Unexplained interannual oscillations of cyanobacterial blooms in the Baltic Sea

Mati Kahru,Emanuele Di Lorenzo,Oleg Savchuk,Ragnar Elmgren

doi:10.1038/s41598-018-24829-7

Abstract

Population oscillations in multi-species or even single species systems are well-known but have rarely been detected at the lower trophic levels in marine systems. Nitrogen fixing cyanobacteria are a major component of the Baltic Sea ecosystem and sometimes form huge surface accumulations covering most of the sea surface. By analysing a satellite-derived 39-year (1979–2017) data archive of surface cyanobacteria concentrations we have found evidence of strikingly regular interannual oscillations in cyanobacteria concentrations in the northern Baltic Sea. These oscillations have a period of ~3 years with a high-concentration year generally followed by one or two low-concentration years. Changes in abiotic factors known to influence the growth and survival of cyanobacteria could not provide an explanation for the oscillations. We therefore assume that these oscillations are intrinsic to the marine system, caused by an unknown, probably mainly biological mechanism that may be triggered by a combination of environmental factors. Interactions between different life cycle stages of cyanobacteria as well as between predator-prey or host-parasite are possible candidates for causing the oscillations.

Highlights

Population oscillations produced by predator-prey or host-parasite interactions[1,2,3] or even in single species[4] are well-known but have rarely been detected at the lower trophic levels in marine systems
As near-coast turbidity and resuspended sediments from the bottom can interfere with our method of cyanobacteria detection, we evaluate Frequency of cyanobacteria accumulations (FCA) in central parts of the Baltic Sea deeper than 15 m (Fig. 2)
FCA time series described in Kahru & Elmgren[9] was extended to 2017 by using data from MODIS-Aqua, MODIS-Terra and VIIRS satellite sensors

Summary

Introduction

Population oscillations produced by predator-prey or host-parasite interactions[1,2,3] or even in single species[4] are well-known but have rarely been detected at the lower trophic levels in marine systems. Nitrogen fixing cyanobacteria have been an important component of the Baltic Sea ecosystem for millennia[5,6,7,8] During the summer they often form massive surface blooms[9], in some years covering as much as 200,000 km[2] (Fig. 1). Their ability to fix nitrogen makes cyanobacteria an important driver in the nitrogen cycle by stimulating overall primary production and contributing to anoxic conditions at the bottom of the sea[6,8,10]. We are not aware of similar 3-year oscillations in the Baltic or any other marine systems in the World

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Results

Conclusion