Degradation of copepod faecal pellets in the upper layer: role of microbial community and Calanus finmarchicus

Abstract

Copepod faecal pellets (FP) are considered important contributors to vertical carbon flux, but investigations comparing FP production with FP export using sediment traps conclude that vertical export is not their only fate. FP are degraded to a large extent in the upper 60 m, and even among large, fast-sinking FP, only a fraction reaches sediment traps deeper than 200 m. Retention mechanisms for copepod FP are still not well understood. In order to investigate the relative importance of the small (<180 µm) compartment of the plankton community versus larger filter-feeding copepods for degradation of large, fast-sinking FP, we incubated FP produced by Calanus finmarchicus (Gunnerus) in 180 µm-filtered water from the chlorophyll a maximum. From a series of experiments, we found that the degradation of large FP is time-dependent, as no degradation was apparent after 20 or 48 h of incubation, but after 72 h FP volume was reduced by 32%. We also found that large filter-feeding copepods may facilitate the degradation process, since FP degradation increased from 0 to 75% after 48 h of incubation in the presence of 5 C. finmarchicus. We conclude that ciliates and dinoflagellates are able to degrade large copepod FP, but that this process is too slow to explain observed retention of large FP in the upper 200 m of the water column due to fast sinking of large particles. Rather than looking for single-factor explanations for flux-regulating processes, we stress the importance of investigating combined effects in relevant time frames to understand the complexity of carbon flux regulation in natural systems. KEY WORDS: Faecal pellet · Faecal pellet retention · Microbial degradation · Faecal pellet fragmentation · Calanus finmarchicus · Carbon cycling Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 462: [39][40][41][42][43][44][45][46][47][48][49] 2012 by copepods. Gon zález & Smetacek (1994) hypothesised that the abundant cyclopoid copepods Oithona spp. play a key role in the retention of copepod FP, but evidence is conflicting, indicating that Oithona spp. may be indi cators for a retention community rather than the retaining organisms (Reigstad et al. 2005, Poulsen & Kiør boe 2006. This shows that organisms other than Oithona spp. could play a major role in FP retention. In an experiment investigating the effect of various size-fractions of grazers, Poulsen & Iversen (2008) concluded that heterotrophic ciliates and dinoflagellates are the main degraders of small FP from the calanoid copepod Acartia tonsa and that the effect of copepods on degradation rates is comparatively minor. At high latitudes, the copepod community is dominated by larger copepod species producing larger FP. It is therefore relevant to ask if the same groups of organisms and mechanisms are important for degradation of larger FP at high latitudes.Sediment trap investigations revealed considerable attenuation of particulate organic carbon (POC) over a limited depth range (30−60 m) in areas whe...

Increased degradation of copepod faecal pellets by co-acting dinoflagellates and Centropages hamatus

“…In this perspective, our experimental exposure time of 48 h for large FP degradation becomes relevant. This conclusion was also drawn from a previous experiment in which the combined effect of Calanus finmarchicus and microorganisms (mainly a combination of dinoflagellates and ciliates) significantly increased the degradation rate of similarly sized copepod FP after 48 h (Svensen et al 2012). The Cspecific degradation rate found in the present experiment (1.12 ± 0.21 d −1 , average ± SD) is an order of magnitude higher than the rate of 0.12 ± 0.03 d −1 obtained from a series of degradation experiments done at 15°C on sinking particles such as aggregates and FP (Iversen & Ploug 2013).…”

“…In this study, we demonstrated that large FP (length ഠ450 µm) are in fact subject to grazing by dinoflagellates, as previously reported for smaller FP (Poulsen & Iversen 2008) but not for large FP (Svensen et al 2012). However, the present experiment was carried out over 48 h, a time frame that might allow large and fast-sinking FP (50 to 300 m d −1 ) to escape the vertical zone where they would be exposed to grazing from dinoflagellates.…”

“…Conversely, in a study conducted in northern Norway, protozooplankton alone did not seem capable of degrading large copepod FP within relevant time-frames, although in concert with filter-feeding copepods, the degradation was significant (Svensen et al 2012).…”

“…To determine FP degradation by various microorganisms, 2 complementary methods can be used: respiration measurements and microscopic counts (Svensen et al 2012). Carbon degradation of copepod FP through bacterial and dinoflagellate respiration (as an indicator of overall metabolism) was measured by 5 replicate incubations of 30 FP in each of 4 ml glass microchamber vials filled with either 0.2 µm FSW or <180 µm chl a max water (collected from the depth of the chl a maximum; i.e.…”

“…The efficiency of this pump is related to primary productivity and degradation of organic material. Copepod faecal pellets (FP) are possibly important contributors to the downward flux of carbon, and mechanisms for FP degradation have received increased attention during the last decade (Poulsen & Kiørboe 2005, Reigstad et al 2005, Iversen & Poulsen 2007, Ploug et al 2008, Svensen et al 2012. However, the contribution of FP to the downward flux of carbon is highly variable both seasonally (Wexels Riser et al 2010) and geographically (Wexels Riser 2007).…”