In the next GE3246 Environmental Pollution lecture (Lecture 6) we will discuss freshwater pollution. In particular, we will cover two processes that commonly impact freshwater ecosytems ~ acidification and eutrophication. As mentioned previously, both processes occur naturally. Human activities have, in effect, brought about an acceleration in both. To distinguish human-accelerated acidification and eutrophication from natural, background, levels we on occasion talk about “cultural acidification” and “cultural eutrophication”. Also as mentioned already, both acidification and eutrophication impact environments globally – aquatic and terrestrial.
We will not have time in the lecture to go into great detail regarding the freshwater effects of acidification and eutrophication. I have already covered ocean acidification in a previous post and in the last lecture (Lecture 5). I have also, briefly, mentioned eutrophication – or enrichment of an ecosystem by nutrients, in particular nitrogen (N) and phosphorus (P) – in past lectures. Under natural conditions, most ecosystems – including freshwater bodies – are nutrient limited. Human activity changes that, however, by making available enhanced levels of nutrients. This results in more primary productivity, which upsets the trophic and chemical balance, and this can cause problems for aquatic organisms and for other organisms, such as humans, who might be dependent on the services provided by the impacted ecosystem.
Excess N and P entering streams, rivers and lakes (including reservoirs) is generally from agricultural runoff (e.g. the wasteful application of fertlizers, which contain N and P) and sewerage (e.g. ineffective waste treatment facilities, broken septic tanks). Run-off from urban areas also often contains high levels of nutrients. Recreational areas, notably golf courses, are also sources of nutrients entering water bodies. As many freshwater ecosystems are connected (drain eventually) to the sea, excessive nutrients from the land also cause eutrophication in coastal areas. Red tides are one consequence of eutrophication of coastal waters. Red tides area a form of Harmful Algal Bloom (HAB). HAB is a bit of a misnomer, because HAB do not just involve algae, they also involve certain forms of cyanobacteria. The error is forgivable, because cyanobacteria used to be known as blue-green algae! HAB deplete oxygen (O) in the water column, effectively suffocating other organisms. Following their death a rupturing of their cells releases toxins, and these toxins can kill aquatic taxa such as fish (e.g. fish that are being farmed in the same waterbody) and be passed on to humans who eat the contaminated fish (Paralytic Shellfish Poisoning (PSP) caused by the neurotoxim saxitoxin can be fatal to humans) . HAB can also directly effect the health of humans in other ways too, e.g. Diarrhetic Shellfish Poisoning (DSP) can as the name suggests cause sub-lethal ailments associated with stomach problems.
For an excellent review of HAB in tropical aquatic environments, see: Tropical Cyanobacterial Blooms: A Review of Prevalence, Problem Taxa, Toxins and Influencing Environmental Factors. The co-authors of the article include Darren Yeo, who some of you might know from the Department of Biological Sciences at NUS.
Most attention on the impacts of eutrophication focuses on the aesthetic (eutrophic water bodies are often unsightly and smelly places!) and direct health (e.g. PSP, DSP) effects. There are other consequences, however, one of which is a link to the emergence of infectious diseases. The paper by Johnson et al (2010), titled “Linking environmental nutrient enrichment and disease emergence in humans and wildlife”, is available in an open access form (hence I can attach it to this blog – below). Johnson et al. link environmental eutrophication to an increased occurrence of diseases such as malaria. They conclude that “available evidence indicates that ecological changes associated with nutrient enrichment often exacerbate infection and disease caused by generalist parasites with direct or simple life cycles. Observed mechanisms include changes in host/ vector density, host distribution, infection resistance, pathogen virulence or toxicity, or the direct supplementation of pathogens. Collectively, these pathogens may be particularly dangerous because they can continue to cause mortality even as their hosts decline, potentially leading to sustained epidemics or chronic pathology. We suggest that interactions between nutrient enrichment and disease will become increasingly important in tropical and subtropical regions, where forecasted increases in nutrient application will occur in an environment rich with infectious pathogens”.
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