NE Atlantic zooplankton wintering in fjord habitats responds to hemispheric climate

Abstract

The study addresses how local abiotic variability links zooplankton stocks to climate systems. Four zooplankton species and abiotic environmental data were sampled annually during October and February 1983-2005 in two northern Norwegian fjord basins. Inter-decadal change in abundance occurred in both fjords but differed between the two communities, presumably resulting from combinations of topography, specific life history strategies and preference for ambient water qualities before advective migration into the habitats. The shallow Mistfjord sill led to accumulation of coastal water in the fjord basin, while the deeper Saltfjord sill allowed frequent seasonal exchange of Atlantic basin water. Changes in basin water temperature and salinity were decadal in the Mistfjord basin, and inter-annual in the Saltfjord basin. Several biotic and abiotic variables covaried with climate indices, which indicates causal relationships between hemispheric climate variability and abundance of wintering zooplankton, but the processes involved are not fully understood. North Atlantic Oscillation apparently forced vernal freshwater discharge from Norway, summer reproduction in neritic waters and geographic distribution of basin-scale population systems. Stratospheric forcing as indicated by Arctic Oscillation seems to be involved in geophysical relationships which influence dynamic processes in zooplankton population systems within the Arctic Mediterranean. arctic regions of the Atlantic sector. NAO above the long-term average (positive NAO) generates southwesterly winds towards Scandinavia (Hurrell, 1995) which discharges latent heat and precipitation over Norwegian coastal landscapes while NAO below average (negative NAO) is associated with dryer climate. Seasonal accumulation of snow and vernal meltwater runoff causes large river discharge in May-July (Tollan, 1976) which generates pronounced geophysical changes in coastal recipients (Saetre, 2007). Several authors have discussed how natural freshwater discharge and lowered salinity during summer may influence neritic biological production (Helland-Hansen

Environmental effects on zooplankton abundance on a sub-Arctic shelf off northern Norway

“…Østvedt 1955, Eiane et al 2002. Such fluctuations likely reflect effects of variability in the environment on the gain and loss processes acting in populations (Hirche et al 2001, Hays et al 2005, Willis et al 2008, Skreslet et al 2015. The mechanisms linking environmental variability with variability in zooplankton populations are only partly understood (Arashkevich et al 2002, Astthorsson & Gislason 2003, Walkusz et al 2009.…”

“…This is because mesoscale heterogeneity in food supply (Young et al 2009, Suchy et al 2016 or predation pressure (Bagøien et al 2001, Eiane et al 2002, Ohman & Hsieh 2008 generate variability in zooplankton abundance, but zooplankton distribution also depends on advection and bathymetry (Tremblay & Roff 1983, Walkusz et al 2009. In coastal waters, mesoscale spatial hetero-geneity in the zooplankton often reflects topographic steering of on-shelf and coastal current systems , Munk et al 2015, Skreslet et al 2015. Thus, zooplankton aggregations can occur in association with the complex hydrography typically forming over shelf breaks (Barange 1994) and in canyons (Macquart-Moulin & Patriti 1996), for example as a combined effect of local upwelling and vertical behaviour (Genin 2004).…”

Ecology of Planktonic Atlantic Cod (Gadus morhua)

“…Several of them fall prey to Atlantic cod and other marine species within the AME. All are subject to ecological changes caused by hemispheric climate variability that causes bottom-up forcing of food webs based on the ecosystem's plankton production [42].…”

“…The abundance of C. finmarchicus immigrating to a fjord habitat near Lofoten during September-October correlated positively with average NAO in March-July (Figure 10), resulting from variable precipitation in Norwegian alpine landscapes [42]. It forces meltwater discharge from rivers to the NCC on the mid-Norwegian shelf during May-August.…”

“…Thus, the Arctic Mediterranean Ecosystem seems to be the prototype of a closed ecosystem that circulates and conserves most of the biogenic energy fluxes within its food web. Hemispheric climate variability as indicated by tropospheric NAO and stratospheric Arctic Oscillation [42] seems to force bottom-up control of biological production from primary production to the trophic levels of Atlantic cod (Figure 13). Roles of seabirds, seals, whales and human fisheries exert top-down control, which also NEA cod older than 0-group do by being cannibalistic.…”