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 109

dispersions (PERMDISP) was used to analyze the multivariate dispersion for each treatment
at all sampling sites. All statistical computations were performed using the statistical
package PRIMER® 6.0 software. The permutational analysis (PERMANOVA) was not
performed on δ 13 C and δ 15 N sediment data in years 2009 and 2010 because, as a
consequence of the low carbon and nitrogen content in the samples, some replicates did not
reach the mass requirements and could not be analyzed in the mass spectrometer.

2.3.2 Partial contributions of aquaculture wastes at the studied food webs
Stable isotope mixing models are an increasingly common approach in environmental sciences.
They are used to determine the proportional contribution of sources to a mixture based on their
respective isotope signatures (Phillips & Gregg, 2001). Amongst its application we can find the
estimation of the relative importance of food sources to animal diets, pollution sources to air or
water bodies and carbon sources to soil organic matter (Michener & Lajtha, 2007).

In this study, we applied the SISUS Bayesian Mixing model (Stable Isotope Sourcing using
Sampling) to quantify the feasible contributions of the potential organic matter sources
(phytoplankton, zooplankton, POM, faecal material, pellet and fresh food) to the filter
feeders bivalves' diet, based on the analyzed stable isotope ratios. Considering the specific
isotope enrichment between prey and consumer tissues, a discrimination of 0.3 ‰ for
carbon was assumed for POM and filter feeders bivalves; while for muscle tissue samples of
Argyrosomus regius a 1.3 ‰ was applied. Previous studies have shown that trophic
fractionation is much larger for δ

15 N than for

δ

13 C, so a correction of 2.3 ‰ per trophic level

was applied (McCutchan et al., 2003).

The model is based on a modification of the Isosource programme (Phillips & Gregg, 2001),
which creates every possible combination of source proportions and compares these
predicted mixtures signatures with the observed mixtures signatures. If they are equal or
within some small tolerance range, this combination represents a feasible solution. The
SISUS software is available for public use at http://statacumen.com/sisus/.

3. Results
3.1 Isotopic composition of the marine food web components at IMTA

The carbon and nitrogen isotopic composition of the marine food components analyzed in
the three years of study in cage and control sites are summarized in Figures 2, 3 and 4.

3.1.1 Potential organic matter sources and environmental elements

Commercial pellet food supplied in the fish farm had similar isotopic values of δ 13 C
(between -20.78 ‰ to -21.87 ‰), with an average value of -21.17 ± 0.09 ‰; however, they
differed greatly in δ 15 N values (between 14.93 ‰ and 6.20 ‰), with pellet 2 being the most
enriched source in 15 N, in contrast to pellet 1 which was the most depleted nitrogen source.
Fresh food were 13 C- enriched compared to pellet food, on average 2 ‰ ( mean -19.1 ± 0.19
‰), in contrast, δ 15 N values were relatively similar. Considering all individual sources, fresh
food sources had similar values of δ 13 C (between -18.82 ‰ to -19.19 ‰), but they showed
differences in δ 15 N values, ranging between 7.92 ‰ to 12.46 ‰, with Loligo vulgaris as the
most enriched source in 15 N and Spicara smaris the most depleted.