A COMPARISON OF TWO MODELS OF PLASTIC FRAGMENT BIOFOULING UNDER THE BALTIC SEA CONDITIONS

Abstract

Information on changes in shape, effective density, and, consequently, buoyancy of plastic fragments during biofouling in the marine environment is necessary for developing realistic models of their transport from the point of entry into the World Ocean. The objective of the current study was to select a model and its parameters that would realistically describe the biofouling process of a cylindrical plastic fragment in a Baltic Sea environment. A one-dimensional approximation was considered. That is, it was assumed that the growth of the microalgae colony on the substrate equaled an increase in biofilm thickness and that all new microalgae were evenly distributed across the cylinder’s lateral surface. It was assumed that biofouling was affected by two mechanisms: microalgae sticking to the substrate due to the fragment’s relative motion in the environment, and a stand-alone growth of a colony of microalgae already sticked to the fragment. The Verhulst-Pearl and Gompertz models were considered as models of the stand-alone growth. The Verhulst-Pearl model has been shown to be preferable because it produces more realistic results, has fewer calibration parameters, and is easier to determine their values. In fact, for a biofilm thickness target value of 0.05 mm (approximately two microalgae diameters) and a target time to achieve this thickness of about 20 days (typically the time after which the biolayer thickness remains practically unchanged), the Verhulst-Pearl model yields a maximum growth rate of about 5 · 10–6 s–1. This value falls within the range of values indicated in the literature (from 1 · 10–6 s–1 to 10 · 10–6 s–1). The model also allows one to calculate changes in the density of the “fragment + biofilm” object. In other words, it provides the necessary geometric and physical information to model the motion of plastic fragments in a marine environment with biofouling taken into account.