Cyanobacterial surface blooms formed by Aphanizomenon sp. and Nodularia spumigena in the Baltic Sea: Small-scale fluxes, pH, and oxygen microenvironments

Ploug, Helle

Limnol. Oceanogr., 53(3), 2008, 914-921 | DOI: 10.4319/lo.2008.53.3.0914

ABSTRACT: Summer blooms of filamentous cyanobacteria, mainly Aphanizomenon sp. and Nodularia spumigena, are characteristic for the Baltic Sea, where they accumulate at the sea surface in calm weather. The chemical microenvironment, and thus the actual growth conditions within these cyanobacterial surface blooms of the Baltic Sea, are largely unknown. Using microsensors, it is shown that photosynthesis is substantial within these millimeter-thin cyanobacterial layers accumulating at the air-water interface. Net oxygen fluxes at the air-sea interface were on average 2.7-fold higher than those at the aggregate-water interface beneath the layers. Net photosynthesis at light saturation was 1.7-2.4 mmol O2 m-2 h-1. Dark respiration varied between 0.21 ± 0.12 mmol O2 m-2 h-1 and 0.50 ± 0.30 mmol O2 m-2 h-1. There is a tight coupling of O2-producing and O2-consuming processes within aggregates of these large, heterocystous, nitrogen-fixing cyanobacteria and their associated heterotrophic microbial community. It is suggested that the close association of autotrophic and heterotrophic organisms and processes creates a pH microenvironment that is favorable for iron uptake for the cyanobacteria, which in turn may release surplus nitrogen to the heterotrophic community. The pH varied between 7.4 and 9.0 between darkness and saturating light intensities. Decaying aggregates were anoxic up to 12 h. The volumetric oxygen consumption rate in an anoxic aggregate was 1.2 µmol O2 cm-3 h-1. During the initial 12 h of decay, aggregate sinking velocity increased 10-fold, concurrent with a decrease in volumetric oxygen consumption to 0.67 µmol O2 cm-3 h-1.

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