Effects of human disturbance on biological traits and structure of macrobenthic communities | 人為擾動對大型底棲動物群落的生物特徵及結構的影響 | Ren wei rao dong dui da xing di qi dong wu qun luo de sheng wu te zheng ji jie gou de ying xiang

xxx, 326 leaves : ill. 30 cm. + 1 CD-ROM (4 3/4 in.)

Thesis (Ph.D.)--City University of Hong Kong, 2010.

Rapid changes in marine biodiversity are occurring globally due to human disturbances, such as fishing and pollution; yet, the ecological impacts of functional features and diversity loss in ecosystems are poorly understood. The effects of trawling on benthic habitat and community structures have drawn much attention in recent years. Trawling is probably the most significant factor affecting the structure of soft sediment communities globally and may lead to large-scale shifts in the functional composition of the marine benthos, with likely effects on the functioning of the entire coastal ecosystem. However, the use of functional features, in combination with traditional methods of analysis of community patterns, based on biodiversity data to detect the effects of trawling is scarce. Although Biological Traits Analysis (BTA) is considered to be a powerful method for evaluating the ecological functioning of benthic assemblages, only a few studies have been reported in temperate waters. Further, the focus of these studies has generally been on the anthropogenic impact. This thesis discusses the impact of trawling on different coastal systems based on the combined use of traditional biodiversity analysis and BTA methods, taking into account the amount of rare species and their total contribution to ecosystem functioning. There have been no previous studies published using this methodology. In this study, BTA was used together with traditional biodiversity analysis to investigate how the structure and function of macrobenthic communities are affected by: 1) non- or low- (known) trawling frequency in two different water masses (Arctic and Atlantic); 2) high-trawling frequency with annual hypoxia (hypoxic gradient on infauna and epifauna) and from three coastal systems with different controls (infauna): (a) a fjord system in Norway where trawled sites were compared to non-trawled sites, (b) an upwelling system in the southern part of Africa (coastal South Africa and Namibia) where heavily trawled sites were compared to lightly trawled sites and (c) a subtropic system in Hong Kong where heavily trawled sites were compared to a marine protected area (MPA). 3) recovery from trawling inside the MPA in Hong Kong where past and present data were compared. All these systems showed changes in structure and functioning to some different degree. As reviewed in literature, the role that rare species play in ecosystem functioning is not well understood. Traditional biodiversity data analysis methods tend to underestimate the importance of rare species. However, the present study showed that rare species are very important when considering the total pool of biological traits (BTs) and, therefore, should not be ignored. In the study of Norwegian water masses, taxonomic composition, abundance of taxa and BTAs were used to investigate differences in structure and functional diversity. Two distinct marine macrobenthic assemblages were considered: the Arctic (cold water) and the southern part of Norway (relatively warm water). Multivariate analysis techniques were used to examine each assemblage’s structure and functioning at 60 sampling stations. The data from seven BTs were divided into 36 categories, for 284 common marine benthic taxa. The two areas showed clear differences in taxonomic composition and relative species abundance. However, when BTs information was taken into account (weighted), the differences between the two areas in the ordination plot were not so apparent. When only the presence or absence of species in the BTs data was considered, there was no significant difference between the assemblages. All of the above suggested that the same BTs are represented in both water masses, but to different degrees, depending on the community dominance of species adapted to each system. It was also noteworthy that in these two Norwegian water masses, several species within the same genus and family had exactly the same combination of BTs. The results thus indicated that different species possessed the same trait combination, even though they came from different water masses. This finding emphasized a balance in functional traits and indicated that different species contributed equally to the BTs for this analysis. However, the effects that species composition and diversity have on ecosystem functioning are difficult to distinguish, and this observation is not mutually exclusive of an idiosyncratic pattern. In the Norwegian (Oslofjord) study, taxonomic and BTs compositions of communities from sampling stations collected in trawled and non-trawled areas were compared. Surprisingly, there were significantly higher numbers of species, individuals and BTs diversity at the trawled locations compared to the non-trawled locations in the Oslofjord. The Intermediate Disturbance Hypothesis may explain this finding since repeated trawling will act as an occasional community disturbance. The hypothesis predicts that a certain degree of disturbance may enhance diversity, provided that the disturbance is not too severe. In cases of severe disturbance, a reduced diversity would result. The South Africa study explored the use of BTA to assess differences in the ecological functioning of infaunal communities between areas exposed to heavy and light-trawling intensities in the southern Benguela region of the south-east Atlantic. Multivariate analyses of biomass were employed to investigate differences in infaunal community composition