Grazer-mediated chloropigment degradation and the vertical flux of spring bloom production in Conception Bay, Newfoundland

Redden, A.M. (1994) Grazer-mediated chloropigment degradation and the vertical flux of spring bloom production in Conception Bay, Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The fate of spring phytoplankton production at subzero temperatures in Conception Bay, Newfoundland, was traced using chloropigments as biomarkers of phytoplankton biomass and flux, and as indicators of biological processes. The study included conventional fluorometric and HPLC analyses of seston, zooplankton and bivalve gut tracts and faeces, trap collections, and sediments. Chloropigment conservation during copepod grazing was examined in experiments conducted at 0°C for 24 hr, using various concentrations of natural seston and cultured diatoms. Transformation to nonfluorescent products was negligible or low (<35%) at seston concentrations >3 μg chlorophyll α/l. At lower food levels, chloropigment destruction was high (>80%). Losses were primarily due to post-starvation feeding and digestive processes. In situ gut and faecal pigment levels in copepods and oikopleurids were highest during mid-bloom production. The dominant copepod grazers exhibited diet feeding rhythms, while oikopleurids showed near-continuous feeding activity. Pyrophaeophorbide a was the primary fluorescent degradation product of copepod grazing; more complex compositions of chlorophyll α derivatives were found in oikopleurid and mussel faeces. Considerable amounts of undegraded chlorophylls were also commonly found in oikopleurid faecal pellets. The contribution of copepod and oikopleurid faecal pellets to the vertical flux of bloom production was highest following the mass sedimentation of primarily senescent diatoms (20-23 m/d) during early May. POC flux to bottom waters was 30-40% of estimated primary production, and 75% of this flux occurred during May. The relative contribution of zooplankton grazing products increased from 35 % at 40 m to 67% at 240 m. Relatively constant carbon/chloropigment ratios at all depths throughout the bloom, and comparable losses of POC and pigment flux between surface and bottom waters, support the use of chloropigments as reliable tracers of vertical flux. C/N ratios of cumulative fluxes were low (7.0-8.8), indicating little decay of the bulk of sedimenting material. High quality particulate material (20% POM) reached near-bottom waters during the terminal phase of the bloom, resulting in elevated carbon and chloropigment levels in the surface sediments. Chloropigment concentrations decreased with sediment depth in the uppermost 5 cm, but composition remained uniform down to 10 cm, presumably due to the bioturbating activities of abundant polychaetes, and pigment preservation processes following burial.

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