Abstract
Abstract. The microbial degradation of dissolved organic carbon and nitrogen (DOC, DON) was studied in three Finnish boreal estuaries with contrasting land use patterns (Kiiminkijoki – natural forest and peatland; Kyrönjoki – agricultural; Karjaanjoki – mixed/urban). Bioassays of 12–18 d long durations were used in 3 seasons at in situ temperatures. Besides the bulk parameters, a suite of dissolved organic matter (DOM) quality parameters were also investigated, including colored DOM (CDOM), fluorescent DOM and the molecular weight of DOM. Bioavailable DOC and DON pools varied significantly between the estuaries, from 7.9 to 10.6% and from 5.5 to 21.9%, respectively. DOM originating from the catchment dominated by natural forests and peatlands (Kiiminkijoki) had the lowest DOC and DON degradation rates, as well as the lowest proportions of biodegradable DOC and DON. A greater proportion of agricultural land in the catchment increased the bioavailability of DON, but not the bioavailability of DOC (Kyrönjoki). Additionally, DOM quality varied significantly between the estuaries, and DOM originating from the agricultural Kyrönjoki catchment sustained higher DOC and DON degradation rates and higher bacterial growth efficiency (BGE) compared to those of the natural forest and peat dominated Kiiminkijoki catchment. The quality of DOM, indicated by differences in CDOM, fluorescent DOM and molecular weight, varied between estuaries with differing land use and was concluded to be major driver of BGE of these systems and thereafter to the microbial CO2 fluxes from the estuaries. The differences in BGE resulted in a 5-fold difference in the calculated daily bacterial CO2 emissions between the study's estuaries due to bacterial activity, ranging from 40 kg C d−1 in the Karjaanjoki estuary to 200 kg C d−1 in the Kyrönjoki estuary. Lower DOC:DON ratios, smaller molecular weight and higher CDOM absorption spectral slope values of DOM resulted in higher proportion of the initial DOC and DON being transferred to microbial growth and therefore to the pelagic food web. The pristine, peatland and forest-dominated Kiiminkijoki catchment had the lowest BGE, and therefore proportionally highest CO2 fluxes.
Highlights
Organic matter consists of particulate and dissolved phases (POM and dissolved organic matter (DOM)), which are traditionally separated on the rather arbitrary criteria of filter pore sizes typically ranging between 0.1 and 0.7 μm
Lower dissolved organic carbon (DOC) : DON ratios, smaller molecular weight and higher colored DOM (CDOM) absorption spectral slope values of DOM resulted in higher proportion of the initial DOC and DON being transferred to microbial growth and to the pelagic food web
Of the fluorescence peaks measured (A, C, M and T), we chose to concentrate on peak C, since the peak C is a proxy for humiclike, terrestrial substances in the DOM pool (Cammack et al, 2004; Stedmon and Markager, 2005)
Summary
Organic matter consists of particulate and dissolved phases (POM and DOM), which are traditionally separated on the rather arbitrary criteria of filter pore sizes typically ranging between 0.1 and 0.7 μm. POM and DOM in aquatic systems are either derived from organisms within the system (autochthonous) or transported into the system via rivers and atmospheric deposition (allochthonous). The DOM pool is vital for the functioning of food webs in aquatic systems since it forms the basis for heterotrophic activity and its breakdown is central to the regeneration of inorganic nutrients (Hansell and Carlsson, 2002; Søndergaard and Thomas, 2004). E. Asmala et al.: Bioavailability of riverine dissolved organic matter
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
