Mesocosm experiments of temperate marine coastal waters: timing of changes in phytoplankton communities and attachment to plastic plates after nutrient addition

Abstract

Abstract Anthropogenic pollutants, such as excessive nutrients and marine plastic debris (MPD), can significantly affect marine ecosystems, but the timing and combined effects of these pollutants are poorly understand. Nutrient enrichment commonly occurs in temperate marine coastal waters during autumn due to terrestrial runoff. We established six 1000-L mesocosms and performed experiments for 15 days during November to examine the effects of nutrient enrichment and MPD. The mesocosms (2 replicates per group) had natural seawater (control), low nutrient (LN) seawater, or high nutrient (HN) seawater with polypropylene (PP) plates to mimic the effect of MPD. We recorded the timing of changes in nutrients and other abiotic factors and in phytoplankton in the water column and in the periphyton communities on the PP plates. Cryptomonas spp. was initially dominant in all three groups, but the LN and HN groups shifted to chain diatoms, such as Pseudo-nitzschia spp., and then to Cylindrotheca closterium. Due to significant nutrient uptake by bloom-forming diatoms, most of the nitrate+nitrite and phosphate were consumed within 4 days in the LN group. In contrast, the some nutrients remained in the HN group until day 15, and this was related to the nutrient requirements of other small solitary diatoms, such as C. closterium. Analysis of the PP plates showed that although small numbers of the planktonic diatom Chaetocerosspp. were present from day 4, there was a shift to the attached diatom C. closterium during the middle and end of the experiment, indicating that C. closterium in the water attached to the PP plates even when nutrients levels were low. In addition, Navicula(>20%) was prominent on the PP plates from the middle to the end of the experiment. Notably, the percentage of total Chl. a in the periphyton community, which was associated with nutrient uptake in the HN group, was higher than in the phytoplankton community. This implies that the periphyton biomass in the HN group effectively utilized the remaining nutrients. Our mesocosm experiments provide important ecological insights regarding the timing of 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.