Connecting flux, deposition and resuspension in coastal debris surveys
Graphical abstract
Introduction
Marine debris is currently regarded as one of the greatest threats to global biodiversity (UNEP, 2012). Millions of tonnes of solid anthropogenic waste enters the marine environment from land each year (Jambeck et al., 2015; Lebreton et al., 2017), 80% of which are plastic items (Eriksen et al., 2014; van Sebille et al., 2015). However, the estimated amount of discarded of mismanaged waste entering the sea from land is an order of magnitude greater than the estimated amount of debris floating on the ocean's surface (Jambeck et al., 2015; Lebreton et al., 2017; Thompson et al., 2004). There are millions of tonnes of marine debris that is missing in ocean sea surface estimates (Jambeck et al., 2015; Thompson et al., 2004; van Sebille et al., 2015). Marine conservation groups and scientists alike frequently conduct surveys to estimate the distribution and amount of anthropogenic debris on the coast (Nelms et al., 2017; Hardesty et al., 2017; Willis et al., 2017) and on the ocean's surface (Eriksen et al., 2014; Law et al., 2014; Maximenko et al., 2012; Reisser et al., 2013; van Sebille et al., 2015; Zettler et al., 2017).
Ongoing efforts by community groups and scientists to document and clean up coastal waste have resulted in extensive data collected on the shores and in coastal regions of the world (Van Der Velde et al., 2017; Zettler et al., 2017). Coasts are dynamic landscapes, where debris can arrive on shores by washing up in wave transport or being blown ashore by winds. This debris remains ashore for a period: The debris that is on the coast at a given point can be described as the “standing stock” of debris. Debris is mobile however, and can depart the shoreline through waves washing it away, people removing the debris, or wind blowing it out of the littoral zone – either further onto land or back into the sea. “Resuspension” is the process where debris in the standing stock on shore is transported back into the ocean – either by wave or wind transport. The continuous arrival and departure of debris over time can be described as the “flux” of debris in a littoral environment.
The effort required to coordinate marine debris surveys on a national and international scale means that surveys are often conducted once in a “snapshot” or at smaller scales on a monthly or annual basis (Nelms et al., 2017; Zettler et al., 2017). In “snapshot surveys” – where debris items are recorded, removed and the sample site is not surveyed again (Hardesty et al., 2017) or when there are long time gaps between surveys (Nelms et al., 2017), it is not feasible to reliably estimate the arrival or departure of debris (Smith and Markic, 2013). Debris interacts with the dynamics of a coastline (Kataoka et al., 2013; Kataoka et al., 2015). Estimating the arrival and departure of debris is important to find trends in how debris accumulates and moves near shores. More than half of debris washed into the ocean from estuaries can be immediately washed back onto shore (Willis et al., 2017), and an estimated 1.15–2.41 million tonnes of debris enters the ocean from river discharge each year (Lebreton et al., 2017). Understanding how debris interacts with coastal processes through its arrival and departure along the shore is important to identify where debris accumulates, and the spatial, social and temporal factors that influence its distribution.
Even when shorelines are surveyed and items removed on a regular basis (daily to weekly), there are challenges to estimate debris arrival and departure rates (Blickley et al., 2016; Eriksson et al., 2013; Smith and Markic, 2013). Using a standard rate (items m−2 day−1) means that resuspension is often underestimated, as the probability of each item being resuspended increases at a linear rate each day (Kataoka et al., 2013). Arrival and departure are continuous processes that can only be feasibly recorded in discrete intervals. This has led to criticism of the “snapshot” approach - that this style of survey does not account for coastline dynamics that influence the arrival and departure of debris over time, and systemically underestimates the debris loads in adjacent waters (Smith and Markic, 2013; Eriksson et al., 2013). Alternatively, a one-off survey may be the long-term equilibrium of arrival and departures of debris in a certain environment. Each removal survey is a record of the standing stock of debris - which can then be interpreted as a discrete observation of a continuous flux. This means that factors that affect the density of debris in a single survey may also be reflected in the arrival and departure of debris, and vice versa.
