If one had to select one fixed sampling location for physicochemical parameters in a large lake such as lake Malawi, how could he/she go about it?

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In the 1940s a British scientist by name of Beauchamp located a fixed sampling station for physicochemical parameters 5 km north-east of Nkhata Bay in the northern offshore waters of Lake Malawi. The lake is about 400 metres deep at that Beauchamp's sampling station, but no details were given on why that location was considered representative of the whole lake.   I'm intending to do my sampling at that same location. However, as several years have lapsed after Beauchamp did his work and temporal changes may have occurred since then, and no clear reasons were given why that one location was considered representative I tend to hesitate. Could anyone help with a suggestion?

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25 Answers

  1. Sampling location for monitoring

    Because of the time since the initial sampling, that location is the most important in the lake. It is immaterial whether it is the right locations or not, it provides historical data which allows changes over time to be evaluated. As others have said, what is the purpose of the monitoring. If it is a comparison for assessing climate change or land use management effects on the lake then use the historical site. If you now want to understand how the lake works, then use the historical site as the reference point and add another site (or more) at locations that will show how different lake processes, especially hydrodynamics, affect the water quality parameters being measured at the historical monitoring site.

  2. Seems more of an assumption/ presumption

    Hi! Elias,

    From your narration, Beauchamp neither seemed to have bothered to give an explanation nor claimed the point to be give representative results. In the absence of Beauchamp's clarification or his objective of having located the sampling station at a particular location; it seems more of an assumption/ presumption.

    If your objective is to validate results of Beauchamp's sampling location; it would be a research study worth taking up and conclude your hypothesis accordingly.

      

  3. Sampling method seems ok to me

    You could make it representative and all - I mean take it from several locations instead of one. But more importantly, what are you doing with this testing? I'm surprised to know Lake Malawi has a regular monitoring system. That is good. The current method - well, I think it would give you some idea of which way the water contamination is moving - you know it is water not land (things diffuse pretty fast in it). So, the method set up by him seems ok to me really.

  4. As many have said, I would sample different parts of the lake at mid depth. If the lake has an effluent point, you could sample that to make sure that the effluent is okay.

  5. Sampling site

    As already mentioned by previous contributors, the selection of sampling sites is directly dependent upon your monitoring objectives. If your objective is to assess general water quality or to track water quality trends, it could be argued that one station could be used to represent any lake. In most cases, a site over the lake's the deepest area best represents "average" conditions in the lake. Lake Malawi is a long lake, additional sites can be located in transect (line) along its midsection. In many cases, water quality differences will be noticeable between the transect sites. Avoid sampling near shore, near inflows, or in downwind areas (unless specifically targeting these areas to document worst case conditions.

    Conversely, to characterize an entire lake, a large lake with a number of shallow bays, inflowing streams, or other distinguishing characteristics, you may need more than one station to provide an adequate characterization. 

    To learn how the lake functions, stations in physically diverse locations and at different depths should be selected.

    I refer you to the document attached.

  7. Many have provided right responses. You may again think of going to Chikwina, Chiweta, Monkebay, and try to do it all corners of our LAKE North, Centre in Salima, easten region in monkebay and southren region to compare your findings. Methodologies as regards stakeholders involvement is also crucial in this case.

  8. I agree with all of the above: why the sampling is being undertaken meaning what are you trying to establish. Is it is to gain an understanding of the lake's structure/stratification eg temp, ph, nutrient load, sources of pollutants, suitability of specific uses? Depth with impact on nutrient loading temp potentially pH. vicinity to inlet outlet, turbidity will impact on the "quality" of water. I would continue to use the previous sites with potentially new ones to have something to compare and to establish a future baseline with a summary of reasoning behind site choice as you mentioned you dont know why the previous sites were chosen - it may have been accessability, politics, vicinity to a then existing factory any number of reasons.

  11. Doubtless there were reasons for choosing that location. As has already been stated, the primary rationale for using that same location is to be able to examine whether there have been changes in the lake conditions. Contrary to previous comments, sampling from the drainage point will only tell you the discharge conditions, which is not necessarily (in fact not likely) representative of the lake conditions - for what I hope are obvious reasons. You need to start with a hypothesis and then establish a sampling program or strategy to evaluate that hypothesis. What are you hoping to determine?

