Seasonally and Spatially Variable Organic Matter Contributions From Watershed, Marine Macrophyte, and Pelagic Sources to the Northeast Pacific Coastal Ocean Margin

Abstract

Globally, coastal waters are considered biogeochemical hotspots because they receive, transform, and integrate materials and waters from both land and the open ocean. Extending from northern California to southeast Alaska, the Northeast Pacific Coastal Temperate Rainforest (NPCTR) region is no exception to this, and hosts a diversity of watershed types (old-growth rainforest, bog forest, glaciers), and tidal (sheltered, exposed) and pelagic marine (deep fjord, shallow estuary, well-mixed channel) environments. With large freshwater fluxes to the coastal ocean, cross-ecosystem connectivity in the NPCTR is expected to be high, but seasonally variable, with pulses in runoff from rainfall, snowmelt and glacial melt, and primary production associated with changes in ocean upwelling and incident light. However, the relative contribution of each ecosystem to surface ocean organic matter pools over time and space remains poorly constrained, despite their importance for the structure and function of coastal marine ecosystems. Here, we use a four-year dataset of particulate organic matter (POM) chemical composition (δ13C, δ15N, C:N ratio) to quantify the relative contributions of watershed materials via riverine inputs, marine phytoplankton, and macrophytes (macroalgae and seagrass) to surface waters (0-10 m) at 11 stations representing fjord, shallow non-fjord estuary, sheltered channel and well-mixed coastal environments at the heart of the NPCTR in British Columbia, Canada. Watershed, marine phytoplankton, and macrophyte contributions to surficial POM ranged between 5-78%, 22-88%, and 0.1-18%, respectively, and varied by season and station. Watershed inputs were the primary source of POM across all stations in winter and were important throughout the year within the fjord. Marine phytoplankton were the principal source of POM in spring and at all stations outside of the fjord through summer and autumn, while macrophyte contributions were greatest in summer. These results demonstrated high, but seasonally and spatially variable, connectivity between ecosystems that are often considered in isolation of one another and highlight the need to consider coastal waters as integrated land-ocean meta-ecosystems. Future work should investigate how heterogeneity in POM sources determines its fate in coastal ecosystems,and the relative importance of different basal organic matter sources for the marine food web.

Small, Coastal Temperate Rainforest Watersheds Dominate Dissolved Organic Carbon Transport to the Northeast Pacific Ocean

“…As a result, the residence time and potential for biological processing of DOC in estuarine ecosystems adjacent to the NPCTR is substantially higher compared to coastlines where runoff from rivers enters the open ocean and is rapidly transported offshore (Edwards et al., 2021). Riverine DOC in the NPCTR has been shown to readily metabolized by marine microbes (Fellman et al., 2010) and serves as a primary source of organic matter in near‐shore ecosystems (St. Pierre et al., 2022). Moreover, at upper trophic levels terrestrial OC has been shown to account for a substantial proportion (12%–50%) of the biomass C of copepods, birds, and fish in CTR fjord ecosystems in Chile and Alaska (Arimitsu et al., 2018; Vargas et al., 2011), however it is unclear what proportion of this C enters marine food webs as DOC compared to POC.…”

Fjord circulation permits a persistent subsurface water mass in a long, deep mid-latitude inlet

“…In F12, all ungauged rivers were estimated as the Homathko River discharge multiplied by their watershed area ratio (i.e., area of ungauged river divided by area of Homathko River). However, the new version of the model took advantage of a recent watershed characterization and classification (Giesbrecht et al, 2022) to select a representative donor gauge for each ungauged watershed. Specifically, each ungauged watershed was assigned a donor gauge from a nearby watershed of the same type and with similar climate and hydrology.…”

“…High-and mid-latitude coastlines are beset with innumerable fjords, remnants of glacial periods. The importance of these coastal geomorphological features extend to many realms, since they provide habitats for multiple species (Arimitsu et al, 2012;Keen et al, 2017;Mathews and Pendleton, 2006;Frid et al, 2021); receive inputs from both the watersheds and the neighbouring ocean (Bianchi et al, 2020;St Pierre et al, 2022); and offer protected waters for transportation, aquaculture, fisheries, and other human activities (Iriarte et al, 2010;Bergh et al, 2023). They are also important sites for traditional cultures (e.g., Ball, 2021;Brattland, 2010).…”