Perspectives of Aquatic Toxicology/Section I: INTRODUCTION
Aquatic species are vital to our planet. Phytoplankton, algal plankton, and kelp are major sources of the planet’s oxygen. They absorb and store carbon dioxide, and maintain a hospitable climate. They also play an important role in the global nitrogen cycle and support aquatic animals such as fish, mollusks, sponges, and corals. Aquatic species help maintain the earth’s ecosystem and help preserve its rich biodiversity as well as providing food, medicine, livelihoods, tourism, and recreational opportunities1.
It is therefore essential to protect the planet’s rich and diverse aquatic life, and combat the many threats facing aquatic organisms including climate change, habitat destruction, overfishing, the introduction of invasive species, and chemical pollution2. This chapter will focus on chemical pollution. The risks to aquatic life can be minimized and better managed by understanding how chemicals impact it.
There are more than 140,000 man-made chemicals in the environment3, with the United States alone producing 2000 new chemicals every year4. It is conceivable that aquatic species are exposed to many of these chemicals on an acute (short-term) and chronic (long-term) basis, although there is an absence of data to indicate how many of these chemicals are released into various water bodies. Chemical exposure can affect organisms’ growth, development, fecundity, behavior, and survival, among other biological processes. Hence, it is important to test chemical toxicity before it is released into the environment in order to determine maximum acceptable toxicant concentrations (see section II of this chapter) and to protect species from potential harm.
Toxicity testing is done to identify the degree to which chemicals can damage living organisms in a controlled environment. It has four major objectives:
a. To obtain toxicity and exposure data for various chemicals
b. To aid in estimating and managing risks posed by various chemicals
c. To aid in setting chemical regulations and environmental standards
d. To classify chemicals based on how toxic they are to various species
The dose makes the poison in toxicology. It is possible to determine safe and unsafe doses, or concentrations, for nearly every chemical. For example, the most toxic substance on earth, the bacteria-produced botulinum toxin, can kill humans with a very small dose, but it can be used safely in Botox5.
Risk is a function of toxicity and exposure. A chemical can be very toxic, but it will have zero risk to aquatic organisms if it never enters water bodies (i.e., there is no exposure). The maximum allowable concentration for a chemical in the environment is based on the risk it poses to various species. An acceptable “safe” concentration is usually one that does not harm 95% of the species.
Many questions can be answered by carrying out toxicity tests:
a. At what concentration is a chemical non-toxic to an organism? At what concentration is it toxic?
b. What effects can be observed from short-term and long-term chemical exposure?
c. Which chemicals are the most and least toxic to an organism?
d. Which organisms are the most or least sensitive to a chemical?
e. Are some life stages of an organism more sensitive?
f. Do certain environmental conditions make a chemical more toxic?
g. Is the toxicity of a chemical similar in lab and in the outside environment?
h. What is the effect of a mixture of chemicals?
Aquatic organisms can be exposed to chemicals when effluents and sewage are released into water bodies. Sometimes chemicals inadvertently enter water through oil spill or runoffs from agricultural fields. Chemicals present in the air can be deposited into water bodies either directly (dry deposition) or through rainfall, snowfall, and fog (wet deposition). Some of the chemicals commonly found in water bodies include detergents, fertilizers, pesticides, pharmaceuticals, food and cosmetic preservatives, chemicals used in kitchenware and plastic, and metals6-8. Aquatic animals such as fish can take up these chemicals via their gills, absorb them through their integument, and/or ingest them. Aquatic plants that have vascular systems can absorb chemicals through their epidermal surface and/or roots. Plants that are not completely submerged in water can take up chemicals in the air through their stomata.
Chemical properties and type of aquatic species determine how chemicals are taken up, distributed, stored, metabolized, and excreted. Hydrophobic (fat-loving) chemicals are more likely to enter a fish’s body, and warm temperatures typically increase the uptake as the fat become more fluid-like. Smaller, uncharged molecules also cross membranes more easily. Hydrophilic (water-loving) chemicals are more likely to be transported by the circulatory system. On the other hand, hydrophobic chemicals are more likely to bind to molecules and accumulate in fat bodies. While chemical storage is protective in the short term (they are not free to move and act), they can be released later and cause toxicity. This usually happens when an organism breaks down fat for greater energy needs, i.e., during illness, starvation, or reproduction.
A species’ metabolic enzymes often modify a chemical in order to detoxify its effects, but this modification can sometimes make a chemical more toxic. Chemicals with many halogen atoms such as chlorine, and fluorine are often difficult to modify. Many aquatic animals eliminate chemicals through their gills or skin. Further details on chemical biotransformation can be found in the Biotransformation of Xenobiotics Chapter of this book.
Chemical exposure can kill or harm aquatic organisms directly through such means as growth reduction, delayed development, decreased fertility, and behavioral changes, or can reduce or eliminate its food supply by killing its prey, or limiting its shelter through habitat destruction. This can lead to increased competition for food and shelter, disrupting the food web, and altering the ecological balance.