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In order to lower the phytoplankton biomass in lakes it is theoretically most effective to reduce the nutrient that is actually limiting. However, it is widely assumed that the abundance and N₂-fixation rate of N₂-fixing cyanobacteria (Nostocales) would increase in response to reduced N loading, and thereby render efforts to improve water quality by N reduction ineffective. Nostocales N₂-fixation has a huge energy demand and consequently the light intensity may affect the response of Nostocales biovolume and N₂-fixation to varying N additions. This led to the following aims, for which three different sets of experiments were conducted.
(i) Determination of the seasonal dynamic of N- and P-limitation for four lakes of differing lake types in the German lowlands and testing the power of four N:P ratios to predict the limiting nutrient: Biweekly experiments were performed in 4 lakes differing in lake. For the shallow lakes there was a trend from P limitation in spring to N or light limitation in summer and autumn, while the deep lake remained predominantly P limited. The ability of in-lake N:P ratios to predict the relative strength of N vs. P limitation was tested using linear regression. All four N:P ratios had significant positive relationships with the response ratio to N and P additions, but the TN:TP and DIN:TP ratios performed best. Nitrogen limitation was predictable, frequent and persistent, suggesting that nitrogen reduction could play a role in water quality management.
(ii) Determination of the response of Nostocales biovolume and N₂-fixation to varying N additions: To achieve this, an experiment with water from an N limited lake (LAN) was conducted. While the Nostocales biovolume did not respond to varying N additions, the N₂-fixation increased in low N microcosms. To quantify the extent to which Nostocales compensated for the varying N addition rate, we calculated a compensation rate (CR). By the end of the experiment a CR of 36 % was reached. However, at biovolumes typical for summer in LAN the CR would be much lower. Therefore, in shallow polymictic lakes like LAN, reduced N loading may lower both in-lake N concentrations and biovolumes of non-fixing phytoplankton without significantly impacting Nostocales biovolume.
(iii) Determination of the effect of the light intensity on the response of Nostocales biovolume and N₂-fixation to varying N additions: To accomplish this aim another microcosm experiment with water from LAN was conducted. While at low and intermediate light intensities the reduction of N addition had no effect on Nostocales biovolume, at high light intensities it led to an increase. The N₂-fixation increased at all light intensities when N addition got reduced. This positive response to a reduction in N addition increased with light, showing that Nostocales may take advantage of being able to fix N₂ mainly at high light intensities.
The ability to fix molecular nitrogen is considered to be a competitive advantage of Nostocales to overcome periods of nitrogen shortage but it is unclear to what extend these cyanobacteria import nitrogen into freshwaters and if they are able to compensate the efforts of reducing anthropogenic nitrogen input.
We studied nitrogen fixation, cyanobacterial biovolume and species composition and abiotic parameters in two polymictic lakes (Germany) over three years. Although Nostocales were present from April to November N2-fixation was found only from June/July to September. In the summer months, it amounted up to 40 mgN m-²d-¹ or up to 500 mgN m-²d-¹ resulting in rather low annual N-inputs between 0.1 and 8 gN m-²a-¹. We found a high variation in N2-fixation rates between the two lakes and the years, which could neither be explained by total Nostocales biovolume nor heterocyte numbers. N2-fixation rates measured in the field will be analyzed on Nostocales species level, be compared to those of laboratory cultures and be discussed in the context of possible phosphorus or light limitation of Nostocales.