Search

In aquatic ecosystems, light penetrating the sediment surface in shallow lakes may regulate the internal phosphorus (P) release through benthic primary production, which subsequently affects oxidation, pH levels, and alkaline phosphatase activity in the upper sediment. To study the effects of light exposure on the P dynamics at the sediment-water interface under eutrophic conditions, a two-month mesocosm experiment was conducted in twelve cement tanks (1000 L each). The tanks were equipped with Light-Emitting Diode (LED) lights, and surface sediments collected from eutrophic Lake Nanhu (China) were exposed to four different light intensities (0, 50, 100, 200 μmol m -2 s -1 ). The results revealed that: 1) Both the total phosphorus concentration and the phosphorus release flux from the sediment were lower in the light treatments (mean value, 0.59-0.71 mg L -1 and 0.00-0.01 mg m -2 d -1 , respectively) than in the control treatment (0.77 mg L -1 and 0.01 mg m -2 d -1 , respectively), indicating that light supplement could decrease the internal P release. 2) Benthic primary production promoted by light directly absorbed soluble reactive phosphorus and decreased the internal P release. The resulting improved production could also increase dissolved oxygen concentrations at the sediment-water interface, thus indirectly inhibiting internal P release. 3) The relative contributions of direct absorption and indirect inhibition on the internal P release ranged between 23% to 69% and 31% to 77% depending on the light intensity., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

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 (ChlaPhyt) and periphyton (ChlaPeri) 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 ChlaPhyt, while none were significantly related to ChlaPeri. Neither ChlaPhyt nor ChlaPeri 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., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

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]

Published research suggests that the total phosphorus ( TP) thresholds for the regime shifts between a clear-water state dominated by submersed macrophytes and a turbid-water state dominated by phytoplankton in shallow lakes vary with forms of lake basins and climates. However, such hypotheses remain untested by direct field evidence. We therefore tested the hypotheses with empirical data from subtropical lakes on the Yangtze floodplain and also from other lakes in temperate to tropical zones., TP thresholds were found to vary little at moderate depths, but to decrease notably when depth exceeds a level of probably 3-4 m, and increase sharply when depth is below a level of around 1-2 m., TP thresholds were found to be nearly equal in shallow lakes (1-2 m

Dolman (2014) argued that our recent finding (Wang et al., 2014) of no marked effect of submerged macrophytes on the chlorophyll a (Chl a) to total phosphorus ( TP) relationship in Yangtze subtropical lakes may not necessarily be valid due to the statistical procedure used to analyse the relationship between these two variables., We agree with Dolman (2014) that a likelihood ratio test could provide an alternative statistical approach to analyse the effect of macrophytes on Chl a versus TP. However, we do not consider it would be appropriate to use the test on only a limited sub-data set corresponding to where TP values overlap for the vegetated and non-vegetated states, as suggested by Dolman (2014)., Besides testing slopes of the log10(Chl a)-log10( TP) relationship, we compared log10(Chl a)/log10( TP) values between vegetated and non-vegetated lakes at a range of overlapping TP concentrations (Wang et al., 2014)., The conclusion drawn by Dolman (2014) from a highly cited work on subtropical lakes in Florida was shown to be invalid. Re-analysis of these lakes demonstrated similar results to those found in Yangtze subtropical lakes. We therefore consider that our original conclusions are valid and that submersed macrophytes in subtropical lakes do not necessarily exert the same strong effects on algal turbidity as in temperate lakes. [ABSTRACT FROM AUTHOR]

• Nitrate has a dual effect on sediment phosphorus release in shallow lakes • Nitrate can reduce sediment phosphorus release through oxidation • Nitrate can promote phosphorus release by stimulating phytoplankton growth • Alkaline phosphatase secreted by phytoplankton explains the phosphorus release • The effects of nitrate loading on sediment phosphorus release are dose-dependent Phosphorus (P) release from sediment is a key process affecting the effectiveness of eutrophication mitigation. We hypothesized that high nitrate (NO 3 −) input may have dual effect on sediment P release: reduce the sediment P release by improving the oxidation of sediment or promote P release by stimulating the growth of phytoplankton and increase the decomposition rates and oxygen consumption at the sediment water interface. To test the effect of different NO 3 − concentrations, we conducted a three-month experiment in 15 cement tanks (1 m3), with five targeted concentrations of NO 3 −: control, 2 mg L−1, 5 mg L−1, 10 mg L−1, and 15 mg L−1. The results showed that: i) when NO 3 − was maintained at high levels: NO 3 −≥5–7 mg L−1 (range of median values), there was no effect of NO 3 − on net P release from the sediment, likely because the positive effects of NO 3 − (increasing oxidation) was counteracted by a promotion of phytoplankton growth. ii) after NO 3 − addition was terminated NO 3 − dropped sharply to a low level (NO 3 −≤0.4 mg L−1), followed by a minor P release in the low N treatments but a significant P release in the high N treatments, which likely reflect that the inhibition effect of NO 3 − on P release decreased, while the promotion effects at high NO 3 − concentrations continued. The results thus supported our hypotheses of a dual effect on sediment P release and suggest dose-dependent effect of NO 3 − loading on stimulating P release from the sediment, being clear at high NO 3 − exceeding 5–7 mg L−1. [Display omitted] [ABSTRACT FROM AUTHOR]

• N-induced p release was most pronounced in summer and from sediment rich in organic matter. • Sediment acidification by long-term n application partly explained the p release. • A positive feedback loop of "P-algae-fish" was found. • In this loop, algae-stimulated fish-biomass increased disturbance, promoting p release. The unprecedented global increase in the anthropogenic-derived nitrogen (N) input may have profound effects on phosphorus (P) dynamics and may potentially lead to enhanced eutrophication as demonstrated in short-term mesocosm experiments. However, the role of N-influenced P release is less well studied in large-scale ecosystems. To gain more insight into ecosystem effects, we conducted a five-year large-scale experiment in ten ponds (700–1000 m2 each) with two types of sediments and five targeted total N concentrations (TN) by adding NH 4 Cl fertilizer (0.5, 1, 5, 10, and 25 mg N L−1). The results showed that: (ⅰ) The sediment P release increased significantly when TN exceeded 10–25 mg N L−1. (ⅱ) The most pronounced sediment P release increase occurred in summer and from sediments rich in organic matter (OM Sed). (ⅲ) TN, algal biomass, fish biomass, non-algal turbidity, sediment pH, and OM Sed were the dominant factors explaining the sediment P release, as suggested by piecewise structural equation modeling. We propose several mechanisms that may have stimulated P release, i.e. high ammonium input causes a stoichiometric N:P imbalance and induce alkaline phosphatase production and dissolved P uptake by phytoplankton, leading to enhanced inorganic P diffusion gradient between sediment and water; higher pelagic fish production induced by the higher phytoplankton production may have led increased sediment P resuspension through disturbance; low oxygen level in the upper sediment caused by nitrification and organic decomposition of the settled phytoplankton and, finally, long-term N application-induced sediment acidification as a net effect of ammonium hydrolysis, nitrification, denitrification; The mechanisms revealed by this study shed new light on the complex processes underlying the N-stimulated sediment P release, with implications also for the strategies used for restoring eutrophicated lakes. [Display omitted] [ABSTRACT FROM AUTHOR]