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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. [Display omitted] • Light decreased internal phosphorus release (L Int) under eutrophic condition. • Benthic algae promoted by light directly absorbed phosphorus and decreased L Int. • Higher benthic primary production indirectly inhibited L Int by enhancing dissolved oxygen. • Direct and indirect effects contributed 23-69% and 31-77% to the L Int , respectively. [ABSTRACT FROM AUTHOR]

Highlights from the article: Although P is the common limiting factor of land and wetland plants [6], it is almost impossible to control weeds and invasive plants through reducing P supply, while chemical, mechanical and biological control is widely used. In the Experimental Lakes Area (ELA) of Canada (49°N), long-term fertilization experiments in Lake 304 (1971-1976), Lake 226 (1973-1980) and Lake 302 (1982-1986) [1] have proved that P-only reduction can control algae blooms and eutrophication. 2a); the heterocyst density in the +P pond was 4.5 times as much as that in the +N + P pond, and TN in the former was close to that in the latter from the 4th month [3]. Considering that very high N levels (5 mg L SP -1 sp -10 mg L SP -1 sp ) have some stresses on organisms (submerged macrophytes, fishes, etc.) and can promote sediment phosphorus release [13], we advocate loosening N control and focusing on P abatement in eutrophication mitigation so as to reduce costs substantially.

There is a heated debate over the necessity of nitrogen (N) reduction, in addition to phosphorus (P) reduction, for the control of eutrophication. Whole-lake fertilization experiments and lake restoration practices in high latitudes have demonstrated that P is the primary factor regulating total phytoplankton. Recognizing the limited large-scale evidence in warmer climatic zones, a fertilization experiment was conducted in four ponds located in the subtropical Yangtze River Basin, China. Total phytoplankton abundance in a pond receiving P (+P) was similar to that in a pond receiving both N and P (+N+P). Both had higher phytoplankton than a pond receiving no additional nutrient (Control). Total nitrogen concentration (TN) in the +P pond increased with the appearance of N-fixing cyanobacteria. Total phytoplankton abundance was similar in the ponds without P addition (+N, Control), and both ponds had lower phytoplankton levels than the +N+P pond. These results showed that P, not N, determines total phytoplankton abundance and that N deficiency is offset by N fixation in subtropical lakes. This experiment supports the idea that attention should be mainly focused on P reduction in mitigating eutrophication. [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]

Urban lakes are important natural assets but are exposed to multiple stressors from human activities. Submersed macrophytes, a key plant group that helps to maintain clear-water conditions in lakes, tend to be scarce in urban lakes, particularly when they are eutrophic or hypertrophic, and their loss is linked, in part, to impaired underwater light climate. We tested if enhancing the underwater light conditions using light-emitting diodes (LEDs) could restore submersed macrophytes in urban lakes. Twelve mesocosms (1000 L each) were each planted with tape grass (Vallisneria natans) and monitored over three months (22 August–7 November), using a control and three artificial light intensity treatments (10, 50, and 100 µmol m-2 s-1). Compared with the control, the high light treatment (100 µmol m-2 s-1) had higher leaf number, maximum leaf length, and average leaf length (3.9, 5.8, and 2.8 times, respectively). Shoot number, leaf number, leaf dry mass, root dry mass, and photosynthetic photon flux density in the high-light treatment were significantly greater than the control, but root length and phytoplankton chlorophyll a were not related to plant growth variables and were low in all treatments. Periphyton chlorophyll a increased significantly with the plant growth variables (i.e., shoot number, leaf number, and maximum leaf length) and was high in the light treatments but did not hamper the growth of the macrophytes. These results indicate that LED light supplementation enables the growth of V. natans under eutrophic conditions, at least in the absence of fish as in our experiment, and that the method may have potential as a restoration method in urban lakes. Lake-scale studies are needed, however, to fully evaluate LED light supplementation under natural conditions where other stressors (e.g., fish grazing) may need to be controlled for successful restoration of urban lakes. [Display omitted] • Light addition could enable macrophyte growth in eutrophic waters. • Light addition did not promote growth and shading effects of phytoplankton. • Light addition enabled periphyton growth, but with little shading effects on plants. [ABSTRACT FROM AUTHOR]

Eutrophication of lakes is becoming a global environmental problem, leading to, among other things, rapid reproduction of phytoplankton, increased turbidity, loss of submerged macrophytes, and the recovery of these plants following nutrient loading reduction is often delayed. Artificial light supplement could potentially be a useful method to help speeding up recovery. In this study, three common species of submerged macrophytes, Vallisneria natans, Myriophyllum spicatum and Ceratophyllum demersum, were exposed to three LED light treatments (blue, red and white) and shaded (control) for 100 days (from 10 November 2016 to 18 January 2017) in 12 tanks holding 800 L of water. All the three LED light treatments promoted growth of the three macrophyte species in terms of shoot number, length and dry mass. The three light treatments differed in their effects on the growth of the plants; generally, the red light had the strongest promoting effects, followed by blue and white. The differences in light effects may be caused by the different photosynthetic photon flux density (PPFD) of the lights, as indicated by an observed relationship of PPFD with the growth variables. The three species also responded differently to the light treatments, V. natans and C. demersum showing higher growth than M. spicatum. Our findings demonstrate that artificial light supplement in the low-growth winter season can promote growth and recovery of submerged macrophytes and hence potentially enhance their competitiveness against phytoplankton in the following spring. More studies, however, are needed to elucidate if LED light treatment is a potential restoration method in small lakes, when the growth of submerged macrophytes are delayed following a sufficiently large external nutrient loading reduction for a shift to a clear macrophyte state to have a potential to occur. Our results may also be of relevance when elucidating the role of artificial light from cities on the ecosystem functioning of lakes in urban areas. [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]