Driving Factors of Geosmin Appearance in a Mediterranean River Basin: The Ter River Case

Abstract

In recent decades, human activity coupled with climate change has led to a deterioration in the quality of surface freshwater. This has been related to an increase in the appearance of algal blooms, which can produce organic compounds that can be toxic or can affect the organoleptic characteristics of the water, such as its taste and odor. Among these latter compounds is geosmin, a metabolite produced by certain cyanobacteria that confers an earthy taste to water and which can be detected by humans at very low concentrations (nanogram per liter). The difficulty and cost of both monitoring the presence of this compound and its treatment is a problem for drinking water treatment companies, as the appearance of geosmin affects consumer confidence in the quality of the drinking water they supply. In this field study, the evaluation of four sampling sites with different physicochemical conditions located in the upper part of the Ter River basin, a Mediterranean river located in Catalonia (NE Spain), has been carried out, with the aim of identifying the main triggers of geosmin episodes. The results, obtained from 1 year of sampling, have made it possible to find out that: (i) land uses with a higher percentage of agricultural and industrial activity are related to high nutrient conditions in river water, (ii) these higher nutrient concentrations favor the development of benthic cyanobacteria, (iii) in late winter-early spring, when these cyanobacteria are subjected to both an imbalance of the dissolved inorganic nitrogen and soluble reactive phosphorus ratio, guided by a phosphorus concentration increase, and to cold-mild temperatures close to 10°C, they produce and release geosmin, and (iv) 1-2 weeks after cyanobacteria reach a high relative presence in the whole biofilm, an increase in geosmin concentration in water is observed, probably associated with the cyanobacteria detachment from cobbles and consequent cell lysis. These results could serve as a guide for drinking water treatment companies, indicating under what conditions they can expect the appearance of geosmin episodes and implement the appropriate treatment before it reaches consumers' tap.

Keywords: Mediterranean river; biofilm; cyanobacteria; field study; geosmin; global change.

FIGURE 1

Ter River basin and sampling sites located in upper part (El Ripollès and Osona regions): T1 = Ter at Vilallonga de Ter municipality, T2 = Ter downstream Ripoll municipality, T3 = Ter at Colónia de Borgonyà (Sant Vicenç de Torelló municipality), and T4 = Ter at Gurb municipality.

FIGURE 2

Mean value and standard deviation of geosmin concentration in water for different sampling sites (T1, T2, T3, and T4) in upper part of Ter River basin (NE Catalonia, Spain) at each sampling day.

FIGURE 3

Redundancy discriminant analysis showing response parameters distribution based on drivers evaluated in four different sampling sites in upper Ter River basin (NE Catalonia, Spain) in 2019. Axes 1 and 2 combined explain 89% of variance. Drivers: pH, temperature (Temp), electrical conductivity (EC), dissolved oxygen (DO), turbidity, nitrite (N-NO₂^–), nitrate (N-NO₃^–), ammonium (N-NH₄⁺), and phosphate concentration (P-PO₄^–3), and nitrogen to phosphorus ratio (DIN:SRP). Response variables: geosmin, minimum fluorescence yield (F₀), photosynthetic efficiency (Yeff), chlorophyll-a concentration (Chla), ash free dry mass (ADFM), Margalef index (MI), cyanobacteria (Cb), diatoms (Dt) and green algae (GA) biomass, and relative abundance of cyanobacteria (R_Cb) and diatoms (R_Dt).

FIGURE 4

Redundancy discriminant analysis showing biological parameters distribution at T4 sampling site based on physicochemical variables evaluated in upper Ter River basin (NE Catalonia, Spain) in 2019. Axes 1 and 2 combined explain 83.3% of variance. Seasons are winter (W), spring (Sp), summer (Su), and autumn (Au). Drivers: pH, temperature (Temp), electrical conductivity (EC), dissolved oxygen (DO), turbidity, nitrite (N-NO₂^–), nitrate (N-NO₃^–), ammonium (N-NH₄⁺), and phosphate concentration (P-PO₄^–3), and nitrogen to phosphorus ratio (DIN:SRP). Response variables: geosmin, minimum fluorescence yield (F₀), photosynthetic efficiency (Yeff), chlorophyll-a concentration (Chla), ash free dry mass (ADFM), Margalef index (MI), cyanobacteria (Cb), diatoms (Dt) and green algae (GA) biomass, and relative abundance of cyanobacteria (R_Cb) and diatoms (R_Dt).

FIGURE 5

Relative importance (in %) of main predictors [temperature, in degrees Celsius; nitrates concentration, in N-NO₃^– mg/L; phosphate concentration (SRP), in P-PO₄^3– mg/L; DIN:SRP ratio, in mol:mol; and turbidity, in NTU] at different time lags (in weeks) in random forest model.

FIGURE 6

(A) Mean value and standard deviation of geosmin concentration (nanogram per liter), and cyanobacteria relative abundance (%) and (B) mean value and standard deviation of geosmin concentration (nanogram per liter) and DIN:SRP ratio (mol/mol), water temperature (°C) in T4 sampling site for different sampling in 2019, being Jan = January, Feb = February, Mar = March, Apr = April, May = May, Jul = July, Sep = September, Oct = October, Nov = November, and Dec = December. Blue boxes indicate time when cyanobacteria relative abundance in biofilm was high.