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).