Annual nitrate drawdown observed by SOCCOM profiling floats and the relationship to annual net community production

Abstract

Annual nitrate cycles have been measured throughout the pelagic waters of the Southern Ocean, including regions with seasonal ice cover and southern hemisphere subtropical zones. Vertically resolved nitrate measurements were made using in situ ultraviolet spectrophotometer (ISUS) and submersible ultraviolet nitrate analyzer (SUNA) optical nitrate sensors deployed on profiling floats. Thirty‐one floats returned 40 complete annual cycles. The mean nitrate profile from the month with the highest winter nitrate minus the mean profile from the month with the lowest nitrate yields the annual nitrate drawdown. This quantity was integrated to 200 m depth and converted to carbon using the Redfield ratio to estimate annual net community production (ANCP) throughout the Southern Ocean south of 30°S. A well‐defined, zonal mean distribution is found with highest values (3–4 mol C m−2 yr−1) from 40 to 50°S. Lowest values are found in the subtropics and in the seasonal ice zone. The area weighted mean was 2.9 mol C m−2 yr−1 for all regions south of 40°S. Cumulative ANCP south of 50°S is 1.3 Pg C yr−1. This represents about 13% of global ANCP in about 14% of the global ocean area.

Nutrient Controls on Export Production in the Southern Ocean

“…Except for the STZ, oxygen‐based ANCP estimates are lower than nitrate‐based estimates, which may be due to some important fraction of organic carbon remineralization occurring above 100 m south of the STF and therefore missed in our oxygen‐based estimation of ANCP. The general latitudinal trend of our estimates is similar to that inferred from a compilation of previous observations based on oxygen drawdown (Hennon et al, ; Martz et al, ; Riser & Johnson, ), as well as on changes in nutrients, dissolved inorganic carbon, and pCO 2 (Bender & Jönsson, ; Johnson et al, ; Lourey & Trull, ; MacCready & Quay, ; McNeil & Tilbrook, ; Munro et al, ; Shadwick et al, ). An important caveat of most of these estimates is that they have a large degree of uncertainty, which prevents us from obtaining clear statistical differences between zones.…”

“…Meridional pattern of ANCP estimates (and associated errors) in the Southern Ocean from previous studies (Bender & Jönsson, ; Johnson et al, ; Hennon et al, ; Lourey & Trull, ; MacCready & Quay, ; Martz et al, ; McNeil & Tilbrook, ; Munro et al, ; Shadwick et al, ; Riser & Johnson, ) and from our analysis of nitrate (black filled circles) and oxygen (gray filled squares) drawdown measured by Southern Ocean Carbon and Climate Observations and Modeling floats. ANCP estimates from this study are inferred from 5° latitudinally binned estimates (± standard deviation).…”

“…We expect most of the oceanic export production to occur during this period, although minor export events could also occur during fall and winter months. In an effort to detect float periods where is not driven mainly by biological respiration, we remove periods following the same criteria as in the analysis of seasonal nitrate drawdown carried out by Johnson et al (): If the change in salinity (Δ S ) at 500 m from winter to summer is > 0.05, if the float moved > 8° in longitude, or if the float moved > 5° in latitude. Finally, we estimate ANCP as depth‐integrated respiration between 100 and 500 m, multiplied by the number of days of the productive period of each float ( D prod ): The upper boundary of the vertical integration was selected based on preliminary analyses showing that most of the primary productivity as well as the spring–summer depth of the mixed layer are constrained to above 100 m. The mixed layer depth (MLD) is calculated using float in situ temperature and salinity profiles following de Boyer Montégut et al ().…”

“…Based on this seasonality in oxygen levels, respiration rates have been estimated from the temporal evolution of oxygen measured by autonomous profiling floats in the Southern Ocean (Martz et al, ), as well as other ocean regions (Hennon et al, ; Riser & Johnson, ). Similarly, the net production of organic matter can be assessed from the seasonal drawdown of the upper ocean nitrate reservoir (Johnson et al, ). In this study, we build upon these ideas to estimate annual net community production (ANCP) based on the seasonal drawdown of surface nitrate and mesopelagic oxygen, using data obtained from 123 profiling floats deployed by the SOCCOM program.…”

Sustainable Observations of the AMOC: Methodology and Technology

“…In the last decade, however, novel biogeochemical sensors and autonomous samplers have begun to overcome the substantial technical difficulties that have restricted their application from remote environments. Optical fluorescence‐based optodes for dissolved oxygen have become an integral addition to profiling floats (Bushinsky et al, ; Körtzinger et al, ), while sensors for nitrate (based on ultraviolet spectrophotometry, e.g., Johnson et al, ), pH (based on ion sensitive field effect transistors technologies, e.g., Martz et al, ; Bresnahan et al, or spectrophotometry, e.g., Cullison Gray et al, and pCO 2 (based on infrared/colorimetric spectrometry or optodes, e.g., Clarke et al, ) have matured to the extent they are now optionally included on floats, at fixed‐point observatories or on VOSs. Sensors for dissolved inorganic carbon, total alkalinity, and the nutrients phosphate and silicate are also undergoing rapid development that means they will soon also become available for deployment on diverse platforms.…”