Impact on lake development of changed agricultural watershed exploitation during the last three centuries

The history of the development of Lake Hejrede Sø (Denmark) and the related history of the watershed management were studied based on analyses of macrofossil content, chemical composition and Clostridium perfringens content in sediment cores. Depth-age relations of ecological changes were established through Pb²¹⁰ analyses, and a systematic search for written sources describing the watershed history was carried out.Lake Hejrede Sø used to be a humic, acid but relatively clearwater lake with an extensive submersed vegetation. Owing to deforestation and reclamation of bogs and meadows, the accumulation of nutrients in these ecotones vanished. Subsequently, in early 1800 the lake developed into a clearwater alkaline lake, and later became more eutrophic which resulted in the decline of the submersed vegetation. At the time the annual sediment phosphorus accumulation increased more than twofold, and influx of mineral matter increased drastically. In mid-1900, submersed vegetation had disappeared. The Clostridium perfringens analysis and the historical evidence show that no sewage was discharged to the lake until mid-1900, which implies that changed agricultural watershed exploitation and ecotone reclamation during the 19th century was the cause of the increased nutrient influx, and the deterioration of the lake.Written historical sources confirm the palaeolimnological interpretations and describe the agricultural practices responsible for the development.     

A Century of Change in Macrophyte Abundance and Composition in Response to Agricultural Eutrophication

Clear Lake, Iowa, USA is a shallow, agriculturally eutrophic lake that has changed drastically over the past century. Eight macrophyte surveys since 1896 were pooled and examined to characterize long-term impacts of eutrophication on macrophyte community composition and relative abundance. Surveys in 1981 and 2000 revealed few submergent and floating-leaved species and a dominance in emergent species (Scirpus, Typha). Over the past century, however, species richness has declined from a high of 30 species in 1951 to 12 found today, while the community composition has shifted from submergent-(99%) to emergent-dominated floras (84%). Potamogeton praelongus was the first emergent species to disappear but was followed by several other clear water Potamogeton species. Several floating leaved and emergent genera increased in relative abundance with eutrophication, notably Nuphar, Nymphaea, Phragmites, Polygonum, Sagittaria, Scirpus, and Typha. P. pectinatus was present over the entire century due to its tolerance of eutrophic conditions. Macrophyte growth was generally light-limited, with 93% of the variance in relative abundance of submergent species explained by changes in water transparency. Clear Lake exhibits signs of alternative stable states, oscillating between clear and turbid water, coupled with high and low submerged species relative abundance. The maximum macrophyte richness occurred as the lake oscillated between submergent- and emergent-dominated states. Changes in the water level have also impacted macrophyte growth since the area of the lake occupied by emergent macrophytes was negatively correlated with water level. Strongest correlations indicated that macrophytes respond to water level variations with a 2-year time-lag.     

The occurrence of internal phosphorus loading in two small,eutrophic, glacial lakes in Northeastern Ohio

Internal loading of phosphorus for the summers of 1972–1974 in the eutrophic Twin Lakes, Ohio, USA was calculated from nutrient budgets, and was found to account for 65–100% of the increase in phosphorus content of the lakes during this period. Recovery of lakes of this type after nutrient diversion may be delayed by internal loading and chemical inactivation of phosphorus may be needed. A discussion of sources of this internal loading is presented.     

Effects of the Sand Bar Breaching on Typha domingensis (PERS.) in a Tropical Coastal Lagoon

Coastal lagoons are usually subjected to several kinds of human impacts, especially eutrophication. The breaching of the sand bar, which separates the lagoon from the ocean, by human action, is a common process used to decrease the negative effects of eutrophication. The aims of this research were to evaluate the effects of the artificial sand bar breaching on the populations of the aquatic macrophyte Typha domingensis and the subsequent effects on nutrients concentration in a tropical coastal lagoon. Samplings were carried out monthly from February/01 to January/02 in a monospecific stand of T. domingensis at Imboassica lagoon (Rio de Janeiro/Brazil). Two sampling sites, in the middle and in the border of the stand, were marked in three different transects. Water depth was measured and the aerial biomass sampled with a 0.25 m² quadrat. The macrophyte samples were separated into live and dead material and the shoot length, shoot density and number of leaves were analyzed. All plant material was oven dried till constant weight and net primary production, dead stand crop production and the nutrients release through decomposition were estimated at each site. The decrease in water level due to sand bar opening affected negatively T. domingensis populations, but the most intense effects were observed in the middle of the stand. The shoot mortality was highly enhanced after the sand bar breaching and the nutrients were released through decomposition to the water column. It reflected on an input of 11.5 kg C m⁻², 0.22 kg N m⁻² and 0.13 kg P m⁻² into the lagoon, which represent from 22.5 to 44.8 ton P and 35.9 to 71.8 ton N to the lagoon. The decrease of nutrients concentration after the sand bar breaching was not successfully accomplished. The decay of T. domingensis stands due to the sand bar breaching neutralized the exportation of nutrients to the ocean, and contributed to the phosphorous increase in the water column. Thus, to a better management of aquatic ecosystems subjected to human eutrophication, the role of aquatic macrophytes decomposition on internal fertilization of aquatic ecosystems should be accounted.     

