Bioavailable dissolved organic matter and biological hot spots during austral winter in Antarctic waters

Primary production and heterotrophic bacterial activity in the Antarctic Ocean are generally low during the austral winter. Organic carbon is considered to be a major factor limiting bacterial metabolism, but few studies have investigated the bioavailability of organic matter during winter. Herein, the chemical composition and bioavailability of dissolved organic matter (DOM) were investigated in surface (5-100 m) and mesopelagic (200-750 m) waters off the northwestern Antarctic Peninsula during August 2012. Concentrations of dissolved organic carbon (DOC) were low (424 mu mol L-1) and showed no apparent spatial patterns. By contrast, the composition of DOM exhibited significant spatial trends that reflected varying ecosystem productivity and water masses. Surface distributions of chlorophyll-a and particulate organic carbon depicted a southward decline in primary productivity from open waters (60.0 degrees S-61.5 degrees S) to ice-covered regions (61.5 degrees S-62.5 degrees S). This trend was evident from concentrations and DOC-normalized yields of dissolved amino acids in the surface waters, indicating decreasing DOM bioavailability with increasing latitude. A different pattern of DOM bioavailability was observed in the mesopelagic water masses, where amino acids indicated highly altered DOM in the Circumpolar Deep Water and bioavailable DOM in the Transitional Weddell Water. Depth distributions of amino acid yields and compositions revealed hot spots of elevated bioavailable DOM at approximate to 75 m relative to surrounding waters at most ice-free stations. Relatively low mole percentages of bacterially derived d-amino acids in hot spots were consistent with an algal source of bioavailable DOM. Overall, these results reveal the occurrence and spatial heterogeneity of bioavailable substrates in Antarctic waters during winter.