The present paper explores (i) the way silicate concentrations may control the switch from diatom to non-diatom dominated phytoplankton, (ii) the consequence of mixing intensity and nutrient loading on phytoplankton composition (and potentially harmful species), (iii) to what extent trophic interactions may interfere with this abiotic control. The data used for this paper were selected from experiments run in 3-m3 land-based mesocosms (SW Netherlands) since 1993. Various nutrient and/or mixing regimes were applied to seawater incubated for periods varying from 1 to five months. The termination of the initial diatom bloom was induced by silicate concentrations below 2 µM, whereas the subsequent flagellate bloom was limited by N or P availability. Combined high nitrogen loadings and stratification events promoted the development of potentially harmful dinoflagellates but other factors such as grazing and temperature played a decisive role in the appearance of such blooms. Despite their relatively low intrinsic growth rate, copepods were able to control diatoms and microzooplankton development. Microzooplankton efficiently incorporated blooms of small phytoplankton species. These results, combined with previous experiments where nitrogen enrichment induced increases in small phytoplankton biomasses, stress the positive effect of N-enrichment on the microbial food web. Conversely, blooms of large diatom species exemplified the trophic mismatch leading to the accumulation of algal biomass without transfer to higher trophic levels. [KEYWORDS: diatoms, Phaeocystis, Cerataulina marina, stratification, ciliates, copepods]
Original languageEnglish
Journal publication date2002

ID: 102923