MEPS 367:1-13 (2008)  -  doi:10.3354/meps07611

Degradation of copepod fecal pellets: key role of protozooplankton

Louise K. Poulsen1,2,*, Morten H. Iversen3

1Danish Institute for Fisheries Research, Department of Marine Ecology and Aquaculture, Kavalergården 6,
2920 Charlottenlund, Denmark
2Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark
3Alfred-Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany

ABSTRACT: Copepod fecal pellets are often degraded at high rates within the upper part of the water column. However, the identity of the degraders and the processes governing the degradation remain unresolved. To identify the pellet degraders we collected water from Øresund (Denmark) approximately every second month from July 2004 to July 2005. These water samples were divided into 5 fractions (<0.2, <2, <20, <100, <200 µm) and total (unfractionated). We determined fecal pellet degradation rate and species composition of the plankton from triplicate incubations of each fraction and a known, added amount of fecal pellets. The total degradation rate of pellets by the natural plankton community of Øresund followed the phytoplankton biomass, with maximum degradation rate during the spring bloom (2.5 ± 0.49 d–1) and minimum (0.52 ± 0.14 d–1) during late winter. Total pellet removal rate ranged from 22% d–1 (July 2005) to 87% d–1 (May). Protozooplankton (dinoflagellates and ciliates) in the size range of 20 to 100 µm were the key degraders of the fecal pellets, contributing from 15 to 53% of the total degradation rate. Free-living in situ bacteria did not affect pellet degradation rate significantly; however, culture-originating bacteria introduced in association with the pellets contributed up to 59% of the total degradation rate. An effect of late-stage copepod nauplii (>200 µm) was indicated, but this was not a dominating degradation process. Mesozooplankton did not contribute significantly to the degradation. However, grazing of mesozooplankton on the pellet degraders impacts pellet degradation rate indirectly. In conclusion, protozooplankton seems to include the key organisms for the recycling of copepod fecal pellets in the water column, both through the microbial loop and, especially, by functioning as an effective ‘protozoan filter’ for fecal pellets.

KEY WORDS: Bacteria · Copepod · Degradation rate · Dinoflagellates · Fractionated plankton community · Protozoan filter · Nauplii

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Cite this article as: Poulsen LK, Iversen MH (2008) Degradation of copepod fecal pellets: key role of protozooplankton. Mar Ecol Prog Ser 367:1-13

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