The importance of microbiota and terrestrial inflows in controlling seston C:N:P:O:Si:Ca:Mn:Mg:Fe:K:Na:Cl:S:Cu:Zn stoichiometry of a deep coastal fjord

Abstract

Comprehensive fjord-systems represent major extensions of the coastline and are therefore important transfer zones of materials from land to ocean. Despite increased terrestrial inflows to fjords due to climate changes, we know little about the effects on the ecosystem, especially biogeochemical cycling. We present novel data on spatiotemporal variations of seston multielement stoichiometry in the Sognefjord, the second longest (204 km) and deepest (1308 m) fjord in the world, relative to environmental conditions and microbiota. Concentration of major elements was highest in the upper brackish layer whereas trace metals and minor elements were highest close to the bottom. Seasonally varying microbiota was an important part of the seston in surface waters. None of the seston C:N:P (molar) annual means at specific depths corresponded to the Redfield ratio (106:16:1). At 5 m, annual means of N/P and C/N were 8.4 and 6.5, respectively, while at depth (50–1220 m) N/P were on scale 3 times higher (21–31) and C/N 3 times lower (1.6–2.6), suggesting alternative N-sequestration mechanisms. Overall, correlations between C-Ca and C-S indicate a strong influence from calcite (CaCO3) and organosulfur producing microorganisms, while correlations between particulate Si and Mg–K–Ca–O at depth are consistent with clay and sinking diatom frustules. Mn concentrations increased strongly towards the bottom, likely from resuspension of MnO2 rich sediments and clay particles. Based on seston concentrations, we arrived at the following stoichiometric relationship: C55N16P1Si3.6Ca3.4O16Fe0.74Mn0.51Zn0.33S0.21Cu0.08Cl1.7Na0.68Mg0.71K0.37, although rarely measured, such information is a prerequisite for evaluating environmental impact on coastal ecosystems, biogeochemical cycling, pollution risk analysis and monitoring guidelines.