Decrease In Water Transparency Research Articles

Temporal Changes in the Physico chemical Composition of Lake Geneva from 1957 to the Present

This manuscript presents an exhaustive analysis of a physicochemical dataset collected from Lake Geneva since 1957, offering a detailed exploration of its long-term environmental evolution. This study encompasses an array of cycles and trophic states that the lake has undergone. A critical finding is the significant influence of human activities within the catchment area, notably the introduction of substantial quantities of nutrients, especially phosphorus, into the lake’s ecosystem. This anthropogenic impact culminated in a marked ecological decline between the 1960s and 1970s, characterized by a phase of eutrophication. This period saw a surge in algal blooms and a consequent decrease in water transparency. In the 1970s, strategic interventions were implemented to address this environmental challenge. These measures, including the enhancement of phosphorus treatment at sewage facilities in the catchment area and the nationwide ban on phosphated detergents in Switzerland, proved instrumental in reducing the influx of nutrients. Consequently, a significant reduction in the lake’s phosphorus levels was observed by the 1980s. The lake is currently undergoing a re-oligotrophication process, with phosphorus concentrations indicating a mesotrophic state. However, it is noteworthy that the biological components of the lake are exhibiting a slower recovery rate in adapting to reduced nutrient levels, in contrast to the more immediate response noted during the eutrophication phase. Despite the progress achieved, ongoing vigilance and efforts are essential to continuously monitor, identify, and manage potential sources of phosphorus pollution. This proactive approach is vital for maintaining the lake’s ecological balance and ensuring it remains free from significant algal proliferation.

Tree dieback and subsequent changes in water quality accelerated the climate-related warming of a central European forest lake

Abstract The water temperature of many lakes has recently risen as a result of climate change. We evaluated trends in the cloudiness, solar radiation, wind, air and water temperatures, ice cover, thermocline depth, transparency, and composition of two Bohemian Forest lakes (Czech Republic) from 1998 to 2022. Lake water temperatures increased by 0.32–0.47 °C decade−1, and the ice cover periods decreased by 11.7–14.8 days decade−1. These changes were mostly associated with increasing air temperatures during most months and increasing solar radiation (due to decreasing cloudiness) especially in March and November (the months preceding ice-on/off). Decreasing snow cover in winter (by 3.8 cm decade−1) further accelerated the earlier ice melt. The number of days with water temperature ≥4 °C increased similarly in both lakes by 12–13 days decade−1. However, the number of days with water temperature ≥20 °C increased and the depth of the summer thermocline decreased more in the lake with tree dieback in its catchment. Tree dieback accelerated the leaching of organic carbon and phosphorus, increasing water brownification, algal production, and decreasing water transparency. Solar radiation was absorbed in shallower water layers. Changes in catchment forest thus contributed to the variability in the response of lake water temperatures to climate change.

Community structure, environmental conditions and anthropogenic pressure on the habitat of the European endemic aquatic plant Luronium natans (L.) Raf.

Luronium natans (L.) Raf. is a European endemic species and is becoming increasingly rare and endangered in most countries. This study aimed to compare the community structure and environmental conditions of shallow and deep-water habitats of Luronium, and related anthropogenic influences. A total of 21 Luronium lake habitats were surveyed at Pomerania Lakeland (NW Poland). Luronium occurs mainly with other isoetids, as well as bryophytes, specifically Sphagnum denticulatum. It can also be found in oligotrophic lakes at a depth of 1.0 ± 0.6 m and in water with a large pH range (4.52 – 8.76), as well having a low conductivity (38.3 ± 20.9 µS cm−1; 19.0 – 106.1) and calcium concentration (3.9 ± 2.4 mg dm−3; 1.6 – 11,7).The largest Luronium cover occurs at a depth of 1.5 m (44.8 ± 35.3%), but occasionally as deep as 3.5 m. In the depth gradient, the structure of underwater vegetation and environmental conditions exhibit obvious changes, which presents a clear distinction between shallow and deep-water habitats of Luronium. The differences mainly pertain to the abundances of Isoëtes lacustris and Elodea canadensis in the community, as well as environmental factors, such as water calcium, nitrogen and phosphorus concentrations, PAR, conductivity, and water color.Compared to other isoetids, Luronium usually occurs in habitats with intermediate features, which are characterized by values between the typical, but deep-water, Isoëtes and shallow water Lobelia and Littorella. However, Luronium clearly prefers waters with higher temperatures (23.8 ± 2.7 °C), which are thus less oxygenated (96.6 ± 20.0%). In terms of pH, conductivity, and calcium concentration, Luronium occurs in waters having slightly lower values compared to other isoetids. Therefore, Luronium is a species that significantly expands the diversity of habitat number 3110 in the Natura 2000 network. Therefore, it can be considered as an indicator species of lobelia lakes.An increased anthropogenic pressure primarily results in an increased water conductivity and a decreased water transparency. Consequently, Luronium increasingly inhabits shallower waters that are more oxygenated. Moreover, Luronium abundance is decreasing, while the abundances of species comprising underwater communities are also decreasing, e.g., S. denticulatum and I. lacustris, with a concurrent increase in Myriophyllum alterniflorum and E. canadensis cover.

