Seasonal vertical strategies in a high-Arctic coastal zooplankton community

Abstract

We studied the larger (>1000 µm) size fraction of zooplankton in an Arctic coastal water community in Billefjorden, Svalbard (78°40' N), Norway, in order to describe seasonal vertical distributions of the dominant taxa in relation to environmental variability. Calanus spp. numerically dominated the herbivores; Aglantha digitale, Mertensia ovum, Beroë cucumis, and Parasagitta elegans were the dominant carnivores. Omnivores and detritivores were numerically less important. Descent to deeper regions of the water column (>100 m) between August and October, and ascent to the shallower region (<100 m) between November and May was the overall seasonal pattern in this zooplankton community. In contrast to other groups, P. elegans did not exhibit pronounced vertical migrations. Seasonal vertical distributions of most species showed statistical associations with the availability of their main food source. The vertical distribution of later developmental stages of Calanus spp. was inversely associated with fluorescence, indicating that they descended from the shallower region while it was still relatively productive, and ascended before the primary production had started to increase. Strong associations between the vertical distributions of secondary consumer M. ovum and Calanus spp., and tertiary consumer B. cucumis and M. ovum indicated that these carnivores seasonally followed their prey through the water column. We conclude that seasonal vertical migrations are a widespread trait in the high Arctic community studied, and predator−prey interactions seem particularly central in shaping the associations between the seasonal vertical strategies of adjacent trophic levels.

Characterisation of large zooplankton sampled with two different gears during midwinter in Rijpfjorden, Svalbard

“…m -3 ), which is in accordance with Falkenhaug (1996) who observed very high biovolumes of this species in winter in northern Norway fjord. Otherwise, generally low abundances of gelatinous zooplankton we observed in MIK samples is consistent with the results of the seasonal study of macrozooplankton in Billefjorden, Svalbard (78°40'N) (Bandara et al 2016), in which the vertical distributions of the larger (>1000 μm) zooplankton were assessed using vertical hauls (WP-3 net; 1000 μm mesh size) from three depth strata (0-50, 50-100 and 100-180 m) between August 2008 and May 2009. In interpreting the results of this study, it was assumed that both ctenophore species had descended to occupy deeper depths between August and October and then ascended back up to shallower depths from November.…”

“…It seems that the tiny but abundant population of B. cucumis was possible to be caught only by the net with the fine mesh size (e.g. MPS in our study), contrary to coarser WP-3 net used by Bandara et al (2016). On the other hand, the size distributions of the last mentioned species inhabiting the northern Norway fjord (Falkenhaug 1996) demonstrated its size peak in winter (between 20 and 30 mm length), which seems to be in accordance with our MIK measurements (average 27 mm).…”

“…In Rijpfjorden our study shows that the abundance of M. ovum peaked at the intermediate depths (50-100 m depth layer in MPS samples and 75 m in MIK samples). Whereas the results of both nets showed that B. cucumis was concentrated at the surface 20 m. The autumn descent (and spring-summer ascend) is likely a response to the migrations of their prey (Bandara et al 2016). Mertensia ovum feeds predominantly on Calanus spp., which during our Rijpfjorden study was also concentrated in the upper water column, whereas B. cucumis is a tertiary consumer that feeds specifically on M. ovum (Swanberg and Båmstedt 1991).…”

“…2a-b). Bandara et al (2016), in their ten-month study in Billefjorden, reported that the populations of ctenophores were dominated by small specimens (mean annual body lengths were 6.7 mm for M. ovum and 2.9 mm for B. cucumis) but increased to 11 mm for M. ovum and 6 mm for B. cucumis in January 2009. In case of M. ovum the average value obtained in the same month as in our study in Billefjorden, is in agreement with our mean measurements from MPS and MIK, while it is much lower for B. cucumis sampled by MPS.…”

“…Although there have been a few recent investigations of the mesozooplankton communities in the European Arctic during wintertime (e.g. Daase et al 2014;Grigor et al 2014;Båtnes et al 2015;Błachowiak-Samołyk et al 2015;Lischka and Hagen 2016) and some macrozooplankton (e.g., Søreide et al 2003;Zhukova et al 2009;Webster et al 2015;Grenvald et al 2016;Bandara et al 2016) more comprehensive data on a broader spectrum of zooplankton size fractions is needed if our current level of understanding continues to be based solely on data for the more abundant and readily sampled size fractions (e.g. mesozooplankton).…”