Impacts of nutrient and marine plastic debris influx on the microalgal community (phytoplankton and periphyton) in Korean temperate coastal waters: Mesocosm study

Abstract

Anthropogenic pollutants, such as excessive nutrients and marine plastic debris (MPDs), can significantly affect marine micro-ecosystems, including phytoplankton and periphyton. However, there have been few studies about the impacts of these pollutants on microalgal community. We aimed to assess these impacts by 1000-L mesocosm experiments that simulated the influx of nutrients and MPDs in autumn (natural seawater: control, low nutrient: LN group, high nutrient: HN group). The phytoplankton biomass in water increased rapidly in the LN group (maximum 21.7 μg L−1) and HN group (maximum 14.6 μg L−1), resulting in a succession from Cryptomonas spp. to Pseudo-nitzschia spp.. Interestingly, although the threefold difference in nutrient addition between the LN and HN groups, the phytoplankton biomass was similar (cumulative biomass during the entire experiment: 68.9 μg L−1 in LN group and 70.7 μg L−1 in HN group). On the other hand, on day 15 the periphyton biomass on the PP plates was 12.7-times higher in HN group than in LN group. Also, the estimated periphyton biomass of mesocosm wall was significantly different between LN group (4,696 µg cm−2) and HN group (59,835 µg cm−2), in relation to the lack of phytoplankton biomass relative to nutrient addition. In particular, the species-specific characteristics of Cylindrotheca closterium, which grow well even in low nutrients, and the relatively weak solar irradiance in autumn helped to increase the periphyton biomass. Therefore, these findings suggest that the influx of MPDs and nutrients can change the community structure of microalgae, leading to an increase in the periphyton ratio, and this change can have negative consequences on the marine ecosystem by potentially reducing primary productivity. Our mesocosm experiments provide important ecological insights regarding the time-series changes of phytoplankton in the water column and of microalgae attached to MPD when nutrients and plastic debris are introduced into coastal waters through river discharge after rainfall during autumn in temperate regions.