Measuring pollution in Hong Kong's rivers
by David Gallacher
Anyone who has spent a little time in Hong Kong may have noticed it has a few pollution problems.... from air pollution that often reaches levels high enough to pose a health risk, to the tons of sewage dumped daily into Victoria Harbour. Water quality of Hong Kong's streams and rivers has improved since the 1980s, but many watercourses, particularly in the New Territories, remain heavily polluted.
Across much of East Asia, including Hong Kong, river water quality is usually monitored with abiotic measures of pollution (things like pH, nutrient levels and dissolved oxygen concentrations). Although essential in any monitoring scheme, using just these measures has limitations. For example, water chemistry samples can only indicate pollution levels at one point in time. The use of biological monitoring may help solve this problem. As organisms are present in rivers for extended periods of time, they can provide a good indication of long term pollution trends.
In flowing waters, macroinvertebrates are commonly used as biomonitors. The communities found in the river bed (the benthos) are made up of insect, snails, bivalves, crustaceans and annelids. They make good monitors as they are easy to sample, are found in virtually all types of rivers, and most importantly, the whole community responds to lots of different impacts. For example, in my final year undergraduate project in the UK, I investigated how benthic macroinvertebrate communities responded to both a saline mine discharge and an organic pollution source. Here in Hong Kong, I am looking at how best to use macroinvertebrates to detect impacts in local streams and rivers. My project is divided into 2 parts; how to sample the communities, and how to interpret the data collected.
Appropriate sampling programmes are essential in any study of natural communities. As many macro-invertebrate monitoring schemes involve sampling a large number of sites, the emphasis is usually placed on collecting a representative sample of taxa from a site for the least amount of effort. Many of the studies I have looked at from East Asia contain no explanation as to why a particular sampling programme has been chosen, or else the programme used had been based on European or N. American studies. I therefore thought it worthwhile to look at what sampling regime would be most efficient in Hong Kong streams. 1 chose 2 sites on different rivers in the New Territories for this part of my study, taking 10 replicate 1 min, kick-samples at each site on 3 occasions in both the wet and dry seasons. Once I have finished processing the samples, I'll be able to decide how many replicates one should take at a site to gain a representative taxa list, how many times you need to sample in a season and whether it's necessary to sample in both the wet and dry seasons.
Even with an efficient sampling program, biomonitoring surveys can generate a lot of raw data, which can take eons to process, and is often difficult to interpret. This has led researchers in Europe and N. America to develop simple measures of community structure or function that are sensitive to pollution and other impacts. In Europe, biotic indices are amongst the most popular of these measures. These indices are based around tolerance values: scores given to each species or family in a community based on their tolerance to pollution. The index is calculated by summing the tolerance values of all the scoring taxa found at a site. Such measures are quick and easy to calculate, and produce results easily understood by non-specialists who may have to base important decisions on biomonitoring survey results.
In the second part of my study, I am developing a biotic index specific to local benthic invertebrates. To do this, I'm using data collected from over 60 sites around Hong Kong. About half of these sites were sampled by Tony Chan for his M. Phil., and are mostly free from pollution. The rest of the data I collected myself from sites with various levels of pollution. Once I've processed all the samples, I'll rank the sites according to their pollution status using physical, chemical and biological data collected at each site. By looking at the distribution and abundance of invertebrate taxa through these rankings, I can allocate each taxon a score between 1 and 10, based on their tolerance to pollution. An index value can then be calculated by summing all the scores from a site. The effectiveness of this simple scoring system will then be tested on a different data set (which I'll collect in 1999). P.15
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