| Growth parameters and toxicity of the herbicide diuron were assessed for the macrophyte E. canadensis under both field and laboratory conditions. Under controlled laboratory conditions, non-axenic E. canadensis shoots were grown in AAP, Hutner's, Hoagland's, Smart and Barko, and Andrew's media as well as in DI water with endpoints of biomass (wet/dry), plant length, root (mass, length, number), chlorophyll-a content, percent of plants survived, and relative growth rate (RGR) (based on wet/dry mass and plant length) measured at the end of 14-d to determine which media type promoted strongest growth. AAP media performed most consistently and exhibited significantly higher growth compared to many of the other media. This was likely due to a N:P ratio that did not provide an excess of N and P to encourage the growth of algal contamination, which was a critical issue in this laboratory study. A toxicity test with E. canadensis using AAP as growth media showed a definite toxicological response to diuron and higher toxicological sensitivity compared to L. gibba and L. minor. Although this showed promise for the use of E. canadensis in further laboratory testing, caution needs to be taken in data interpretation since the endpoints measured exhibited higher variation compared to Lemna species. No response in E. canadensis was observed using the herbicide atrazine, likely due to interference caused by algal contamination. A microcosm study exposing E. canadensis to diuron was also conducted to determine the field efficacy of the laboratory toxicity test. E. canadensis grown at low population densities and in young, non-established stands showed higher relative growth rates (RGR) compared to high density, established populations. Individual shoots in cone-tainers showed the lowest RGRs. This was likely due to intraspecific competition and established stands reaching carrying capacity, thus experiencing a reduction in growth. A clear toxicological response was observed in the field, with higher populations densities and non-established stands showing slightly higher sensitivity compared to lower densities and established populations. Individually grown shoots in cone-tainers showed lower sensitivity compared to populations. These results support the hypothesis that higher RGRs can lead to greater toxicological sensitivity, a phenomenon previously described in literature. A comparison between laboratory and field toxicity data for diuron exposure showed no consistent trends in sensitivity between the two test systems, thus laboratory data were not predictive of field data. This indicates that if E. canadensis is to be used as a surrogate for other macrophytes for further laboratory testing, work needs to be done to reduce variation in these laboratory assays. |