Nitrate availability and hydrophyte species richness in shallow lakes

1. Submerged plant richness is a key element in determining the ecological quality of freshwater systems; it has often been reduced or completely lost. 2. The submerged and floating‐leaved macrophyte communities of 60 shallow lakes in Poland and the U.K. have been surveyed and species richness related to environmental factors by general linearised models. 3. Nitrogen, and more specifically winter nitrate, concentrations were most important in explaining species richness with which they were inversely correlated. Phosphorus was subsidiary. Such an inverse relationship is consistent with findings in terrestrial communities. Polish lakes, with less intensively farmed catchments, had greater richness than the U.K. lakes. 4. The richest U.K. communities were associated with winter nitrate‐N concentrations of up to about 1–2 mg L^?1 and may correspond with ‘good’ ecological quality under the terms of the European Water Framework Directive. Current concentrations in European lowlands are often much higher.     

Evaluation of a eutrophic coastal lagoon ecosystem from the study of bottom sediments

Morphological reconstruction and biogeochemical characterisation of the lagoon of Comacchio (Italy) were carried out in order to provide recommendations for the recovery, conservation and sustainable management of the Fattibello-Spavola coastal lagoon ecosystem. Samples were taken in two seasons: July 1997 and November 1998. The irregular morphology of the Fattibello lagoon affects depositional processes and seawater exchange (tidal currents). Several depressions retain part of the dense water of the saline wedge; these stagnant waters became sinks for fine terrigenous and organic matter. The basin is already extensively supplied with N and P compounds from land. The inflow is demonstrated by the large quantity of organic C, N and organic and inorganic P compounds in the superficial sediment. The ratios between the various forms of macronutrients indicate that the organic matter is primarily of autochthonous origin, with relatively low C/N ratios (8.4±0.6 and 8.1±0.6 in July and November, respectively). Shallow areas were almost always oxygenated by tidal currents and thus rich in organisms, with a predominance of molluscs and Ficopomatus enigmaticus. However, the trophic equilibrium of the ecosystem was affected by the accumulation of organic matter in the depressions, favoured by the increased hydrodynamics during the autumn. These accumulations generate high oxygen consumption and release considerable quantities of nutrients into the water column, with the risk of serious dystrophy throughout the basin during the summer. Local dredging and an improvement of the circulation have been suggested and carried out to contain these processes. Reclamation measures in the longer term were proposed.     

The organic carbon budget of a shallow arctic tundra lake on the Tuktoyaktuk Peninsula,N.W.T., Canada

The organic carbon cycle of a shallow, tundra lake (mean depth 1.45 m) was followed for 5 weeks of the open water period by examining CO₂ fluxes through benthic respiration and anaerobic decomposition, photosynthesis of benthic and phytoplankton communities and gas exchange at the air-water interface. Total photosynthesis (as consumption of carbon dioxide) was 37.5 mmole C m^–2 d^–1, 83% of which was benthic and macrophytic. By direct measurement benthic respiration exceeded benthic photosynthesis by 6.6 mmole C m^–2 d^–1. The lake lost 1.4 × 10⁶ moles C in two weeks after ice melted by degassing C0₂, and 6.8 mmole C m^–2 d^–1 (1.5 × 10⁶ moles) during the remainder of the open water period; 2.2 mmole C m² d^–1 of this was release Of CO₂ stored in the sediments by cryoconcentration the previous winter. Anaerobic microbial decomposition was only 4% of the benthic aerobic respiration rate of 38 mmole C m^–2 d^–1. An annual budget estimate for the lake indicated that 50% of the carbon was produced by the benthic community, 20% by phytoplankton, and 30% was allochthonous material. The relative contribution of allochthonous input was in accordance with measurement of the ¹⁵N of sedimented organic matter.     

An assessment of the importance of emergent and floating-leaved macrophytes to trophic status in the Loosdrecht lakes (The Netherlands)

The potential importance of the six major emergent and floating-leaved macrophyte species in recycling of sediment phosphorus in the Loosdrecht lakes was studied. Representative plant samples were collected at the time of maximum biomass, and analysed for biomass and carbon, nitrogen and phosphorus contents. Species cover was determined by aerial photography.Total cover in the seven lakes studied ranged between 2 and 26 percent. For the four main species, biomass per unit area increased with lake trophic status. Consistent differences in C, N and P contents per unit biomass were not observed. Although cover values were small, significant amounts of C, N and P were contained in the macrophytes when compared with maximum sestonic content.Potential P loads from macrophyte decay were calculated. In Lake Loosdrecht, the P load represented 15 percent of current external P inputs. The potential importance of macrophyte decay to P recycling in the other lakes is greater.Decay of macrophyte species at the end of the growing season appears to affect autumnal nutrient and chlorophyll a levels in the water column of some lakes. The re-establishment of submerged species following lake restoration may increase the importance of this pathway in the lakes.     

