In the Southern Ocean, polynyas exhibit enhanced rates of primary productivity and represent large seasonal sinks for atmospheric CO2. Three contrasting east Antarctic polynyas were visited in late December to early January 2017: the Dalton, Mertz, and Ninnis polynyas. In the Mertz and Ninnis polynyas, phytoplankton biomass (average of 322 and 354 mg chlorophyll a (Chl a)/m2, respectively) and net community production (5.3 and 4.6 mol C/m2, respectively) were approximately 3 times those measured in the Dalton polynya (average of 122 mg Chl a/m2 and 1.8 mol C/m2). Phytoplankton communities also differed between the polynyas. Diatoms were thriving in the Mertz and Ninnis polynyas but not in the Dalton polynya, where Phaeocystis antarctica dominated. These strong regional differences were explored using physiological, biological, and physical parameters. The most likely drivers of the observed higher productivity in the Mertz and Ninnis were the relatively shallow inflow of iron‐rich modified Circumpolar Deep Water onto the shelf as well as a very large sea ice meltwater contribution. The productivity contrast between the three polynyas could not be explained by (1) the input of glacial meltwater, (2) the presence of Ice Shelf Water, or (3) stratification of the mixed layer. Our results show that physical drivers regulate the productivity of polynyas, suggesting that the response of biological productivity and carbon export to future change will vary among polynyas.
The Chesapeake Bay, a large coastal plain estuary, has been studied extensively in terms of its water quality, and yet, comparatively less is known about its carbonate system. Here we present discrete observations of dissolved inorganic carbon (DIC) and total alkalinity from four seasonal cruises in 2016-2017. These new observations are used to characterize the regional CO 2 system and to construct a DIC budget of the mainstem. In all seasons, elevated DIC concentrations were observed at the mouth of the bay associated with inflowing Atlantic Ocean waters, while minimum concentrations of DIC were associated with fresher waters at the head of the bay. Significant spatial variability of the partial pressure of CO 2 was observed throughout the mainstem, with net uptake of atmospheric CO 2 during each season in the upper mainstem and weak seasonal outgassing of CO 2 near the outflow to the Atlantic Ocean. During the time frame of this study, the Chesapeake Bay mainstem was (1) net autotrophic in the mixed layer (net community production of 0.31-mol C m −2 ·year −1 ) and net heterotrophic throughout the water column (net community production of −0.48-mol C m −2 ·year −1 ), (2) a sink of 0.38-mol C m −2 ·year −1 for atmospheric CO 2 , and (3) significantly seasonally and spatially variable with respect to biologically driven changes in DIC. Plain Language SummaryWater quality in the Chesapeake Bay, the largest estuary in the continental United States, has been extensively monitored for over 30 years, yet relatively less is known about the cycling of carbon in these waters. The data collected in this study demonstrate considerable seasonal and spatial variability of dissolved carbon dioxide (CO 2 ) in the Chesapeake Bay mainstem. Much of this variability is driven by the physical setting: Waters have lower salinity in the northern Bay due to riverine inputs and higher salinity in the southern Bay due to exchange with the Atlantic Ocean. Changes in salinity driven by estuarine circulation patterns throughout the mainstem have a large influence on the seasonal and spatial variability of CO 2 , as do biological processes. In surface waters of the mainstem, photosynthesis is greater than respiration over a complete seasonal cycle. In the years studied, there is also large spatial variability with respect to the uptake of atmospheric CO 2 . Through the combination of changes in salinity and biological processes, the mainstem of the bay acts as a net sink of atmospheric CO 2 .
Particle fluxes at the Southern Ocean time series (SOTS) site in the Subantarctic Zone (SAZ) south of Australia (∼47 • S, ∼142 • E, 4600 m water depth) were collected from 1997-2017 using moored sediment traps at nominal depths of 1000, 2000, and 3800 m. Annually integrated mass fluxes showed moderate variability of 14 ± 6 g m −2 yr −1 at 1000 m, 20 ± 6 g m −2 yr −1 at 2000 m and 21 ± 4 g m −2 yr −1 at 3800 m. Particulate organic carbon (POC) fluxes were similar to the global median, indicating that the Subantarctic Southern Ocean exports considerable amounts of carbon to the deep sea despite its high-nutrient, low chlorophyll characteristics. The interannual flux variations were larger than those of net primary productivity as estimated from satellite observations. Particle compositions were dominated by carbonate minerals (>60% at all depths), opal (∼10% at all depths), and particulate organic matter (∼17% at 1000 m, decreasing to ∼10% at 3800 m), with seasonal and interannual variability much smaller than for their flux magnitudes. The carbonate counter-pump effect reduced carbon sequestration by ∼8 ± 2%. The average seasonal cycle at 1000 m had a two-peak structure, with a larger early spring peak (October/November) and a smaller late summer (January/February) peak. At the two deeper traps, these peaks became less distinct with a greater proportion of the fluxes arriving in autumn. Singular value decomposition (SVD) shows that this temperate seasonal structure accounts for ∼80% of the total variance (SVD Mode 1), but also that its influence varies significantly relative to Modes 2 and 3 which describe changes in seasonal timings. This occurrence of significant interannual variability in seasonality yet relatively constant annual fluxes, is likely to be useful in selecting appropriate models for the simulation of environmental-ecological
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