Correlation Analysis of Various Water Quality Parameters of the Tiru Reservoir, Maharashtra, India

The present study was conducted in the department of Aquatic Environment Management, College of Fishery Science, Udgir, District Latur, Maharashtra, India during August, 2017 to January, 2019 to investigate trophic status of the Tiru reservoir. Twenty (20) water quality parameters had been collected monthly from five sampling sites of Tiru reservoir, Udgir, Latur district, Maharashtra, India. Seasonal trends were analysed and discussed in this chapter. Pearson’s correlation coefficient (r) of eighteen physico-chemical parameters from five study locations were analyzed to know the pollution causing parameters. Water quality analysis showed that Total Phosphorus concentrations was highest in the summer, when reservoir water levels were lowest. Correlation between Total phosphorus and Chl-a was highest at 0.926. Eutrophication was mainly generated by phosphorus available in the water, which increases chlorophyll-a concentration (algal blooms). A positive correlation between temperature and DO might be possible because higher summertime water temperatures boost photosynthetic activity. SDD has shown a substantial negative connection with Chl-a throughout all seasons, since an increase in phytoplankton abundance yields a decrease in water transparency. The negative correlation between SDD and turbidity is primarily caused by the high levels of productivity in the summer, and high levels of suspended organic load due to surface runoff in the monsoon season. The majority of the time throughout the research period, Tiru reservoir was found in the eutrophic state, and in some cases even hyper-eutrophic.

Browning of boreal lakes: Do public perceptions and governance meet the biological foundations?

Browning of surface waters, also known as brownification, is a process of decreasing water transparency particularly in boreal lakes surrounded by intensively managed forests and wetlands. In this paper, we review the ecological consequences and ecosystem-based management of browning by using a systematic review approach and adopt an interdisciplinary approach to build new governance on this complex phenomenon. To understand the effect of browning on recreational values of freshwaters, we present primary survey data on public perceptions of recreational fishing tourists on water quality in Finland. We identify a need to develop ecosystem-based management beyond the EU's Water Framework Directive (WFD) to fully account for the extensive implications of browning. We also detect a need for a better understanding of the within-lake microbial processes to estimate the browning-associated changes in the greenhouse gas balance of lakes. Tourist perceptions of the quality of waterbodies in Finland were largely in agreement with the general proportion of waterbodies classified to a good or excellent ecological status class, but these perceptions may be detached from biological quality assessment criteria. Consequently, we suggest that the ecosystem-based management of inland waters should improve the utilization of information on not only biogeochemical processes but also users' perspectives on aquatic ecosystems beyond the EU WFD.

Manure Acidification and Air Cleaners for Ammonia Abatement: A Holistic Assessment of the Costs and Effects on Terrestrial, Freshwater and Marine Ecosystems

Manure acidification has been introduced as an abatement to reduce ammonia (NH3) emissions to improve air quality and protect terrestrial and aquatic environments from nitrogen deposition. A successful regulation of NH3 emissions using manure acidification might, however, result in increased nitrogen leaching from fertilized fields with adverse effects on freshwater and marine ecosystems, if the overall fertilizer application rate in the fields is not adjusted according to the increased fertilizer value of the manure. We apply a holistic model framework encapsulating all important environmental compartments to assess the ecological and economic consequences of a specific agricultural practice or a combination of these. The results show that manure acidification combined with air cleaners reduces NH3 emission and atmospheric nitrogen deposition with substantial positive effects on the terrestrial environment. Although manure acidification results in a slight increase in total nitrogen input into freshwater and marine ecosystems, the subsequent increase in chlorophyll a concentration and decrease in water transparency is insignificant. Hence, according to the model results, manure acidification will improve terrestrial nature quality, with no significant adverse effects on the aquatic environments.

