I am looking to address independence between sample sites that are downstream from each other in rivers. I intend to sample microplastic concentrations along sections of two different rivers that are ephemeral in hydroperiods (so they are connected for short periods of times following rain events). The reaches that are ephemeral in each river are upstream of perennial sections and I am hoping to compare the microplastic concentrations between the ephemeral pools, and between the pools and perennial reaches over time. I am trying to figure out how to statistically analyze the sites but am wondering if there is a way to account for independence of each location?
It's hard to imagine nearby up- vs. downstream sites being independent of each other, even if they're disconnected part of the yr. That’s b/c of the importance of hyporheic flow, which could be transporting microplastics downstream. And migratory fishes could also be carrying the eaten microplastics upstream. This classic paper is relevant, though:
Stewart-Oaten, A., W.W. Murdoch, and K.R. Parker. 1986. Environmental impact assessment: “pseudoreplication” in time? Ecology 67: 929–940 (http://www.tidalmarshmonitoring.net/pdf/Stewart-Oaten et al. 1986.pdf).
-Bob
http://www.tidalmarshmonitoring.net/pdf/Stewart-Oaten%20et%20al.%201986.pdf
I agree with Robert that upstream to downstream sites are connected through many flows. One way to analyze is to specify a dynamic model with reaches as segments spatially connected. This way you can account for fluxes and evaluate interaction among these sites.
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good luck
Andre--it sounds like one of the things you would like to do is assess whether or not the microplastic concentrations in any pool are independent from the microplastic concentrations in each of the other pools--is this correct?
That is correct Dr. Trask? I am thinking that they are independent when not connected (which is most of the time) but during and immediately following (
It's not binary re: connectedness, but rather at what temporal scale based on flow velocity. At time durations that are (a) shorter, surface flow happens (esp. quick during storms that increase water velocity); (b) intermediate, hyporheic flow & upstream fish migrations happen; & (c) longer, groundwater flow happens. There is no true spatial independence, although you could do empirical tests of spatial autocorrelation:
Griffith, D.A. 1987. Spatial autocorrelation: a primer. Association of American Geographers, Resource Publications in Geography. Washington, DC. 82 pp. & app. (cf. https://books.google.com/books/about/Spatial_autocorrelation.html?id=JYspAQAAMAAJ).
-Bob
Andre, it is an interesting approach. I want to give you some suggestions, and you may further research. It is better if you compare your results at the sites, both when river reaches are connected and not. Also, if you think those sites are independent, you need to answer how it is possible, whether there are localized sources of microplastic ??, etc. And the concentrations of those localized pools will vary with time, water evaporation rates, microenvironmental conditions at sites, etc.
If you want to compare results, one-way ANOVA and flowed by mean separations may help with assumptions.
And check the possibility of two way ANOVA arrangement to assess the interaction between sites with assumptions.
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