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

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 delta C-13 and delta N-15 signatures in relation to sea surface water temperature (SST), nutrients and plankton. The correlation of delta C-13 with SST was highly significant for the entire transect but less obvious within Subantarctic shelf ecosystems. Stable isotopes of POM varied from delta C-13 similar to-12 parts per thousand and delta N-15-8 parts per thousand in Subantarctic shallow waters to delta C-13 similar to-32 parts per thousand and delta N-15 similar to-2 parts per thousand in the sector including the oceanic Subantarctic waters and the Antarctic region. In Argentine shelf waters delta C-13 was >-24 parts per thousand (on average -20.9 parts per thousand) and more variable than in oceanic Subantarctic and Antarctic waters (average of -27.6 parts per thousand). High isotopic variability of POM in northern Argentine shelf waters is probably due to a pronounced nutrient gradient. There, a sharp delta C-13 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 delta C-13, and the influence of the sea surface temperature (SST) was more evident. delta N-15 in POM of Argentine shelf waters averaged 6.3 +/- 2.4 parts per thousand, and the lowest delta N-15 values (-1.7 parts per thousand) occurred in the northern Drake Passage, where they build, together with delta C-13 around -27 parts per thousand, a clearly distinct pattern in the western South Atlantic. For the whole transect, SST alone accounted for 74% of the delta C-13 variability. A multiple regression including SST, ammonium and POC explained 83% of delta C-13 variance. The fit improvement by ammonium involved the nutrient-poor, regenerative system in the northernmost shallow sector and the Subantarctic shelf delta N-15 showed a strong inverse relationship with the fraction of unutilized nitrate, probably due to isotopic enrichment in the nitrate pool by phytoplankton uptake. (C) 2009 Elsevier B.V. All rights reserved.