Organic enrichment or eutrophication results in an increase in nitrogen and/or hydrogen sulphide in marine sediments, which, in turn, leads to changes in benthic communities. Most previous studies only demonstrated such changes in qualitative terms. In this study, response of a soft shore benthic community to eutrophication was followed in a field manipulation study and a microcosm experiment in order to quantify community changes with respect to the amount of nutrients added. The field manipulation experiment examined the long-term effects of nutrient enrichment of nitrogen (N) and phosphorus (P) on (i) sediment characteristics, (ii) inter-tidal benthic community structure, and (iii) vertical distribution of marcofauna. Twelve 1 m2 plots were randomly selected on the mid shore of a sandy beach and divided into four treatment groups, each with 3 replicates: 0x (control), 1x (1.5 g N/m2, 0.1 g P/m2), 2x (i.e., 3 g N/m2, 0.2 g P/m2) and 4x (i.e., 6 g N/m2 and 0.4 g P/m2) of nutrient enrichment using slow-release gypsum matrix fertilizer stakes. Core sediment samples (D: 8 cm H: 20 cm) were collected from each plot at monthly intervals for the first 4 samplings and then quarterly for the last 3 sampling over a period of 1 year, for analysis of infauna at intervals of 0-5 cm, 5-10 cm and 10-20 cm depth. Sediment particle characteristics and levels of total organic carbon (TOC), total Kjeldahl nitrogen (TKN), total phosphorus (TP) and acid volatile sulphide (AVS) were also measured. Over the field study period, sediment particle size did not significantly alter with nutrient enrichment, but nutrient levels in deeper sediment increased with time. A total of 45 species, including nemerteans, polychaetes, molluscs, crustaceans, brachipods and sipunculids, were recorded and mean species diversity and evenness of infaunal community showed a decrease with increase in nutrient levels. Among the faunal groups, polychaetes showed significant changes among treatments and with time. Responses of infaunal composition changes to nutrient enrichment in 0-5 cm and 5-10 cm sediment depth were more pronounced than that in 10-20 cm depth. In the treatment plots with 4x nutrient enrichment, species were dominated by capitellid polychaetes, which are typical indicators of organic pollution. The microcosm experiment tested the short-term effects of nitrogen and sulphide on soft shore sediment infauna over a period of 12 weeks. The experiment consisted of 4 treatments: (A) sediment cores (D: 8 cm H: 20 cm) dosed with nitrogen to an in-situ mean level of 0.48 mg N/g dw, (B) sediment cores dosed with sulphidic solution to an in-situ mean level of 310.6 μM S/g dw, (C) sediment cores dosed with nitrogen and sulphidic solution to in-situ mean levels of 0.46 mg N/g dw and 340.0 μM S/g dw, respectively, and (D) control sediment cores with no addition of N and S. Sediment cores were retrieved for faunal and sediment (ammonia and total organic carbon) analyses after weeks 2, 4, 8 and 12. From the results of two-way ANOVA, no significant difference in treatment and time was observed in the total organic carbon content in the sediment cores over the experimental period. For ammonia, however, significant difference in treatment and time was noted. All treatment groups showed an increase in ammonia levels towards the end of week 12. In terms of faunal diversity, 1,431 specimens belonging 30 species were recorded from the cores, with polychaetes being the most abundant with respect to species and numbers. Mean species and individual numbers, species richness, diversity and evenness indices of the cores were statistically different among treatments, with a decrease in species number, richness, diversity and evenness, and an increase in individual numbers over time. Multivariate analyses of the faunal data suggested that benthic composition of treatments A (with nitrogen) and D (control) did not differ during the first 8 weeks, but changes in benthic structure in treatments B (with sulphide) and C (with nitrogen and sulphide) were evident. All treatments showed relatively similar species composition towards week 12, i.e., end of the experiment, although abundance of the dominant species varied. Results of both field and laboratory experiments suggested that response of benthic infauna to nutrient enrichment largely followed a direct dose-response relationship. There were also synergistic, adverse effects of N and S on the faunal community composition. However, factors such as hydrographic situations and the availability of opportunistic species may affect the outcome of the changes of the infaunal community structure over time.
| Date of Award | 15 Jul 2005 |
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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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