The Response Of Marine Phytoplankton Community To Iron Enrichments

Iron plays a very important role in marine biogeochemical cycle. It is more essential than the other trace elements especially for phytoplankton growth. Iron limitation has obvious impacts in many oceanic domains, which has been one of the hotspots in the field of oceanography study nowadays. In this thesis, phytoplankton communities response to iron enrichment has been discussed: The influence of different light intensions, nitrogenous species and their concentrations on iron uptaking by different marine diatoms was investigated by batch culture experiments of three common diatom species in the Jiaozhou Bay, Cylindrotheca closterium(Ehrenberg) Reimann et Lewin, Pseudo-nitschia pungens (Grunow et Cleve) Hasle and Chaetoceros debilis Cleve. Marine phytoplankton natural communities response to iron enrichments was studied by three in vitro iron enrichment experiments at station C3 in the Jiaozhou Bay, coastal station E4 and oceanic station A2 in the East China Sea. The impact of iron, nitrogen and phosphorus on phytoplankton growth was compared by the dilution experiment coupling with nutrients "all-minus-one" enrichment. The grazing impact of increased mesozooplankton abundance in response to an iron-induced enhancement of algal biomass was also simulated by performing mesozooplankton grazing manipulations during in vitro iron enrichment studies.The main results are as folio wings:The batch culture experiments show a faster iron uptaking rate in the higher nitrate concentration regime. However, the different ammonium concentrations have no significant impact on iron uptaking. Moreover, the diatom Chaetoceros debilis Cleve has higher iron uptaking rates in nitrate medium than in that of ammonium at the same concentration, suggesting a higher iron requirement in nitrate substrates. In therange of the experimental light intensions, the higher light intension promotes iron uptaking in three diatoms.All the three in vitro iron enrichment experiments indicate that iron enrichments significantly promote the growth of phytoplankton and change the communities structure. After iron enrichments, chlorophyll a concentrations increase faster than the controls, without iron addition, both in the coastal area and oceanic oligotrophic area. The biomass of diatoms, especially the big and chain form ones, is promoted much faster. And diatom becomes the predominant species in the community soon. It is suggested that iron induced phytoplankton growth may contribute significantly to the CO2 draw-done and climate change, caused by the contribution of such marine diatoms to carbon flux down to the deep waters. Iron enrichments also promote the phytoplankton utilization of nitrate and phosphate, reduce the phytoplankton requirement of silicate. Consequently, the bioavailability of iron may influence the biogeochemical cycle of these macro-nutrient elements in the seawater.The natural phytoplankton community at station C3 in the Jiaozhou Bay of December, 2003 was cultured by combining dilution technique and method of nutrients "all-minus-one" enrichment. The ranking of the calculated net growth rates of phytoplankton in the culture bottles is: All>-N>-P>-Fe. This result shows that in this station, the phytoplankton growth is limited by iron. The phosphate is also a restrictive element for phytoplankton growth. And there is almost no nitrate limitation. Iron favors the multiplication of Thalassiosira rotula, Entomoneis alata, Coscinodiscus sp and Detonula pumila most, which are big and chain form cells.After simulation of the vertical migration cessation of mesozooplankton by adding extra stocks of mesozooplankton to the culture bottles after certain lag times, it is found that the increasing grazing pressure only delays the start of phytoplankton exponential growth period, reduces the short-term growth rate of phytoplankton and the iron-mediated increases in algal biomass are not found to be prevented by the increasing grazing pressure. On the other hand, in this study, because of some kind roles of zooplankton metabolism, the n...