  12. It should be at the deepest point. This will allow you to do a depth profile of temperature/dissolved oxygen. Changes in the lake's stratification will be one of the clearest indications of environmental change.

  13. If your funding is limited, then move the sample point to the outfall and sample what is flowing out of the lake. If the lake serves as a drinking water supply then sample from the water plant's inlet. As already pointed out, it depends upon what your reasons for sampling it are. If the lake has no outlet for flow, then you would want to sample at several points spanning the area of the lake in order to properly characterize it.

  14. A thought...

    Depends of yo0ur purpose...

    You may want to go ahead and use the same location... o perhaps you could ask around,  gather multiple stakeholders to define where should be done... Is traditional knowledge relevant for your purpose?

    Greetings

    Luis Ore

     

  16. Sampling in the lake

    As said already, what are your objectives? If it is about water quality, what parameters do want to determine? sampling at a single spot will only represent that sampling point for a large lake. You can sample at the same point as the earlier research and compare, if both of you have same result or other physical, chemical and biological processes may have caused a change/transformation. Your analyses can then be carried out in reference to the particular point (noting the geographical references). Sorry that will not be representative of the lake. If logistics are available and with the map of the watershed, you can make effective planning considering other entry points into the watershed. 

    If you want to sample at the outlet of the watershed also take into consideration the above-mentioned processes which can impact the target parameter depending on the distance of travel.

  17. You can't do it

    There is no single sample that would be "representative" of a large body of water.  You can replicate the location and then the sample would be comparable to Beauchamp's previous work - in time.  Beauchamp's assumption of representativity should be assumed incorrect until proven out.

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  18. One sampling point for Lake Malawi?

    Elias, I think the first question you need to answer is why are you doing the sampling? What is the purpose of the sampling programme you want to establish? If this question is answered then you can proceed to locate your sampling points.

    If it is your wish to investigate the water quality of the Lake then one sampling point is inadequate. You must know what comes in and what goes out. A whole lot of activities take place along the length and breadth of the Lake and inside the lake so get the first question answered and the rest will follow.

     

  19. I would agree with Dennis. To start a sampling program you should first to decide what is your objective, then you have to select parameters to be analysed, then you have to evaluate resources availability (logistics, sample preservation, time between sampling and analysis, etc) and after that you decide sampling point. Probably the sampling program used before is important as reference but will not represent actual situation.

  20. representative site for the sampling

    The only site that can be seen as "representative" of the lake would be at the exit point - thus where water leaves the lake at the wall/sluice gates. For a proper study (if you have time,logistics etc) one would sample at each inlet point, a few points along the length of the lake, and the exit point.

  21. why you want to do this?

    If you want to compare the first (1940) results and yours....you have to do it in same site...for obvious reason.

    If your study has other aims....then you consider the suggestions given...

    sampling should either representative eg represent the lake water as a whole.

    Many contributing factors are there affecting/contributing to water quality or other aspects.

    M.I.Zuberi, Ethiopia

  22. I would start with why test at all? What are you trying to confirm or understand? Then you can make logical choices about where to locate (but I would agree, one fixed spot seems like of limited use, so preferably increase or make mobile with rotation to various fixed points on a schedule).

  23. sampling location

    Hello Beachamp,

    the lake is quite big and it looks like there are several rivers feeding it. there is also a city near by, the influence of which can significantly modify your results if you will sample near Maleri island. If I would be you, I would sample at few locations and at different depth to see the differences with longitude.

    good luck!

  24. is the lake closed? is there any feeding river or regular rain? so you may need to consider water add-on rate. And what is the purpose of the tests? try to think about locations where they are close to the uses of the lake (maybe water intake?!).

  25. generally, you don't use one point. but i guess, he was doing a Q and D or quick and dirty. if i were to do it, you want an established point. doing it at the same point is better than starting at a new spot. perhaps its away from population..of around average depth.. and he thought it was representative of the lake..