Although plastic is ubiquitous in marine systems, our current knowledge of transport mechanisms is limited. Much of the plastic entering the ocean sinks; this is intuitively obvious for polymers such as polystyrene (PS), which have a greater density than seawater, but lower density polymers like polyethylene (PE) also occur in sediments. Biofouling can cause large plastic objects to sink, but this phenomenon has not been described for microplastics <5 mm. We incubated PS and PE microplastic particles in estuarine and coastal waters to determine how biofouling changes their sinking behavior. Sinking velocities of PS increased by 16% in estuarine water (salinity 9.8) and 81% in marine water (salinity 36) after 6 weeks of incubation. Thereafter sinking velocities decreased due to lower water temperatures and reduced light availability. Biofouling did not cause PE to sink during the 14 weeks of incubation in estuarine water, but PE started to sink after six weeks in coastal water when sufficiently colonized by blue mussels Mytilus edulis, and its velocity continued to increase until the end of the incubation period. Sinking velocities of these PE pellets were similar irrespective of salinity (10 vs. 36). Biofilm composition differed between estuarine and coastal stations, presumably accounting for differences in sinking behavior. We demonstrate that biofouling enhances microplastic deposition to marine sediments, and our findings should improve microplastic transport models.

No abstract

No abstract

No abstract

In the tidal inlet of the back barrier area of Spiekeroog Island (Southern North Sea), nutrient concentrations (silica, phosphate, and nitrite plus nitrate) were determined hourly by an autonomously analysing system on a permanently installed time-series station from April 2006 to December 2008. Based on the high frequency of analyses we studied nutrient dynamics on annual, seasonal, and tidal time scales. By comparing the nutrient input to the tidal flat area via freshwater through a flood-gate and pore water discharge from tidal flat sediments, we conclude that nutrients are primarily supplied to the water column by pore water advection, while the freshwater contribution is negligible. To assess the annual nutrient contribution of our study area to the German Bight, we used a numerical Euler-Lagrangian model (EcoTiM) to calculate annual budgets of silica and phosphate. The model results indicate that the back barrier area of Spiekeroog Island exports inorganic silica (128 * 10 6 mol a -1 ), phosphate (3 * 10 6 mol a -1 ), and nitrite plus nitrate (29 * 10 6 mol a -1 ) to the North Sea. Extrapolation of these data to the entire Wadden Sea along the southern North Sea reveals that the back barrier areas export silica and phosphate in the same order of magnitude and nitrite plus nitrate one order of magnitude lower than the combined rivers Elbe, Weser, and Ems.