Bioindicator-based monitoring of heavy metals in agricultural runoff using Spirogyra sp. and visible-near infrared (VNIR) spectral analysis

Background Heavy metal water pollution in agricultural ecosystems is a major threat to ecosystem functioning and human health; however, rapid and cost-effective monitoring tools are limited. In this study, freshwater algae, Spirogyra sp., were evaluated as bioindicators of heavy metal contamination using Visible-Near Infrared (VNIR) spectral indices and algal bioaccumulation data from canal and rice paddy systems at Mae Hia Agricultural Research Centre, Chiang Mai, Thailand. Results Water quality varied among habitats, with pH ranging from 6.8 to 8.8. Rice paddy systems supported higher algal biomass, reaching up to 17.62 g dry weight, compared with canal environments, likely reflecting greater nutrient availability and hydrological stability. Spirogyra sp. bioaccumulated all seven metals tested lead (Pb), copper (Cu), zinc (Zn), aluminium (Al), nickel (Ni), cadmium (Cd), and mercury (Hg) with bioaccumulation factors (BAFs) exceeding 1 at every site; Al reached the highest BAF of 2.35×10 6 L/kg in canal samples, followed by Zn, Pb and Cu (2.2–2.5×10 4 L/kg), confirming strong and active metal uptake from the surrounding water. Strong positive correlations among Pb, Cu, Zn, Al, and Ni in algal tissues (Pearson r = 0.95–1.00) indicated shared non-specific biosorption pathways, while mercury exhibited a distinct accumulation pattern. VNIR spectral indices reliably differentiated Spirogyra sp. from surrounding water. Vegetation- and water-related indices were both negatively correlated with Pb and several co-accumulating metals in algal tissue (Kernel Normalised Difference Vegetation Index, KNDVI r = − 0.88 to − 0.95; Modified Normalized Difference Water Index, MNDWI r = − 0.76 to − 0.85). For log-transformed BAFs, the KNDVI gave the clearest spectral response, decreasing as the bioaccumulation of Al (r = − 0.97), Zn (r = − 0.95), Pb (r = − 0.94), Cu (r = − 0.92) and Ni (r = − 0.92) increased, whereas the Normalised Difference Water Index (NDWI) increased with Hg bioaccumulation (r = 0.87). Conclusions VNIR spectroscopy cannot directly measure heavy metal concentrations in water, but Spirogyra sp. showed excellent correlations with several heavy metals, mainly Pb, Cu, Zn, Al, Ni, Cd and Hg, suggesting its potential as bioindicator and bioaccumulative organism in freshwater ecosystems. The biomass of algae was highly related to the increasing concentrations of heavy metals in water. Additionally, the use of VNIR spectroscopy for algae has shown potential for estimating heavy metal concentrations from spectral analysis. As far as we are aware, this is the first report combining VNIR spectroscopy with Spirogyra sp. for heavy metal assessment in agricultural freshwater.