Dynamics of Calanus species linked to Oceanographic processes, Predators and carbon Sequestration (COPS)

This project (i) established and analyzed new zooplankton time series based on stored samples collected by the Faroe Marine Research Institute along three standard transects, radiating from the Faroe shelf in May and August 1994-2023, (ii) studied zooplankton influence on selected planktivorous fish on the Faroe shelf and slope, and (iii) explored advection of overwintering Calanus finmarchicus with the overflow through the Faroe Bank Channel (FBC), their fate and lipid sequestration.

C. finmarchicus (CI–CVI) accounted for up to 40% of the zooplankton community in May, but its dominance had declined to around 5% in August. Despite this marked shift in abundance, seasonal differences in dry weight were relatively small, reflecting an increase in smaller copepod abundances, which compensate for the decline in C. finmarchicus. The overall copepod abundance was higher in the open ocean compared to the shallow Faroe shelf. In May, C. finmarchicus dominated both offshore and shallow regions. In August, Temora and Acartia were common within the shallow region while Acartia and Pseudocalanus dominated offshore. The overall mean offshore C. finmarchicus (CIV-CVI) abundance in May differed between regions, with sections E and V having the highest and lowest abundance, respectively. However, section V showed sharpest pulses of increased abundance, which in turn showed a positive association with increased abundance within the shallow area (r = 0.45). Section E showed less dramatic differences in abundance between the shallow and offshore enviornment and although the abundance flucuated interannually, it appeared more stable compared to section V. The time-series of the offshore abundance in the eastern, western and southern regions showed high variability throughout the study period, with no clear temporal trend.

Stomach content analysis in Norway pout and juvenile blue whiting on the Faroe shelf and slope in February-March and in August showed that Norway pout (mean length 16.0 and 17.1 cm respectively mainly fed on copepods while juvenile blue whiting (mean length 28 and 25 cm respectively) preferred juvenile euphausiids. In both cases their food mainly seem to rely on advection from the surrounding oceanic environment.

Overwintering C. finmarchicus (mainly CV individuals) were observed in the deep overflow water in the FBC already in beginning of July. In August the abundance in the upper layer had decreased markedly and most individuals that gone into diapause in deep cold water. Ascendance started in February, and the abundance of diapause individuals in the overflow water until start of ascendance ranged between ~100 and 250 (mean 158) individuals per m³. Assuming an overflow volume transport of 2.2 Sv through the FBC, this corresponds to a mean C. finmarchicus transport of about 6000 tonnes per day and a total annual transport of about 1.4 mill. tonnes DW. On passing through the FBC, the bulk of these copepods are entrained into relatively cold and dense water that spills into the Iceland basin to a depth of between 800 and 1200 m and subsequently flow along the western slopes of the Iceland Faroe Ridge. Here they come into warmer environment and their lipid content reduces. We estimate the potential contribution of this to the overall carbon sequestered in the oceans by the biological carbon pump to be of the order 30 MtC with a residence time of approximately 200 years.