Fate of production in the Arctic seasonal ice zone. An investigation of suspended biomass, vertical export and the impact of grazers during the onset of the spring bloom north of Svalbard

Abstract

In the Arctic Ocean, biological productivity is largely determined by sea ice, making the seasonal sea ice zone (SSIZ) its most productive region. The current study is a combined investigation of the suspended biomass, vertical export of organic material, and potential retention processes by zooplankton, during a crucial period of bloom development in the Eurasian SSIZ north of Svalbard, where few studies have previously been done. To evaluate the magnitude and composition of the bloom and subsequent vertical export, short-term sediment traps, at five depths between 30 and 200m, were deployed at eight sea ice stations. Daily patterns of chlorophyll a, particulate organic carbon (POC) and contribution of zooplankton fecal pellets (FP) were discovered in distinct assemblages – conditions ranging from pre- to mid-bloom development. Daily loss rates of POC increased from 0.6 to 2.7% as the bloom progressed from a pre- to mid-bloom phase, but the vertical carbon export rates in the shallower depths exceeded those in the deeper layers as the bloom developed accordingly. Phytoplankton carbon (PPC) was found to be a more important component to the vertical POC flux than FP carbon (FPC), especially as the bloom progressed. PPC and FPC contributed 5-75% and 0.5-24% to POC export respectively. The contribution of FPC flux to total POC flux was found to be in line with previous studies, revealing that the relative contribution FPC flux to vertical carbon export is variable but may diminish northward with the SSIZ. The impact of grazers was further investigated through FP production experiments of key Calanus species. The proportion of Calanus finmarchicus community-produced FPC exported to 40m decreased from 36% to 4% from early- to mid-bloom conditions, suggesting stronger zooplankton-mediated retention as the bloom intensifies. Additionally, under slower bloom development, grazers appeared to be effectively controlling and inhibiting the accumulation of biogenic biomass and subsequent vertical flux. The current study reveals that the northern ice-covered Barents Sea shelf break can provide comparable vertical export rates of organic material during the spring bloom to the productive and shallower central Barents Sea.