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

51 GFCM:XXXV/2011/Dma.9
The complex dynamics of nutrient dispersion and uptake, and the bottom up and top down control and
co-limitation of primary productivity by various trace elements make it difficult to establish nutrient
levels as ‘safe’ or ‘critical’ in the vicinity of fish farms. However, the UK ‘Comprehensive Studies
Task Team’ on coastal eutrophication (CSTT 1997) has recommended EQSs for winter nutrients
Scottish lochs; these are 168 µg l -1 for dissolved inorganic Nitrogen (DIN) and 62 µg l -1 for dissolved
available inorganic phosphorus (DIP). Although in some sheltered areas of the Western
Mediterranean these values may be approached at sampling stations close to fish farms (Pitta et al
2006), the recorded background levels in most typical (semi-exposed or even moderately sheltered)
sites in the Mediterranean are 50% lower than these EQSs (Pitta et al. 1999, 2005).
Table 5 - Sediment quality criteria in Scotland: action levels within and outside the Allowable
Zone of Effects (AZE), modified from Henderson & Davies (2000)
Component Determinant Action level within AZE Action level outside AZE
Benthos Number of taxa Less than two polychaete taxa
present (sample replicates
bulked)
Benthos Number of taxa Two or more replicate samples
with no taxa present
Benthos Abundance Organic enrichment polychaetes
present in abnormally high
densities
Benthos Shannon-Wiener diversity
index
Benthos Infaunal trophic index
(ITI)
51
Must be at least 50% of
reference station value
Organic enrichment
polychaetes must not exceed
200% of reference station value
N/A Must be at least 60% of
reference station value
N/A Must be at least 50% of
reference station value
Sea bed Beggiatoa N/A Mats present
Sea bed Feed pellets Accumulations of pellets Pellets present
Sediment Copper 289 mg kg -1 (dry wt)
Sediment Zinc 169 mg kg -1 (dry wt)
Sediment Free sulphide 4800 mg kg -1 (dry wt) 3200 mg kg -1 (dry wt)
Sediment Organic carbon 9%
Sediment Redox potential Values lower than -150mV (as
a depth average profile OR
Values lower than -125mV (in
surface sediments 0-3 cm)
Sediment Loss on ignition 27%
The CSTT (1997) group has also suggested a critical value of 10 µg l -1 for Chlorophyll a in water,
assuming that above this level there is a high risk for phytoplankton sedimentation to cause sediment
anoxia. These values are quite difficult to find in most typical Mediterranean mariculture sites but,
nonetheless, this EQS value is a useful stimulus for research on the potential impact of eutrophication,
even if in extreme cases. The fact that the water quality standards proposed for Scotland are currently
quite high in comparison to what has been measured in the Mediterranean does one necessarily mean
that the water quality is not a relevant issue. The concentration of fish farms in specific zones
dedicated for aquaculture, and/or the increase in production levels to achieve an economy of scale,
might lead to an increase in levels to ones that are much higher than present day ones. To this end, the
establishment of efficient and long term monitoring programs in areas with an increasing
concentration of fish farms and/or increased aquaculture production will allow collection of data on
dose-response that will be useful for the assessment of the ultimate ecological thresholds is such a
scenario. Very large scale production e.g. aquaculture parks (such as the ‘polygonos’ in Spain) with
high farm densities may increase the potential for eutrophication. The Phytoplankton Community