Environmental Project, 1041 – Pulse Effects of Herbicides on Periphyton in Streams and Recovery

Pulse Effects of Herbicides on Periphyton in Streams and Recovery

4 Discussion

4.1 Effects of herbicides on the photosynthesis of epiphyton
4.2 Effects of metribuzin on the individual algal groups

4.1 Effects of herbicides on the photosynthesis of epiphyton

Periphyton are highly relevant as test organisms in toxicity tests since they constitute the most important food item for benthic fauna in streams, and effects on the productivity and biomass of the algae caused by toxic substances may have impact on the entire food web. Bonilla et al.(1998) found in experiments with episammon (mikroalgae on sandgrains), periphyton and phytoplankton that the different algal communities showed similar sensitivity to the herbicide simazine, but different sensitivity to the herbicide paraquat, where periphyton was the most sensitive.

Generally only few data for pulse effect of herbicides on periphyton communities is published. In the present study isoproturon affected the photosynthesis activity severely at quite low concentrations. NEC in 24-hour test was below 0.02 µgl^-1, lowest observed effect concentration (LOEC) was 0.4 µgl^-1 (Fig. 1), and EC₅₀ were the lowest detected in these experiments, i.e., 0.53-1.74 µgl^-1 (Fig. 1 & Table 1). All values are distinctly lower than EC₅₀ value published for standard single-species growth-test over 72 or 96 hour: EC₅₀ between 12-40 µgl^-1 has been published for the planktonic microalgae Chlorella pyrenoidosa, Scenedesmus subspicatus and Chlamydomonas reinhardtii (Traunspurger et al. 1996; Anton et al. 1993). These results illustrate the difficulty to predict what the effects under natural conditions will be from the standard single-species toxicological tests, since these tests do not include influences which occur on community level, for instance displacements in species composition, competition between species, etc.

Compared to the frequent occurrence and concentration of isoproturon detected in Danish and Swedish streams, the effect concentration for periphyton is low. In Danish streams (Lillebæk and Odder Bæk) isoproturon is found in about 25% of the water samples and in concentration between 0.011-0.068 µgl^-1 (NERI, Denmark 2001). In Swedish streams isoproturon has been detected in concentration up to 10 µgl^-1 (Krueger 1998).

Metribuzin was highly toxic to the periphyton community at low concentrations. In the experiment September 1999 NEC was 0.11 µgl^-1 and EC₅₀ was 5.57 µgl^-1 in 24 hour-test (Table 1, Fig. 1). Only very few data on the toxicity of Metribuzin have been published. In a single-species test with Selenastrum capricornutum an EC₅₀ (96 hours) of 43 µgl^-1 was found (Fairchild et al. 1997). This value is distinctly higher than the effect concentration found in the present study in September 1999 for periphyton communities.

The effect concentration EC₅₀ of isoproturon and metribuzin decreased by 1-2 orders of magnitude when exposure time increased from 1-2 hours to 24 hours (Table 1 & 2). This result implies that the effects of pesticides on periphyton communities largely depend on the duration of the exposure to the herbicides. This result is important to take into account in the risk evaluation of pulses of pesticides. This was also the case for hexazinone, although the effect concentration could not be estimated for the short-term exposure experiment because of inconsistency in the dose-response curves and lack of points in the calculation of NEC and EC₅₀. The inconsistency was caused by stimulation at the three lowest concentrations (0.4, 2, and 10 µgl^-1, Fig. 1) of hexazinone compared to controls and inhibition at the highest concentration. After 24 hours exposure the stimulation of the photosynthesis disappeared (Fig. 1), the dose-response curve was consistent, and NEC and EC₅₀ for hexazinone was 2.29 and 32.88 µgl^-1 (Table 1). Such stimulation in a short-term experiment is commonly found as a short-term response to toxic stress. However, the stimulation is obviously a response to the toxicant, and it is difficult to evaluate its effect.

Pendimethalin did not show any effects on the natural community of periphyton at concentrations up to 50 µgl^-1. The solubility of pendimethalin in water is relative low, i.e., about 275 µgl^-1 and pendimethalin has a relatively high affinity for particles (Pesticide Manual), both properties that indicate the bio-availability for algae may be low. Swedish investigations indicated that the transport of pendimethalin from agriculture to streams probably is small (Kreuger 1998). In contradiction to the Swedish results pendimethalin has been found in Danish stream in concentrations up to 0.077 µgl^-1 (NERI, Denmark 2001).

