Low microzooplankton grazing rates in the Arctic Ocean during a Phaeocystis pouchetii bloom (Summer 2007): fact or artifact of the dilution technique?

• Published in: Journal of Plankton Research
• Main Authors: Calbet, Albert, Saiz, Enric, Almeda, Rodrigo, Movilla, Juan Ignacio, Alcaraz, Miquel
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press 2011
• Subjects: Arctic Ocean; Microzooplankton; Phaeocystis pouchetii; Dilution technique; Food web; Arctic; Greenland; Svalbard; East Greenland; Greenland Sea; Phytoplankton
• Online Access: http://hdl.handle.net/10261/47438; https://doi.org/10.1093/plankt/fbq142

15 pages, 7 figures, 4 tables.-- This is a pre-copy-editing, author-produced PDF of an article accepted for publication in Journal of Plankton Research following peer review. The definitive publisher-authenticated version Journal of Plankton Research 33 (5): 687-701 (2011)is available online at: https://doi.org/10.1093/plankt/fbq142 We studied the structure and dynamics of the microbial community of Arctic waters during July 2007 using a microzooplankton grazing dilution approach. The sampling covered a latitudinal transect along the East Greenland Sea, and a series of stations in the high Arctic (up to 808500N), west and north of the Svalbard Islands. A main feature of the area was the presence of Phaeocystis pouchetii, which formed dense blooms. Despite the considerable biomass of microzooplankton (mostly large ciliates and dinoflagellates), their grazing impact on phytoplankton, assessed as total chlorophyll a, was significant in only 6 out of 16 experiments, which resulted in 8% of the standing stock being consumed on average. Overall, phytoplankton instantaneous growth rates were very low and even negative at times (range: 20.24 to 0.14; average: 20.04 for total chlorophyll), which could not be attributed to nutrient limitation nor the estimated microzooplankton grazing. We present three non-exclusive explanations for this fact: (i) we were facing a senescent community in which many organisms were dying either as a result of virus infections or for other natural causes, as corroborated by parallel estimates of natural cell mortality using membrane permeability probes; (ii) the widespread and abundant P. pouchetii was probably deterring grazing and adversely affecting the entire planktonic community at the time of the study; and (iii) the dilution technique failed to give a real estimate of grazing (i.e. either non- significant or positive slopes), likely as a consequence of trophic cascades (decline of major grazers in the more concentrated treatments) combined with saturated-feeding responses. This last ...