Seasonal variations in C:N:Si:Ca:P:Mg:S:K:Fe relationships of seston from Norwegian coastal water: Impact of extreme offshore forcing during winter-spring 2010

Abstract

The aim of this study was to reveal the relative content of C, N, Ca, Si, P, Mg, K, S and Fe in seston particles in Norwegian coastal water (NCW), and how it relates to biological and hydrographic processes during seasonal cycles from October 2009–March 2012. The following over all stoichiometric relationship for the time series was obtained: C66N11Si3.4Ca2.3P1Mg0.73S0.37K0.35Fe0.30, which is novel for marine waters. A record-breaking (187-year record) negative North Atlantic Oscillation (NAO) index caused extreme physical forcing on the Norwegian Coastal Current Water (NCCW) during the winter 2009–2010, and the inflow and upwelling of saline Atlantic water (AW) in the fjord was thus extraordinary during late spring-early summer in 2010. The element concentrations in fjord seston particles responded strongly to this convection, revealed by maximum values of all elements, except Fe, exceeding average values with 10.8 × for Ca, 9.3 for K, 5.3 for S, 5.1 for Mg, 4.6 for Si, 4.0 for P, 3.8 for C, and 3.3 for N and Fe. This indicates that the signature of the Atlantic inflow was roughly two times stronger for Ca and K than for the others, probably connected with peaks in coccolithophorids and diatoms. There is, however, 1.5 × more of Si than Ca contained in the seston, which could be due to a stronger dominance of diatoms than coccolithophorids, confirming their environmental fitness. In total our data do not indicate any severe nutrient limitation with respect to N, P and Fe, but accumulation of iron by Fe-sequestering bacteria might at times reduce the availability of the dissolved Fe-fraction. There is a high correlation between most of the measured elements, except for Ca, which together with Fe only weakly correlated with the other elements. It is to be expected that environmental alterations in NCW related to climate change will influence the seston elemental composition, but the full effect of this will be strongly dependent on the future dominance of the high pressure versus low pressure systems (i.e. NAO index), since they are key regulators for the direction of wind driven vertical convection (i.e. upwelling or downwelling). Changes in stratification, temperature, light, pH (ocean acidification), CaCO3 concentrations (carbon pump) and availability of nutrients in the euphotic zone (biogeochemical cycling) are essential for the future dominance of coccolithophorids versus diatoms.