Potential effect of an anchovy-mediated pump on the vertical availability of nitrogen for primary production in the Catalan Sea (northwest Mediterranean)

The potential effect on the pelagic ecosystem of nitrogen recycling by anchovies (Engraulis encrasicolus) in a major spawning area of the Catalan Sea (NW Mediterranean) was assessed by comparing estimates of N excretion in the form of ammonia and urea with the N requirements of primary production. The influence of diel vertical migrations of anchovies on primary production in the stratified layer above the thermocline was also addressed. Results showed that N released by anchovy accounted for almost 2% of phytoplankton total demand in the entire water column during the thermal stratification period. Diel trophic dynamics in the area related to vertical migrations led to a net upward nutrient transport through the thermocline, which might contribute to the vertical reallocation of production within the photic layer towards the stratified layer. If nocturnal N excretion occurs in the topmost 10 m, where spawning takes place, the net production increase in this surface layer could be >5%. This could enhance survival in the first stages of larval development of the species.