The arrival and departure of debris can be affected by many social, spatial and temporal factors (Blickley et al., 2016; Kataoka et al., 2013). Social factors that may affect the arrival of debris include the proximity to urban infrastructure and tourist activity (Willis et al., 2017), shipping lanes (Critchell et al., 2015) and river mouths (Lebreton et al., 2017), whilst local stewardship may affect the departure of debris (Hardesty et al., 2017; Slavin et al., 2012). Different substrates accumulate different densities and compositions of debris (Hardesty et al., 2017; Willis et al., 2017). From “snapshot surveys”, sandy beaches have had lower densities of debris than boulders, gravel, mudflats and rock slabs on Australian coasts (Hardesty et al., 2017; Willis et al., 2017). Coves have exhibited higher densities of debris than straight or convex shores (Hardesty et al., 2017). Beaches with dunes have significantly higher densities of debris than vegetated backshore types – like tussock, mangrove or woody backshores (Hardesty et al., 2017). The gradient of the coast has had a negative correlation with the density of marine debris (Hardesty et al., 2017). The aspect of a coastline affects its exposure to swell and wind, which in turn affects the daily arrival rate of marine debris (Blickley et al., 2016; Eriksson et al., 2013). Seasonal rainfall that affects urban runoff (Cheung et al., 2016) as well as changes in ocean currents (Agustin et al., 2015; Ribic et al., 2012) can significantly affect coastal debris loads. Stokes drift is the capacity of wave transport to move a suspended object (Bever et al., 2011). The wave transport of debris is a result of exposure of the coast to swell (Blickley et al., 2016) and the movement of debris items in the waves (Kataoka et al., 2015). The stokes drift and exposure of our sites to swell is what we used to estimate the wave transport of debris in its arrival and departure.
We tested the spatial and temporal factors that affect the arrival and departure of debris on shorelines. The case study took place in Murramarang National Park (NP), New South Wales (NSW), Australia. We utilized transect methods that have previously been applied at national scales to estimate debris densities in Australia (Hardesty et al., 2017) and elsewhere. We conducted repeated surveys with a mixture of removal or mark and recapture of debris items. These observations enabled us to statistically model the flux and departure of debris independently, and to identify the important variables that drive these processes. Additionally, we used flux and departure estimates to deduce the arrival of debris in the study area which is a reflection of the debris loads in the adjacent waters.
Section snippets
Site selection
The 20 km northernmost section of coastline in Murramarang NP was selected as the study site. Murramarang NP is on the south coast of NSW between the towns of Batemans Bay (pop. 7692) and Ulladulla (pop. 15,152) (Australian Bureau of Statistics (ABS), 2017a). It is 240 km south of Sydney (pop. 4,823,991), with several regional centres between Sydney and Murramarang NP (ABS, 2017a). Murramarang NP has a heterogeneous coastline exhibiting a wide range of substrates, gradients, shapes, and
Results
The mean number of new items sampled for each replicated transect was 0.82 (±2.23) items. There was no trend between the number of items sampled and the day that the transect survey was replicated (Fig. 2). Different substrates exhibited different compositions of debris types (Fig. 3). With respect to size, debris items larger than 6 cm were the most frequently sampled, with items between 1 and 2 cm the second most abundant size class observed (Fig. 4).
Based on AIC, the best model for the
Discussion
We found that the important factors influencing the standing stock and departure of debris from coastal sites are substantially different. Shape, onshore wind, and the backshore vegetation were all important for departures, but not for standing stock. The variables previously identified to affect debris distribution on Australian shores (Hardesty et al., 2017; Willis et al., 2017) only accounted for 6.50% of the variance in departure. This suggests that the departure of debris from shores is
Conclusion
We used repeated sampling of the same sites and using a mark-recapture experiment adjacent to a removal transect so we could estimate the equilibrium flux (or stock) and departure of debris. Measuring the stock and departure in each area allowed us to predict the unobserved arrival of debris whilst removing the bias due to resuspension. This approach enables coastal dynamics to be accounted for, without time and cost-intensive daily surveys. We found that seasonal changes in debris loads
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Chris Wilcox and Britta Denise Hardesty were supported by CSIRO Oceans and Atmosphere.
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