I' am not familiar with streams, but in the Dutch drainage ditches, quality parameter definitely change during summer when throughflow ceases. Especially oxygen-related parameter change, such as BOD O2 NO3 NH4. Of course, water level decrease drastically some completely dry out. Due to the low water level temperature can reach 35 degrees Celsius in summer. Maybe CH4 production also fluctuates, but I' am not sure. Furthermore, I assume the conductivity also rises but this depends on the amount of water evaporating and inflow of surrounding water. However, I' am not sure if this can be translated to streams.

Wim has given some good pointers. Changes in pH, salinity and DO may also affect metals specification and bioavailability if changes in basic phy-chem are significant. One other factor that might significantly affect water chemistry during different periods of flow is the source of the water. During stormflows much of the contribution will be overland flow. After rainfall ceases and that water has passed through the system, it is possible that residual flows in the creek are increasingly due to either subsurface flow through the soil or from groundwater discharge if the bed of the creek intercepts groundwater. The chemistry of this water that has been in residence in the soil or aquifer will likely be quite different to run off generated flow. You can look for major ion differences (variations will be more likely be affected by source than by evapo-concentration) or stable isotopes signatures.

Thankyou gentlemen - I suppose the real reason for the question is this - is taking a surface sample going to provide a fair representation of the ionic signature of the waterhole or is there a requirement to sample multiple depths?

Andrew its probably going to be one of those horses for courses problems where it depends on the site conditions, geology, surface/groundwater interaction, etc etc. Here is a link to one of Frans papers that covers some aspects of your question (Article Water quality in two Australian dryland rivers: Spatial and ...

Seqwater use various sampling protocols to account for spatial scale variabiity, more driven by the parameter needs. than location, e.g. algae via integrated tube, bacto grab sample, physchem profiles. But edge samples are generally taken away from the shoreline but below the surface to avoid possible localised differences associated with the shoreline. I would image that if you mapped a waterhole for the parameters of interest, you would find some bottom vs surface differences, but do these differneces matter for your study objectives. A worthy side project at least.

It depends on the site. You should expect at least 4 principal factors that could change chemistry: 1. baseflow contribution (regional/local groundwater discharge) that affects mixing reactions, 2. evaporation that changes mineral concentrations and associated equilibria, 3.gassing/degassing associated with bacterially derived reactions related to plant root zones/soils (e.g. pCO2 and pH that will affect carbonate equilibrium), and 4. changing redox states as the source of oxygen rich surface water flow trickles down to nothing. Of course, these are all inter-related. Think about what conservative and reactive analytical parameters you would have to measure to gain insight on these processes. Focus on CHONS chemistry and stable isotopes.

I think David has made a very important point to quote " I would image that if you mapped a waterhole for the parameters of interest, you would find some bottom vs surface differences, but do these differences matter for your study objectives. You have to have a clear reason for the collection of the data (what is it that you really need to know to answer the objective) and also think about what you are going to do with the data if you sample from multiple depths. You can gather lots of data but does it provide you with information?

when water flow is ceases ,Chemical and physical changes are more obvious . But this type of problem depend on various parameters which may vary place to place. However such type of study may provide valuable information.

The focus is on condition and trend monitoring of the 'ambient' conditions of sites on streams. To date there has been inconsistent approaches to sampling during no-flow in waterholes in streams with some sites being sampled/where others are not becasue of ambiguity in the overall project aims and officers not understanding the purpose.

Really the aim is the longer term condition and trend to look at water management and landuse impacts over time. This data has been collected over for a couple of decades now on a monthly or quarterly basis - both insitu parameters, total ions and nutrients.

By doing a review, I wanted to understand if taking a total ion sample from just below the surface is reflective of the general chemistry at a site to understand whether a single surface sample was sufficient. Also for nutrients as well, but I understand that there may be differences in active forms based on depth of sample (eg. NO3, CH4).

Does anyone know of any references that have sampled multiple levels in smaller waterbodies for differences in total ions/nutrients at depth? There are numerous examples for lakes/dams - but these are generally waterholes