Effects of seasonal variations in phytoplankton on the bioenergetic responses of mussels (Mytilus galloprovincialis) held on a raft in the proximity of red sea bream (Pagellus bogaraveo) net-pens

The seasonal variability of the physiological components of the Scope for Growth (SFG) of mussels Mytilus galloprovincialis was investigated in a raft adjacent (170 m) to fish net-pens and compared with a raft 550 m distant from the cages in Ría Ares-Betanzos (Galicia, Spain). Chlorophyll and phytoplankton size-classes were determined in the field, simultaneously with SFG. Average chlorophyll-a was 0.65 ± 0.24 μg l− 1, while nanophytoplankton (2-20 μm) was the most abundant size-class, ranging from 50 to 70% of the total chlorophyll. The temporal pattern found for chlorophyll-a and phytoplankton size-classes reflected the upwelling-downwelling events and were correlated with the feeding, digestive and metabolic rates. Nanophytoplankton and microphytoplankton were preferentially cleared and ingested by mussels. There were no significant differences between the chlorophyll and phytoplankton size-classes among rafts. The lack of any enhancement in food availability resulted in no significant increase in the SFG of mussels beside the fish cages. Maximum SFG corresponded with the autumn (16.60 ± 7.90 J h− 1) and spring (12.72 ± 9.32 J h− 1) chlorophyll maximums. An abnormally hot summer and reduced chlorophyll levels resulted in lower energy intake, significantly higher metabolic expenditure and a negative SFG (− 34.57 ± 12.55 J h− 1). Any particulate wastes and potential fish-derived chlorophyll enhancement would be rapidly diluted by the currents, while the placement of bivalves too distant from the fish farm in an environment with high supplies of natural seston may explain the lack of an augmented SFG of the co-cultured mussels.