Kevin R. Arrigo

Impacts of Atmospheric Anthropogenic Nitrogen on the Open Ocean

Increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for up to about a third of the oceans external (nonrecycled) nitrogen supply and up to ∼3% of the annual new marine biological production, ∼0.3 petagram of carbon per year. This input could account for the production of up to ∼1.6 teragrams of nitrous oxide (N2O) per year. Although ∼10% of the oceans drawdown of atmospheric anthropogenic carbon dioxide may result from this atmospheric nitrogen fertilization, leading to a decrease in radiative forcing, up to about two-thirds of this amount may be offset by the increase in N2O emissions. The effects of increasing atmospheric nitrogen deposition are expected to continue to grow in the future.

Massive phytoplankton blooms under Arctic Sea ice

In midsummer, diatoms have taken advantage of thinning ice cover to feed in nutrient-rich waters. Phytoplankton blooms over Arctic Ocean continental shelves are thought to be restricted to waters free of sea ice. Here, we document a massive phytoplankton bloom beneath fully consolidated pack ice far from the ice edge in the Chukchi Sea, where light transmission has increased in recent decades because of thinning ice cover and proliferation of melt ponds. The bloom was characterized by high diatom biomass and rates of growth and primary production. Evidence suggests that under-ice phytoplankton blooms may be more widespread over nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in these waters may be underestimated by up to 10-fold.

Primary production in Southern Ocean waters

The Southern Ocean forms a link between major ocean basins, is the site of deep and intermediate water ventilation, and is one of the few areas where macronutrients are underutilized by phytoplankton. Paradoxically, prior estimates of annual primary production are insufficient to support the Antarctic food web. Here we present results from a primary production algorithm based upon monthly climatological phytoplankton pigment concentrations from the coastal zone color scanner (CZCS). Phytoplankton production was forced using monthly temperature profiles and a radiative transfer model that computed changes in photosynthetically usable radiation at each CZCS pixel location. Average daily productivity (g C m−2 d−1) and total monthly production (Tg C month−1) were calculated for each of five geographic sectors (defined by longitude) and three ecological provinces (defined by sea ice coverage and bathymetry as the pelagic province, the marginal ice zone, and the shelf). Annual primary production in the Southern Ocean (south of 50°S) was calculated to be 4414 Tg C yr−1, 4–5 times higher than previous estimates made from in situ data. Primary production was greatest in the month of December (816 Tg C month−1) and in the pelagic province (contributing 88.6% of the annual primary production). Because of their small size the marginal ice zone (MIZ) and the shelf contributed only 9.5% and 1.8%, respectively, despite exhibiting higher daily production rates. The Ross Sea was the most productive region, accounting for 28% of annual production. The fourfold increase in the estimate of primary production for the Southern Ocean likely makes the notion of an “Antarctic paradox” (primary production insufficient to support the populations of Southern Ocean grazers, including krill, copepods, microzooplankton, etc.) obsolete.

Agricultural runoff fuels large phytoplankton blooms in vulnerable areas of the ocean

Biological productivity in most of the worlds oceans is controlled by the supply of nutrients to surface waters. The relative balance between supply and removal of nutrients—including nitrogen, iron and phosphorus—determines which nutrient limits phytoplankton growth. Although nitrogen limits productivity in much of the ocean, large portions of the tropics and subtropics are defined by extreme nitrogen depletion. In these regions, microbial denitrification removes biologically available forms of nitrogen from the water column, producing substantial deficits relative to other nutrients. Here we demonstrate that nitrogen-deficient areas of the tropical and subtropical oceans are acutely vulnerable to nitrogen pollution. Despite naturally high nutrient concentrations and productivity, nitrogen-rich agricultural runoff fuels large (54–577 km2) phytoplankton blooms in the Gulf of California. Runoff exerts a strong and consistent influence on biological processes, in 80% of cases stimulating blooms within days of fertilization and irrigation of agricultural fields. We project that by the year 2050, 27–59% of all nitrogen fertilizer will be applied in developing regions located upstream of nitrogen-deficient marine ecosystems. Our findings highlight the present and future vulnerability of these ecosystems to agricultural runoff.

Distributions of phytoplankton blooms in the southern ocean.

A regional pigment retrieval algorithm for the Nimbus-7 Coastal Zone Color Scanner (CZCS) has been tested for the Southern Ocean. The pigment concentrations estimated with this algorithm agree to within 5 percent with in situ values and are more than twice as high as those previously reported. The CZCS data also revealed an asymmetric distribution of enhanced pigments in the waters surrounding Antarctica; in contrast, most surface geophysical properties are symmetrically distributed. The asymmetry is coherent with circumpolar current patterns and the availability of silicic acid in surface waters. Intense blooms (>1 milligram of pigment per cubic meter) that occur downcurrent from continental masses result from dissolved trace elements such as iron derived from shelf sediments and glacial melt.

Rapid and early export of Phaeocystis antarctica blooms in the Ross Sea, Antarctica.

The Southern Ocean is very important for the potential sequestration of carbon dioxide in the oceans and is expected to be vulnerable to changes in carbon export forced by anthropogenic climate warming. Annual phytoplankton blooms in seasonal ice zones are highly productive and are thought to contribute significantly to pCO2 drawdown in the Southern Ocean. Diatoms are assumed to be the most important phytoplankton class with respect to export production in the Southern Ocean; however, the colonial prymnesiophyte Phaeocystis antarctica regularly forms huge blooms in seasonal ice zones and coastal Antarctic waters. There is little evidence regarding the fate of carbon produced by P. antarctica in the Southern Ocean, although remineralization in the upper water column has been proposed to be the main pathway in polar waters. Here we present evidence for early and rapid carbon export from P. antarctica blooms to deep water and sediments in the Ross Sea. Carbon sequestration from P. antarctica blooms may influence the carbon cycle in the Southern Ocean, especially if projected climatic changes lead to an alteration in the structure of the phytoplankton community.