between sites and differences in intensities of trawling. Multivariate analyses showed significant differences among sampling sites, as well as between heavily and lightly-trawled areas (ANOSIM, p < 0.05). The analysis of infauna biomass weighted by BTs showed significant differences between heavily and lightly trawled areas for 17% of the traits investigated (non-parametric Mann-Whitney U test, p < 0.05). BTs were also shown to differ significantly between areas having larger or smaller proportions of sand (12% traits differed significantly) and mud (7% traits differed significantly). This suggested that in the coastal region of southern Africa, the disturbances caused by trawling contributed more to the observed differences in BTs than sediment composition. In the Hong Kong study, heavily trawled sites inside Tolo Channel were compared to a Marine Protected Area (MPA) in Hoi Ha Wan, which has been closed to trawling for approximately 12 years. There were significant differences in community structure and biological functioning between the wet and dry seasons. However, the BTs results showed that there were no significant differences between the trawled area and the non-trawled area (MPA). It was noteworthy that seasonal changes appeared to play a more important role in determining both the structure and functioning of the two macrobenthic communities (trawled and non-trawled) than that played by the effects of trawling. Prior to this study, it was assumed that biodiversity would increase after the MPA was established (i.e., after trawling has ceased) and that larger and long-lived species would dominate. However, when the author’s benthic data from the MPA was compared to historical data (i.e., prior the closure of the area to trawling), it was found that the opposite was the case, i.e., the biodiversity and abundance had decreased dramatically inside the protected area since trawling had ceased. Regarding the MPA in Hoi Ha Wan, there are three important factors to consider: the rate of recovery from the cessation of trawling, the hydrodynamics of the protected area and the presence of artificial reefs deployed in the MPA. Given the difficulty of assessing the individual effects of these factors, it is hard to deduce any clear cause for the decreasing trend in biodiversity after the closure of the area for trawling. It is suggested that further, long-term research is carried out on the structural and functional diversity inside the MPA. This research should include changes around the artificial reefs and comparisons of the community structures over time between trawling sites and the MPA. The second study in Hong Kong was related to heavily trawled sites with annual hypoxia problems. Organic pollution and eutrophication arising from poor water circulation and dispersion is a known problem in the Tolo Harbour area and has caused major changes in the structures of phytoplankton, fish and benthic communities. The differences in the macrobenthic communities between the wet and dry season are significant in Hong Kong waters. Data taken at the end of the wet season showed that there was a clear increase in both the hypoxic gradient and the total organic carbon (TOC) gradient moving inland from coastal areas (i.e., from Mirs Bay towards the Tolo Channel and the inner harbour). In general, the dissolved oxygen increased after trawling for all four of the layers measured (1 cm below the sediment surface, and 1 cm, 50 cm and 1 m above the sediment surface). The biodiversity of the infauna decreased with increasing levels of TOC in the sediment. The epifauna followed a similar pattern. In the dry season, the level of dissolved oxygen (DO) was high for all the stations, and the differences after trawling were not as clear in the upper layers (50 cm and 1 m above the sediment surface) as for the wet season. High mortality occurred in the summer due to the low oxygen content in the inner part of the Tolo Harbour. However, in the winter (dry season), the community managed to revert to normal due to the higher oxygen content, and rapid re-colonization occurred. There were significant differences in BTs composition for the infauna between the two seasons. A closer examination of the traits showed significant differences for 14% (five categories) of the 36 categories considered. These five categories were: size < 5 mm: medium mobility; dorsal flat body form; permanent tube habitat and scavenger feeding type. For the epifauna, 58% (21 out of 36) of the categories showed significant differences. It was anticipated that opportunistic and small-body-size species would be abundant under more hypoxic conditions (summer/wet season). However, the significant BT characteristics of the few species which remained under the hypoxic summer conditions (i.e., no mobility, cylindrical body, permanent and sessile attachment) suggested adaptation rather than opportunism to the low DO levels. In this thesis, the BTs under different environmental stressor conditions (e.g., different levels of trawling and hypoxia) were examined. The differences in BTs which were observed have led to a better understanding of the impact due to changes in some environmental conditions. A similar examination of the differences in BTs may also help with the future assessment of the effects of different environmental changes (stressors) on the soft benthic community. Study of the changes in the relative proportions of BTs considered in this thesis complements traditional methods of biodiversity and community structure analyses. This combined approach may be helpful in identifying impact-driven alterations to ecological functioning and may also offer more information on ecosystem monitoring, management and conservation.

Includes bibliographical references (leaves 257-319)