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The response of Vallisneria spinulosa (Hydrocharitaceae) to different loadings of ammonia and nitrate at moderate phosphorus concentration: a mesocosm approach
Author(s) -
LI WEI,
ZHANG ZHAO,
JEPPESEN ERIK
Publication year - 2008
Publication title -
freshwater biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.297
H-Index - 156
eISSN - 1365-2427
pISSN - 0046-5070
DOI - 10.1111/j.1365-2427.2008.02053.x
Subject(s) - mesocosm , macrophyte , eutrophication , phosphorus , hydrocharitaceae , nitrate , zoology , chlorophyll a , aquatic plant , environmental chemistry , biomanipulation , chemistry , botany , nutrient , biology , ecology , organic chemistry
Summary 1. While phosphorus (P) is often considered the most important growth limiting factor for plants in lakes, recent studies of shallow lakes indicate that nitrogen (N) may be of greater importance than realized hitherto and that submerged macrophytes may be lost when the N concentration exceeds a certain threshold, as long as the concentration of P is sufficiently high. 2. We studied the effects of different loadings of NH 4 ‐N and NO 3 ‐N on chlorophyll a and on a macrophyte tolerant of eutrophication, Vallisneria spinulosa (Hydrocharitaceae). In outdoor mesocosms we used water from a pond as control and created four levels of NH 4 ‐N and NO 3 ‐N (approximately 2.5, 5, 7.5 and 10 mg L −1 ) by dosing with NH 4 Cl and NaNO 3 , respectively. After the experiment, the plants were transferred back to a holding pond to study their recovery. In contrast to previous research, we used a low background concentration of phosphorus (TP 0.024 ± 0.003 mg L −1 ) so we could judge whether any effects of N were apparent when P is in short supply. 3. Chlorophyll a increased significantly with N dosing for both forms of N, but the increase was highest in the NH 4 ‐N dosed mesocosms (maximum 58 μg L −1 versus 42 μg L −1 in the NO 3 ‐N mesocosms), probably due to a higher total inorganic N concentration (part of the added ammonia was converted to nitrate during the experiment). 4. Although the number of ramets of V. spinulosa was not affected by the N treatment, the biomass increased up to concentrations of 7.5 mg L −1 , while biomass at 10 mg L −1 remained at the control level for both N ions treatments. A similar pattern was apparent for the content of N and soluble sugar of the plant, while there were no differences in the plant P content among treatments. Five months after transplantation back to the pond no difference was found in the number of ramets or in biomass, except that the biomass of plants grown at 10 mg N L −1 during the experiment was greater than that in the control, while the N and P contents of plants were similar to those of the controls. 5. Nitrogen concentration had little influence on the growth of the eutrophication tolerant submerged macrophyte at moderately low concentrations of phosphorus. Moreover, the two N ions showed no toxic effects, suggesting that loss of macrophytes observed in other studies, run at higher phosphorus concentrations, was probably related to enhanced shading by periphyton and/or phytoplankton rather than to any toxic effects of N.