Thresholds of Gross Primary Production for the Metabolic Balance of Marine Planktonic Communities

The notion that less productive marine planktonic communities tend to be heterotrophic was tested by synthesizing reported estimates of the relationships between the net community production or community respiration and gross primary production (GPP), allowing calculation of the threshold GPP separating less productive, heterotrophic communities from more productive, autotrophic ones. A total of 35 estimates of the threshold GPP were assembled, derived from reports of comparative analyses of individual regions (Mediterranean Sea, Atlantic Ocean, Southern Ocean, Pacific Ocean, and Indian Ocean) and global comparative analyses for open-ocean and coastal environments, time- series analyses of changes in planktonic metabolism at individual locations, experimental manipulations in mesocosms, and a semi- empirical modeling exercise. Planktonic communities of the open ocean and continental shelf showed threshold GPP values ranging 30-fold, from 0.34 mmol O₂ m⁻³ d⁻¹ to 9.45 mmol O₂ m⁻³ d⁻¹, with those for estuarine and coastal locations reaching 50.60 mmol O₂ m⁻³ d⁻¹. Antarctic and ultra-oligotrophic ecosystems showed the lowest threshold GPP values (< 2.2 mmol O₂ m⁻³ d⁻¹), with a general consistency across approaches for a given ecosystem. Plankton community respiration in the absence of or under low primary production is not negligible and is supported by semi-labile dissolved organic carbon. The analysis of GPP thresholds suggests that allochthonous organic inputs to the less productive regions of the ocean must be in the order of 5–6 mmol C m⁻² d⁻¹, consistent with recent estimates of allochthonous inputs of organic carbon to the ocean.