may be a result of the biodeposits produced by the filter-feeding epifauna on the AR surface. With regard to physico-chemical characteristics, TOC showed significant spatial difference at different transect stations. TOC measured in sediments sampled at 1 m (mean: 0.47%), 5 m (mean: 0.47%) upstream, and 5 m (mean: 0.48%) downstream of the ARs were significantly lower than that in sediments of 50 m upstream (mean: 0.71%) and downstream (mean: 0.75%) of the ARs. However, two-way ANOVA results showed no significant spatial differences in water content (mean: 35.5-52.2%), silt/clay fraction (mean: 51.5-72.1%), TKN (mean: 557.3-1154.9 mg/kg) and TP (mean: 312.3-497.7 mg/kg) of sediments sampled at different transect stations. TOC, TKN and TP in sediments showed temporal differences during the sampling periods. All of them were found significantly lower in July-August 2007 (mean values of TOC: 0.54%, TKN: 680.3 mg/kg, TP: 344.4 mg/kg) than that in May-June 2008 (mean values of TOC: 0.66%, TKN: 1021.8 mg/kg, TP: 426.7 mg/kg). However, no significant temporal changes were found in water content and silt/clay fraction. A total of 26 fatty acids, including 9 saturated fatty acids (SFAs), 7 monounsaturated fatty acids (MUFAs) and 10 polyunsaturated fatty acids (PUFAs), were determined in the samples. The present results showed that in general the fatty acid profiles in seabed sediments sampled at 1, 5, 20 and 50 m upstream and downstream of the ARs were similar. The fatty acid profiles in the sediment collected in upper sediment traps were also similar to that in lower sediment traps. The fatty acid profiles of oyster tissue and total particulate matter, however, showed unique patterns when compared with the profiles of sediment samples. This unique fatty acid profile of oyster tissue was attributed to the presence of some fatty acids, especially the PUFA docosahexaenoic acid C22:6n3 (mean: 23.0%) which was only found in oyster tissue samples. For the total particulate matter samples, the unique pattern was contributed by the high abundance of some SFAs such as C16:0, C18:0, C22:0 and C24:0 (mean: 21.6-42.7%) in different sampling periods. For detailed comparison of each fatty acids, ANOVA results showed that for SFAs, C17:0 was detected the highest in upper sediment trap samples (mean: 13.2%); C18:0 was significantly higher in 20 m upstream sediments (mean: 36.3%) when compared with that at 50 m upstream of the ARs (mean: 24.8%); C21:0 was found in particulate, sediment trap and oyster tissue samples (mean: 0.94-6.51%) but not in sediments; and C24:0 was significantly lower in 50 m downstream sediments (mean: 2.23%) and particulate matter (mean: 10.8%) than that in upper sediment traps (mean: 14.6%). For MUFAs, C14:1 was significantly higher in 50 m downstream sediments (mean: 6.8%) than that in the other sediment and tissue samples (means: 0.0-0.2%); whereas C16:1n7 was measured the highest in lower sediment traps (mean: 17.7%) than in particulate, sediment and oyster tissue samples (mean: 3.4-10.7%). For PUFAs, C20:2 was significantly higher in 1 m upstream sediments (mean: 8.3%) than that in 5 m downstream sediments of the ARs (mean: 0.0%); C20:3n3 was only detected in the lower sediment traps and oyster tissues (mean: 0.7-1.6%); C20:4n6 was found in upstream 5 m sediments, particulates and oyster tissues (mean: 1.3-6.1%); C20:5n3 had higher content in oyster tissues (mean: 6.6%) than that in upper sediment traps and downstream sediments (0.0-0.7%); C22:2 was only measured in 20 and 50 m downstream sediments of the ARs (mean: 0.4-0.5%); and C22:6n3 was only detected in oyster tissues (mean: 23.0%). In comparing samples collected in July-August 2007 and May-June 2008, significant temporal differences were observed in some of the fatty acids from ANOVA results. C14:0, C14:1, C15:1, C17:0, C18:1n9c, C18:2n6c, C18:2n6t, C20:2 and C22:6n3 in samples collected in July-August 2007 were significantly lower than that in May-June 2008. However, C16:0, C18:0, C18:1n9t, C18:3n3 and C20:3n3 in samples collected in July-August 2007 were significantly higher than that in May-June 2008. In summary, the presence of ARs has shown to alter the physico-chemical characteristics of the nearby sediments. The epifauna colonized on the AR surface and/or fish aggregating at the reefs can also enhance sedimentation due to the production of biodeposits and biological wastes, which may serve additional food sources to the infauna living in the adjacent areas of the ARs. Based on the analysis of the nutrient levels and fatty acid profiles in the environmental and biological samples collected, the effects of the ARs on the nearby benthic habitat appear to be localized, which is only evident in close proximity to the AR deployment area.
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