RESEARCH ARTICLE Variable predator–prey relations in zooplankton overwintering in Subarctic fjords Stig Skreslet , Marina Espinasse , Ketil Olsen & Boris D. Espinasse Faculty for Biosciences and Aquaculture, Nord University, Bodø, Norway Abstract Keywords Zooplankton predator–prey relations in northern Norwegian fjords are highly Crustacea; Polychaeta; Chaetognatha; variable in time and space, and the mechanisms driving this variability are still Coelenterata; advection; predation poorly understood. Replicate Juday net sampling in October and February from 1983 to 2005, which included five repeated tows from bottom to surface, was Correspondence conducted in Saltfjord and Mistfjord, northern Norway. The time-series pro- Stig Skreslet, Faculty for Biosciences and vided evidence of in situ variability in species abundance, as well as seasonal Aquaculture, Nord University, PO Box 614, NO-8622 Mo i Rana, Norway. E-mail: stig. and interannual changes in standing stock abundance. The shallow sill of one [email protected] fjord caused accumulation of coastal water in the fjord’s basin, while the other fjord’s deeper sill selected denser water of Atlantic origin from the same open Abbreviations shelf habitat. The selective advection caused differences in the immigration CIV, CV, CVI: copepodid stages of species recruiting to the fjords’ specific overwintering communities of zoo- DI: deviation index plankton. Statistical analyses of the cumulated replicate data indicated signifi- DVM: diurnal vertical migration cant in situ variability in the spatial density of species. Cases with an abundance NAC: Norwegian Atlantic Current NCC: Norwegian Coastal Current of carnivores relating positively to other species probably resulted from the carnivores’ attraction to patches with concentrations of prey. Interspecific neg- ative density relations likely indicated either predator avoidance or substantial trophic activity during the sampling. During years of high abundance, some wintering stocks of carnivores evidently reduced the local stocks of overwinter- ing prey. We conclude that predator–prey interactions and stock variability in Subarctic fjords result from complex bio-geophysical interactions that occur on the scales of local habitats and basin-scale population systems. To access the supplementary material, please visit the article landing page Introduction are a balance between local mortality and the exchange of water with outside habitats. While some mortality The Subarctic fjords of northern Norway provide win- results from predation by a wide range of benthonic and tering habitats for the zooplankton biota of mixed boreal nektonic planktivores in fjords, some is due to predator– and Arctic species. Many species establish local stocks by prey relations on different spatial scales within defined immigrating from extensive population systems in the zooplankton communities. Studies of interspecific trophic Arctic Mediterranean (Tchernia 1980), where the ecosys- relations on zooplankton community levels are rare and tem has been evolving since the first glaciation of the Pleis- not easily achieved; in this study, we applied an approach tocene period (Dunbar 1968). The relative abundance of that to our knowledge is new to plankton science. wintering stocks in fjords changes with the hemispheric Saltfjord and Mistfjord are two fjords situated just climate by its effects on large-scale thermohaline circula- above the Arctic Circle in northern Norway and are sep- tion within the Arctic Mediterranean ecosystem, which arated by the Bodø peninsula (Fig. 1). Both fjords lock eventually influences the local shelf-to-fjord advection of mesopelagic basin water behind the sills at their entrances, both oceanic NAC water and shelf water of lower salin- but their differing sill depths permit the selective import ity (Skreslet et al. 2015; Espinasse et al. 2018). Temporal of water and zooplankton species from the outside shelf changes in a fjord’s abundance of wintering zooplankton habitat. This causes their basin water quality, as well as Polar Research 2020. © 2020 S. Skreslet et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 1 International License (http://creativecommons.org/licenses/by-nc/4.