How could anthropogenic inputs of Se in ecosystem affect the quality of the environment?

Question

and the well being of living species?

Answer 1

Se is a complex element environmentally, both in terms of its chemistry and effects.  I can't cite you chapter and verse about speciation, transformation, etc.  But here are some impacts and references about Se and environmental quality that you might not find easily.

Irrigation in California's San Joaquin valley has caused elevated Se levels in surface water.  I believe that some Se occurs naturally because of leaching from the bedrock, and evaporation of irrigation water caused significant elevation of Se levels in irrigation return flows.  In the Kesterson Wildlife Refuge downstream from areas of heavy irrigation, Se was responsible for death and deformities in birdlife.

Ref 1 describes efforts at control of Se by Environmental Defense, a US environmental group.

Environmental Defense also writes about Se in seafood having a beneficial effect (Ref 2).

Q:  I've heard that the high amount of selenium in seafood counteracts the harmful effects of mercury. Is this true?

A:  There is limited evidence to date that selenium in seafood provides significant protection against the negative effects of methylmercury (the toxic form of mercury found in fish).

Selenium, an essential nutrient, is present in the cells of all mammals. When bound to certain proteins, selenium acts as an antioxidant by detoxifying free radicals. (Free radicals are highly-reactive atoms or molecules that can damage cells.) Organ meats and seafood are the best sources of selenium, and the USDA ranks 16 different seafood sources in the top 25 selenium-containing foods.

A form of selenium – selenide – has also been shown to neutralize the toxicity of some forms of mercury. As part of its 2006 report Seafood Choices: Balancing Benefits and Risks, the Institute of Medicine reviewed the scientific evidence that selenium reduces the risks associated with methylmercury in seafood.

The expert panel concluded that although selenium may diminish some of the toxic effects of some forms of mercury and other heavy metals, the mechanisms for these interactions are poorly understood. In addition, there was little or no evidence showing that selenium affected the toxicity of other seafood contaminants such as PCBs or dioxins. Therefore, it is premature to conclude that selenium acts as a safeguard against methylmercury. Choosing fish that are low in contaminants is still the best course of action (see our list of best and worst choices).

So there you have two examples, one negative, one positive, of Se's effect on env. quality.