Water residence time: A regulatory factor of the DOM to POM transfer efficiency

Abstract

The pools of dissolved (DOM) and particulate organic matter (POM) and of transparent exopolymeric particles (TEP) were studied along two sampling gradients in the lagoon of New Caledonia in relation to the residence time of the water masses. The efficiency of the transfer of material from the dissolved to the particulate phase via TEP formation, indicating the physicochemical reactivity of organic matter, was investigated. DOM, POM, and TEP concentration increased along the sampling gradients, but their relative proportions varied. The contribution of the TEP pool to POM increased from 20% to 60%, from the most oligotrophic stations to the more anthropogenically affected bays. According to the low density of TEP and to the observed variations of the proportion of TEP compared with more conventional and solid particles, the aggregates formed inside the bays would be either neutrally or positively buoyant, whereas in the vicinity of the coral barrier, they would be negatively buoyant. As a result, the downward export of organic matter inside the bays might be greatly reduced, thereby prolonging the residence time of organic matter in the water column. The efficiency of the DOM/TEP transformation and the TEP turnover rate dropped drastically when the residence time increased from 0 to 50 d, suggesting that the reactivity of organic matter is reduced as it ages. The very high residence time of the water mass inside the bays, constrained by the hydrodynamic circulation inside the lagoon, favors the installation of a feedback system in which organic matter is not exported and is continuously degraded, leading to the formation of refractory DOM with a low physicochemical reactivity. In contrast, organic matter produced in areas in which water mass has a low residence time (i.e., near the coral barrier) is rapidly exported because of its high physicochemical reactivity.

Depleted dissolved organic carbon and distinct bacterial communities in the water column of a rapid-flushing coral reef ecosystem

“…Many previous studies of DOC and bacterioplankton have focused on atoll lagoon systems with relatively long residence times and potential accumulation of organic material, explaining the widespread perception that reefs exhibit elevated levels of organic matter and bacteria (Linley and Koop, 1986;Yoshinaga et al, 1991;Torréton et al, 1997;Sakka et al, 2002). Our results fit well with observations that indicators of eutrophication (concentrations of DOM and particulate organics, bacterial and phytoplankton biomass and production, and rates of organic aggregate formation) increase along a continuum of increasing reef residence time and declining oceanic connectivity (from rapid-flushing fringing reefs to isolated atoll lagoons; Pagès and Andréfouët, 2001;Torréton et al, 2002;Mari et al, 2007). Further development of comparative models integrating reef habitats of varying residence time would help clarify the degree to which different reefs are supported by oceanic DOM inputs and planktonic microbial recycling (Torréton, 1999).…”

“…DOC and bacterioplankton depletion in the backreef could be caused by aggregation of organic particles (Passow and Alldredge, 1994;Verdugo et al, 2004;Mari et al, 2007) and subsequent flux to the sediment or adsorption onto reef structures. However, increased aggregation should be reflected in elevated concentrations of particulate organic carbon on the reef (which is not observed; Supplementary Figure S2) unless aggregates are rapidly consumed by metazoans within the reef.…”

Effect of solar ultraviolet radiation on bacterio- and phytoplankton activity in a large coral reef lagoon (southwest New Caledonia)

“…In the present study, phytoplanktonic and bacterial standing-stock and production yielded values in the range reported for the area (Jacquet et al 2006, Mari et al 2007). UV-R greatly inhibited phytoplanktonic production and BP.…”

“…The southwest lagoon of New Caledonia is characterized by severe oligotrophic conditions near the coral barrier, whereas the studied bay is subject to terrestrial and urban inputs (Jacquet et al 2006, Mari et al 2007). The lagoon is characterized by a clear gradient of water mass residence times from oligotrophic to inshore enriched stations (Jouon et al 2006).…”

Antifungal Metabolites Produced by Chaetomium globosum No.04, an Endophytic Fungus Isolated from Ginkgo biloba

“…Over several decades, various methods had been used to prevent, control, oreradicate plant diseases, and development of synthetic fungicides were primarily investigated [1,2]. However, the global trend appears to be shifting towards reduced use of fungicides on produce and hence, there is a strong public and scientific desire to seek safer and eco-friendly alternatives to treat plant diseases in agriculture for reducing the decay loss [3]. Biological control provides an alternative to the use of synthetic fungicides with the advantages of greater public acceptance and reduced environ environmental impact [4].…”