Impact of the Construction of New Port Facilities on the Biomass and Species Composition of Phytoplankton in the Neva Estuary (Baltic Sea)

The construction of new port facilities and the creation of new lands is a natural consequence of the rapid growth in the population of ocean coastlines. Despite the fact that such human activity is becoming more and more widespread, its impact on various components of aquatic ecosystems, including phytoplankton, is still poorly understood. The aim of the study was to assess the effect of the large-scale construction of new port facilities on biomass and taxonomic composition of phytoplankton in the Neva Estuary (northeastern Baltic Sea). Studies have shown that digging and displacing large amounts of bottom sediments during these works led to a significant increase in suspended mineral matter and a decrease in water transparency in the estuary. This significantly reduced the species richness and biomass of phytoplankton. However, the analysis of beta diversity did not show significant changes in the phytoplankton community during the periods of port construction and the periods when no works were carried out. The changes mainly concerned rare species, while the occurrence and biomass of dominant and subdominant species changed to a lesser extent. Due to various adaptations, the phytoplankton species common in the estuary are apparently able to survive under prolonged shading and successfully compete with species that are unable to withstand the lack of light for a long time. To correctly take into account the effects of the construction of new port facilities on phytoplankton and, if possible, minimize it, additional studies of the ecology of certain phytoplankton species, their relationships, and physiological responses to various environmental factors are required.

Seaward migration of pink salmon Oncorhynchus gorbuscha (Walbaum) fry in the Malaya Khuzi River (North-Eastern Sakhalin) in 2018

Counting pink salmon Oncorhynchus gorbuscha fry, migrating seaward in the Malaya Khuzi River (NorthEastern Sakhalin), revealed the total half-million stock of the migrants in 2018. Forming such generation from extremely poor parental stock, entered the river in 2017 had been possible due to a high survival rate in the course of incubation. Seaward migration was about 1.5 month long and finished 7–10 days earlier than in previous years. Major part of the fry stock emigrated during the I decade of June. Dense aggregations of ice on the seashores during the mass seaward migration could bring negative effects on survival of the fry. Migration occurred at night. Day time migration was observed in case of decreasing water transparency on flooding. The migrants in most cases did not have yolk sack residuals. Mean values of the body length and weight of the migrants did not exhibit extensive changes for the period of migration.

First report about toxic cyanobacterial bloom occurrence in Lake Sevan, Armenia

AbstractLake Sevan, Armenia, is the largest freshwater body in the Caucasus region. Cyanobacteria have become increasingly dominant in summer in Lake Sevan, reflecting the eutrophication of the lake and formed a massive bloom event in 2018. These recent observations mark the transition of this previously oligotrophic high mountain lake into an eutrophic lake with scum‐forming cyanobacterial blooms. A bloom of Dolichospermum in July caused a sudden increase in chlorophyll‐a concentrations up to, on average, 20 µg/L and a strong decrease in water transparency. The cyanobacterial genera Dolichospermum, Aphanizomenon, Anabaena, Cyanobium, and Synechococcus were detected by metagenomic analysis of the lake bacterioplankton. A qualitative and quantitative assessment of peptide‐based secondary metabolites revealed the first detection of cyanotoxins in Lake Sevan. Ten types of microcystin congeners were found in Lake Sevan. The total concentration of microcystins in the phytoplankton varied from 0.34 to 2.49 µg/L. This first record of cyanotoxins in the largest lake in the Caucasus region calls for an urgent need for sustainable nutrient management and a systematic assessment of the ultimate causes that lead to the reoccurrence of scum‐forming Cyanobacteria in this large Alpine lake.

The Effect of Human Impact on the Water Quality and Biocoenoses of the Soft Water Lake with Isoetids: Lake Jeleń, NW Poland

Soft water lakes with isoetids (SLI) are ecosystems prone to degradation due to the low buffer capacity of their waters. One of the main threats resulting from human impact is eutrophication due to agriculture, catchment urbanization and recreational use. In this paper, changes in the water chemistry and transformation of biocoenoses of one of the largest Polish SLI, Lake Jeleń, over the past 30 years are presented. The lake is located within the borders of a city, and a significant part of its catchment is under agriculture and recreation use. The physicochemical (concentration of nutrients, organic matter, electrical conductivity, oxygen saturation and water pH) and biological parameters (macrophytes and phytoplankton) were measured in summer 1991, 2004, 2013 and 2018. Since the beginning of the 1990s, a gradual increase in the trophy of the lake has been observed as indicated by increased nutrient availability, deterioration of oxygen conditions and a decrease in water transparency. The alterations of water chemistry induce biological transformations, in particular, an increase in phytoplankton abundance (4-fold increase of biomass in epilimnion) as well as a gradual reduction in the range of the phytolittoral (from 10 to 6 m), a decrease in the frequency of isoetids, Lobelia dortmanna and Isoetes lacustris, and expansion of plant species characteristic for eutrophy.

