Biophysical controls on dissolved organic carbon concentrations of Alaskan coastal temperate rainforest streams

Abstract

Coastal carbon cycling models remain incomplete in key continental margins worldwide. Large quantities of labile terrestrial DOC are transferred to the Gulf of Alaska in a flow of freshwater discharge from thousands of watersheds that equals the discharge of the Mississippi River. The coastal margin of southeast Alaska and British Columbia is a potential hotspot of worldwide DOC metabolism and the mass and reactivity of DOC in rivers and estuaries of the region make quantifying and modeling DOC export a priority. Scaling DOC export requires a well-constrained model of streamwater DOC concentrations. We established models for prediction of DOC streamwater concentrations through a broad sampling of streams across a large, diverse landscape in 61 independent watersheds. Stream DOC concentrations were significantly related to the amount of wetlands in a watershed confirming the necessity of understanding the fate of the mass of carbon stored within them. Several measures of slope were useful in predictive models of streamwater DOC concentrations suggesting that slope predicts the presence of wetland soils better than current wetland map layers. We also provide evidence that suggests watersheds with large salmon spawning runs have distinct characteristics that influence the relationship between wetlands and carbon cycling that may provide additional insights into how carbon is processed.

A global hotspot for dissolved organic carbon in hypermaritime watersheds of coastal British Columbia

“…In comparison to global models of DOC export (Mayorga et al, 2010) and DOC exports quantified for southeastern Alaska (D'Amore et al, 2015a(D'Amore et al, , 2016Stackpoole et al, 2017), our estimates of freshwater DOC yield from Calvert and Hecate island watersheds are in the upper range predicted for the perhumid rainforest region. When compared to watersheds of similar size, DOC yields from Calvert and Hecate island watersheds are some of the highest observed (see reviews in Hope et al, 1994;Alvarez-Cobelas et al, 2012), including DOC yields from many tropical rivers, despite the fact that tropical rivers have been shown to export very high DOC (e.g., Autuna River, Venezuela, DOC yield: 56 946 kg C km −2 yr −1 ; Castillo et al, 2004) and are often regarded as having disproportionately high carbon export compared to temperate and Arctic rivers (Aitkenhead and McDowell, 2000;Borges et al, 2015).…”

“…In Alaska, for example, riverine DOC concentrations vary with wetland cover (D'Amore et al, 2015a) and glacial cover (Fellman et al, 2014). Previous studies have shown that streams in southeast Alaska can contain high DOC concentrations (Fellman et al, 2009a;D'Amore et al, 2015a) and produce high DOC yields (D'Amore et al, 2015a, b;Stackpoole et al, 2016), but no known field estimates have been generated for the perhumid CTR of British Columbia, an area of approximately 97 824 km 2 (adapted from Wolf et al, 1995).…”

“…In Alaska, for example, riverine DOC concentrations vary with wetland cover (D'Amore et al, 2015a) and glacial cover (Fellman et al, 2014). Previous studies have shown that streams in southeast Alaska can contain high DOC concentrations (Fellman et al, 2009a;D'Amore et al, 2015a) and produce high DOC yields (D'Amore et al, 2015a, b;Stackpoole et al, 2016), but no known field estimates have been generated for the perhumid CTR of British Columbia, an area of approximately 97 824 km 2 (adapted from Wolf et al, 1995). Within the perhumid CTR of British Columbia, terrestrial ecologists have defined a large (29 935 km 2 ) "hypermaritime" subregion where rainfall dominates over snow, seasonality is moderated by the ocean, and wetlands are extensive (Pojar et al, 1991; area estimated using British Columbia Biogeoclimatic Ecosystem Classification Subzone/Variant mapping Version 10, 31 August 2016, available at https://catalogue.data.gov.bc.…”

Impact of Wetland Decline on Decreasing Dissolved Organic Carbon Concentrations along the Mississippi River Continuum

“…This is particularly surprising when considering it is well known that DOC concentrations vary with climate zone (Schlesinger and Melack, 1981) and land use -land cover (LULC) (Butman et al, 2015). Wetland coverage is often the best predictor of DOC concentrations in rivers and streams (Gergel et al, 1999;D'Amore et al, 2016). However, LULC (e.g., wetland distribution) is seldom used to interpret DOC changes in large river systems.…”

High salmon density and low discharge create periodic hypoxia in coastal rivers

“…Within the sub‐region of southeastern Alaska alone, there are nearly 3000 coastal watersheds with drainage areas >1.2 km 2 that empty directly into saltwater (D'Amore et al. ). Although climate change modeling in this region predicts an overall increase in future annual precipitation, snowfall in the winter and rainfall in the summer are likely to decrease (Shanley and Albert , Shanley et al.…”