To identify the seasonal pattern of nitrogen N and phosphorus P limitation of phytoplankton in four different lakes, biweekly experiments were conducted from the end of March to September Lake water samples were enriched with N, P or both nutrients and incubated under two different light intensities. Chlorophyll a fluorescence Chla was measured and a model selection procedure was used to assign bioassay outcomes to different limitation categories. N and P were both limiting at some point.
For the shallow lakes there was a trend from P limitation in spring to N or light limitation later in the year, while the deep lake remained predominantly P limited. To determine the ability of in-lake N:P ratios to predict the relative strength of N vs. Nitrogen limitation was predictable, frequent and persistent, suggesting that nitrogen reduction could play a role in water quality management.
However, there is still uncertainty about the efficacy of N restriction to control populations of N 2 fixing cyanobacteria. This is an open-access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Anthropogenic eutrophication is one of the biggest threats to freshwater ecosystems. Its consequences include changes in phytoplankton species composition and increases in biovolume that are accompanied by unpleasant odors, oxygen depletion, decreases in water transparency and a loss of biodiversity [1] , [2].
There has been an extended debate over whether nitrogen N or phosphorus P is the nutrient that ultimately determines productivity in lakes [3] โ [5]. Early work emphasized P as the main nutrient controlling phytoplankton biovolume in most lakes based on inferences from the stoichiometry of N and P in phytoplankton and the relative availability of these elements in nature [4].
