This dataset contains biogeochemical measurements of surface water, sampled across Loch Etive, Scotland, between the 3rd and 7th of October 2022. The dataset encompasses 23 locations including nine open loch stations, four fish-farm adjacent loch sites, six river inflows, and four process experiment river sites, enabling a comparison of water chemistry across marine, freshwater, and anthropogenically influenced environments. Surface water samples were collected and analysed for an extensive range of biogeochemical parameters. In-field measurements included water temperature, pH, dissolved oxygen, and specific conductivity. Laboratory analyses comprised: - Alkalinity - Major nutrients, including nitrate, nitrite, ammonium, soluble reactive phosphorus (SRP), and silica - Dissolved and total organic and inorganic carbon and nitrogen (DOC, TIC, TDN) - Trace elements and rare earth elements (REEs), including lithium, barium, aluminium, and a full suite of lanthanides - Stable isotope composition of oxygen (δ18O), determined via isotope ratio mass spectrometry - Chromophoric and fluorescent dissolved organic matter (CDOM/FDOM), with derived indices such as fluorescence index (FI), biological index (BIX), and humification index (HIX) In addition to water chemistry, particulate samples were collected and analysed for total suspended solids (TSS), particulate organic matter (POM), particulate inorganic matter (PIM), chlorophyll-a concentration, and particulate organic carbon/nitrogen (POC/PON). Phytoplankton community composition was assessed by flow cytometry, quantifying major groups including diatoms, pico- and nano-chlorophytes, cryptophytes, and cyanobacteria. This comprehensive dataset is intended to inform biogeochemical process understanding in polar analogue environments and to validate analytical workflows and inter-institutional protocols. Full details about this dataset can be found at https://doi.org/10.5285/50f4344e-a79a-4e1f-a9ee-e7985ed847cd