Incorporation of diazotrophically fixed nitrogen by juvenile fish in a coastal sea

Abstract Nitrogen‐fixing cyanobacterial blooms worldwide have been shown to support production in primary consumers such as zooplankton and benthic invertebrates, but there have been few estimates of how much of the cyanobacterial nitrogen ends up in secondary consumers, such as fish. Using compound‐specific analysis of δ 15 N in amino acids, we have investigated whether cyanobacterially fixed (diazotrophic) nitrogen (N) can be traced in higher trophic level organisms such as zooplankton and mysids, and detected in juvenile Atlantic herring ( Clupea harengus ). At the eutrophied station, the biovolume of N‐fixing cyanobacteria was higher than at the reference station, and the autotrophic population was comprised of a higher proportion of cyanobacteria palatable to zooplankton. The δ 15 N ‰ of source amino acids in juvenile herring significantly decreased over the season at the eutrophied station. Bayesian mixing models were used to calculate the percent of diazotrophically fixed N found in herring muscle tissue in relation to prebloom N, and results showed that herring at the eutrophied station had a higher percent (≈ 34 %) of diazotrophically fixed N in their muscle tissue than fish at the reference station (≈ 18 %). A significant relationship was found between the percentage of diazotrophically fixed N in herring muscle tissue and the biovolume of N‐fixing cyanobacteria in the environment two months prior to the capture date of the fish. This study adds to the mounting evidence that cyanobacteria blooms, a global occurrence, support production at higher trophic levels, including fish.