Cyanobacterial blooms have increased in magnitude and distribution in the Baltic Sea in recent decades. Cyanobacteria have poorer nutritional quality than other phytoplankton taxa (i.e., diatoms) and are expected to negatively affect pelagic food web efficiency (FWE) and quality, but these potential effects are yet to be verified. We conducted a 29 days mesocosm experiment to contrast the effects of cyanobacteria (Aphanizomenon sp.) and diatoms, combined with water mixing intensity (high vs low), on pelagic FWE and food web quality. The experiment consisted of four treatments with three replicates: diatoms with high mixing, diatoms with low mixing, cyanobacteria with high mixing and cyanobacteria with low mixing. Food webs are based on the natural plankton community in the northern Baltic Sea. We found lower zooplankton production and FWE (i.e., zooplankton production: (phytoplankton + bacterial production) ratio) in the cyanobacterial treatments at the end of the experiment. Higher mixing led to higher FWE. Food web quality measured in terms of the ratio between ω3 and ω6 fatty acids (ω3:ω6) in zooplankton was lower in the cyanobacterial treatments than in the diatom treatments. Sediment in the diatom treatments had higher EPA, ω3 polyunsaturated fatty acid and ω3:ω6 fatty acids ratio. The N₂ fixing Aphanizomenon induced decreases in δ¹⁵N isotopic signals of zooplankton that indicate assimilation and trophic support of diazotrophic N in zooplankton in the cyanobacterial treatments. Overall, our results imply that climate-change induced increases in cyanobacterial blooms likely will lower pelagic FWE and food web quality in the northern Baltic Sea.

Primary Presenter: Tharindu Herath, Umeå University (tharindu.bandara@umu.se)

Authors:

Tharindu Bandara Herath, Umeå University  (tharindu.bandara@umu.se)

Sonia Brugel, Umeå University  (sonia.brugel@umu.se)

Danny Lau, Swedish University of Agricultural Sciences  (danny.lau@slu.se)

Agneta Andersson, Umeå University  (agneta.andersson@umu.se)