The role of microzooplankton grazing in trace element biogeochemistry
Student thesis: Doctoral Thesis
Microzooplankton consume a significant fraction of phytoplankton and bacterioplankton production, remineralising macronutrients and providing a major trophic link to larger protozoan and metazoan consumers. Trace elements are essential for the growth of phytoplankton and bacteria and play a pivotal role in regulating primary productivity and microbial diversity in many areas of the ocean. To quantify the role of microzooplankton grazing in trace element biogeochemistry, we modified the dilution method to provide the first ever simultaneous measurements of microzooplankton grazing impacts on bacterioplankton and phytoplankton, and size fractionated biogenic particulate trace elements. In the Arctic Ocean we found microzooplankton obtained the majority of their nutrition (0.48 – 1.68 μg C L⁻¹ d⁻¹) from nanophytoplankton, on which they exerted a strong top down control (15 – 122% standing stock ingested). We subsequently applied our method in the North Atlantic Ocean. Microzooplankton grazing consumed a significant proportion of bacteria (97 – 165%) and particulate trace element standing stocks (0.4 fM d⁻¹ – 5 pM d⁻¹), thus playing a significant role in the flow of carbon and trace elements. Our results suggest the rate and quantity at which microzooplankton grazing mediates trace element biogeochemistry depends strongly on the structure and function of microbial assemblages. Iron enrichment assays supported the hypothesis that the high latitude North Atlantic is seasonally iron limited. Importantly, microzooplankton grazing had a significant effect in cropping expected iron stimulated biomass. Serendipitously we conducted assays in an area of ocean influenced by volcanic ash deposition from Eyjafjallajökull’s eruption. Our data are the first to quantify microzooplankton grazing in such an environment and revealed deposition of volcanic ash had significant short term local effects upon microbial ecology and carbon cycling. Results were analogous to iron enrichment assays and comparable to data from mesoscale iron enrichment experiments. Assays in a coastal environment showed microzooplankton played an important role in the control of bacterioplankton and cycling of carbon through the microbial food web. At all locations, ingestion rates of biogenic particulate trace elements suggest microzooplankton grazing has the potential to reduce the pressure of trace element deplete conditions at the microbial level by increasing residence times in the upper ocean through remineralisation. Our results provide important new parameterizations of trace element cycling that until now had been hypothesised but not observed. These data will allow us to better constrain biogeochemical models and those predicting the Ocean’s response to climate change and geoengineering. Likewise, the temporal and spatial nature of our data may allow the elucidation of inherent interannual and annual variability to enduring change at our study locations.
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