Dealing With Environmental Variability 421 Independent samples

Good sampling design requires that each level of your design, and each replicate sample, is independent of each other. Dividing a plankton sample in half is not a replicate. For example, samples should not be collected simultaneously - and replicates sampled immediately downstream of another sample are not independent. The paired nets of a bongo net (see Section 4.7) are not independent replicates. Moving the vessel away from the initial sample does satisfy the requirement of independent sampling.

To assess water quality you may need an additional level of analysis -that of an independent estuary or lake. This is because the water body of interest may be changing throughout all bays and coves due to changes in its catchment (of major concern to a manager), or due to global or regional

Figure 4.1 Detection of an effect at independent reference sites. A possible monitoring design of an estuary or river, illustrating the importance of replicating sites in each region (with a replicated sample from each), and possibly an external reference estuary. The external site is to ensure that changes in your estuary are not due to some global or climate phenomenon. The external data could be another municipality's monitoring program - it does not have to be pristine. Perhaps the town or village was wishing to install an artificial wetland or perhaps fencing along one of the rivers. This design could assess the environmental cost-benefit. Your sampling needs to have replicate sites to ensure that the changes you are observing are not just peculiar to one site and unrelated to the wetland or fence.

Figure 4.1 Detection of an effect at independent reference sites. A possible monitoring design of an estuary or river, illustrating the importance of replicating sites in each region (with a replicated sample from each), and possibly an external reference estuary. The external site is to ensure that changes in your estuary are not due to some global or climate phenomenon. The external data could be another municipality's monitoring program - it does not have to be pristine. Perhaps the town or village was wishing to install an artificial wetland or perhaps fencing along one of the rivers. This design could assess the environmental cost-benefit. Your sampling needs to have replicate sites to ensure that the changes you are observing are not just peculiar to one site and unrelated to the wetland or fence.

changes (also of concern, but not within a manager's mandate). Consequently a parallel sampling program should be conducted in two related or similar water bodies (Figure 4.1). It is difficult to convince managers to invest funds outside their constituency, despite the need to benchmark your own investigation. One approach is to do the regional comparison during a particular summer month only - in a separate analysis - or by collaborating with other groups.

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