Metribuzin affected the periphyton sampled in September 1999 and in May 2000 very differently. The community sampled in May was less sensitive to metribuzin than the community sampled in September, and the photosynthesis in May was even stimulated at the lowest concentrations (Table 1 & 2, Figure 1 & 2). These results confirm the hypothesis that natural communities are highly variable due to influence by physical, chemical, and biological parameters, and toxic substances have different impact on and result in different effect concentrations of the algal communities depending on season, species composition, nutrient status etc. Unfortunately the group composition was not analysed in September. Communities typically consist of many different species that differ largely in sensitivity to toxicants. The difference in sensitivity found may be explained by the dominance of metribuzin tolerant/sensitive species in the communities sampled in May. In the case that tolerant species dominated the community the risk of the toxicant will be underestimated. Because of the variations in sensitivity of phytoplankton communities at different locations and at different times of the year, it has been advocated that ¹⁴C-assimilation tests with phytoplankton communities are unsuitable in hazard evaluations of toxicants (Kusk & Nyholm 1991). However, single-species tests are not a good alternative since variability in sensitivity for toxicants may differ up to three orders of magnitude between different species and no general sensitive algae species have been identified (Blanck et al. 1984, Wängberg and Blanck 1988, Källqvist and Romstad 1994).

In the recovery experiment following metribuzin exposure, EC₅₀ did not reveal any increasing or decreasing trend in toxicity as function of the duration of the exposure, but varied in a range between 9.57 and 34.00 µgl^-1 and NEC and LOEC were not differing much throughout the 48 h exposure experiment (Figs. 2 and Table 2). The inhibition of the photosynthesis due to metribuzin exposure was therefore not depending on the duration of the exposure to this toxicant. This was also found in the study by Bonilla et al (1998) in experiments with natural communities of microalgae and the herbicide simazine, while the inhibition due to exposure to the herbicide paraquat was dependent on the exposure time, which increased from 18 to 76 % between 30 min and 24 h of exposure.

The recovery of the primary production was almost complete after 48 hours in clean water even at the highest concentrations where the metribuzin inhibited the primary production by 80%. The result indicates that the pulse effect of metribuzin on periphyton is reversible even at very high concentrations. However this was not the case since the composition of the periphyton was affected even by short-term exposure (2 hours) at the lowest concentration tested (0.4-µgl^-1).

4.2 Effects of metribuzin on the individual algal groups

Since the concentration of accessory pigments are related the Chl a concentration at equal light intensity (Schlüter et al. 2000), the change in the concentrations of the detected pigments can be used for assessing the development of the biomass of the respective groups as effects of the treatment with the herbicide metribuzin. For chlorophytes and diatoms, several specific pigment were detected (neoxanthin, violaxanthin, lutein (chlorophytes) and diadinoxanthin and diatoxanthin (diatoms), data not shown), and the development of these pigments followed the diagnostic pigments, chlorophyll b and fucoxanthin, respectively, closely, indicating that this method is very robust. The chlorophytes were the most affected group by exposure to metribuzin and were almost always reduced due to the metribuzin treatment, while especially cyanobacteria, but also diatoms at the lowest concentrations, i.e., 0.4, 2, and 10 µgl^-1 (Fig. 4), were stimulated due to the metribuzin exposure. This gives evidence of a different response to metribuzin within the periphyton community, where the negative impact on chlorophytes and positive impact on diatoms and cyanobacteria causes displacements in the composition of the algae community within 48 h. Displacements in the biomass of such functional different algae groups, result in changed food availability and quality for the grazers which ultimately will have impact on the entire food web. At the highest applied metribuzin concentration, 50-µgl^-1, the biomass of all phytoplankton groups was reduced compared to the control. Consequently, potential indirect impact on higher trophic levels of metribuzin and other pesticides that may affect the composition and biomass of periphyton warrant further investigations.

The induced changes in composition of the communities and the fast recovery of the photosyntethic activity are in good agreement to the responses found in other studies including effects of toxicants on algae communities e.g. tributyl-tin (Blanck and Dahl, 1996, Petersen and Gustavson 1998), atrazine (Gustavson & Wängberg 1995), arsenate Blanck and Wängberg 1988). The most likely direct effects of herbicides on periphyton communities in streams may be exclusion and inhibition of sensitive species.

We would like to thank Alexander Nielsen, Merete Allerup and Kristian Møller Christensen for excellent technical assistance.