Trophic-Level Interactive Effects of Stoichiometric Nutrient Modulation and Heavy Metal Exposure in Algae and Daphnia
Awoyemi, Olushola Moses
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The freshwater ecosystems including lakes, streams, and rivers are constantly being faced with stressors that may impair the structure and function of the ecosystems. There are various anthropogenic activities that alter the levels of essential mineral elements including phosphorus (P) and nitrogen (N) while also introducing other contaminant (e.g. heavy metals) that are toxic to aquatic organisms. Furthermore, this may impair food quality and/or quality available for keystone species and other organisms up the food chain. There are currently very limited data on ecological risk assessment (ERA) studies that couple both dietary quality (or nutrient stoichiometry) and mixed contaminant exposure (e.g. heavy metals). While there are various assessments involving single metal exposures in aquatic systems that do not consider the interactive effects of mineral elements on toxicological endpoints, toxicities of contaminant mixtures are difficult to assess. Hence, several laboratory-derived assessment endpoints derived from single contaminant exposure under single media nutrient condition may be less predictive of the influence of media and/or dietary nutrients stoichiometry on the toxicity outcomes during an ERA, in setting realistic limits for contaminant exposure. Hypotheses for toxicological effects of media-dietary metals include alterations in growth and productivity of freshwater algae, a primary producer occupying the first trophic level, as well as survival, growth, reproduction, physiology, and behavior of zooplanktonic crustacean, a grazer occupying the second trophic level, under varying stoichiometric (P- and N-based) conditions. This dissertation research investigated the following: (1) toxicological effects of media treated cadmium (Cd), arsenic (As), and binary mixture (Cd/Asmix) at concentrations 0, 5, 25, 125, 625, and 3125 µg/L (for Cd and As) and 0/0, 5/5, 25/25, 125/125, 625/625, and 3125/3125 µg/L (for Cd/Asmix) on Scenedesmus acutus, and (2) toxicological effects of dietary treated Cd- (0, 18.75, 37.5, and 75 µg/L), As- (0, 625, 1250, and 2500 µg/L), and Cd/Asmix-treated (0/0, 18.75/625, 37.5/1250, and 75/2500 µg/L) on Daphnia pulex. Both exposure assessments were carried out under varied media and dietary P [low (LP), median (MP), and optimum (COMBO)] and N [low (LN), median (MN), and optimum (COMBO)] conditions. Results showed significant (p<0.05) interactive effects of media nutrients changes (P and N) and dietary metal (Cd, As, and/or Cd/Asmix) coupled with concentration-dependent inhibition of S. acutus growth and productivity. For media P modulation, the 7-d toxicity of Cd against S. acutus was 2x, 11x, and 4x that of As in LP, MP, and COMBO conditions, respectively; while the joint toxicity effects of Cd/Asmix were partial additive in LP and COMBO, and synergistic in MP media. The 7-d toxicity of Cd against S. acutus was 4.3x, 5.3x, and 1.2x that of As in low, median and optimum-N conditions, respectively; while the toxicity of Cd/Asmix showed partial additive effects in low and optimum-N, and synergistic effect in median-N. Furthermore, acute lethal toxicity (96 h) of Cd in D. pulex was ~60x that of As, while Cd/Asmix joint toxicity was synergistic. For stoichiometric P changes, chronic toxicity (14 d) in D. pulex showed significant (p<0.05) interaction of As and P-availability on survival, reproduction and behavior (distance moved, velocity, acceleration and mobility), while Cd and P availability showed significant interactive effect on rotational behavior. For stoichiometric N changes, 14-d chronic toxicity showed significant (p<0.05) interactive effects of media N and dietary metal (As and Cd/Asmix) on D. pulex reproduction but not on behavior while survival was significantly impaired by dietary Cd, As, and Cd/Asmix. Moreover, there were significant (p<0.05) interactive effects of the stoichiometric nutrient imbalance (both P- and N-based) and metal exposures (Cd, As, and Cd/Asmix) on the growth and physiological response of D. pulex including feeding, oxygen (O2) consumption and/or heartrate. More specifically, feeding rate was significantly reduced by diets containing Cd, As, and Cd/Asmix in LP and LN media, and so is growth rate, body length, and abdominal width. Additionally, metal uptake and bioaccumulation in S. acutus and D. pulex varied with stoichiometric nutrient imbalance and metal treatments while dose-response effects of Cd, As, and Cd/Asmix in S. acutus and D. pulex were either monophasic (e.g. linear, concentration dependent) or biphasic (e.g. hormesis) under varying nutrient stoichiometry. This dissertation provides detailed empirical evidence of the interactive effects of media/dietary nutrients stoichiometry (P- and N-based) and dietary metals (Cd, As, and Cd/Asmix) exposure at environmentally relevant concentrations and higher, emphasizing the need for consideration of such interactions during ERA and in setting realistic limits for metal exposures in aquatic ecosystems.