Estimates of marine debris accumulation on beaches are strongly affected by the temporal scale of sampling

Abstract

Marine debris is a global issue with impacts on marine organisms, ecological processes, aesthetics and economies. Consequently, there is increasing interest in quantifying the scale of the problem. Accumulation rates of debris on beaches have been advocated as a useful proxy for at-sea debris loads. However, here we show that past studies may have vastly underestimated the quantity of available debris because sampling was too infrequent. Our study of debris on a small beach in eastern Australia indicates that estimated daily accumulation rates decrease rapidly with increasing intervals between surveys, and the quantity of available debris is underestimated by 50% after only 3 days and by an order of magnitude after 1 month. As few past studies report sampling frequencies of less than a month, estimates of the scale of the marine debris problem need to be critically re-examined and scaled-up accordingly. These results reinforce similar, recent work advocating daily sampling as a standard approach for accurate quantification of available debris in coastal habitats. We outline an alternative approach whereby site-specific accumulation models are generated to correct bias when daily sampling is impractical.

Figure 1. Map of the study area showing Charlesworth Bay and the extent of the urbanised area of Coffs Harbour (shaded).

Figure 2. Plot of estimated daily accumulation rate of marine debris against interval between sample periods (days) for Charlesworth Bay.

The regression is a power function with an r ² of 0.872 and the following equation: estimated daily accumulation (y)  = 749.81interval(x) ^−0.67.