Bioavailability of dissolved organic matter in coastal waters

Abstract

Dissolved organic matter (DOM) is the largest reservoir of organic matter in the ocean containing the same amounts of carbon as atmospheric carbon dioxide. DOM consists of a mixture of compounds ultimately originating from terrestrial and aquatic plant, and microbial materials. The complex chemical composition of DOM results in varying availability to microbes, containing a pool that is readily utilized by heterotrophic bacteria and a refractory part that resists biological degradation. In this study the seasonal variation in bioavailability of DOM was quantified in two contrasting marine systems: Loch Creran (Scotland) and the Ría de Vigo (Spain). The bioavailable DOM (BDOM) varied temporally at both stations and controlled the seasonal variations in DOM. Dissolved organic phosphorus (DOP) was found to be more bioavailable than dissolved organic nitrogen (DON) and DON was more bioavailable relative to dissolved organic carbon (DOC). The variation in BDOM was, in both areas, associated with changes in chlorophyll a. The DOM rate constants were observed to vary seasonally and they demonstrated that higher BDOM concentrations would lead to faster mineralization rates. Calculations from both areas suggested that they exported BDOM, which could contribute to bacterial production in the adjacent waters. It is shown that BDOM produced in the Ría de Vigo, with a half life time of 1.9 - 3.3 days, is significantly more labile than that produced in Loch Creran, with a half life time of 6 - 13 days. A fraction of DOM is coloured containing a part that absorbs (CDOM), and a part that absorbs and reemits the energy again (Fluorescence, FDOM). The relation between the optical properties of DOM and the bioavailability of DOC was tested in the Ría de Vigo. This study demonstrated that it was possible to derive both the bioavailable DOC and its rate constants from the DOM protein-like fluorescence, whereas refractory DOC could be estimated from in situ CDOM and humic-like substances in the Ría de Vigo. Heterotrophic microbes have often been viewed as the main consumers of DOM, but it is now evident that they can also produce DOM. The heterotrophic microbial production of DOM was studied using microbial communities from Loch Creran. The experiments demonstrated that the microbes produced refractory and labile DOM with a coloured signal.