Increased anthropogenic pressures
on coastal marine ecosystems
in the last century are threatening their biodiversity and functioning.
Global warming and increases in nutrient loadings are two major stressors
affecting these systems. Global warming is expected to increase both
atmospheric and water temperatures and increase precipitation and
terrestrial runoff, further increasing organic matter and nutrient
inputs to coastal areas. Dissolved organic nitrogen (DON) concentrations
frequently exceed those of dissolved inorganic nitrogen in aquatic
systems. Many components of the DON pool have been shown to supply
nitrogen nutrition to phytoplankton and bacteria. Predictions of how
global warming and eutrophication will affect metabolic rates and
dissolved oxygen dynamics in the future are needed to elucidate their
impacts on biodiversity and ecosystem functioning. Here, we experimentally
determine the effects of simultaneous DON additions and warming on
planktonic community metabolism in the Baltic Sea, the largest coastal
area suffering from eutrophication-driven hypoxia. Both bacterioplankton
community composition and metabolic rates changed in relation to temperature.
DON additions from wastewater treatment plant effluents significantly
increased the activation energies for community respiration and gross
primary production. Activation energies for community respiration
were higher than those for gross primary production. Results support
the prediction that warming of the Baltic Sea will enhance planktonic
respiration rates faster than it will for planktonic primary production.
Higher increases in respiration rates than in production may lead
to the depletion of the oxygen pool, further aggravating hypoxia in
the Baltic Sea