Effects of sporophyll storage on giant kelp Macrocystis pyrifera (Agardh) bioassay

Abstract

AbstractThe giant kelp Macrocystis pyrifera (Agardh) is a U.S. Environmental Protection Agency (U.S. EPA)‐approved west coast marine species for chronic toxicity monitoring and compliance in the National Pollution Discharge Elimination System (NPDES). The protocol allows field‐collected sporophylls to be stored for up to 24 h at 9 to 12°C prior to use. However, the effects of sporophyll storage on the bioassay results have not been fully investigated, particularly with kelp collected from beds south of Point Conception, CA, USA. Therefore, 13 matched‐pair reference toxicant bioassays using fresh and stored sporophylls collected from a subtidal kelp bed near Laguna Beach, CA, USA, were performed and compared. The results indicate that a lower percentage of spores germinate and the germ tube lengths are reduced when stored sporophylls are used. The intratest precision of the germination endpoint decreased as evidenced by significant increases in the percent minimum significant difference (%MSD) statistic. The intertest precision also decreased in the germination endpoint as demonstrated by significant increases in the coefficient of variation (CV) values at four effect levels. Conversely, a significant reduction in the CVs was observed in the germ tube length data, possibly as a consequence of the decrease in germ tube length associated with storage. Finally, significant decreases in mean effect concentrations in the germination endpoint in tests using stored sporophylls indicated that storage increased the sensitivity of the spores to the toxic effects of CuCl2. The toxicological sensitivity and intratest precision of the germ tube length endpoint were not significantly affected by storage of the sporophylls. The effects of sporophyll storage resulted in a high frequency of invalid tests, lower statistical power, less effective quality assurance standards, and apparent bias in the observed toxicity of CuCl2.