Pelagic-benthic coupling in a northern Norwegian fjord over winter : considering seasonality in high-latitude aquaculture

Abstract

Global aquaculture is projected to double by 2050 to meet the demand of a growing human population. Norway has stated its interest in expanding its aquaculture sector to supply this growing international and domestic demand. The environmental impact of aquaculture by-products is determined by their concentration and distribution, which are affected by seasonal signals in pelagic-benthic coupling. Spring and summer studies of pelagic-benthic coupling in Norwegian fjords are significantly more abundant than those focusing on late autumn and winter. This study compared meteorology, hydrography, concentration of suspended and sinking biomass, and total particulate matter flux from October 2017 to February 2018 in Kaldfjorden, Norway (69.746ºN, 18.683ºE) to explore the physical and biological drivers of pelagic-benthic coupling. Stratification of the water column in Kaldfjorden weakened between October and December before disappearing completely in January and February, identifying winter as a time of high mixing. Changes to the physical environment coincided with a steep decline in suspended chlorophyll a concentration (Oct: 0.09-3.15 mg m-3, Dec-Feb: 0.03-0.12 mg m-3) and zooplankton abundance (Nov: 4502.83 ind. m-3, Jan-Feb: <101.72 ind. m-3). Sinking material was sampled using short-term sediment traps (24hrs). The downward biomass flux decreased throughout winter and particulate matter became more degraded, most likely due to zooplankton grazing. Sediment trap samples also showed evidence of resuspension following episodic winds throughout winter. The observed decrease in stratification and biological activity in this study is considered characteristic of a Northern Norwegian fjord, and supports the importance of including seasonally appropriate environmental baselines in the management of open circuit aquaculture to mitigate environmental impacts.