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Ana Catarina Silva, Ana Margarida Ribeiro, Tiago Silva

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We are students studying for a degree in Environmental Sciences at the University of Minho. We hope this padlet inspires you!

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This study showed that microalgae have low tolerance to the salinity of the environment.

Salinity does not affect Daphnia, neither in body area nor in length, with no differences or trends observed in these parameters according to NaCl concentration.

It is concluded that organisms react differently depending on the salinity of the environment, with some being more tolerant and others less so.

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Human activities, including agriculture and mining, have significantly increased freshwater salinity levels worldwide, harming aquatic organisms adapted to stable salinity conditions (Schuler et al. 2019).

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Exposure to increasing NaCl concentrations disrupts osmoregulatory mechanisms in freshwater organisms, leading to energetic trade-offs that reduce growth, reproduction, and survival, especially in ion-sensitive species (Hintz & Relyea, 2019).

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This study intended to evaluate the growth rate of Daphnia magna and the absorbance of Raphidocelis subcapitata in response to increasing salinity.

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Daphnia magna is a freshwater cladoceran crustacean widely used as a test organism in aquatic toxicology due to its short life cycle, parthenogenetic reproduction, and high sensitivity to contaminants (OECD, 2004).

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USEPA (United States Environmental Protection Agency), 2002. Short-term Methods for Estimating the Chronic Toxicity of Effluents and Receiving Waters to Freshwater Organisms (EPA-821-R-02-013)

OECD (2004). Guidelines for the Testing of Chemicals: Daphnia sp. Acute Immobilisation Test (OECD 202).

(Schuler, M. S., et al. (2019). Freshwater salinization increases survival of Escherichia coli and risk of bacterial impairment. Water Research, 152, 200-209).

Hintz, W. D., & Relyea, R. A. (2019). A review of the species, community, and ecosystem impacts of road salt salinisation in fresh waters. Freshwater Biology, 64(6), 1081-1097.

Posit team (2025). RStudio: Integrated Development Environment for R. Posit Software, PBC, Boston, MA.

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Raphidocelis subcapitata (CCAP 278/4). Micrografia obtida do Culture Collection of Algae and Protozoa (CCAP). Fonte: CCAP (n.d.).

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Raphidocelis subcapitata is a unicellular, freshwater green alga belonging to the class Chlorophyceae. It reproduces asexually by autospores, has a crescent shape, and is widely used in ecotoxicological assays due to its high sensitivity to a broad range of environmental pollutants (USEPA, 2002).

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Figure 1. Preparation of microalgae assays.

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Raphidocelis subcapitata were exposed to different salt concentrations (0, 1.6, 2.0, 2.5, 3.1, 3.9, 4.9, 6.1, 7.7, 9.6, 12.0 g/L) for 7 days. For the microalgae, cell growth was assessed by measuring color intensity at 440 nm.

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What are the effects of salinization on freshwater organisms?

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Daphnia magna were exposed to different salt concentrations (0, 0.5, 0.7, 0.8, 1.0, 1.3, 1.6, 2.0, 2.6, 3.2, 4.0 g/L), and survival and growth were assessed over a 7-day period. The organisms were fed, measured at the beginning and end (using ImageJ), and somatic growth rates were calculated based on body area and length:

garea= ( ln(Af) - ln(Ai) ) / ∆t

glenght= ( ln(Lf) - ln(Li) ) / ∆t

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Performed in R (Posit team 2025), using RStudio (R version 4.4.2).

Packages used:

multcomp (Hothorn et al. 2008): used to perform Dunnett's test (comparisons with the control).

drc (Ritz et al. 2015): used to fit a log-logistic model and calculate EC50 values.

ggplot2 (Wickham 2016): used for creating plots.

cowplot (Wilke 2020): used for arranging and refining plots.

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Figure 3. Absorbance of microalgae over NaCl concentration.

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A sharp decrease in absorbance was observed as the NaCl concentration increased (ANOVA, p < 0.001), indicating a significant reduction in the growth of the alga Raphidocelis subcapitata.

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Although a log-logistic model was fit to the data, no EC50 could be estimated.

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Figura 4. Daphnia body area over NaCl concentration.

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The area-based growth of Daphnia was unaffected by NaCl concentrations (ANOVA, p = 0.93), suggesting that, within the tested range, increasing salt concentrations were harmless.

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Figure 5. Daphnia length over NaCl concentration.

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Similar to the previous graph, the body area values show variations without a clear trend in response to increasing NaCl concentration. The p-value of 0.93 confirms that salinity did not have a statistically significant effect on the body area growth of Daphnia.