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Pulse Effects of Herbicides on Periphyton in Streams and Recovery

3 Results

• 3.1 Effects of herbicides on the photosynthesis
• 3.2 Recovery
• 3.3 Effect on the algae group composition

3.1 Effects of herbicides on the photosynthesis

Isoproturon, metribuzin and hexazinone had all distinct effects on the photosynthetic activity, which was reduced at increasing concentrations, while pendimethalin did not show any effects (Fig. 1). Short-term and long-term exposure affected the algae community differently: exposure to isoproturon for 1 hour had no effect at the lowest concentration, but after 24 h of exposure the photosynthesis was reduced even at the lowest concentration used, i.e. 0.4 µgl^-1.

Click here to see Figure 1.

NEC was determined to 1.00 µgl^-1 for the periphyton in short-term exposure to isoproturon, while NEC was 0.019 and <0.01µgl^-1, after 24 hours exposure to isoproturon (Table 1). The calculated effect concentration EC₅₀ of isoproturon was higher in short-term exposure (11.3 µgl^-1) than in the two long-term exposure experiments (1.74 and 0.53 µgl^-1, respectively) (Table 1). The response to hexazinone during short-term exposure stimulated the photosynthesis at the three lowest concentrations (0.4, 2, and 10 µgl^-1, Fig .1), and NEC and EC₅₀ could not be determined. This effect was not found in long-term exposure (Fig. 1), where NEC of hexazinone was 2.29 µgl^-1, and EC₅₀ was relatively high, i.e. almost 33 µgl^-1 (Table 1). Metribuzin was highly toxic to the periphyton at low concentrations; NEC was 0.11 µgl^-1 and EC₅₀ was 5.57 µgl^-1 after long-term exposure (Table 1, Fig. 1).

Table 1. No Effect Concentrations (NEC) and Effect Concentrations (EC₅₀) for the herbicides investigated during 1 and 24 h of exposure.* Stimulating effect.

The epiphytic algal community sampled in May 2000 was less sensitive to metribuzin than the community sampled in September 1999 with regard to their photosynthetic response. NEC increased from 0.11 µgl^-1 in September to 2.35 µgl^-1 in May and EC₅₀ was 5.57 µgl^-1 in September and 15.23 µgl^-1 in May after 24 and 23 hours, respectively (Table 1 & 2). The metribuzin treatment had even in some instances a stimulating effect on the photosynthesis in May at the lowest concentrations (Table 1 & Fig 2). EC₅₀ varied during the exposure period between 9.57 µgl^-1 and 34.00 µgl^-1 (Table 2) and was in average 24.54 ± 10.24 (S.D.), and there was no trend of increasing or decreasing toxicity as function of duration of exposure (Fig. 2 & Table 2).

Table 2. No Effect Concentrations (NEC) and Effect Concentrations (EC₅₀) for metribuzin during different exposure periods. * Stimulating effect.

Click here to see Figure 2.

3.2 Recovery

The photosynthesis activity of the periphyton recovered almost completely after 48 hours in fresh water. The pulse effects of metribuzin on the photosynthetic activity was only short-term and apparently reversible (Fig. 3).

3.3 Effect on the algae group composition

The phytoplankton pigments detected indicated that especially diatoms (detected by fucoxanthin, diadinoxanthin and sporadically presence of diatoxanthin) and chlorophytes (detected by chlorophyll b, lutein, violaxanthin, and neoxanthin) were abundant on the glass discs. Also cyanobacteria (detected by zeaxanthin) were present, although in lower abundances (comparing the concentrations of the algae pigments on the plates) (Fig. 4). Metribuzin had a stimulating effect at 0.4 and 2 µgl^-1 on the production of chlorophyll a (Chl a) after exposure for two hours (Fig. 4). However after 23 h, the Chl a concentration was slightly increased at 0.4 µgl^-1 but reduced at 2 µgl^-1, and at 48 h the Chl a concentration was reduced even at 0.4 µgl^-1 (Fig. 4). During recovery the Chl a concentration increased in all experiments, and although Chl a was visibly affected by metribuzin during the exposure period, the periphyton resumed growth at all concentration after transfer to herbicide free medium (Fig. 4). Only at the highest metribuzin concentrations after 48 h, the Chl a concentration was still reduced in the recovery experiments compared to the lower concentrations, but had increased from 6 µg Chl a l-1 to 15 µgl^-1 during the 48 hours of recovery.

Click here to see Figure 3.

Especially chlorophytes were negatively influenced by exposure to metribuzin. While chlorophytes generally decreased due to metribuzin exposure, diatoms in particular, but also cyanobacteria apparently were the groups responsible for the general increase in Chl a at lower metribuzin concentrations, since they increased at all concentrations except at the highest concentration, 50 µgl^-1 (Fig. 4). During the recovery experiment both diatoms and especially cyanobacteria recovered well, i.e. no difference in the concentration in the controls compared to the metribuzin treatments, and only diatoms were affected at 50 µgl^-1 after long-term exposure. The chlorophytes, however, were affected at all metribuzin concentrations even after 48 hours in fresh water, also after only short-term exposure to metribuzin (Fig. 4).

Click here to see Figure 4.

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Version 1.0 October 2005, © Danish Environmental Protection Agency