Large scale importance of sea ice biology in the Southern Ocean

Despite being one of the largest biomes on earth, sea ice ecosystems have only received intensive study over the past 30 years. Sea ice is a unique habitat for assemblages of bacteria, algae, protists, and invertebrates that grow within a matrix dominated by strong gradients in temperature, salinity, nutrients, and UV and visible radiation. A suite of physiological adaptations allow these organisms to thrive in ice, where their enormous biomass makes them a fundamental component of polar ecosystems. Sea ice algae are an important energy and nutritional source for invertebrates such as juvenile krill, accounting for up to 25% of total annual primary production in ice-covered waters. The ability of ice algae to produce large amounts of UV absorbing compounds such as mycosporine-like amino acids makes them even more important to organisms like krill that can incorporate these sunscreens into their own tissues. Furthermore, the nutrient and light conditions in which sea ice algae thrive induce them to synthesize enhanced concentrations of polyunsaturated fatty acids, a vital constituent of the diet of grazing organisms, especially during winter. Finally, sea ice bacteria and algae have become the focus of biotechnology, and are being considered as proxies of possible life forms on ice-covered extraterrestrial systems. An analysis of how the balance between sea ice and pelagic production might change under a warming scenario indicates that when current levels of primary production and changes in the areas of sea ice habitats are taken into account, the expected 25% loss of sea ice over the next century would increase primary production in the Southern Ocean by approximately 10%, resulting in a slight negative feedback on climate warming.

Phytoplankton taxonomic variability in nutrient utilization and primary production in the Ross Sea

Patterns of nutrient utilization and primary productivity (PP) in late austral spring and early summer in the southwestern Ross Sea were characterized with respect to phytoplankton taxonomic composition, polynya dynamics, and upper ocean hydrography during the 1996–1997 oceanographic program Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea. Phytoplankton biomass in the upper 150 m of the water column ranged from 40 to 540 mg chlorophyll a (Chl a) m−2, exceeding 200 mg Chl a m−2 everywhere except the extreme northern and eastern boundaries of the Ross Sea polynya. Diatom biomass was greatest in the shallow mixed layers of Terra Nova Bay, while the more deeply mixed waters of the Ross Sea polynya were dominated by Phaeocystis antarctica. Daily production computed from the disappearance of NO3 (1.14 g C m−2 d−1) and total dissolved inorganic carbon (TDIC, 1.29 g C m−2 d−1) is consistent with estimates made from an algorithm forced with satellite measurements of Chl a (1.25 g C m−2 d−1) and from measurements of 14C uptake (1.33 g C m−2 d−1). Phytoplankton PP in the Ross Sea averaged 100 g C m−2 yr−1 during 1996–1997. Despite the early formation of the Terra Nova Bay polynya the diatom bloom there did not reach its peak PP until middle to late January 1997 (most likely because of more intense wind mixing in November), ∼6 weeks after the P. antarctica bloom in the Ross Sea polynya had reached the same stage of development. From 70 to 100% of the C and N deficits in the upper 150 m could be accounted for by particulate organic matter, indicating that there had been little dissolved organic matter production or export of particulate material prior to our cruise. This suggests that early in the season, PP and zooplankton grazing are decoupled in the southwestern Ross Sea. The NO3∶PO4 disappearance ratio in waters dominated by P. antarctica (19.0±0.61) was significantly greater than in waters where diatoms were most common (9.52±0.33), and both were significantly different from the Redfield N∶P ratio of 16. Vertical profiles of TDIC suggest that P. antarctica took up 110% more CO2 per mole of PO4 removed than did diatoms, an important consideration for climate models that estimate C uptake from the removal of PO4.

Physical forcing of phytoplankton dynamics in the southwestern Ross Sea

Coastal zone color scanner (CZCS) imagery of phytoplankton pigments and passive microwave imagery of sea ice distributions in the southwestern Ross Sea are presented for three different seasons (1978–1989, 1979–1980, and 1981–1982) and were analyzed in conjunction with meteorological data obtained from a series of automatic weather stations (AWS). Dynamics of the phytoplankton bloom in Terra Nova Bay differed from those in the Ross Sea owing to spatial differences in katabatic wind fields which determine when the surface waters stratify. Interannual variation in the timing of formation of the Ross Sea polynya appears to be controlled by winter temperatures, which determine sea ice thickness and integrity, rather than variability or intensity in wind stress. Together, CZCS, AWS, and passive microwave data suggest that when the Ross Sea polynya forms early, stronger and more frequent katabatic winds result in increased advective losses of phytoplankton in surface waters and a delay in the phytoplankton bloom. If polynya formation is delayed until after the winds diminish in frequency, the phytoplankton bloom will develop earlier. The observation that diatoms dominate both the marginal ice zone and Terra Nova Bay, which are hydrographically similar, while Phaeocystis antarctica is found in unstable waters north of the Ross Ice Shelf, suggests that stratification plays an important role in determining species composition in the Ross Sea.

Spring phytoplankton production in the Western ross sea.

Coastal zone color scanner (CZCS) imagery of the western Ross Sea revealed the Presence of an intense phytoplankton bloom covering >106,000 square kilometers in early December 1978. This bloom developed inside the Ross Sea polynya, within 2 weeks of initial polynya formation in late November. Primary productivity calculated from December imagery (3.9 grams of carbon per square meter per day) was up to four times the values measured during in situ studies in mid-January to February 1979. Inclusion of this early season production yields a spring-to-summer estimate of 141 to 171 grams of carbon per square meter, three to four times the values previously reported for the western Ross Sea.