Importance of carbon as limiting nutrient for bacterioplankton in a boreal sub-arctic coastal system.

Much debate has revolved around nutrient limitation in estuaries. A common view today is that nitrogen (N) and phosphorus (P) act as co-limiting nutrients, both for processes carried out by phytoplankton and by bacterioplankton, although recent studies show that also carbon (C) may control estuarine bacterioplankton, even in systems with large inputs of terrestrial organic matter (tOM). We conducted full factorial experiments in which C, N, P and salt (S) were added and assessed as factors controlling bacterioplankton respiration in a tOM-rich estuary (Öre estuary, northern Sweden). No nutrient limitation was found at high flow episodes, but during base flow each of the elements C, N and P generally boosted the respiration individually and with synergetic interaction effects when added in combination. Interestingly, salt additions increased tOM respiration in the high-salinity part of the continuum, but decreased respiration in the freshwater end. Our findings highlight the combined importance of C, N and P for oxygen consumption rates in coastal systems, suggesting that also C may need to be considered in nutrient management plans for coastal marine areas.