Potential effects of elevated sea-water temperature on pelagic food webs.

• The effect of temperature changes on the marine pelagic food web was studied in three successive mesocosm experiments, performed during the spring bloom 2001 in the northern Baltic Sea. The temperature was varied from 5 to 20 °C in each experiment, running over a 3-week period. The experiments included food webs of at least four trophic levels: (1) phytoplankton-bacteria, (2) flagellates, (3) ciliates and (4) metazooplankton. The results showed that heterotrophic to autotrophic biomass ratio (H/A) increased 5 times when temperature was raised from 5 to 10 °C. In agreement, the carbon fixation to respiration ratio indicated a decrease of six times over the same temperature range. Furthermore, the sedimentation decreased by 45% when the temperature was elevated from 5 to 10 °C, probably as a consequence of the increased respiration losses and bacterial biodegradation of settling material. Analyzed parameters, thus, indicated that the degree of heterotrophy increased in the temperature interval of 5–10 °C. Above 10 °C, the analyzed parameters in general were more stable. Our results indicate that moderately elevated seawater temperatures, due to climate change or weather alterations, may affect the entire ecosystem function in temperate sea areas by altering the balance between autotrophy and heterotrophy.

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