Site selection and carrying capacity in Mediterranean ... - FAO Sipam

38
3.2 Use of chemical and physical attributes to assess status of sediments
38
GFCM:XXXV/2011/Dma.9
Sediment sampling
Station positions should reflect gradients of impact, with one or more stations situated in the
area of maximum impact. Stations may be chosen by modelling or by means of a pre-survey
using redox potential, video, etc. It is preferable to maximise the chance of detecting a
gradient and this is generally manageable by taking a minimum of 5 stations per transect. A
reference station (or preferably 2) should be allocated, which should have similar water depth
and sediment characteristics to those of the farm but would be at a sufficient distance from the
farm so as not to be influenced by the farming activities. For chemical analyses, three
replicates taken at each sampling station are generally sufficient to determine variability
among samples.
Visual assessment of sediment cores
This should be carried out prior to processing the core samples and undertaking redox
analysis, and consists of a written description of changes in sediment appearance,
composition and colour with depth. This may provide valuable information on the degree of
organic enrichment, sediment bioturbation and other important characteristics.
Time required: cores taken by divers – 1 - 4 h; sampling with box corer – 0.5 - 1 day
Necessary equipment and infrastructure: sediment sampling by means of: SCUBA divers
(hand-held core samplers) or box cores (sediments in box core are subsampled with small
cores)
Level of expertise required: SCUBA divers experienced in taking sediment cores, or ship
with box-corer
Redox potential
This is one of the most popular proxies for determining the level of oxygen in marine
sediments (Bagander 1978). Redox potential is generally performed in sediment cores
(Hinchey and Schaffner 2005) and indicates the degree of organic loading in the sediments
(Pearson and Stanley, 1979). It is often correlated with changes in benthic community
structure. A limitation concerns coastal fine sediments (mud, silt) that are not necessarily
organically enriched and often have relatively lower values of Eh even in the top few cm of
the sediment, thereby limiting comparability with coarse sediment sites. However, for sand or
coarse grain sediments, redox is a good means to detect changes in Eh at sites where
aquaculture is practiced and where an impact is present e.g. beneath cages, and therefore
should be retained as an analytical tool for environmental monitoring. Aquaculture carried out
in locations having fine-grained sediments is rapidly becoming the exception rather than the
rule (especially in the Mediterranean) because such sediments generally occur in depositional
bodies of water that are highly sensitive to organic loading. As such, redox potential is a very
useful tool for rapidly establishing the potential for benthic impact. Hargrave (1994) designed
the Benthic Enrichment Index (BEI) which is calculated as the product of the flux of organic
carbon to the seabed and redox potential in surface sediments. This index was successfully
used to assess the status of sediments in Canadian waters and can be applied in the
Mediterranean. Displacing aquaculture offshore (at depths >40m) is likely to involve
deployment of cages over bottoms characterised by finer sediment and therefore calibration is
needed to obtain “optimal” Eh values for such environments as well as to establish thresholds
for acceptable deviation.
Time required: 15 min per core.
Necessary equipment and infrastructure: redox potential electrodes, plastic sediment corers
with holes in sides to insert redox electrodes.