Silicon composition and stoichiometric ratios with other nutrients in the lower Minjiang River, Southeast China.

Over the past decades, rivers have delivered imbalanced nutrient loads to coastal marine ecosystems due to human activities, which leads to serious regional or global eutrophication problems. The Minjiang River is heavily influenced by human activities. To understand the changing characteristics of nutrient transport ratios in the Minjiang River waters, we measured the seasonal variations of carbon, nitrogen, phosphorus and silicon nutrients in the lower surface waters of the Minjiang River between July 2019 and July 2020. The results showed that the annual average contents of dissolved silicon (DSi), lithogenic silicon (LSi) and biogenic silicon (BSi) in the surface waters of the lower Minjiang River were 5.30, 4.58 and 2.37 mg·L-1, respectively. There were large seasonal differences among these parameters, with higher content of DSi than LSi and BSi in summer, higher content of DSi than BSi and LSi in autumn and higher content of LSi than DSi and BSi in winter. The proportions of DSi in total silicon tended to decrease gradually from land to sea, while the proportion of BSi was on the contrary. In term of stoichiometric ratios, the Minjiang River mostly presented carbon or phosphorus limitation and was unlimited by silicon or nitrogen. About 1.03×1010 mol DSi and 0.46 ×1010 mol BSi were delivered via the Minjiang River to the ocean yearly, showing a decreasing trend year by year. Based on the data in recent years, the nutrient loads of carbon, nitrogen, and phosphorus transported by Minjiang River showed an increasing trend. The imbalanced nutrient loads may lead to changes in the structure and function of the river, estuary, and offshore ecosystems. The study of nutrient stoichiometric ratios can provide a theoretical basis for solving the problems in structural balance of nutrients and eutrophication in Minjiang River estuary and adjacent marine waters.

[Influences of tide on silicon and nitrogen contents in soil and porewater in the Minjiang Ri-ver estuary, Southeast China]. / æ½®æ±å¯¹é—½æ±Ÿå£æ„Ÿæ½®æ¹¿åœ°å­”éš™æ°´åŠåœŸå£¤ä¸­ç¡ ã€æ°®æµ“åº¦çš„å½±å“.

Taking Shanyuntan wetland in the Minjiang River estuary as test object, the dissolved silicates (DSi) and inorganic nitrogen contents in porewater and the biogenic silica (BSi) and total nitrogen contents in surface soil of the Phragmites australis wetland, Cyperus malaccensis wetland and Spartina alterniflora wetland were measured in October 2014 (spring tide month) and April 2015 (neap tide month), respectively, to illuminate the influence of tide on silicon and nitrogen contents in soil and porewater of estuarine wetland. Results showed that the DSi content in porewater and the BSi content in surface soil in spring tide month were slightly higher than those in neap tide month, with the highest being observed on neap tide day and the lowest occurring on spring tide day. In contrast, the BSi content in surface soil on spring tide day showed an opposite trend with that on neap tide day. The contents of NH4+-N and NO3--N in porewater of different wetland soils in spring tide month were higher than those in neap tide month, while the content of NH4+-N on spring tide day was significantly higher than that on neap tide day (P<0.05). The study found that hydrological conditions such as flooding duration and drying-wetting alternation caused by tide had great influences on silicon and nitrogen contents in porewater and surface soil, and vegetation types also showed great influences on their distributions in intertidal wetland of the Minjiang River estuary.

[Modelling nitrogen and phosphorus transfer in Potamogeton malaianus Miq. decompostion].

