Microzooplankton bacterivory and herbivory in oceanic and coastal environments: comparisons of the subarctic Pacific with Newfoundland coastal waters

Putland, Jennifer Nancy (1998) Microzooplankton bacterivory and herbivory in oceanic and coastal environments: comparisons of the subarctic Pacific with Newfoundland coastal waters. Masters thesis, Memorial University of Newfoundland.

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Abstract

A comparative analysis was used to estimate top-down control by microzooplankton on microbial prey and the relative ingestion of various prey by microzooplankton. Using the dilution technique (Landry and Hassett 1982), the rates of ingestion of autotrophic and heterotrophic picoplankton and total (>0.7μm) phytoplankton by microzooplankton were measured during spring, summer, and winter from 1995 to 1996 in the surface layers of both the subarctic Pacific (SAP) and Newfoundland coastal waters (NCW). In the SAP, size fractionated dilution assays were also conducted to determine the ingestion by small (<35 μm) and large (<202 μm) microzooplankton. The microzooplankton grazing impact (measured as the percentage of prey potential production ingested) in the SAP was primarily by <35 μm protists and was greatest on bacteria, Synechococcus, and <35 μm phytoplankton. The mean values of potential production ingested by microzooplankton were ca. 109%, 123%, 65%, and 28% for bacteria, Synechococcus, <35 μm and 5-202 μm phytoplankton, respectively. In NCW, the mean values of potential production ingested by microzooplankton were ca. 88%, 167%, and 97% for bacteria, autotrophic picoplankton, and >0.7 μm phytoplankton, respectively. Total ingestion of microbial carbon varied seasonally in the SAP and NCW, with minimum (ca. 2 to 4 μg C• L⁻¹ • d⁻¹) and maximum (6 to 13 μg C• L⁻¹ • d in the SAP and ca. 20 μg C•L⁻¹ • d⁻¹ in NCW) total ingestion rates during winter and summer, respectively. In the SAP and NCW, autotrophic and heterotrophic picoplankton generally represented a large portion (>40%) of the carbon ingested by microzooplankton. This was because microzooplankton generally ingested prey proportional to their availability and autotrophic and heterotrophic picoplankton represented most (>40%) of the microbial biomass. The comparative approach used in this thesis led to two important quantitative generalizations about microbial food web processes in subarctic waters. First, microzooplankton, on average, ingest ca. 80% and 100% of picoplankton (autotrophic and heterotrophic) and > 0.7 μm phytoplankton potential production, respectively. Second, autotrophic and heterotrophic picoplankton represent ca. 69% of the carbon ingested by microzooplankton. These quantitative generalizations represent important advances in microbial ecology as they facilitate hypothesis testing and the detection of unusual systems.

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