Description: Nitrogen (N) is the major limiting nutrient for phytoplankton growth and productivity in large parts of the world's oceans. Differential preferences for specific N substrates may be important in controlling phytoplankton community composition. To date, there is limited information on how specific N substrates influence the composition of naturally occurring microbial communities. We investigated the effect of nitrate ( math formula), ammonium ( math formula), and urea on microbial and phytoplankton community composition (cell abundances and 16S rRNA gene profiling) and functioning (photosynthetic activity, carbon fixation rates) in the oligotrophic waters of the North Pacific Ocean. All N substrates tested significantly stimulated phytoplankton growth and productivity. Urea resulted in the greatest (〉300%) increases in chlorophyll a (〈0.06 μg L−1 and ∼0.19 μg L−1 in the control and urea addition, respectively) and productivity (〈0.4 μmol C L−1 d−1 and ∼1.4 μmol C L−1 d−1 in the control and urea addition, respectively) at two experimental stations, largely due to increased abundances of Prochlorococcus (Cyanobacteria). Two abundant clades of Prochlorococcus, High Light I and II, demonstrated similar responses to urea, suggesting this substrate is likely an important N source for natural Prochlorococcus populations. In contrast, the heterotrophic community composition changed most in response to math formula. Finally, the time and magnitude of response to N amendments varied with geographic location, likely due to differences in microbial community composition and their nutrient status. Our results provide support for the hypothesis that changes in N supply would likely favor specific populations of phytoplankton in different oceanic regions and thus, affect both biogeochemical cycles and ecological processes.

Description: Dissolution of calcium carbonate neutralizes anthropogenic CO2. An upward shift of the calcite and aragonite saturation horizons exposes carbonate deposits to dissolution which is an important carbon sink reaction on a time scale of several thousand years for the world oceans. In the Southern Ocean, the surface calcite and aragonite saturation states are naturally low due to cold temperatures. They are further reduced by the uptake of anthropogenic carbon which is strongest in the top 1000 m. Undersaturation at the surface might occur even before the underlying water column is completely undersaturated. Therefore, carbonate sediments on Antarctic shelves are likely to be be among the first to dissolve due to man-made acidification. Obviously, we need to know the inventory of CaCO3 in the bioturbated layer of the Antarctic shelf sediments to quantify the capacity of this negative feedback mechanism. Here, we present a technique that allows us to spatially interpolate CaCO3 data on the Antarctic shelves. We derive quantitative relationships between nearly 400 measurements of CaCO3 on the Antarctic shelves, water depth and satellite-derived primary production in the overlying water column. This confirms that primary production mainly determines the CaCO3 distribution on the Antarctic shelves: On the one hand, there is hardly any CaCO3 production when primary production is low. On the other hand, dissolution due to CO2 produced by remineralization of organic matter dominates in high primary production regions; this constrains CaCO3 accumulation and preservation to regions with an optimum primary production level. These relationships between sedimentary CaCO3, primary production, and water depth are then applied to produce a map of CaCO3 on all Antarctic shelves. The inventory, calculated from this interpolated map of CaCO3, amounts to 4 Pg CaCO3, capable to neutralize about 0.5 Pg C. This, however, is in the same range as estimates of the annual anthropogenic CO2 uptake in the Southern Ocean. The dissolution of CaCO3 is limited by slow reaction kinetics, otherwise CaCO3 could disappear from the Antarctic shelves in only one to a few years. Our analysis suggests that deposits of CaCO3 will dissolve without releasing a significant buffering signal and that Antarctic acidification will proceed without being slowed down by dissolution of carbonates from Antarctic shelves.

Description: Measurements of late springtime nutrient concentrations in Arctic waters are relatively rare due to the extensive sea ice cover that makes sampling difficult. During the SUBICE cruise in May-June 2014, an extensive survey of hydrography and pre-bloom nutrient concentrations was conducted in the Chukchi Sea. Cold (〈 -1.5°C) winter water was prevalent throughout the Chukchi Sea shelf, and the water column was weakly stratified. Nitrate (NO3-) concentration averaged 12.6±1.92 µM in surface waters and 14.0±1.91 µM near the bottom and was significantly correlated with salinity. The highest NO3- concentrations were associated with winter water within the Central Channel flow path. NO3- concentrations were much reduced near the northern shelfbreak within the upper halocline waters of the Canada Basin and along the eastern side of the shelf near the Alaskan coast. Net community production (NCP), estimated as the difference in depth-integrated NO3- content between spring (this study) and summer (historical), varied from 28-38 g C m-2 a-1. This is much lower than previous NCP estimates using NO3- concentrations from the southeastern Bering Sea as a baseline. These results demonstrate the importance of using local profiles of NO3- measured as close to the beginning of the spring bloom as possible when estimating NCP.