5.1. How much particulate organic carbon is delivered by
the two rivers discharging into Bute Inlet?
We have estimated that Homathko and Southgate Rivers carried 23 ± 5 Kt
OC/yr into Bute Inlet based on suspended and bedload sampling conducted
in 2017 (Table 1). This total OC flux is interpreted to contain three
pools (Fig. 6): young terrestrial biospheric (woody debris); old
terrestrial biospheric (soil organic matter); and petrogenic OC
(rock-derived OC). Identification of these pools is based on TOC and
δ13C signatures (Fig. 6), and on ramped oxidation data
combined with radiocarbon dates previously presented in Hage et al.
(2020). To these three pools previously identified, we add a fourth pool
made of extracellular polymeric substances (EPS) associated with
bacterioplankton that occurs when the rivers enter the fjord and create
surface plumes in which the heterotrophic activity of bacterioplankton
can be enhanced (Albright, 1983). This activity appears to result in an
OC pool characterized by high TOC (>3 %) and unusually
high δ13C signatures (Fig. 6). Such signatures cannot
be explained by the presence of carbonates (typically enriched in
δ13C) because they were removed prior to isotope
measurements. These high δ13C signatures may also
correspond to C4 plants, yet C4 plants are almost exclusively found in
tropical and warm environments (Hecky and Hesslein, 1995). Instead, we
suggest that the high TOC and associated δ13C
signatures correspond to bacterial extracellular polymeric substances
(EPS). The EPS hypothesis is further supported by the stickiness of the
plume samples, which is typical for EPS (Underwood et al., 1995). This
fourth pool of OC is not included in our OC river input flux as it is
most likely formed within the fjord water. Further research on the
highly enriched δ13C composition of these plume
samples is required, yet it is beyond the scope of this study. We
further note that the EPS-related OC pool is not found in the fjord
seabed sediment samples (Fig. 6). We propose this is because OC
associated with EPS is labile and not heavy enough to settle, and it
thus degrades rapidly in the water column before reaching seabed
sediments (Albright, 1983).
We note that there are key assumptions behind these estimates of
riverine OC composition. First, we have a limited number of river
samples (n=22) and these samples were collected during a rain-induced
runoff event in fall when river discharge was near the annual average
(Text S1). River discharge peaks driven by the freshet and/or intense
rain likely brought more and coarser terrestrial organic debris than the
rates estimated here (as in Turowski et al., 2016). We thus expect these
river OC fluxes to fluctuate not only seasonally but also following
extreme events such as floods and intense rain, yet the data presented
in this study do not constrain this variability. Second, we have no
constraints on the OC content potentially coming from small streams
entering the fjord along its margins. However, these streams only
represent 6 % of the total sediment and freshwater input coming into
the fjord (Syvitski and Farrow, 1983).