To optimize copepod mass production in semi-extensive, open air, open top concrete enclosures RUC + DTU + AU + FISHLAB + MAXIMUS performed two research campaigns (3 month in 2011 and 1 month in 2012). In the enclosures a linear food chain: phytoplankton-copepods-turbot larvae were established. All abiotic and all lower trophic levels leading to a direct influence on turbot larval survival and growth was monitored in an un-manipulated experiment with triplicate enclosures (2011) and a manipulation experiment with inorganic nitrogen addition in triplicate with controls, full treated and pulse treated enclosures (2012). The effort revealed limited success rate for turbot fry during late summer when MAXIMUS follow un-manipulated standard procedures. In fact the larval survival decreased from an appropriate level of 18% in the first campaign early summer to just 4% during the last campaign in late summer. The decreasing survival throughout the productive season most likely is due to inorganic nitrogen limitation of the phytoplankton in late summer, which was observed to translate into decreased fecundity of the copepods and thereby also food shortage for the turbot larva.
Spiking with inorganic nitrogen during late summer in 2012 caused a significantly increased phytoplankton production and also copepod production and biomass in the enclosures. However, due to elevated pH and turbidity in the enclosures caused by high primary production resulted in that the turbot larvae suffered mass mortality. Our recommendation to MAXIMUS is that they can increase copepod production significantly by spiking with inorganic nutrients, but in designated enclosures from which copepods can be feed out to turbot rearing enclosures.