Effects of daily nitrogen and phosphorus input on planktonic community metabolism in a semi-enclosed bay by mesocosm experiment

Planktonic metabolism plays an important role in affecting the energy transportation and carbon cycle of the marine ecosystem.However,its regulation mechanism remains unclear under the continuously exogenous nutrient inputs in nearshore waters.In this study,a mesocosm experiment was conducted in a semi-enclosed bay,the Daya Bay,to explore the responses of plankton metabolic balance and community structure to a concentration gradient of daily nitrogen and phosphorus inputs.The results showed that nutrient enrichments promoted phytoplankton biomass,total primary production,and community respiration,and the promoting effect enhanced alongwith the increase of nutrient concentration.However,the net community production fluctuated more violently between autotrophic and heterotrophic with the increase of nutrient inputs and tended to be more heterotrophic from the 5 th day to the 10 th day of the experiment.In addition,the daily flux of nitrogen and phosphorus,2μmol/(L·d)and 0.066μmol/(L·d),respectively,could be regarded as a potential threshold for ecosystem stability and health,since most of the ecological characteristics related to plankton structure and function have undergone significant changes when the nutrient level is higher than that.In the nearshore enclosed or semi-enclosed waters,nutrient from continuous terrigenous input is likely to be concentrated and exceed this level,indicating the ecological risks due to the metabolic unbalance and the deterioration of plankton community structure.

Simulation for Water Quality Management in Inohana Lake Estuary, Japan

The Inohana Lake is a branch lake of the Hamana Lake. The Inohana Lake is an estuary rather than a brackish lake, and has suffered environmental problems such as eutrophication and bottom hypoxic water. In this study, the coupled hydrodynamic and ecological models （eco-hydrodynamic model） were used to construct the strategy for preventing the bottom hypoxic water and improving or recovering the water quality in the lake. Using the model input obtained from the summertime data over 1998-2002, the summer-average flow field and oxygen concentration and budget of the standard run were calculated. Remedial measures used in this study are divided into two parts： the biogeochemical and physical changes in the present situation. For the remedial measures including the biogeochemical changes in the present situation, the simulations considering the reductions of the nutrient inputs from the river, main lake （land） and bottom sediment, and the sediment oxygen demand （SOD） were carried out. For the remedial measures including the physical changes, the 50 and 100 m extensions of the inlet width were considered in the model runs. These simulated results were compared in terms of changes in the dissolved oxygen （DO） concentration and oxygen budget in the bottom layer in the Inohana Lake. There was no significant change in the DO concentration and oxygen stock in the simulations for the reduction of the nutrient inputs from the land and bottom sediment, however increases in those in the simulations for the reduction of SOD. When SOD was reduced by 50%, the bottom DO concentration increased by approximately 2 mg/L and the oxygen stock in the bottom layer increased by 47% comparing the present situation （the standard run） of the lake. The simulation results for inlet width extension showed that the extension of width makes the DO concentration and oxygen stock lower. The remedial measures for the sediment control were proposed to prevent the bottom hypoxia and manage the water quality.

Nitrogen addition increased resistance of resident plant community to Solidago canadensis invasion by altering allelopathic effect

Allelopathy plays an important role in the interaction between invasive and resident plants.Atmospheric nitrogen(N)deposition has become a global problem,but it is unclear whether N enrichment affects the interaction between invasive and resident plants by affecting their allelopathy.Thus,we performed a greenhouse experiment in which the resident plant community was grown under two levels of invasion by S.canadensis(invasion vs.no invasion)and fully crossed with two levels of allelopathy(with or without adding activated carbon)and two levels of N addition(with or without).The resident plant communities were constructed with eight herbaceous species that often co-occur with S.canadensis.The results showed that both allelopathy of S.canadensis and the resident plants had obvious positive effects on their own growth.Nitrogen addition had more obvious positive effects on the resident plants under invasion than those that were not invaded.Moreover,N addition also altered the allelopathy of resident plants.Specifcally,N addition improved the allelopathy of resident plants when they were invaded but decreased the allelopathy of resident plants when they grew alone.Although nitrogen addition had no obvious effect on S.canadensis,it reduced the allelopathy of S.canadensis.These results suggest that N addition could improve the resistance of resident plants to invasion by improving the allelopathy of resident plants and reducing the allelopathy of S.canadensis.These fndings provide a scientifc basis to manage and control the S.canadensis invasion.

Deposition of Nutrients From Harmattan Dust in Ghana, West Africa

In order to measure dust's nutrient input on farmland in different agro-ecological zones, Harmattan dust was sampled by mats with plastic straw in Ghana between 2002–2006. The inputs of total nutrients by Harmattan dust in Ghana per Harmattan period were about 1–2 kg Ca ha-1, 0.5–2 kg K ha-1, 0.5–1.5 kg Mg ha-1and less than 0.5 kg P ha-1. Compared with the annual input of nutrients by precipitation, the dust accounted for 10% or less of Ca, Mg and K but approximately 20%–40% of P. The input of nutrients by dust was only valid for areas with vegetation, because in areas with none or sparse vegetation, loss of soil due to wind erosion and hereby loss of nutrients might be significant. In farmland areas with bare and vegetated fields there seemed to be an internal redistribution of the nutrients and not a net gain of nutrients from outside the area(long-range transported dust). The input of P by dust might be of some importance in the traditional shifting cultivation systems, while the inputs of other three nutrients of Ca, Mg and K were so low that they must be considered insignificant. In the intensive agriculture systems with huge inputs of manures and fertilizers the nutrient input by dust is insignificant and could be neglected.

Broussonetia papyrifera controls nutrient return to soil to facilitate its invasion in a tropical forest of Ghana

Aims Non-native invasive plants can alter soil chemistry through litter production and decomposition to facilitate their invasion.However,the important roles of these underlying processes in plant invasion remain poorly understood,particularly in tropical forest ecosys-tems.Here,we compared litter production,quality and decompos-ition of two invasive species(Broussonetia papyrifera and Cedrela odorata)and two co-occurring native species(Celtis mildbraedii and Funtumia elastica),and soil properties under them to elucidate their roles in the invasion of a tropical forest in Ghana.Methods Leaf litter production rates were determined using 36 mesh traps installed in the study area,while litter quality and soil physico-chemical properties were determined using standard protocols.A 6-month decomposition experiment using the litterbag technique was conducted to compare the decomposition rates of the species.Important Findings Litter production varied among the species and over time,with B.papyrifera producing 0.35-4.27 tons ha−1 y−1 from October to January;the other species produced 0.03^(-1).74 tons ha−1 y−1 over the same period.In the litterbag experiment,B.papyrifera recorded the lowest mass remaining(11-36%),followed by C.odorata(17-51%),F.elastica(31-55%)and C.mildbraedii(48-62%)in that order.Broussonetia papyrifera had the highest nitrogen(3.91%)and phosphorus(0.24%)but lowest lignin(12.20%)concentrations and the lowest C:N(10.87)ratio,indicating higher litter quality com-pared to the other species.Soil under B.paprifera was richest in phosphorus and nitrogen compared to the other species.Overall,our results indicate that the production of more nutrient-rich and rapidly decomposing leaf litter by B.papyrifera may constitute an important positive feedback mechanism driving its invasion and impacts in this tropical forest.