Tyler D. Geer

Cellular and aqueous microcystin-LR following laboratory exposures of Microcystis aeruginosa to copper algaecides

Microcystin release from algal cells influences use of copper-algaecides in water resources. Accurate data regarding relationships between copper-algaecide exposures and responses of microcystin-producing algae are needed to make informed management decisions. Responses of Microcystis aeruginosa were measured in terms of cellular microcystin-LR (MC-LR), aqueous MC-LR, and chlorophyll-a following exposure to CuSO4 and copper-ethanolamine. Comparisons were made between treated and untreated samples, and copper formulations. EC50s and slopes for M. aeruginosa responses to copper exposures were calculated. Algal responses followed a sigmoidal exposure-response relationship, and cellular MC-LR and chlorophyll-a were negatively related to copper concentrations. Aqueous MC-LR increased with copper concentrations, although the increase in aqueous MC-LR was not proportional to decreases in cellular MC-LR and chlorophyll-a. Cellular MC-LR and chlorophyll a declined at a greater rate than aqueous MC-LR increased. Total MC-LR was less than untreated controls following copper exposure. Differences were measured between copper formulations in terms of aqueous and total MC-LR concentrations at concentrations of 0.5 and 1.0 mg Cu L-1. Aqueous and total MC-LR were greater (10-20%) following exposure to CuSO4 compared to copper-ethanolamine one day following exposure. The positive relationship between copper concentration and aqueous MC-LR at 0.07-1.0 mg Cu L-1 demonstrates that lower copper concentrations were as effective as higher concentrations in controlling M. aeruginosa while decreasing the total amount of MC-LR, and minimizing the proportion of MC-LR released to the aqueous-phase. Results serve to support more accurate risk evaluations of MC-LR concentrations when M. aeruginosa is exposed to copper-algaecides and when it is untreated.

Comparative toxicity of sodium carbonate peroxyhydrate to freshwater organisms

Sodium carbonate peroxyhydrate (SCP) is a granular algaecide containing H2O2 as an active ingredient to control growth of noxious algae. Measurements of sensitivities of target and non-target species to hydrogen peroxide are necessary for water resource managers to make informed decisions and minimize risks for non-target species when treating noxious algae. The objective of this study was to measure and compare responses among a target noxious alga (cyanobacterium Microcystis aeruginosa) and non-target organisms including a eukaryotic alga (chlorophyte Pseudokirchneriella subcapitata), microcrustacean (Ceriodaphnia dubia), benthic amphipod (Hyalella azteca), and fathead minnow (Pimephales promelas) to exposures of hydrogen peroxide as SCP. Hydrogen peroxide exposures were confirmed using the I3(-) method. SCP margins of safety for these organisms were compared with published toxicity data to provide context for other commonly used algaecides and herbicides (e.g. copper formulations, endothall, and diquat dibromide). Algal responses (cell density and chlorophyll a concentrations) and animal mortality were measured after 96h aqueous exposures to SCP in laboratory-formulated water to estimate EC50 and LC50 values, as well as potency slopes. Despite a shorter test duration, M. aeruginosa was more sensitive to hydrogen peroxide as SCP (96h EC50:0.9-1.0mgL(-)(1) H2O2) than the eukaryotic alga P. subcapitata (7-d EC50:5.2-9.2mgL(-1) H2O2), indicating potential for selective control of prokaryotic algae. For the three non-target animals evaluated, measured 96-h LC50 values ranged from 1.0 to 19.7mgL(-1) H2O2. C. dubia was the most sensitive species, and the least sensitive species was P. promelas, which is not likely to be affected by concentrations of hydrogen peroxide as SCP that would be used to control noxious algae (e.g. M. aeruginosa). Based on information from peer-reviewed literature, other algaecides could be similarly selective for cyanobacteria. Of the algaecides compared, SCP can selectively mitigate risks associated with noxious cyanobacterial growths (e.g. M. aeruginosa), with an enhanced margin of safety for non-target species (e.g. P. promelas).

Relationship among aqueous copper half-lives and responses of Pimephales promelas to a series of copper sulfate pentahydrate concentrations

Copper algaecide exposures in situ are often of shorter duration than exposures for static toxicity experiments because aqueous concentrations in situ dissipate as a function of site-specific fate processes. Consequently, responses of organisms to static copper exposures may overestimate effects following in situ exposures. To understand the role of exposure duration for altering responses, Pimephales promelas survival was compared following static (96 h) and pulse (1.5, 4, 8, and 15 h half-lives) exposures of CuSO4•5H2O. Copper concentrations sorbed by fry indicated a consequence of different exposures. Responses of P. promelas to static exposures resulted in 96 h LC50s of 166 µgCu/L (95% confidence interval [CI], 142–189 µgCu/L) as soluble copper and 162 µgCu/L (CI, 140–183 µgCu/L) as acid soluble copper. Relative to static 96 h LC50s, exposures with half-lives of 1.5, 4 and 8 h resulted in LC50s 10, 3 and 2 times greater, respectively, for responses measured 96 h after exposure initiation. Copper concentrations extracted from fry exposed for 1.5, 4 and 8 h half-lives were less than the static experiment. However, copper sorbed by fry in the 15 h half-life experiment was not different than the static experiment. The relationship between 96 h LC50 and 1/half-life was expressed using the equations y = 116 + 1360 × (R2 = 0.97) for soluble copper and y = 147 + 1620 × (R2 = 0.98) for acid soluble copper. Incorporation of exposure duration for predictions of P. promelas responses to copper pulse exposures increases prediction accuracy by an order of magnitude.

Cell density dependence of Microcystis aeruginosa responses to copper algaecide concentrations: Implications for microcystin-LR release

Along with mechanistic models, predictions of exposure-response relationships for copper are often derived from laboratory toxicity experiments with standardized experimental exposures and conditions. For predictions of copper toxicity to algae, cell density is a critical factor often overlooked. For pulse exposures of copper-based algaecides in aquatic systems, cell density can significantly influence copper sorbed by the algal population, and consequent responses. A cyanobacterium, Microcystis aeruginosa, was exposed to a copper-based algaecide over a range of cell densities to model the density-dependence of exposures, and effects on microcystin-LR (MC-LR) release. Copper exposure concentrations were arrayed to result in a gradient of MC-LR release, and masses of copper sorbed to algal populations were measured following exposures. While copper exposure concentrations eliciting comparable MC-LR release ranged an order of magnitude (24-h EC50s 0.03-0.3mg Cu/L) among cell densities of 106 through 107 cells/mL, copper doses (mg Cu/mg algae) were similar (24-h EC50s 0.005-0.006mg Cu/mg algae). Comparisons of MC-LR release as a function of copper exposure concentrations and doses provided a metric of the density dependence of algal responses in the context of copper-based algaecide applications. Combined with estimates of other site-specific factors (e.g. water characteristics) and fate processes (e.g. dilution and dispersion, sorption to organic matter and sediments), measuring exposure-response relationships for specific cell densities can refine predictions for in situ exposures and algal responses. These measurements can in turn decrease the likelihood of amending unnecessary copper concentrations to aquatic systems, and minimize risks for non-target aquatic organisms.