Response of Mytilus edulis to enhanced phytoplankton availability by controlled upwelling in an oligotrophic fjord

Abstract

The controlled upwelling of nutrient-rich deep water in oligotrophic coastal regions has been proposed as a means of increasing phytoplankton and, subsequently, bivalve aqua culture production. This was tested as part of a large-scale upwelling experiment in an oligo trophic environment (Lysefjord, Norway). The mean chlorophyll a concentration in the upwelling area was consistently higher than at the control site (mean ± SD: 3.3 ± 1.9 and 1.5 ± 0.6 mg chl a m-3, respectively) during the 4 mo of controlled upwelling. After 2 mo with upwelling, the dry flesh weight of 1 yr old and 2 yr old mussels was 95% and 24% higher, respectively, than that of the mussels at the control site. The 1 yr old mussels (Mytilus edulis) at the upwelling site achieved up to 2.4-fold higher dry flesh weight compared to the control. Reproductive output was also higher at the upwelling site and only there, spawning of 1 yr old mussels was detected. Standardized clearance and respiration rates showed maximum values during the most intense period of tissue growth. Average ingestion rates were 40% higher at the upwelling than at the control site. Tissue growth and clearance rates were not correlated with the measured seston parameters, suggesting that food acquisition was responsive to other exogenous parameters and/or to increased endogenous energy demands. It was concluded that the sustained upwelling of nutrient-rich deep water in an oligotrophic fjord can increase phytoplankton biomass, resulting in improved mussel growth performance and increased aquaculture production carrying capacity. Thus, controlled upwelling represents a simple but effective ecosystem engineering approach for enhancing human food production.