Biological hot spots and the accumulation of marine dissolved organic matter in a highly productive ocean margin

Abstract

Concentrations of dissolved organic carbon (DOC) and major biochemicals (amino acids and carbohydrates) were measured during five cruises (2009)(2010) to the Louisiana margin in the northern Gulf of Mexico. Concentrations of amino acids and carbohydrates were elevated at mid-salinities and were indicative of plankton production of dissolved organic matter (DOM) in surface waters. Hot spots of two compositionally distinct types of labile DOM were identified based on the relative abundances of amino acids and carbohydrates. Amino acidrich hot spots occurred sporadically in regions of high phytoplankton biomass and were mostly observed between dusk and dawn, reflecting a grazing source. In contrast, carbohydrate-rich hot spots were more widespread and were often found in nutrient-poor waters, indicating the production of carbon-rich DOM associated with nutrient limitation. Major biochemical indicators and bioassay experiments indicated labile DOM comprised a relatively small fraction of the DOC. Most DOM was degraded and had a semi-labile nature. Substantial accumulations of marine (plankton-derived) DOC were observed in surface waters, particularly at mid-salinities during the summer. Microbial alteration of marine DOC and nutrient limitation of microbial utilization of carbon-rich DOM appeared largely responsible for the accumulation of DOC. The reservoir of accumulated marine DOC in the shelf surface mixed layer ranged from 0.11 Tg C to 0.23 Tg C, with the lowest and highest values occurring during winter and summer. Substantial cross-shelf export of semi-labile marine DOM occurred during the summer and provided a major carbon and energy subsidy to microbial food webs in offshore waters.

Bacterial Carbon Cycling in the River Plume in the Northern South China Sea During Summer

“…In addition, an increase in bacterial production in the plume‐impacted region, in combination with a significant correlation between bacterial production and Chl a concentrations (Figure b and Table ), suggested that the improvement of phytoplankton biomass was primarily responsible for enhanced bacterial production. A recent study has shown that high concentrations of amino acids and carbohydrates released from phytoplankton, which are the common components of labile DOC, occurred at midsalinities with high phytoplankton biomass in the Louisianan margin in the northern Gulf of Mexico (Shen et al, ). Changes in the DOC composition were closely tied to shifts in bacterial community composition (Buchan et al, ; Cottrell & Kirchman, ; Luria et al, ).…”

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

“…Maximal concentrations and yields of amino acids were observed near a depth of 75 m at most sea ice‐free stations and the values were twice those in surrounding waters. The maximal amino acid yields (1.3–1.7% DOC) were comparable to those (1–2% DOC) in coastal and open ocean surface waters [ Kaiser and Benner , ; Shen et al ., ], indicating the occurrence of bioavailable DOM in these polar waters. Low mole percentages of glycine, nonprotein amino acids ( β ‐Ala and γ ‐Aba) and d ‐amino acids further indicated the fresh biochemical nature of the DOM and minimal diagenetic alterations.…”

“…Molecular indicators of bioavailable DOM can be directly measured in seawater and can provide insights about the occurrence of bioavailable DOM without the need for lengthy incubations. Amino acids have been used as qualitative and quantitative indicators of bioavailable DOM over broad spatial and temporal scales in various aquatic systems [ Davis and Benner , ; Shen et al ., ]. Certain amino acids, such as glycine and nonprotein amino acids ( β ‐alanine and γ ‐aminobutyric acid), increase in relative abundance during DOM degradation, thereby providing additional insights into the extent of DOM alteration [ Kaiser and Benner , ; Shen et al ., ].…”

Stable and radiocarbon isotopic composition of dissolved organic matter in the Gulf of Mexico

“…Using the DOC abundance and Δ 14 C and δ 13 C isotopic signatures of these end-members (equations (2)-(4)), a S = 31.53 predicts MRP 2 m DOC = 114.2 ± 1.0 μM with Δ 14 C = À184 ± 4‰ and δ 13 C = À22.7 ± 0.2‰. Thus, the ΔDOC = 48 μM between P2, 26 m and offshore P3 waters at the same depth (79 μM) likely results from the accumulation marine DOC (as opposed to a removal of tDOC) with low Δ 14 C signatures during microbial alteration of DOC in biological "hot spots" on the shelf [Shen et al, 2016]. This simple mixing model does not account for known major DOC removal mechanisms occurring in the MRP (e.g., flocculation, microbial remineralization, and photo-oxidation).…”

“…GULF [Gardner et al, 1996;Guo et al, 2009;Santschi et al, 1995]. Microbial alteration of this DOM can result in the seasonal accumulation of 0.11-0.23 Tg DOC [Shen et al, 2016]. This large, episodic delivery of terrestrial DOC (tDOC) from this river system is rapidly removed in the northern GOM [Bianchi et al, 2004;.…”