Seasonal Variation in Plankton Community Responses of Mesocosms Dosed with Pentachlorophenol

Abstract

Seasonal variations in plankton community response to pentachlorophenol (PCP) were studied in four mesocosm experiments using enclosures in a small lake. The mesocosms (860 l) were dosed with single applications of technical grade PCP (0, 4, 10, 24, 36, 54, 81 and 121 microg/l PCP) and monitored for 20 days. Multivariate statistical analyses showed that plankton community taxonomic composition varied with season. In winter and spring, communities were most stable in time; species diversity and abundance were lowest in winter. Seasonally, the communities varied little with respect to the dominant species, which were the copepod Calamoecia lucasi, the alga Peridinium sp. and the rotifer Ascomorpha ovalis. The direct effects of the PCP additions varied little between seasons, but indirect effects were evident at lower treatment levels in autumn. Indirect effects were not evident in winter. Minor variations in plankton community responses to PCP with season were apparent in the following order of decreasing sensitivity; autumn > or = winter/spring > or = summer. At the species level, C. lucasi showed the largest response. The responses observed were greatest in autumn, with decreased abundance at PCP concentrations > or = 24 microg/l. In the other seasons, effects were observed at levels of 54 or 81 microg/l and higher. Ascomorpha ovalis was the most responding rotifer in winter and spring, whereas Anuraeopsis fissa responded more strongly in autumn and summer. The dinoflagellate alga Peridinium sp. had the largest negative response in all but winter, when Dinobryon cylindricum did. Cryptomonas sp. responded positively to PCP in all seasons, increasing in abundance in the highest treatments, possibly due to reduced grazing pressure, reduced competition, or increased decomposition. The plankton community no-observed effect-concentration (NOEC) was 24-36 microg/l PCP. Results reported here suggest that the Australian and New Zealand water quality guideline values for PCP are sufficient to protect plankton communities against adverse effects.

Ecological Effects of Spring and Late Summer Applications of Lambda-Cyhalothrin on Freshwater Microcosms

“…At higher concentrations, recovery took more time in late summer. Similar observations have been reported for an enclosure experiment with pentachlorophenol (Willis et al 2004).…”

“…On the basis of outdoor model ecosystem experiments (Willis et al 2004; present study), it seems that exposure concentrations around threshold levels for direct effects observed in early-season studies are reasonably predictive for effects later in the season. Above these threshold concentrations, however, severity, duration, and type of direct as well as indirect effects may vary much more during different periods of the year because of seasonal variation in population densities, recovery potential, and food web interactions.…”

“…However, a study with pentachlorophenol in lake enclosures, which also included concentrations around the threshold level, indicated that direct effects on planktonic communities varied little with season. It was suggested that season was not a particularly important factor for ecologic risk assessment and the determination of ecological threshold levels (Willis et al 2004).…”

Guidance on tiered risk assessment for plant protection products for aquatic organisms in edge‐of‐field surface waters

“…In these lake enclosure experiments (n = 4) the variability in ecological threshold concentration varied by approximately a factor of 2 (Willis et al, 2004). …”

Application of functional data analysis to investigate seasonal progression with interannual variability in plankton abundance in the Bay of Fundy, Canada

“…This general state space method has been extended by Godsill et al (2004) to the non-linear and non-Gaussian case, in which a Monte Carlo approach to smoothing was established, improving the overall ability to allow for more realistic distributions in the data. Other studies have examined the seasonal variation in taxonomic composition through cluster analysis based on nonmetric multidimensional scaling (Salmaso 1996) and also by a multivariate approach with principal response curves (Willis et al 2004). There is also interest in describing trends of plankton biomass (Li et al 2006).…”