Benthic indicator - fauna and sediment

Benthic fauna indicators

The ECASA study sites extended both over the Mediterranean and the Atlantic and covered an ample gradient of latitudinal locations, water depths, and different target species. Moreover, the complementary sediment analyses made in these locations differ from each other. Only few variables are similar across all sites e.g. Total Organic Matter (TOM) or redox potential. These factors make comparison among sites and variables very difficult to undertake on a global basis. However, the availability of a data set of species composition and density of a broad set of locations makes it possible to check some of the benthic indicators proposed within this project.

The biological data sets, containing the density and species composition, per replicate and station, were homogenised and the structural parameters (density, biomass, etc.) were calculated on the basis of square meter. Then, other univariate variables were derived (Shannon’s diversity, AMBI, etc.) and comparisons were made between sites.

The list of ECASA benthic fauna indicators

The most important problem when analysing the data is the absence of the same variables at all locations. However, when studying those variables which are common to all datasets, several conclusions can be obtained:

• Some locations do not show clear gradients of impact, in terms of the benthic indicators selected in ECASA. In these cases it seems that the good dispersion of the discharges from the cages avoids any damage to the benthic communities. This good dispersion seems to be related with: (i) the situation of the cages on high water depths (>40-50 m), such as in Spain or Norway; (ii) the strong dynamics of the area e.g. currents, such as in Spain and ICRAM.

• The gradients of impact are related with the distance to the cage and depth, being the most impacted area that extending until 25-50 m from the cages.

• Most structural parameters are correlated between them.

• AMBI seems to be a good indicator of the benthic stress, related with organic matter percentages. This is also related with redox potential (showing the oxygen consumption under the cages).

• Multiple regression using AMBI as dependent variable and distance to the cage, depth, and TOM as independent, explains 53.6% of the total variability.
  

For the full document, click here (pdf).

Sediment indicators