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 120

which are present in smaller amounts or not even present in the cage site and control site
1. Aquatic plants give up dissolved nitrogen in waters, meaning that there are smaller
amounts in the aquatic mean available for filter feeders, phytoplankton and zooplankton,
resulting in a decrease of the isotopic signature for these compared to the isotopic
signature values given in the other two study sites.
Regarding the filter feeder bivalves, Mytilus galloprovincialis has lower δ 15 N and δ 13 C values
than Chlamys varia (Figures 5, 6, 7 and 8). Difference in δ 13 C between species are smaller than
differences in δ 15 N, the reason being that nitrogen is more stable than carbon. Nitrogen is used
in amino acids and lipids while carbon is used in carbohydrates which are constantly being
used up. Nitrogen therefore has a longer persistence in tissues than carbon, showing up with a
higher isotopic signature. Moreover, nitrogen-rich fish waste affects δ 15 N values, while δ 13 C
shows less variation as already stated by previous studies (Vizzini & Mazzola, 2004).
For all study sites, SOM always gives lower δ 15 N values and higher δ 13 C values in contrast
to the other samples. Low values of SOM might be attributed to the fact that only the fine
fraction of the sediment (<125 mm) was analysed for stable isotope determination. Another
explanation could be a larger filtering capacity and high waste assimilation by filter feeders
therefore less organic matter will be available for incorporation at SOM. Other studies (Sarà
et al., 2004, 2006) have addressed effects of farming wastes to POM and SOM, but no
inclusion of all organic sources and consumers have been considered. Our study improves
knowledge on fish farms effects by integrating filter feeders bivalves.
The enrichment in 15 N exhibited by Holothuria forskalii in the cage site is derived by the
incorporation of part of aquaculture wastes that sink to the seafloor, as it has been suggested
by the Bayesian mixing model, and observed by other authors, as Dolenec et al. (2007). Even
though it has been demonstrated that filter feeders are efficient in the assimilation of part of
these products, there is still a fraction which accumulates in the sediment affecting the δ 15 N
signature of the sedimentivorous feeders. This is also reflected in control 1, which is 15 N
enriched relative to control 2, where 15 N enrichment is probably due to the effect that the
presence of cleaner waters and aquatic plant communities might have on sedimentivorous
species.
Higher δ 13 C values for Chlamys varia and Mytilus galloprovincialis in the cage site compared
to the reference sites are probably linked to higher water residence time in the inner bay
where the fish farm is deployed, while control sites are at open areas. Therefore,
phytoplankton blooms at cage are more probable resulting in an increase of the δ 13 C isotopic
signature for both filter feeders.
4.2 Partial wastes contributions
The existing annual variability of feasible contribution of fresh food and pellet food to
Argyrosomus regius´ s diet (Figure 9) most probably depends on the nutritional regime that
fin fish have been fed on; that is if they have been given more or less fresh food in contrast
to pellet food. For the three years of study, the feasible contribution of pellet food compared
to fresh food is considerable higher. During 2008, Argyrosomus regius shows a much higher
feasible contribution of fresh food than during the other two years. The reason being is
attributed merely to distinctions in the feeding mode and not due to a change in the intake
nutrient strategy of Argyrosomus regius.