Bioavailability of riverine dissolved organic matter in three Baltic Sea estuaries and the effect of catchment land use

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 CO 2 fluxes from the estuaries. The differences in BGE resulted in a 5-fold difference in the calculated daily bacterial CO 2 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 CO 2 fluxes.

High bacterial carbon demand and low growth efficiency at a tropical hypereutrophic estuary: importance of dissolved organic matter remineralization

“…Indeed, previous studies performed in tropical estuarine waters showed lower BR rates (BR max =0.7 µM C h LEE et al, 2009;RAM et al, 2003). Nutrient availability also plays a major role in regulating bacterial carbon metabolism (ASMALA et al, 2014;BERGGREN et al, 2010;GASOL et al, 2009;KEMP, 2003). Bacterial uptake of DOM follows a rather constant ratio: C:N:P ~ 50:10:1 (FAGERBAKKE et al, 1996;GOLDMAN et al, 1987).…”

“…Moreover, the N-NH 4 contents, the most readily available inorganic N form for bacterioplankton (CHURCH, 2008;VALLINO et al, 1996), represented, on average, a minor fraction of total DIN (38%). Inorganic nitrogen limitation of bacterial production and growth efficiency has been widely reported in both batch cultures and natural systems experiments (CARLSSON et al, 2012;CHURCH et al, 2000;KROER, 1993;ORTEGA-RETUERTA et al, 2012), some of them showing a higher dependency of BGE on inorganic nitrogen than on DOM quality (ASMALA et al, 2014;LØNBORG et al, 2011). The direct correlation between BP and N-NH 4 (r=0.53; p=0.01) and the absence of correlation between BP and the N-oxidized forms (r=0.25; p=0.23) suggests a higher reliability of bacterioplankton on ammonium contents in this system.…”

“…The direct correlation between BP and N-NH 4 (r=0.53; p=0.01) and the absence of correlation between BP and the N-oxidized forms (r=0.25; p=0.23) suggests a higher reliability of bacterioplankton on ammonium contents in this system. The higher BGE observed in the more productive estuarine systems is usually correlated to the nutrient availability and/or DOM quality (APPLE; DEL GIORGIO, 2007;ASMALA et al, 2014;NEWELL, 2012;GUENTHER et al, 2008) which support bacterial growth (BP) with less maintenance energy costs (BR). The DOM quality (or lability) is defined by its nutritional value (stoichiometric balanced C:N:P ratio), diagenetic state (freshly leached vs. microbial processed), as well as molecular size (low vs. high molecular weight) and shape (degree of aromaticity), irrespective of its allochthonous or autochthonous origin (AMON; BENNER, 1996;ATTERMEYER et al, 2014;GOLDMAN et al, 1987).…”

“…Since then, the vast majority of studies still corroborate this pattern: BGE >30% on average in more productive areas, such as upwelling and estuarine systems (ASMALA et al, 2014;NEWELL, 2012;FUENTES et al, 2012;GUENTHER et al, 2008), and BGE < 20% on average at oceanic sites (ALONSO-SÁEZ et al, 2007;DEL GIORGIO et al, 2011;LØNBORG et al, 2013;. This means that in most eutrophic systems, bacterioplankton act more as a source of organic carbon for the higher consumers.…”

“…Nevertheless, several studies showed a great spatial and temporal variability in BGE within and among systems, due to several abiotic and biotic factors like temperature, inorganic nutrients availability, dissolved organic matter (DOM) quality, light incidence and grazing pressure (APPLE; DEL GIORGIO, 2007;APPLE et al, 2006;ASMALA et al, 2014;BERGGREN et al, 2010;CORY et al, 2010;RAM et al, 2003). The need for simultaneous in situ estimates of bacterial production and respiration rates became thus imperative to accurately evaluate the bacterial carbon metabolism in the aquatic systems and understand its regulation.…”