Potamogeton malaianus Miq. is one of the dominant species of submerged aquatic vegetations in Lake Taihu, China. The decomposition of its debris and metabolic detritus is an important part of nutrients cycling in the lake water. Nitrogen and phosphorus transfer model in P. malaianus Miq. decomposition has been set up based on an indoor P. malaianus Miq. decomposition experiment to quantitatively characterize the decomposition process. It mainly focuses on the dissolving process of inorganic nitrogen and phosphorus in P. malaianus Miq., the degradation process of its organic nitrogen and phosphorus, and the boundary's adsorbing process of nitrogen and phosphorus in water. There are eight state variables in the model, including inorganic and organic nitrogen in P. malaianus Miq., inorganic and organic phosphorus in P. malaianus Miq., total nitrogen and total phosphorus in water, and nitrogen and phosphorus adsorbed on container boundary. The model calibration showed a good accordance with the observed results of P. malaianus Miq. decomposition experiment. The dissolve rates of inorganic nitrogen and phosphorus in P. malaianus Miq. are 0.04 d(-1) and 0.06 d(-1) respectively. And the decompose rates of these two state variables are 0.005 25 d(-1) and 0.010 44 d(-1) respectively. Model outputs show that 6.7% nitrogen and 35.8% phosphorus can release from P. malaianus Miq. in the former 5 days. Phosphorus release is prior to nitrogen due to the bigger inorganic/organic ratio of phosphorus than that of nitrogen in P. malaianus Miq., Decomposition of P. malaianus Miq. could be affected by water temperature, and the affection is slight when water temperature is lower according to the model. The model also showed that P. malaianus Miq. decomposition process has influences on water quality in the former days, which can be eliminated by adsorbing process later.

[Effects of elevated CO2 concentration on the growth of submerged macrophyte Potamogeton malaianus in Taihu Lake].

An outdoor culture experiment was conducted to study the growth and nutrient contents of submerged macrophyte Potamogeton malaianus in Taihu Lake under different CO2 concentrations. As affected by elevated CO2 concentration (1000 micromol x mol(-1)), the biomass per plant increased by 44.3% (P < 0.01), but the shoot biomass decreased by 5.5% (P < 0.05). The N content in root and leaf decreased by 18.1% (P < 0.05) and 6.4% (P < 0.05), respectively, but that in stem was less affected (P > 0.05). The P content in root, shoot, and leaf increased by 22.2% (P < 0.05), 26.6% (P > 0.05) and 38.8% (P < 0.05) , soluble sugar content increased by 27.3%, 18.3% and 37.5% (P < 0.05), and total C content increased by 4.6%, 5.3% and 2.0%, respectively. Elevated CO2 concentration decreased the N and P concentrations in water body by 7.9% and 5.1% (P < 0.05), respectively, but had less effects on the N and P contents in sediment. It was suggested that elevated CO2 concentration had definite effects on the growth and habitat of submerged macrophytes.

[Spatial distribution characteristics and environmental effect of N and P in water body of Taihu Lake].

The characteristics of spatial distribution of nitrogen and phosphorus in water were studied. The results showed that concentrations of total nitrogen/phosphorus in water from Meiliang Bay, Zhusan Bay, Gonghu Bay, West and Central Taihu Lake were obviously higher than those from any other region of Taihu Lake. The spatial distribution of dissolved nitrogen/phosphorus and detritus nitrogen/phosphorus was consistent with that of total nitrogen/phosphorus. The concentrations of dissolved nitrogen and detritus nitrogen significantly correlated with those of total nitrogen with rDN = 0.8192 and rDeN = 0.6969, respectively; the concentrations of dissolved and detritus phosphorus in water also significantly correlated with that of total phosphorus with rDP = 0.7477 and rDeP = 0.9260, respectively. The discrepancy in spatial distribution of chlorophyll a (Chla) in water was distinctly obvious, the highest level appeared in West Taihu Lake (179.2 microg x L(-1) +/- 25.9 microg x L(-1)), and the lowest in East Taihu Lake (11.3 microg x L(-1) +/- 2.7 microg x L(-1)). There were significantly linear relationships between total nitrogen, detritus nitrogen, total phosphorus, dissolved phosphorus, detritus phosphorus, permanganate index, pH value and suspended solids and the content of Chla with rTN = 0.6622, rDeN = 0.8739, rTP = 0.8130, rDP = 0.4077 and rDeP = 0.8781, rCOD = 0.8689, rpH = 0.5173 and rSS = 0.5334, respectively; while dissolved nitrogen, electric conductivity and alkalinity did not affect Chla in water from Taihu Lake.