Strong Seasonality of Marine Microbial Eukaryotes in a High-Arctic Fjord (Isfjorden, in West Spitsbergen, Norway).

The Adventfjorden time series station (IsA) in Isfjorden, West Spitsbergen, Norway, was sampled frequently from December 2011 to December 2012. The community composition of microbial eukaryotes (size, 0.45 to 10 µm) from a depth of 25 m was determined using 454 sequencing of the 18S V4 region amplified from both DNA and RNA. The compositional changes throughout the year were assessed in relation to in situ fjord environmental conditions. Size fractionation analyses of chlorophyll a showed that the photosynthetic biomass was dominated by small cells (<10 µm) most of the year but that larger cells dominated during the spring and summer. The winter and early-spring communities were more diverse than the spring and summer/autumn communities. Dinophyceae were predominant throughout the year. The Arctic Micromonas ecotype was abundant mostly in the early-bloom and fall periods, whereas heterotrophs, such as marine stramenopiles (MASTs), Picozoa, and the parasitoid marine alveolates (MALVs), displayed higher relative abundance in the winter than in other seasons. Our results emphasize the extreme seasonality of Arctic microbial eukaryotic communities driven by the light regime and nutrient availability but point to the necessity of a thorough knowledge of hydrography for full understanding of their succession and variability.

Continuous daylight in the high-Arctic summer supports high plankton respiration rates compared to those supported in the dark.

Plankton respiration rate is a major component of global CO2 production and is forecasted to increase rapidly in the Arctic with warming. Yet, existing assessments in the Arctic evaluated plankton respiration in the dark. Evidence that plankton respiration may be stimulated in the light is particularly relevant for the high Arctic where plankton communities experience continuous daylight in spring and summer. Here we demonstrate that plankton community respiration evaluated under the continuous daylight conditions present in situ, tends to be higher than that evaluated in the dark. The ratio between community respiration measured in the light (Rlight) and in the dark (Rdark) increased as the 2/3 power of Rlight so that the Rlight:Rdark ratio increased from an average value of 1.37 at the median Rlight measured here (3.62 µmol O2 L-1 d-1) to an average value of 17.56 at the highest Rlight measured here (15.8 µmol O2 L-1 d-1). The role of respiratory processes as a source of CO2 in the Arctic has, therefore, been underestimated and is far more important than previously believed, particularly in the late spring, with 24 h photoperiods, when community respiration rates are highest.

Partitioning of polychlorinated biphenyls between Arctic seawater and size-fractionated zooplankton.

Concentrations of hydrophobic organic contaminants in zooplankton have been hypothesized to be governed by either near-equilibrium partitioning with surrounding water, growth dilution, or biomagnification. Concentrations of 17 polychlorinated biphenyls (PCBs) were measured in size-fractionated zooplankton, in phytoplankton (> 0.7 microm), and in the dissolved water phase (< 0.7 microm) in the surface water of the northern Barents Sea marginal ice zone east and north of Spitsbergen (Norway) and in the central Arctic Ocean at 89 degrees N. The linear partition model was used to indirectly assess if PCBs were equilibrated between water and the extractable organic matter (EOM) of zooplankton. As an independent test, the relation between the EOM-normalized partition coefficient (log K(EOM)) and trophic level (TL) of the zooplankton (based on delta 15N) was investigated. All log K-log K(OW) regressions were significant (n=18, p < 0.05, r2 = 0.65-0.95), being consistent with near-equilibrium partitioning and indirectly suggesting the absence of biomagnification. No correlation was found between log K(EOM) and TL, further supporting the apparent absence of biomagnification in zooplankton. One implication of these results is a reduced uncertainty in modeling of food web uptake, in which kinetic parameterizations of biodilution or biomagnification in zooplankton may be replaced by a simpler parameterization based on equilibrium partitioning.

New production regulates export stoichiometry in the ocean.

The proportion in which carbon and growth-limiting nutrients are exported from the oceans' productive surface layer to the deep sea is a crucial parameter in models of the biological carbon pump. Based on >400 vertical flux observations of particulate organic carbon (POC) and nitrogen (PON) from the European Arctic Ocean we show the common assumption of constant C:N stoichiometry not to be met. Exported POC:PON ratios exceeded the classical Redfield atomic ratio of 6.625 in the entire region, with the largest deviation in the deep Central Arctic Ocean. In this part the mean exported POC:PON ratio of 9.7 (a:a) implies c. 40% higher carbon export compared to Redfield-based estimates. When spatially integrated, the potential POC export in the European Arctic was 10-30% higher than suggested by calculations based on constant POC:PON ratios. We further demonstrate that the exported POC:PON ratio varies regionally in relation to nitrate-based new production over geographical scales that range from the Arctic to the subtropics, being highest in the least productive oligotrophic Central Arctic Ocean and subtropical gyres. Accounting for variations in export stoichiometry among systems of different productivity will improve the ability of models to resolve regional patterns in carbon export and, hence, the oceans' contribution to the global carbon cycle will be predicted more accurately.