Effects of water level fluctuations on the trophic state of a connected lacustrine system (Southern Andes, Patagonia, Argentina): applicability of the CDOM method as a tool for monitoring of eutrophication

Abstract

Hydrological connectivity plays a central role in lake functioning and is strongly influenced by water level fluctuations (WLFs), which occur naturally or due to human activities and can induce changes in the trophic states of lakes. Traditionally, the trophic state has been assessed using the Trophic State Index (TSI), but recently, chromophoric dissolved organic matter (CDOM) has emerged as a new indicator of eutrophication. We conducted a 2-year seasonal survey in three interconnected Andean-Patagonian lakes (Argentina), with different hydrogeomorphic features and macrophyte cover, considering both low-water (LWP) and high-water (HWP) phases. We aimed to determine the influence of WLFs and macrophyte cover on TSI dynamics, and to evaluate the use of CDOM for monitoring the trophic state of these Andean-Patagonian lakes. TSI ranged from ultraoligotrophy to mesotrophy and showed significant changes between rainy and dry seasons, particularly in shallow lakes. WLFs were revealed to be an important regulatory factor in this connected system, since a pronounced differentiation of physicochemical variables was found among lakes during the LWP, and homogenization during the HWP. Submerged macrophytes strongly affected nutrient dynamics, especially nitrogen (N), and consequently TSI. During the LWP, nutrient release associated with macrophyte senescence resulted in elevated TSI values in the shallowest lake. During periods of macrophyte growth and higher hydrological connectivity, a dilution effect developed, with reduced TSI values. Strong positive correlations found between TSI and CDOM (a₂₅₄) highlighted CDOM as a reliable indicator of TSI, even in ultra-oligotrophic lakes. The positive relationship of TSI–a₃₅₅ suggests the potential use of optical satellites to estimate TSI, increasing spatial and temporal monitoring capabilities.