How is this typically done in terms of location on the lake/river. There will obviously be different concentrations at the edges/middle or depths? How is all this information conformed into a clear picture of the concentration within a lake/river? Links to journal studies would be appreciated or a clear explanation.
Thanks
To make things more clear, I would like to compare excess nutrient concentrations in various lakes or rivers in relation to the population density of a certain aquatic plant. Would it be fair to take water samples near the plant and compare it to various other locations?
Here's what I would be tempted to do, obviously it's dependent on your budget.
Grid the water body {neccessary for a lake, not so much for a stream}.
Within each grid use a floating quadrat to take a random sample.
Take the sample from the quadrat, have it tested for Total oxidised nitrogen, and dissolved reactive phosphorus {not all catchments are Nitrogen limited, as I recall, most aren't}.
Measure the cover of the plant within the quadrat.
The size of the grid squares is up to you - more squares means better resolution, but will also cost more because you will have more samples to test.
You can take more than one quadrat per grid square if you want. This will enable you to derive an average calue for each grid-square, however, again it will cost more.
You will want to take a hand held GPS with you to record the location of the quadrat, but remember that it can take a couple of minutes to get an accurate reading. You'll also want to take a camera with you to try and get a photo of the quadrat in place on the water surface.
With regards to sampling protocols.
First and foremost, if you have a laboratory in mind for doing the tests, contact them. They may be able to provide you with bottles for storing your sample in and will be able to give you the details of any pre-treatment of the bottles that may be neccessary (some labs require you to filter the sample before you submit it for testing, for example).
Secondly. Label everything rigourosly.
Thirdly, if possible take your samples from approximately 20cm under the surface of the water (about 8 inches if you're american). If the water body is not deep enough to achieve this, then try to take your sample from as close to the middle as possible.
Fourthly, if you're working in a stream. BE SAFE. I can not emphasize this enough. Sampling from streams can be dangerous, I know of someone who was trying to get a sample to submit to the organization I work for that slipped, fell, and drowned to to get a sample. Consider a life-jacket and an EPLB as saftey equipment - depending, of course, on the circumstance. Take someone with you if you can, if you can't, select someone to act as a buddy - someone you can say in advance to "I should be back by this time" who can contact you and potentially raise the alarm should you fail to check in.
Fifthly when you're sampling from a stream, move from the most down stream location to the most up stream location. When you sample you risk disturbing the stream bed, in some cases disturbing the stream bed is inevetable. Sampling the water when it's carrying the stream bed sediment can give you a misleading result. The easiest way of avoiding this is to start downstream and work upstream, that way any sediment plumes you create will be behind you and affect only areas you have already sampled.
Finally, and this is perhaps the most important point write
EVERYTHING down. You don't know in advance which results might be anomalous, and what you write down might be key to explaining any anomaly. The sample forms that we use when we submit a sample for testing at work include things like the weather (wind direction and speed, is it cloudy sunny or raining), water characteristics (turbidity, colour, flow rate) and other things like odour.
When things are a little less crazy here at home I'll see if I can track down some standardized macrophyte survey protocols for you.
