N limited herbivore consumer growth and low nutrient regeneration N:P ratios in nutrient poor Swedish lakes along a gradient in DOC concentration

Abstract:

Nutrient limitation of primary producers and their consumers can have a large influence on ecosystem productivity. The nature and strength of nutrient limitation is driven both by external factors (nutrient loading) and internal processes (consumer-driven nutrient regeneration). Here we present results from a field study in 16 unproductive headwater lakes in northern subarctic and boreal Sweden where N deposition is low. We assessed the C:N:P stoichiometry of lake water, seston and zooplankton and estimated the consumer driven nutrient regeneration N:P ratio. The elemental imbalances between seston and zooplankton indicated that zooplankton were mainly N limited and regenerated nutrients with low N:P ratios (median 9.7, atomic ratio). The N:P regeneration ratios declined with increasing DOC concentrations, suggesting that catchment release of DOC accentuates the N limitation by providing more P to the lakes. The N:P regeneration ratios were related to responses in phytoplankton bioassays in mid-summer with low N:P regeneration with N limited phytoplankton, and high N:P regeneration with P limited phytoplankton. During other seasons, increased nutrient loading from the surrounding catchments during periods of greater water throughput had stronger effects on phytoplankton nutrient limitation. Our results suggest that herbivore zooplankton are N limited and recycle nutrients with low N:P ratio in low productive lakes with low N deposition. This will, at least during seasons when in-lake processes play an important role in nutrient turn over, contribute to continued N limitation of phytoplankton in these systems. We anticipate that increased N deposition and changes in climate and hydrology may affect this feedback and result in qualitative changes in these ecosystems, changing both autotroph producers and herbivore consumers from N- to P-limitation, eventually affecting important ecosystem characteristics such as productivity and turnover of energy and nutrients.