Coastal Biogeochemical and Microbial Responses to Exports from a Small Rain-Dominated River Plume in the Northeast Pacific Coastal Temperate Rainforest

Abstract

The Northeast Pacific Coastal Temperate Rainforest (NPCTR) spans over 2000 km from Alaska to California and includes 1000's of diverse small rain-dominated coastal watersheds that, despite covering 24% of the surface area, deliver 33% of discharge to the Pacific Ocean. Coastal ecosystems receiving these inputs are potential hotspots of biogeochemical transformation but remain understudied due to logistical and historical biases of the fields of oceanography and limnology. This lack of integration across these fields limits our understanding of how terrestrial exports from small catchments affect coastal carbon and nutrient dynamics, microbial community structure, and overall ecosystem ecology.

This thesis addresses this problem by examining a river-ocean estuary on Quadra Island (British Columbia, Canada) to assess how rain-dominated watershed exports shape coastal microbiology and biogeochemistry. Water samples collected within a small river plume (0-27.9 PSU) at 0, 15, and 50 cm depth across seven sites from Hyacinthe Creek into Hyacinthe Bay were analyzed for phosphate, silica, carbon and nitrogen species and isotopes, dissolved organic matter (DOM) composition, and putative active (RNA) and present (DNA) microbial communities. The core of this thesis is a manuscript that analyses carbon and nutrient concentrations, stable isotope signatures, and microbial community diversity to investigate how vertical and horizontal salinity gradients affect dispersion of organic and inorganic matter, and its potential effects on microbial ecology.

Surface samples and salinity mapping revealed distinct biogeochemical and microbial signatures relative to underlying waters, dissipating with increasing distance from the creek mouth. Nutrient and organic matter concentrations and signatures increased with distance from the creek mouth, deviating from conservative mixing and suggesting active reprocessing. Microbial communities exhibited distinct horizontal and vertical structures, supporting the existence of a river-to-sea microbial continuum, and revealing taxa that may be disproportionately more active compared to the total community in transitional waters. Salinity, organic carbon, condensed aromatic compounds, and isotopic signatures (δ¹³C-DOC, δ¹³C-POC, δ¹⁵N-PN) were highlighted as key drivers of coastal microbial community structure. Cumulatively, this study explores important biogeochemical trends through small-scale vertical and horizontal extents, a scale often overlooked in oceanography and larger river plume systems. This work further highlights that small, rain-dominated river plumes may strongly influence biogeochemical cycling and microbial dynamics in NPCTR estuaries, and that their impacts are concentrated in a vertically shallow but active surface layer. This estuary approach advances our current understanding of biogeochemical dynamics in small rain-dominated river plumes of the NPCTR, while suggesting the need for a finer-resolution lens to study small rain-dominated watersheds in future.