Integrated Multitrophic Aquaculture: Filter Feeders Bivalves as Efficient Reducers of Wastes Derived from Coastal Aquaculture Assessed with Stable Isotope Analyses - Aquaculture and the Environment: A Shared Destiny - page 119

4. Discussion
The study is based on an approach of integrated multitrophic aquaculture to minimize the
impact of aquaculture activities to the marine food web and benthic communities through
stable isotope signatures of cultured Argyrosomus regius , filter feeder bivalves, aquaculture
derived products, particulate and sedimentary organic matter at the proximity of cages. The
results demonstrate the efficiency of integrated multitrophic aquaculture as valuable
methods for the reduction of impacts of aquaculture activities at the coastal zone, especially
so at oligotrophic waters such as the Balearic Islands.
Filter feeders bivalves ( Mytilus galloprovincialis and Chlamys varia ) are both enriched in 13 C
and 15 N at the cage treatment when compared to the bivalves studied at the two control
sites. This clearly demonstrates that both bivalves are assimilating the aquaculture derived
wastes in an efficient way precluding that future aquaculture installations should
incorporate species with different feeding strategies (fish, filter feeders) in order to
guarantee a good environmental status by improving water quality at fish farms.
4.1 Food webs at cage and control treatments
The food web quantified by stable isotope signatures at each treatment (cage and two control
sites) depicted different matter fluxes. 15 N isotopic signature can be used as a viable indicator
to place trophic levels in a food web since nitrogen isotope 15 increases amongst trophic
levels; therefore organisms higher in the trophic web will present higher δ 15 N values.
Accordingly, at the cage treatment the fish Argyrosomus regius has higher mean δ 15 N values
than filter feeders bivalves, zooplankton, phytoplankton, Holothuria forskalii , POM and SOM.
Variability in 15 N isotopic signatures of pellet food reflects the pellet composition, which
consists in a mixture of vegetal and fish meals with different proportion; pellet 2 is 15 N
enriched due to its composition with 78 % fish meals and 22 % vegetal meals, while pellet 1 is
15 N-depleted because it is mainly composed of vegetal meals, with 71%. Previous studies
have demonstrated that aquaculture derived waste enters the food web altering the natural
isotopic composition of organic matter sources at the base and the upper trophic levels
(Vizzini & Mazzola, 2004) as it is the case for our results.
In the cage treatment filter feeders bivalves showed a nitrogen isotopic signature similar to
that one for zooplankton, phytoplankton, fresh food, Argyrosomus regius faeces and
Holothuria forskalii. This similarity demonstrates that filter feeders are working efficiently by
getting their nitrogen isotopic signature from the filtered substances. Phytoplankton,
Holothuria forskalii, POM and SOM are carbon enriched compared to the other samples.
Figure 3 and 4 follow the same structure as Figure 2. While in cage treatment, POM is
carbon enriched compared to SOM, in control 1, it is the other way round, SOM has
higher values of δ 13 C than POM. Nevertheless, filter feeders in control site 1 have nitrogen
isotopic signatures higher than POM and SOM, which are in the lower part of the food
chain, and isotopic signatures similar to those for zooplankton and phytoplankton. In
Figure 3 we can observe how POM has δ 13 C values much lower than the POM sampled in
the cage site and control 2 site. We can also observe that in control site 2 all 15 N isotopic
signature values for all samples are slightly lower than values found for samples in both
cage site and control site 1. In control site 2 we find cleaner waters which are purified as
well as by filter feeders by aquatic plants such as Posidonia oceanica (Linnaeus) Delile 1813