Seasonal and diel patterns in sedimentary flux of krill fecal pellets recorded by an echo sounder

Abstract

We used a moored upward-facing 200 kHz echo sounder to address sedimentation of fecal pellets (FPs) from dielly migrating Meganyctiphanes norvegica. The echo sounder was located on the bottom at 150 m depth in the Oslofjord, Norway, and was cabled to shore for continuous measurements during winter and spring. Records of sinking pellets were for the first time observed with an echo sounder. Seasonal patterns of sedimentation of krill FPs were strongly correlated with data from continuous measurement of fluorescence, which illustrate the development of the spring bloom. Sedimenting particles were first observed as fluorescence values started to increase at the end of February and continued to increase until the bloom suddenly culminated at the end of March. This collapse of the bloom was detected on the echo sounder as a pulse of slowly sinking acoustic targets over a 2 d period. Prior to this event, there was a strong diel pattern in sedimentation, which correlated, with some time lag, with the diel migration of krill foraging at night near the surface. Pellet average sinking speeds ranged between 423 m d 21 and 804 m d 21 , with a strong relation to pellet target strength, which is an acoustic proxy for size. This novel approach shows that echo sounders may be a valuable tool in studies of vertical pellet flux and, thereby, carbon flux, providing temporal resolution and direct observation of the sedimentation process, which are not obtained from standard methods.

Migrant biomass and respiratory carbon flux by zooplankton and micronekton in the subtropical northeast Atlantic Ocean (Canary Islands)

“…Below this depth and after defecation, the fast sinking fecal pellets would reinforce the already weakened gravitational flux coming from surface, bypassing carbon from active to passive flux at mesopelagic depths (see Alonso-González et al, 2013). At this point, there are also important differences between zooplankton and micronekton since larger animals produce larger fecal pellets, which are expected to sink faster and to reach greater depths before decomposition (Small et al, 1979;Robison and Bailey, 1981;Røstad and Kaartvedt, 2013). Other exporting mechanisms such as the carbon inputs via excretion (Steinberg et al, 2000) and mortality still remain to be considered.…”

The diel vertical migration patterns and individual swimming behavior of overwintering sprat Sprattus sprattus

“…Another potential food source available was krill, which were particularly common the current winter (Røstad & Kaartvedt 2013. Although they appear to be too large to be preyed upon by most sprat, krill did occur among the stomach contents for the largest individuals the winter of this study (Solberg et al 2015).…”

“…Yet, an increase in measured light levels was apparent in mid-March, with the sprat leaving upper waters nearer to the upcoming sunrise these days. There was a strong build-up of chlorophyll a in mid-March (Røstad & Kaartvedt 2013), so that increased light extinction would cause darker waters relative to surface measurements in this period. The sprat apparently adjusted their timing in upper waters accordingly.…”

“…Fig. 5 in Røstad & Kaartvedt 2013). The distribution of overwintering Calanus in oxygen-depleted waters may have motivated individual excursions into these deep layers.…”

Interaction of sinking behaviour of Saharan dust and lithogenic and biogenic fluxes in the Canary Basin

“…Transparent exopolymer particles (TEPs) and faecal matter are also components of particulate organic carbon (POC). The former are composed of extracellular polymeric substances exuded by microorganisms (Alldredge et al 1993), while the latter are mainly produced by mesoplanktonic, macroplanktonic (Turner 2015), and micronektonic (Røstad andKaartvedt 2013, Ariza et al 2015) organisms, transporting a fraction of their ingested matter. These compounds are thus all expected to sink together with the mineral and organic particles deposited on the ocean during dust events such as those seen in the Canary Current due to the proximity of the Sahara desert.…”