Internal Ecosystem Feedbacks Enhance Nitrogen-fixing Cyanobacteria Blooms and Complicate Management in the Baltic Sea

Abstract

Eutrophication of the Baltic Sea has potentially increased the frequency and magnitude of cyanobacteria blooms. Eutrophication leads to increased sedimentation of organic material, increasing the extent of anoxic bottoms and subsequently increasing the internal phosphorus loading. In addition, the hypoxic water volume displays a negative relationship with the total dissolved inorganic nitrogen pool, suggesting greater overall nitrogen removal with increased hypoxia. Enhanced internal loading of phosphorus and the removal of dissolved inorganic nitrogen leads to lower nitrogen to phosphorus ratios, which are one of the main factors promoting nitrogenfixing cyanobacteria blooms. Because cyanobacteria blooms in the open waters of the Baltic Sea seem to be strongly regulated by internal processes, the effects of external nutrient reductions are scale-dependent. During longer time scales, reductions in external phosphorus load may reduce cyanobacteria blooms; however, on shorter time scales the internal phosphorus loading can counteract external phosphorus reductions. The coupled processes inducing internal loading, nitrogen removal, and the prevalence of nitrogen-fixing cyanobacteria can qualitatively be described as a potentially self-sustaining "vicious circle." To effectively reduce cyanobacteria blooms and overall signs of eutrophication, reductions in both nitrogen and phosphorus external loads appear essential.

Impact of Climate Change on Ecological Quality Indicators and Biogeochemical Fluxes in the Baltic Sea: A Multi-Model Ensemble Study

“…A feedback mechanism is suggested that explains why changing climate counteracts nutrient load abatement strategies in our model experiments. Higher water temperatures reinforce the so-called vicious circle which was suggested by earlier studies (e.g., Vahtera et al 2007;Savchuk 2010). Higher water temperatures are projected to (1) reduce oxygen concentrations in the water column due to lower solubility of oxygen in warmer water and (2) accelerate organic matter mineralization and oxygen consumption.…”

“…Despite high denitrification rates, denitrification removes a smaller fraction of the nitrogen turnover and the growth of the pelagic DIN pool accelerates. This is in contrast to the state of the Baltic Sea during the 1980s and 1990s, when increasing hypoxia reduced the pelagic DIN pool (Vahtera et al 2007;Savchuk 2010). Meier et al (2011b) studied the quasi-steady state response to changing climate in 100-year simulations.…”

“…During the past 100 years, parts of the Baltic Sea have changed from oligotrophic to eutrophic systems, caused by human-induced nutrient load increases (Elmgren 2001;Savchuk et al 2008;Gustafsson et al 2012). As a consequence of eutrophication, environmental problems like spreading of hypoxia and increased frequency and intensity of cyanobacteria blooms have been observed (Vahtera et al 2007). Hypoxia increased both in the deep offshore waters (Conley et al 2009;Savchuk 2010) and in the coastal zone (Conley et al 2011).…”

Extremes of Temperature, Oxygen and Blooms in the Baltic Sea in a Changing Climate

“…We found that the probability for hypoxic and anoxic conditions in the future projections increased in most bottom waters of the central Baltic Sea, and the maximum duration of hypoxic and anoxic events roughly doubled. Anoxia reduces phosphorus sequestration in sediments (Vahtera et al 2007), and more phosphate may be available in the pelagic system. This could intensify nitrogen fixation.…”

“…The likely most crucial effect is the liberation of iron-bound phosphate from the sediment. This is considered a self-eutrophication process (Vahtera et al 2007).…”