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Modification of the N : P ratio in lakes by in situ processes
Author(s) -
Levine S. N.,
Schindler D. W.
Publication year - 1992
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.1992.37.5.0917
Subject(s) - mesocosm , biogeochemical cycle , sediment , redfield ratio , denitrification , environmental science , littoral zone , environmental chemistry , thermocline , oceanography , phytoplankton , nutrient , nitrogen , geology , ecology , chemistry , biology , paleontology , organic chemistry
In situ mesocosms in two Canadian Shield lakes were used to evaluate the contributions of inlake vs. external sources of nitrogen and phosphorus to nutrient budgets and N : P ratios. These mesocosms were designed to have variable exchange with sediments. Half were fertilized with N and P at a ratio great enough to ensure P limitation for most phytoplankton (atomic ratio, 33 : 1); the other half were fertilized at a ratio low enough to cause N limitation (4.4 : 1) in the absence of compensation mechanisms. For littoral mesocosms, sediments were a major source of N, but not of P. A comparison of mesocosms having sediments with one having a plastic floor indicated that sediment N return was derived largely from decomposing material at the sediment surface, rather than from deep sediments. Disproportionate returns of N from sediments, along with lower denitrification, reduced N limitation in the low N: P mesocosms. In pelagic mesocosms, which lacked sediment contact, N 2 fixation and thermocline entrainment late in the experiment were the principal internal N sources at low N : P. Biogeochemical mechanisms for reducing N: P in the water column at high N : P supply ratios were less effective than those that ameliorated N shortages at low N: P. The most important mechanism for reducing N: P was denitrification, while both N 2 fixation and sediment return raised N : P. We conclude that biogeochemical mechanisms allow N shortages to be more readily overcome than P shortages in Canadian Shield lakes. Due to the importance of sediments as an N source, it is important to scale mesocosms so that they have sediment: water ratios similar to those of the lakes they are designed to simulate.