Oxygen Budget of a River Rich in Submerged Macrophytes (River Zschopau in the South of the GDR)

The oxygen budget of the river Zschopau, a flowing water typical of hilly areas in the south of the GDR, was investigated in regard of seasonal variations in the activity of submerged macrophytes. Oxygen concentrations were continuously measured in the river by means of electrodes. Seasonal activity patterns of Ranunculus penicillatus, the dominant macrophyte in the river stretch investigated, were found by laboratory experiments. The oxygen budget was obtained by measuring CO₂ concentrations by infrared gas analysis (IRGA) and CO₂ and O₂ concentriations by means of electrodes.     

Sediment phosphorus release in phytoplankton dominated versus macrophyte dominated shallow lakes: importance of oxygen conditions

Increased discharges of organic matter from different sources in Morales Stream, one of the main tributaries of the Matanza-Riachuelo River, caused not only an increase in its primary production but also drastic changes in the composition of its sediments, thus favoring eutrophication processes. An in situ study was carried out in order to assess the effects of an organic point source contamination (from intensive cattle rearing) on the sediments of Morales Stream. Surface water and sediment samples were analysed to determine the chemical characteristics of the water–sediment system. The amounts and forms of sediment phosphorus were determined using the `EDTA method' (Golterman, 1996) at two sites of the stream having different nutrient loads. The increase in the organic load of Morales Stream waters influences the dynamics of sediment P, producing two main effects: (1) an increase in the organic matter amount of the sediment that leads to an increase in the amount of P associated to organic fractions, which may be released by bacterial activity under anoxic conditions; and (2) a decrease in the concentration of P in the fraction bound to iron. Morales Stream sediments may act as a potential source of P, which can release this nutrient to water under the reducing conditions originated by uncontrolled discharges of organic residues to this water body.     

亚热带杉木林土壤有机碳及其活性组分对氮磷添加的响应

通过野外氮、磷添加,分析N₀(0 kg N·hm^-2·a^-1)、N₁(50 kg N·hm^-2·a^-1)、N₂(100 kg N·hm^-2·a^-1)、P(50 kg P·hm^-2·a^-1)、N₁P和N₂P等6种处理3年后对亚热带杉木人工林土壤有机碳(SOC)、颗粒有机碳(POC)和水溶性有机碳(WSOC)的影响.结果表明:氮、磷添加对0～20 cm土层SOC含量无显著影响.磷添加显著降低0～5 cm土层POC含量,与无磷处理相比,加磷处理POC含量降低26.1%.WSOC含量对氮、磷添加的响应主要表现在0～5 cm土层,低水平氮添加和磷添加显著提高WSOC含量.在0～5 cm土层,氮添加对POC/SOC值无显著影响,而与无磷添加相比,POC/SOC值在磷添加处理下显著降低15.9%.在5～10和10～20 cm土层,氮、磷添加处理对POC/SOC值无显著影响.在亚热带地区,森林土壤碳稳定性主要受磷含量的调控,短期磷添加易导致表层土壤活性有机碳分解,增加土壤碳稳定性.     

The uptake,translocation and release of phosphorus by Elodea densa

Short-term (16 h) laboratory studies of ³²P uptake by Elodea densa rooted in sediment demonstrated both foliar and root uptake, and that translocation occurred acropetally and basipetally. Root absorption is projected to provide 83–85% of total phosphorus uptake during 12–16 h photoperiod days. Measured foliar uptake and excretion rates suggest that there would be no net leakage of phosphorus into the water by undamaged actively-growing E. densa. Foliar uptake decreased and root uptake increased in the dark relative to rates under light.     

Modeling as a tool to manage ecosystems under multiple stresses: an application to Lake Ontario

Present-day ecosystem management involves understanding of the synergistic effect of multiple stressors on multiple and frequently nebulous management end-points. An example is the simultaneous management of nutrient load reductions and salmon stocking in Lake Ontario. In this study, a simple whole-lake annual time scale model was developed to assess the relationship between these two stressors and various ecosystem responses. The model was used to explore the utility of some possible management end-points for ecosystem health. In historical simulations, production per stocked fish and salmon survival appeared to be good indicators, while nutrient recycling rate and average ecosystem-wide food limitation were found to be fairly unresponsive to the two stressors. The model was further used to predict long term averages of salmon biomass and selected health indicators at various sustained loading and stocking rates. Salmon biomass increased with stocking rate at all stocking rates examined, but the rate of increase declined somewhat at high stocking rates. The response of salmon biomass to nutrient loading appeared to be approximately sigmoidal i.e. there was a nutrient threshold below which fish biomass could not be sustained and another nutrient threshold above which salmon biomass either remained constant or even decreased. The response to either stressor was found to be modified by the value of the other stressor, illustrating the importance of ecosystem-level models for aquatic ecosystem management.