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Introduction: Water depth (WD) and snail abundance (SA) are two key factors affecting the growth of submersed aquatic plants in freshwater lake ecosystems. Changes in WD and SA drive changes in nutrients and other primary producers that may have direct or indirect effects on submersed plant growth, but which factor dominates the impact of both on aquatic plants has not been fully studied. Methods: To investigate the dominant factors that influence aquatic plant growth in plateau lakes, a one-year field study was conducted to study the growth of three dominant submersed macrophyte (i.e., Vallisneria natans, Potamogeton maackianus, and Potamogeton lucens) in Erhai Lake. Results: The results show that, the biomass of the three dominant plants, P.maackianus, is the highest, followed by P.lucens, and V.natans is the lowest. Meanwhile, periphyton and snails attached to P.maackianus are also the highest. Furthermore, WD had a positive effect on the biomass of two submersed macrophyte species of canopy-type P.maackianus and P.lucens, while it had a negative effect on rosette-type V.natans. Snail directly inhibited periphyton attached on V.natans and thereby increasing the biomass of aquatic plants, but the effect of snails on the biomass of the other two aquatic plants is not through inhibition of periphyton attached to their plants. Discussion: The dominant factors affecting the biomass of submersed macrophyte in Erhai Lake were determined, as well as the direct and indirect mechanisms of WD and snails on the biomass of dominant submersed macrophyte. Understanding the mechanisms that dominate aquatic plant change will have implications for lake management and restoration. [ABSTRACT FROM AUTHOR]

Due to excess nutrient loading, loss of submersed macrophytes is a worldwide phenomenon in shallow lakes. Phosphorus is known to contribute significantly to macrophyte recession, but the role of nitrogen has received increasing attention. Our understanding of how high nitrogen concentrations affect the growth of submersed macrophytes, particularly under natural conditions, is still limited. In this study, we conducted experiments with canopy-forming Potamogeton crispus in 10 ponds subjected to substantial differences in nitrogen loading (five targeted total nitrogen concentrations: control, 2, 10, 20, and 100 mg L-1) and compared the results with those of our earlier published experiments with rosette-forming Vallisneria natans performed 1 year before. Canopy-forming P. crispus was more tolerant than rosette-forming V. natans to exposure to high NH4 concentrations. This is probably because canopy-forming species reach the water surface where there is sufficient light for production of carbohydrates, thereby allowing the plants to partly overcome high NH4 stress. Both the canopy-forming P. crispus and the rosette-forming V. natans showed clear declining trends with increasing chlorophyll a in the water. Accordingly, shading by phytoplankton might be of key importance for the decline in submersed macrophytes in this experiment. Both experiments revealed free amino acids (FAA) to be a useful indicator of physiological stress by high ammonium but is not a reliable indicator of macrophyte growth. [ABSTRACT FROM AUTHOR]

Loss of submersed macrophytes is a world-wide phenomenon occurring when shallow lakes become eutrophic due to excess nutrient loading. In addition to the well-known effect of phosphorus, nitrogen as a trigger of macrophyte decline has received increasing attention. The precise impact of high nitrogen concentrations is debated, and the role of different candidate factors may well change over the season. In this study, we conducted experiments with Vallisneria natans during the growing season (June–September) in 10 ponds subjected to substantial differences in nitrogen loading (five targeted total nitrogen concentrations: control, 2, 10, 20, and 100 mg L − 1 ) and compared the results with those obtained in our earlier published study from the low-growth season (December–April). Like in the low-growth season, growth of V. natans in summer declined with increasing ammonium (NH 4 ) concentrations and particularly with increasing phytoplankton chlorophyll a (Chl a Phyt ). Accordingly, we propose that shading by phytoplankton might be of key importance for macrophyte decline, affecting also periphyton growth as periphyton chlorophyll a (Chl a Peri ) decreased with increasing Chl a Phyt . Free amino acid contents (FAA) of plants tended to increase with increasing NH 4 concentrations, while the relationships between FAA with growth indices were all weak, suggesting that FAA might be a useful indicator of the physiological stress of the plants but not of macrophyte growth. Taken together, the results from the two seasons indicate that although a combination of high nitrogen concentrations (ammonium) and shading by phytoplankton may cause severe stress on macrophytes, active growth in the growing season enabled them to partly overcome the stress. [ABSTRACT FROM AUTHOR]

Eutrophication of lakes leading to loss of submersed macrophytes and higher turbidity is a worldwide phenomenon, attributed to excessive loading of phosphorus (P). However, recently, the role of nitrogen (N) for macrophyte recession has received increasing attention. Due to the close relationship between N and P loading, disentanglement of the specific effects of these two nutrients is often difficult, and some controversy still exists as to the effects of N. We studied the effects of N on submersed macrophytes represented by Vallisneria natans (Lour.) Hara in pots positioned at three depths (0.4 m, 0.8 m, and 1.2 m to form a gradient of underwater light conditions) in 10 large ponds having moderate concentrations of P (TP 0.03 ± 0.04 mg L −1 ) and five targeted concentrations of total nitrogen (TN) (0.5, 2, 10, 20, and 100 mg L −1 ), there were two ponds for each treatment. To study the potential shading effects of other primary producers, we also measured the biomass of phytoplankton (Chl a Phyt ) and periphyton (Chl a Peri ) expressed as chlorophyll a . We found that leaf length, leaf mass, and root length of macrophytes declined with increasing concentrations of TN and ammonium, while shoot number and root mass did not. All the measured growth indices of macrophytes declined significantly with Chl a Phyt , while none were significantly related to Chl a Peri . Neither Chl a Phyt nor Chl a Peri were, however, significantly negatively related to the various N concentrations. Our results indicate that shading by phytoplankton unrelated to the variation in N loading and perhaps toxic stress exerted by high nitrogen were responsible for the decline in macrophyte growth. [ABSTRACT FROM AUTHOR]

The ecological responses of aquatic macrophytes and benthic macroinvertebrates to deep-release dams in three impounded rivers of the Henares River Basin (Central Spain) were studied, specially focusing on the effects of nutrient enrichment caused by deep releases on these two freshwater communities. Three sampling sites, one upstream and two downstream from the reservoir, were established in each impounded river. Sampling surveys to collect submersed macrophytes and benthic macroinvertebrates at each sampling site were carried out in spring-summer of 2009 and 2011. Water temperature tended to decrease downstream from dams, whereas nitrate and phosphate concentrations tended to increase. These abiotic changes, particularly the downstream nutrient enrichment, apparently affected the macrophyte and macroinvertebrate communities. In the case of submersed macrophytes, total coverage and taxa richness increased downstream from dams. In the case of benthic macroinvertebrates, total density and total biomass also increased downstream, but taxa richness tended to decrease. Scrapers appeared to be the macroinvertebrate feeding group most favored downstream from dams as a probable consequence of the positive effect of nutrient enrichment on periphyton and perilithon abundance. Nutrients would ultimately come from water runoff over agricultural lands and over semi-natural forests and pastures, being subsequently accumulated in the hypolimnion of reservoirs. [ABSTRACT FROM AUTHOR]