Characteristics of suspended particulate organic matter in the southwestern Atlantic: Influence of temperature, nutrient and phytoplankton features on the stable isotope signature

Abstract

Surface particulate organic matter (POM) along a transect from Subantarctic coastal waters on the Argentine shelf to the Bellingshausen Sea was characterized by its organic carbon (POC) and nitrogen (PON) content and δ 13C and δ 15N signatures in relation to sea surface water temperature (SST), nutrients and plankton. The correlation of δ 13C with SST was highly significant for the entire transect but less obvious within Subantarctic shelf ecosystems. Stable isotopes of POM varied from δ 13C ~ − 12‰ and δ 15N ~ 8‰ in Subantarctic shallow waters to δ 13C ~ − 32‰ and δ 15N ~ − 2‰ in the sector including the oceanic Subantarctic waters and the Antarctic region. In Argentine shelf waters δ 13C was > − 24‰ (on average − 20.9‰) and more variable than in oceanic Subantarctic and Antarctic waters (average of − 27.6‰). High isotopic variability of POM in northern Argentine shelf waters is probably due to a pronounced nutrient gradient. There, a sharp δ 13C decrease of ca. 12‰ was associated to an increase of the silicate to nitrate (Si:N) ratio to values > 0.25, and an increase of siliceous phytoplankton. Further south, Si:N ratios > 1 did not significantly affect δ 13C, and the influence of the sea surface temperature (SST) was more evident. δ 15N in POM of Argentine shelf waters averaged 6.3 ± 2.4‰, and the lowest δ 15N values (− 1.7‰) occurred in the northern Drake Passage, where they build, together with δ 13C around − 27‰, a clearly distinct pattern in the western South Atlantic. For the whole transect, SST alone accounted for 74% of the δ 13C variability. A multiple regression including SST, ammonium and POC explained 83% of δ 13C variance. The fit improvement by ammonium involved the nutrient-poor, regenerative system in the northernmost shallow sector and the Subantarctic shelf. δ 15N showed a strong inverse relationship with the fraction of unutilized nitrate, probably due to isotopic enrichment in the nitrate pool by phytoplankton uptake.