App purpose

This app is designed to look for evidence of statistically significant changes in sediment composition between baseline and monitoring surveys.The app can be used for monitoring of marine licences (e.g. marine aggregate, offshore wind, renewables), MPAs or R&D sites. Note that the OneBenthic M-Test Tool can help determine whether sediment changes are likely to be ecologically significant.

Data

The app uses sediment particle size data classified according to the Wentworth scale (i.e. percentages of silt/clay - SC, fine sand - fS, medium sand - mS, coarse sand - cS, fine gravel - fG, medium gravel - mG, coarse gravel - cG). Data used in the app come from the OneBenthic database. The app also uses spatial data polygons (see 'Map Layers' tab) to automatically assign samples to treatment groups (e.g. reference, licensed areas and secondary impact zones).

How it works

Select a sector and choose baseline and monitoring surveys for comparison. For the marine aggregates sector (AGG), users must also select an RSMP survey array and choose whether to use all or only paired sample data (i.e. stations with both a baseline and a monitoring sample). Selected samples are shown in the map. To find surveys for comparison, turn on sample locations (see map 'samples' checkbox) and use the 'Draw a rectangle' tool to highlight an area of interest. Results will appear in the 'Search' tab. Selected data and assigned treatment groups can be viewed under the 'Data' tab. Data are analysed in various ways, with results available, where relevant, at different spatial scales (see 'Results' tab).

The Line Plots tab shows the sediment composition of samples by treatment group. For clarity, individual samples are not shown in regional line plots.

The Means tab includes a table for mean sediment composition by treatment group.

The MDS tab shows a series of non-metric multidimensional scaling ordination (nMDS) plots based on Euclidean distance (untransformed data). Each dot represents a sample, with positions reflecting similarity/dis-similarity in terms of sediment composition. The stress value provides a measure of how well the 2-d plot represents the multidimensional data (stress values of <0.1 are considered good).

The ANOSIM test looks for evidence of statistically significant differences between the two groups of samples (i.e. baseline vs monitoring). Test outputs include R and p-values. The R value indicates the size of the difference between the two groups, with 0 indicating no difference and 1 a large difference. A p-value of <0.05 indicates that results are statistically significant. Both R and p-values should be considered when interpreting test results. Note that where there are large numbers of samples it may be possible to find statistically significant, yet very small differences between the groups. It is therefore important to consider the effect size (R value). For this reason we generally consider differences to only be of interest where p<0.05 and R>0.1. Anosim test outputs include a column for 'interpretation', following criteria in Goss-Souza (2015). Press button to see results displayed in the map. ANOSIM tests are performed using the R vegan package. Where ANOSIM finds a meaningful difference (i.e. p<0.05, R>0.1), relevant data are carried forward for a SIMPER test to identify which sediment fractions are responsible for the differences.

The SIMPER test is carried out using the 'simper' function (vegan) based on Euclidean distances. Finally, under the Change tab, a simple bar chart shows how mean sediment fractions has changed. These plots should be interpreted together with the SIMPER results. Note that analyses only run after clicking on each tab - please be patient. You can change the R value cut-off for SIMPER analysis here:

Contact

This is a beta version of the app. Users should satisfy themselves that comparisons are valid and carefully check the data and results. No liability is accepted by the app developer. For help/advice using the app (or to provide feedback), please get in touch (keith.cooper@cefas.co.uk).