Marine benthic community, especially the macrobenthic infauna (>0.5 mm in body length), have been widely regarded as bioindicator for monitoring coastal pollution.Macrobenthic infauna is relatively sessile and sensitive to environmental changes such as nutrient level and oxygen concentration. They respond quickly in changes of species composition, species diversity, abundance and biomass. Many studies have documented the ways of benthic response to organic pollutionworldwide and a generalized process iswell established.Conversely, studies about benthic response upon recovery fromorganic pollution are limited. In temperate waters, reports have suggested that there is a slow change of species composition (at least 3 years) upon cessation of organic pollution. In sub-tropical waters, such benthic response, however,is speculated to be much faster (in terms of months) due to warmer water and stable climatic conditions. The aim of the present study is to investigate the response of benthic community upon recovery from organic pollution in sub-tropical waters.The study site is Victoria Harbour, Hong Kong where has hadsuffered from severe organic pollution from sewage discharge in the past.The water qualityis, however, recovering after commencement of a large-scale sewagetreatment project in 2001.A two-year benthic survey(June 2004 April 2006) was conducted by taking grab sediment samples at 2 inside-harbour locations (Sai Wan Ho, Causeway Bay) and 3outside-harbourlocations (eastern side: Tung Lung Chau; western side:Tsing Yi, Peng Chau).In general, the benthic recovery was comprehensively assessed from various aspects: (1) physico-chemical parameters of sediments; (2) species composition of macrobenthic community; (3) life-history traits and feeding guilds of polychaete community and; (4) bioassay on sediment quality using larvae of opportunistic polychaete Capitella sp. I. The physico-chemical parameters of sediment included median particle diameter (MD),total organic carbon (TOC), total Kjeldahl nitrogen (TKN), total phosphorus (TP) andammonia (NH3).Bottom water quality data (dissolved oxygen (DO), total nitrogen, total phosphorus, ammonia and 5-day biological oxygen demand (BOD5)) were obtained from Environmental Protection Department, HKSAR Government.At the inside-harbour locations, the organic loading of near-bottom waters decreasedsignificantlyafter the commencement of sewage treatment project but no further improvement could be observed during the present study period.No sign of sediment recovery was detected inside the harbour due to persistent high organic content and NH3level.Relatively, the outside-harbour locations are cleanin terms ofnutrient levels insediments.The results reflected that sediment recovery was not necessarily in phase with improvement of water quality. The sediment recovery could be a slower process requiring more than 2 years.Three factors are suggested for slow sedimentrecovery:organic matter refractory to biodegradation (i.e., high C:N ratio);organic wastes from sewage outfalls with presence of other contaminants (e.g.,heavy metals); and declines of population and diversity of benthic fauna leading to slow consumption and mineralization of organic matter. The species compositions of macrobenthic communities were spatially different among 5 sampling locations. Distinct benthic communities were caused byunique configurationofwater depth, MD,TOC and TKN in the sediment at every location.Macrobenthic community at inside-harbour locations is shown to undergo a specific succession pattern different from outside-harbour locations. Temporally, aweak sign of benthic recovery was detected at inside-harbour locations. The species dominance was shifted from an opportunistic polychaete Prionospio malmgrenito a locally ubiquitouspolychaeteSigambra hanaokai. The abundance of aneutrophic-water flourishingbivalve Ruditapes philipinarumdropped greatly from high number to nearly zero.Increase of Shannon-Weaver Diversity Index ()was observed in a longer time scaleby comparing with previous studies.The weak sign of benthic recovery inside the harbour could be relatedto improved water quality rather than sedimentquality. At Tung Lung Chau, significant change of macrobenthic community was detected with stable water and sediment parameters. Such change is possibly causedby natural recruitment of the polychaete Prionospio malmgreni. At Tsing Yi and Peng Chau,change of species composition isconsidered insignificant due to low total abundance of macrobenthic community. The dominance of an opportunistic polychaete species Mediomastus sp. reflects stressed community and the contamination of heavy metals isapossible reason. The analyses of life-history traits and feeding guilds of polychaete community were employed as a complementary tool for tracing benthic recovery. Proportions of opportunistic species in abundance (opp-abundance) and biomass (opp-biomass), and Wgenerated by Abundance-Biomass Comparison (ABC) method were adopted for representing the life-history traits. Feeding guilds of polychaete species were classifiedaccording to a scheme developed by Fauchald and Jumars (1979) while feeding guild diversity (H-FG) and feeding guild composition were analysed. At inside-harbour locations, there was no temporal change of opp-abundance, opp-biomass and H-FG. At Sai Wan Ho,the originallydominant feeding guilds (surface deposit feeders and omnivores) were replaced by carnivores.Increased proportion of carnivorescan be a sign of benthic recovery since abundance of predators is possibly higher.Asignificant increasing trend of Wprovides further support for benthic recovery insidethe harbour.At Tung Lung Chau, opp-abundance, opp-biomass andW remained steady that showed a stable status of benthic community. The decrease of H-FG and increased dominance of surface deposit-feeders were possibly caused by natural recruitment.AtTsing Yi and Peng Chau, no change of opp-abundance, W and H-FG was detected. The deposit feedersand carnivores were the dominant feeding guilds generally.The contamination of heavy metals was reflected by the increasing trend of opp-biomass.For tracing benthic recovery,analyses of life-history traits and feeding guilds composition appear to be a better approach than that of species composition since the formers reflect overall functionalities of benthic community.However,the results of these analysesshould be carefully assessed by comparing with other biological measures of environmental disturbances such as H.Inthe present study, data of the opp-abundance, opp-biomass and feeding guilds aremore relevant than results of ABC method in determining recovery in benthic community in Hong Kong waters. The bioassay on organic content of sediment was conducted bimonthly from December 2005 to October 2006. The larvae of opportunistic polychaete Capitella sp. I.preferssettling on organically rich sediment. Hence this larva is a useful bioindicator for tracing the changes of organic content insediments. A still-water single choice experiment was conducted that larvae were provided with fine sediment from either 1of 5 sampling locations. The spatial and temporal variations of the larval settlement percentage were used for determiningsediment recovery. According to the results, the percentage of larval settlement was significantly lower at Tung Lung Chau where sediment was lowest in organic content. There was no temporal change of settlement rate at alllocations.Itindicated that the quality of sediment which attracted larval settlement of Capitella sp. I remained unchanged over the study period. Generally, the status in Victoria Harbour is still organically polluted but slow benthic recovery is detected, according to data on life-history traits and feeding guild composition of the macrobenthos.A full benthic recovery may require more than 2years in Victoria Harbour. During the whole study period, the status of macrobenthic community succeeded from pioneer community to early phase intermediate community in the harbour. Further community successions to later phases of intermediate community are thus expected under continual improvement of the benthic environment. According to the present findings, the rates of benthic recovery from organic pollution appearsimilarbetween temperateand sub-tropical waters.However, a direct comparison is not feasible because of different sampling and analysis methods used in the literature.Data from this study suggested that the rate of benthic recovery does not solely depend on latitudinal zonation butthe type of pollution source, and spatial and temporal scales of the pollution.
| Date of Award | 2 Oct 2007 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Kam Shing Paul SHIN (Supervisor) |
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