Date of Award

5-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Committee Member

Dr. Peter van den Hurk, Committee Chair

Committee Member

Dr. Stephen J. Klaine

Committee Member

Dr. Julia L. Brumaghim

Committee Member

Dr. William C. Bridges, Jr

Committee Member

Dr. Thomas Schwedler

Abstract

Dissolved ions are natural components of aquatic systems, the concentration and composition of which are greatly controlled by the surrounding geological material present. Although naturally occurring, many anthropogenic activities, including mountaintop removal mining, road deicing practices, agricultural irrigation and coal-fired power plant effluents, can greatly increase dissolved ion concentrations, as well as alter the ionic composition of freshwater systems. An increase in dissolved ions is positively correlated with an increase in salinity and conductivity. Dissolved ions have many important physiological functions within aquatic organisms, one of which is to create electrochemical gradients within cells. These gradients are necessary for controlling water and ion movement within the organism. To establish these electrochemical gradients, freshwater organisms must balance osmotic gain and passive ion loss by utilizing active transport through a series of pumps and transporters located at the gill. If the external environment exceeds a threshold tolerable for freshwater organisms, they may reallocate more energy for ionoregulation, ultimately reducing the energy available for growth, reproduction and even survival. The overall goal of this research, therefore, was to characterize the chronic toxicity of single ions and ion mixtures to two freshwater organisms, and to investigate the key mechanisms by which they exert these chronic effects.

Reproductive effects of elevated dissolved ions were initially evaluated in Ceriodaphnia dubia, a small cladoceran freshwater invertebrate. Following eight-day static renewal bioassays, divalent ions (calcium, magnesium, sulfate) were determined to have the greatest effect on C. dubia reproduction, while monovalent ions (sodium, chloride, bicarbonate) produced the least. Additionally, binary ion mixtures resulted in additive, less-than-additive, and greater-than-additive responses, depending on the specific ion mixtures. Seven-day static renewal bioassays were also performed utilizing Pimephales promelas (fathead minnow), a small vertebrate species. Similar to reproductive effects described for C. dubia, divalent ions resulted in the largest reduction in growth compared to monovalent ions. Furthermore, P. promelas demonstrated mostly additive and less-than-additive effects following binary ion mixture exposures, with sulfate reducing chloride toxicity, and calcium reducing magnesium toxicity.

A positive correlation between chloride, calcium, magnesium, and sodium EC50 values between C. dubia reproduction and P. promelas growth may indicate similar dissolved ion potencies between an invertebrate and vertebrate species. Although some mixture interaction differences were identified, similarities in concentration-response slopes between the two organisms may indicate similar toxicological modes-of-action. These similarities suggest that C. dubia reproduction may be a useful predictor of P. promelas growth. These results will be useful in the development of future predictive models, which can aid in establishing site-specific water quality criteria. The inclusion of a physiologically-based parameter; however, would greatly improve the accuracy and predictability of such models. For this purpose, it is imperative to identify the underlying modes-of-action for these dissolved ions.

In the present study, the toxicity of ion interactions were identified by comparing the slopes of concentration-response curves. Concentration-response curves exhibiting similar slopes possibly indicate similar modes-of-action between contaminants. It has been suggested that effects of elevated dissolved ions on freshwater organisms may be attributed to the reallocation of energy for ionoregulatory purposes. If the ion concentration in the external environment exceeds a tolerable threshold, freshwater fish may utilize more energy in the synthesis and use of ionoregulatory essential enzymes, including ATPases and carbonic anhydrase. As such, it would be expected that an increase in enzymatic activity would occur, which would reduce the energy available for growth and reproduction. Total ATPase and carbonic anhydrase activity were measured in adult P. promelas gill tissue following 3- and 7-day single ion and multi-ion exposures, as well as a 7-day post-exposure recovery period. Sodium bicarbonate significantly reduced carbonic anhydrase activity, while sodium and chloride significantly increased total ATPase activity. Significant effects on total ATPase and carbonic anhydrase activity in the gills of P. promelas did not result from sodium sulfate exposures. This was somewhat expected, as the fish gill is impermeable to divalent anions, such as sulfate. Instead, alterations in enzymatic activity along the intestinal tract may occur, where sulfate is known to increase the permeability of sodium and chloride. This possibility presents a need for identifying changes in enzymatic activity for other ionoregulatory important tissues, such as the intestines and even kidney.

The results of this research not only add to the limited dataset regarding the chronic toxicity of elevated dissolved ions to freshwater organisms, but also further demonstrate the complex nature of ion toxicity. Differences between previously described acute ion toxicity, and the chronic toxicity established by the present study, indicate that sub-lethal effects cannot be simply explained by their effects on survival. Although acute events, such as fish kills, are very abrupt and easily identified, chronic events should not be discounted. Reproduction and growth are important aspects that contribute to organismal fitness, and ultimately affect the success of an ecosystem. Changes in these aspects can disrupt ecosystem processes over time, and are oftentimes too subtle to take early corrective actions. For this reason, establishing water quality guidelines for elevated dissolved ions based on sub-lethal effects is critical. Overall, these results provide essential information that can help manage water quality by serving as the foundation for the development of future predictive models.

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