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Citation: Polar Research 2020, 39, 3300, http://dx.doi.org/10.33265/polar.v39.3300 (page number not for citation purpose) Zooplankton predator–prey relations in Subarctic fjords S. Skreslet et al. the biodiversity and species abundance of the accumu- shelf sea stratification, sill topography and seasonal cli- lated zooplankton, to correlate differently with hemi- mate variability are among the factors that explain spheric climate variability recorded as North Atlantic observed differences in biodiversity between the two Oscillation, Arctic Oscillation and Atlantic Multidecadal fjord communities of wintering zooplankton (Skreslet et Oscillation (Skreslet et al. 2015). al. 2015). This likely also applies to the meroplanktonic Saltfjord receives a significant amount of Norwegian life stages of various species that establish fjord stocks Sea zooplankton during September and October (Heath of benthic predators of holoplankton. Differences in sill et al. 2000; Skreslet et al. 2000), when the north-run- depth between fjords also structure the recruitment of ning NCC shifts its transversal surface vector of coastal fish to local fjord stocks in this region and lead to a greater water from seaward to landward (Helland-Hansen & abundance of planktivorous species in Saltfjord than in Nansen 1909; Haakstad 1979). This shift is a result of the Mistfjord (Skreslet 1994). Accordingly, the food webs decreased stratification of the NCC caused by reduced of fjord basin communities tend to be fjord-specific and freshwater discharge after summer and increased turbu- complex, as well as variable in time and space. lence forced by winds during autumn (Haakstad 1977), Studies of predator–prey relations in zooplankton are while prevailing south-western winds in September and frequently arranged in dishes and aquaria. Mesocosm October accelerates the surface advection of NCC water enclosures submerged in the sea are scientific arrange- towards the Norwegian coast and fjords in general (Aure ments that are closer approximations to trophic activi- et al. 2007). However, Espinasse et al. (2018) observed ties in real patches of zooplankton (Sullivan et al. 1994). that the inflow frequency and residence time of the water Here, we use empirical field data and statistical methods qualities in Saltfjord and Mistfjord differ according to to investigate the interannual variability and seasonal local winds that change on short temporal scales. change of interspecific trophic relations in zooplankton The 220 m sill depth of Saltfjord allows a deep inflow biota that overwinter in the mesopelagic basins of two of NAC water (≥34.5 psu) into its 382 m deep basin sev- Subarctic fjords. Since the annual import of species varies eral times per year (Skreslet et al. 2000). The 34 m sill in response to combinations of local topography and the depth of Mistfjord selects shelf water (<34.5 psu) to enter hemispheric climate, we hypothesize that interannual its 297 m deep basin from an intermediate layer of the variability in the import of carnivorous plankton shifts same stratified shelf sea habitat (Skreslet et al. 2015). The the biodiversity and predator–prey relations in ways that sampling location in Saltfjord is subject to mixing by a are specific for each fjord. We test this hypothesis on tidal jet that, on average, advects about 3 × 108 m3 of sur- materials obtained from a multiannual time series based face water from the more inshore fjord on every falling on replicate sampling. This method allows for the inter- tide (Eliassen et al. 2001). This generates extensive tur- specific testing of the fjord basin’s stock abundance rela- bulent thermohaline diffusion, which is generally (Aure tionships as well as relationships on smaller spatial scales et al. 2007) understood to reduce the density of resident that might result from the trophic activity at the time of basin water and facilitate frequent deep inflows of new the sampling. NAC water into the basin. In Mistfjord, major replace- ments of new coastal water to its basin occur only at decadal intervals, facilitated by slow turbulent diffusion Ecological rationale mostly forced by internal tidal waves, which are normal in sill fjord basins (Fjeldstad 1964; Skreslet & Schei 1976; In relative terms, zooplankton are normally sparse in Skreslet & Loeng 1977; Skreslet et al. 2015). parts of the water space, occurring mostly in denser In general, immigration of zooplankton from deep aggregations characterized by varying vertical and hori- shelf habitats to sill fjords during winter is related to zontal dimensions. Physical factors, such as discontinuity capacities for vertical migration and facilitated by upwell- layers and turbulence, influence zooplankton distribution ing caused by local katabatic winds (Skreslet & Loeng but are frequently insufficient to explain spatial patterns. 1977) or by regional coastal upwelling that results from There is a biological rationale for zooplankton aggrega- Ekman transport (Aure et al. 2007; Espinasse et al. 2018). tion, which reduces the risk of predation, both collectively Both processes lift dense shelf water residing