Geoscience Reference
In-Depth Information
FDOM signals by degradation of DOM. These amino acid-like FDOM moieties accu-
mulate because bacteria have other preferential sources of carbon and nitrogen available
for growth. In the second case, production of energetically costly fluorescent amino acids
indicates an abundance of labile carbon and nutrients to support bacterial growth.
These proposed scenarios are consistent with conclusions from the literature on studies
aimed at identifying conditions under which bacterial degradation is a sink or source for
amino acid-like FDOM (Cammack et al.,
2004
; Stedmon and Markager,
2005b
, Nieto-Cid
et al.,
2006
; Biers et al.,
2007
). Both Cammack et al. (
2004
) and Nieto-Cid et al. (
2006
) pro-
posed scenarios under which amino acid-like FDOM may accumulate and be subsequently
consumed, as a function of nutrient availability. Consistently, Biers et al. (
2007
) showed
production of amino acid-like FDOM during bacterial incubation in seawater when labile
sources of nitrogen were added. The results from these studies can explain some of the
conflicting trends that have been reported.
In support of the link between amino acid-like FDOM and DOM bioavailability,
Fellman et al. (
2009b
) found that protein-like fluorescence correlated strongly to the per-
cent of biodegradable organic carbon (BDOC) in the DOM pool from a range of Alaskan
freshwaters. A similarly strong relationship was found across a range of coastal water-
sheds in the Gulf of Alaska that vary widely in glacial coverage (Hood et al.,
2009
).
Further, amino acid-like FDOM correlated positively with DOM mineralization in
springs, streams, and permafrost soils (Balcarczyk et al.,
2009
), and with uptake rates of
dissolved organic nitrogen (DON) in temperate streams (Fellman et al.,
2009b
), provid-
ing additional evidence that this fluorescence signal correlates with the presence of labile
carbon and nitrogen. Stedmon and Markager (
2005b
) showed that one amino acid-like
component of FDOM was removed by bacterial degradation and thus was clearly a proxy
for bioavailable material.
In wastewaters and waters receiving wastewater effluent, amino acid-like FDOM was
correlated positively with biological oxygen demand, a measure of bioavailable DOM
(Hudson et al.,
2008
). Wastewaters are enriched in this fluorescence relative to fresh and
marine waters (Reynolds and Ahmad,
1997
), which provides additional support for the link
between elevated amino acid-like FDOM and high rates of biological activity. Together,
these studies strongly support the relationship between amino acid-like FDOM and bio-
logical activity and indicate that it is produced either directly or indirectly during condi-
tions of high bacterial growth; however, because it is widespread and often persists (e.g.,
Vähätalo and Wetzel,
2008
), the relationship between bacterial activity and the consump-
tion or degradation of amino acid-like FDOM is likely more complicated or at least less
well understood at this time.
8.3.1.2 Humic-like Fluorescence
Humic-like fluorescence is predominately but not solely associated with allochthon-
ous organic matter derived from degrading soil and plant matter. Relative to microbial
derived material enriched in amino acid-like FDOM, humic-like FDOM is not expected
to be preferentially degraded by bacteria, and thus considered to be a proxy for slowly
