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 121

Filter feeders bivalves in our study, Mytilus galloprovincialis and Chamys varia, show both
seasonal and annual variability (Figure 10 and 11). During autumn 2009 and 2010 Chlamys
varia has a higher feasible contribution due to pellet food (Figure 7) but it filter less pellet
food during summer (2008 and 2010), with the exception of summer 2009. Chlamys varia
increases the feasible contribution due to phytoplankton during spring and summer, a
reason might be that during this time of the year phytoplankton blooms increase their
presence increasing at the same time their contribution to filtered matter by Chlamys varia.
The feasible contribution of phytoplankton, fish faeces and POM is very small and rather
similar for Chlamys varia during autumn 2009 and 2010. During autumn there is a lower
phytoplankton activity and less particulate organic matter and therefore the contribution of
these will be lower. Therefore, the seasonal variability of Chlamys varia might indirectly
indicate the seasonal variability of phytoplankton, POM and faeces.
Filtering capacity and assimilation of the different components of the food web by filter
feeders bivalves present a remarkable pattern as observed from the mixing models
calculated at Mytilus galloprovincialis and Chlamys varia (Figure 10 and 11). For both species,
the partial contribution are complementary, in the sense that when one specie increases the
feasible contribution due to pellet food, the other specie decreases the contribution due to
this type of food and vice versa. This could be interpreted as a competitive strategy between
both species, possibly due to being in the same bags limited by water income and nutrient
availability. Another feasible explanation is that their diets differ seasonally, and meanwhile
one is relying on phytoplankton, the other can be assimilating more aquaculture derived
wastes and vice versa.
The influence of excess feed in the isotopic signal of the particulate organic matter (POM) of
the water column in the cages is indicative of the impact on the marine ecosystem. Similarly,
the high contribution of the particulate organic matter and debris supply to the isotopic
signal of Holothuria forskalii reinforce the impact on the benthos. Thus, it would be
interesting in future studies to take into account, as components of the food web,
sedimentivores (which may further reduce the organic load in the sediment), opportunistic
fish species (which may alter the carbon fluxes and benthic and pelagic nitrogen) and the
microbial loop (which rejoins the nutrients back to the food web, assuming a renewal of the
carbon and nitrogen present).
5. Acknowledgements
This work was financially supported by the JACUMAR project (Ministerio de Medio
Ambiente y Medio Rural y Marino). The authors thank several people involved in samples
processing such us F. Fuster, S. Sardu and M. Ceglia. Special thanks for collaboration in
stable isotope analyses offered by the SCTI (Scientific-Technical Services) from the Balearic
Island's University and B. Martorell. We appreciate the collaboration of the staff members of
the marine protected area of S'Arenal.
6. References
Bergamino, L.; Lercari, D. & Defeo, O. (2011). Food web structure of sandy beaches:
Temporal and spatial variation using stable isotope analysis. Estuarine, Coastal and
Shelf Science , Vol. 91, No. 4, (March 2011), pp. 536-543, 02727714