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Walter Martin Widmer
Centre for Research on Ecological Impacts of Coastal Cities - The University of Sydney Sydney, New South Wales, AUSTRALIA
Although land-based activities are considered to be the main source of marine debris in coastal areas, marine-based sources may also be significant. This study tested the hypothesis that recreational boating is a contributing source of submerged marine debris. This was done by collecting information about the number of recreational boats anchored at six beaches in Sydney Harbour and the amount of benthic debris off these beaches. A significant positive correlation was found. The greatest amount of debris was found adjacent to the two beaches most visited by recreational boats, both situated inside a marine protected area. The most frequent types of litter were plastic bags, aluminium cans and glass bottles. Implications of these results to management and restoration attempts of littered areas are discussed.
Marine debris is being increasingly recognized as a serious type of pollution (Stefatos et al., 1999) and the detrimental consequences of such debris have been widely documented. Marine debris represents a threat to an array of organisms, including turtles, mammals, birds and fish (Laist, 1987). Fouling invertebrates may also use marine litter as habitat, which may represent a new ecological niche for some species (Winston, 1982). Litter can also be a vector of transportation, with potential implications for the geographic distribution of some species (Gregory 1991; Barnes 2002). For humans, marine debris may be harmful (Dixon and Dixon, 1981) and is aesthetically repulsive (Williams and Nelson, 1997), which may have undesired consequences for areas where the local economy is based on tourism (Corbin and Singh, 1993).
Most studies about marine debris have been done on beaches or in pelagic environments (e.g, Frost and Cullen, 1997). Studies focused on the presence of litter on the seabed were presented by Galil et al. (1995) and Stefatos et al. (1999), among others. Williams et al. (1993) and Backhurst and Cole (2000) have investigated benthic litter in sublittoral areas.
Although land-based activities are considered to be the main source of marine litter in coastal areas (Nollkaemper, 1994), marine-based sources may also be significant (Prutter, 1987). Among several marine-based activities, recreational boating has been suggested as having, in some cases, a significant role as a source of marine debris (Cahoon, 1990; Backhurst and Cole, 2000). Sydney Harbour (33°50' S, 151°16' E) is one of the main areas for recreational boating in the southern hemisphere, with approximately 40 marinas and 35,000 recreational boats (Underwood and Chapman,1999; DUAP, 1999).
Within the harbour, on its northern sector, lies the North Harbour Aquatic Reserve (NHAR). In the Australian State of New South Wales, aquatic reserves were designed to protect fish, habitats for fish, biodiversity and marine invertebrates (Fisheries, 1999). The NHAR was created in 1982 and, because it still lacks a formal plan of management (Bohm. 2000), its official aims are not clear. A placard located on the foreshore of the reserve say that "divers and snorkellers are encouraged to enjoy the natural beauty of the reserve ". Therefore, it seems reasonable to assume that the aims of the NHAR include protection of marine biota and the provision of a rewarding experience for visitors. Some of the beaches inside the reserve (Quarantine beach and Store beach) have been suggested as being popular anchorages for recreational boaters (Mathews, 1997). Considering the detrimental consequences of debris in marine environments, it is reasonable to consider that the presence of anthropogenic benthic litter in a reserve would be contrary to its conservational objectives.
Therefore, the aim of this study was to investigate the potential relationship between recreational boating and the amount of submerged litter inside the NHAR, by studying the anchoring patterns of recreational craft and the quantities of benthic litter off six beaches located in Sydney Harbour. Specifically, the following hypotheses were tested:
(i) Previous observations and literature (Mathews, 1997) suggest that some areas are preferred for anchoring when compared to others. The model proposed is that two beaches inside the NHAR (Quarantine beach and Store beach) are more used as an anchoring ground for recreational boats than are other beaches located inside (Reef beach and Washway beach) and outside (Lady Bay beach and Obelisk beach) the reserve (Fig.1). Therefore, if the number of recreational boats anchored in these beaches were counted, more boats would be expected to be anchored at the first two beaches than at the last four.
(ii) Literature evidence suggests that recreational boating may contribute to the presence of submerged litter in marine habitats (Cahoon, 1990; Backhurst and Cole. 2000). The model proposed is that this pattern is valid for the six beaches chosen for this study. Therefore, if the number of litter items in these beaches were representatively counted, a positive correlation between the mean number of anchored boats and the mean amount of debris items would be expected.
Data on anchored boats
Three independent measurements were taken for each beach from vantage points during weekends in the austral summer of the year 2001 - December 2000 to March 2001, because previous observations indicated a significant increase in recreational boating during weekends. The independence of the data was obtained by taking only one replicate measurement per day. To avoid within-day variability, all measurements in a given month were taken during the mid-afternoon, defined as at three quarters of the time span between sunrise and sunset for the 15th day of that month. Anchored boats were counted from vantage points using binoculars and classified into three categories, according to Adam et al.'s (1992) definitions of recreational boats: speed boats, motor cruisers and sailing yachts.
A few stranded craft on the beach, such as jet skis, rowing boats and sailing dinghies were not included, because they were not anchored. Commercial boats (identified by distinctive characteristics on the hull or on the rigging) were also not abundant and were not included, because this study is focused on recreational boating.
Data on marine debris
Benthic litter present off the six beaches was collected using a constant amount of search effort across beaches. Ten transects (10x4 m) were swum perpendicular to the beach (approximately at the 3 meter isobar) and distributed along the entire length of each beach. All visible submerged litter items along the transects were collected. The material was kept inside plastic bags and brought to the laboratory, where they were counted (e.g. Willoughby et al., 1997) and sorted into the following categories: plastic, metal, glass, paper and others.
Statistical treatment
Three types of beaches were defined, according to the proposed level of boat anchoring and their location. Quarantine beach and Store beach were labelled 'heavy anchoring - inside NHAR'; Reef beach and Washway beach were labelled 'light anchoring - inside NHAR' and Obelisk beach and Lady bay beach were labelled 'light anchoring - outside NHAR'.
Mean numbers of anchored boats on the three types of beaches were compared using analysis of variance. The first, fixed factor was type of beach (with three levels); the second factor was replicate beaches (nested in the first factor and random). The homogeneity of variances was assured by appropriate transformations of data, if necessary. When a factor was found to be significant, Student-Newman-Keuls (SNK) tests were done to test differences between levels within that factor.
The significance of an eventual correlation between the number of anchored boats and the amount of marine debris was tested using Pearson's correlation index r.
Confounding sources of debris
The possibility that the litter found would originally come from sources other than recreational boats is considered to be small, because the six beaches were chosen in a way that their foreshores are protected by a national park (Sydney Harbour National Park, see Fig.1), without residential developments and with restricted access from the land (only walking tracks, no car access). The beaches that were labelled 'heavy anchoring - inside NHAR' are particularly restricted for access other than by boat (only scheduled guided tours run by the National Parks and Wildlife Service for Quarantine beach and no walking tracks to Store beach).
Another possibility is that the litter found in a certain place might have been dumped elsewhere and transported to where it was collected by water currents. This is considered to be unlikely for material that will submerge rapidly as opposed to floating litter and it is reasonable to assume that any litter floating into the sites would represent similar amounts at the six beaches studied.
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Figure 1. The entrance to Sydney Harbour, showing North Harbour Aquatic Reserve and the location of the six beaches used in this study (Light grey areas represent relevant parts of Sydney Harbour National Park)