Abstract
Abstract. In the marine environment, transparent exopolymeric particles (TEP) produced from abiotic and biotic sources link the particulate and dissolved carbon pools and are essential vectors enhancing vertical carbon flux. We characterized spatial and temporal dynamics of TEP during the VAHINE experiment that investigated the fate of diazotroph-derived nitrogen and carbon in three replicate dissolved inorganic phosphorus (DIP)-fertilized 50 m3 enclosures in the oligotrophic New Caledonian lagoon. During the 23 days of the experiment, we did not observe any depth-dependent changes in TEP concentrations in the three sampled depths (1, 6, 12 m). TEP carbon (TEP-C) content averaged 28.9 ± 9.3 and 27.0 ± 7.2 % of total organic carbon (TOC) in the mesocosms and surrounding lagoon respectively and was strongly and positively coupled with TOC during P2 (i.e., days 15–23). TEP concentrations in the mesocosms declined for the first 9 days after DIP fertilization (P1 = days 5–14) and then gradually increased during the second phase. Temporal changes in TEP concentrations paralleled the growth and mortality rates of the diatom–diazotroph association of Rhizosolenia and Richelia that predominated the diazotroph community during P1. By P2, increasing total primary and heterotrophic bacterial production consumed the supplemented P and reduced availability of DIP. For this period, TEP concentrations were negatively correlated with DIP availability and turnover time of DIP (TDIP), while positively associated with enhanced alkaline phosphatase activity (APA) that occurs when the microbial populations are P stressed. During P2, increasing bacterial production (BP) was positively correlated with higher TEP concentrations, which were also coupled with the increased growth rates and aggregation of the unicellular cyanobacterial Group C (UCYN-C) diazotrophs that bloomed during this period. We conclude that the composite processes responsible for the formation and breakdown of TEP yielded a relatively stable TEP pool available as both a carbon source and facilitating aggregation and flux throughout the experiment. TEP were probably mostly influenced by abiotic physical processes during P1, while biological activity (BP, diazotrophic growth and aggregation, export production) mainly impacted TEP concentrations during P2 when DIP availability was limited.
Highlights
The cycling of carbon (C) in the oceans is a complex interplay between physical, chemical, and biological processes that regulate the input and the fate of carbon within the ocean
bacterial production (BP) and primary production (PP) were positively associated during P2, we found no direct correlation between transparent exopolymeric particles (TEP) and PP for either linear (Table S2) or logtransformed regression
Total TEP content was generally stable throughout the experimental period of 23 days and comprised ∼ 28 % of the total organic carbon (TOC) in the mesocosms and lagoon, with uniform distribution in the three sampled depths of the 15 m deep-water column
Summary
The cycling of carbon (C) in the oceans is a complex interplay between physical, chemical, and biological processes that regulate the input and the fate of carbon within the ocean. An essential process driving the flux of carbon and other organic matter to depth, and enabling long-term sequestration and removal of carbon from the atmosphere, is the biological pump that drives organic C formed during photosynthesis to the deep ocean This process, termed export production (Eppley and Peterson, 1979), is facilitated via physical inputs of “new” nutrients Berman-Frank et al.: Dynamics of TEP during the VAHINE experiment, New Caledonia metals) into the euphotic zone from either external sources (deep mixing of upwelled water, river discharge, dust deposition, and anthropogenic inputs) or via biological processes One such process is microbial N2 fixation that converts biologically unavailable dinitrogen (N2) gas into bioavailable forms of nitrogen and enhances the productivity of oligotrophic oceanic surface waters that are often limited by nitrogen (Capone, 2001; Falkowski, 1997)
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