Geoscience Reference
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water temperature change of about 2 °C between the present and the Little Ice Age (about
1350-1850) for oxygen concentrations in the Baltic deep waters. The importance of chan-
ging biogeochemical conditions triggered by temperature variations was highlighted by the
increase in summer period cyanobacteria blooms. The variations in cyanobacteria blooms
during recent decades are coupled with the increase in summer sea surface temperatures.
The Baltic study showed correlation between warmer periods and increased total organic
carbon in laminated sediments. It was estimated that about half of the carbon originated
fromcyanobacteria blooms.Sediment studies likewise showedhighorganic carboncontent
during the Medieval Climate Anomaly in around 950-1250, when the sea surface temper-
ature was comparable with present levels, or about 2 °C higher than during the Little Ice
Age. The results were corroborated by model runs using a three-dimensional circulation
model with an integrated biogeochemical model. The model outputs showed a higher oxy-
gen content in the bottom waters during the cold period than during the warm periods. It
is noteworthy that similar results were obtained when the nutrient loads in the model were
kept as high as at present. The results thus indicate a strong surface layer temperature in-
fluence on the spread of anoxic areas, probably in combination with changing primary pro-
duction. In another study reasonable correlation was suggested between the oxygen content
and a temperature increase in the Baltic deep and bottom waters at times during the 20th
It is has not yet been explained how hypoxia affects the habitat requirements of differ-
ent species or the resilience of an ecosystem. Habitat compression can occur in the water
column and at the sediment-water interface, or in the sediment. Habitat compression and
loss of fauna as a result of hypoxia have very significant effects on the ecosystem func-
tion and energetics. The energy flow can be diverted into microbial pathways, and as the
benthos die off the microbial pathways quickly dominate the energy flow. This has strong
negative consequences for higher trophic levels and growth of predators, with function-
ing of the ecosystem being strongly dependent upon the biodiversity. The importance of
benthic fauna diversity for the functioning of deep-sea ecosystems with oxygen levels well