Phytoplankton Bloom and Photosynthetically Active Radiation in Coastal Waters

The effect of a phytoplankton bloom on the change with depth of photosynthetically available radiation (PAR), the spectral downwelling irradiance in coastal waters, and the phytoplankton ability to absorb PAR in the sea is estimated. The 10-fold increase in the chlorophyll a concentration (from 0.4 to 4.0 mg/m3) during a diatom/ dinoflagellate bloom leads to a decrease in water transparency and a narrowing of the photosynthesis zone. As a consequence, the average PAR within the phytoplankton habitat layer is almost halved. A diatom/dinoflagellate bloom is accompanied by changes in the downwelling irradiance spectrum at the same PAR level (optical depths). The penetrating irradiance maximum near the bottom of the euphotic zone shifts to longer wavelengths (~550–600 nm) as compared to background conditions (~500–550 nm). The increase in diatom/dinoflagellate biomass and the concomitant change in the downwelling irradiance spectrum are accompanied by a decrease in the specific absorption coefficient of phytoplankton pigments up to about three times that under background conditions at the same optical depths. The downwelling irradiance spectrum at fixed optical depths and the ability of cells to absorb light change slightly as compared to background conditions when coccolithophores bloom.

Effects of cyanobacterial detritus on water quality and the biomass of periphyton and <i>Limnodrilus</i> spp. in Lake Taihu

富营养化是现今各国面临的主要水环境问题，其中蓝藻水华暴发是全球富营养化水体最常见的现象之一.蓝藻水华将产生大量的蓝藻碎屑，其对水质及生物的影响还尚不清楚.本研究通过向中宇宙系统添加微囊藻碎屑，分析其对水体不同形态营养盐及水生生物生物量的影响.结果表明：微囊藻碎屑加入后，水体不同形态的营养盐浓度均在短期内迅速增加，其中水体总氮和总磷平均浓度最高分别达到3.86和0.36 mg/L；浮游植物生物量（用叶绿素a表示）在前9天随营养盐浓度的升高而增加，随后逐渐下降至实验初始水平.此外，附着藻类生物量在微囊藻碎屑加入后呈逐渐下降趋势，这可能与浮游植物快速增殖引起的水体透明度下降有关.微囊藻碎屑加入后，水丝蚓生物量随微囊藻碎屑的分解持续增长，在第20天达到生物量最大值.本研究通过模拟太湖梅梁湾生态系统，探讨微囊藻碎屑对水质及水生生物生物量的影响，结果有助于进一步了解蓝藻水华对水生态系统影响的途径及机理，为富营养化湖泊管理提供理论依据.;Eutrophication has become a major water environmental problem all over the world, and cyanobacterial blooms are one of the most common phenomena in eutrophic lakes. Thereafter, a large amount of cyanobacterial detritus will be produced after the blooms, and the effects of these detritus on water quality and biological communities are poorly studied. A 28-day mesocosm experiment was conducted in Meiliang Bay of Lake Taihu to study the effects of cyanobacterial detritus on nutrients dynamics and the biomass of aquatic organisms. We found that the concentration of nutrients increased rapidly after addition of Microcystis detritus, with the maximum mean concentration of total nitrogen (TN) and total phosphorus (TP) up to 3.86 mg/L and 0.36 mg/L, respectively. Meanwhile, phytoplankton biomass (Chl.a) increased dramatically, after detritus addition, in the first nine days, then it gradually declined to the end of the experiment. In addition, the biomass of periphyton decreased gradually after cyanobacterial detritus addition which may be mainly due to the decreasing water transparency caused by the rapid increase of Chl.a limiting the growth of periphyton. Meanwhile, the biomass of Limnodrilus spp. increased gradually, and peaked on day 20, indicating that cyanobacterial detritus facilitates the population development of Limnodrilus spp. Our results have implications for eutrophic lake management and better understanding the effects of cyanobacterial blooms on aquatic ecosystems.

Changes in the soil erosion status in the middle and lower reaches of the Yangtze River basin from 2001 to 2014 and the impacts of erosion on the water quality of lakes and reservoirs

ABSTRACTThe revised universal soil loss equation (RUSLE) was used to obtain the soil erosion intensity distribution in the middle and lower reaches of the Yangtze River basin (MLYB), where the input data included a digital elevation model (DEM) and Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing products. Changes in the soil erosion intensity throughout the MLYB were analysed from 2001 to 2014, and the potential influences of these changes on the local water quality of lakes and reservoirs were revealed. This investigation is the first to reveal the spatial and temporal changes in soil erosion throughout the MLYB. The results indicated that from 2001 to 2014, most of the MLYB was characterized by slight and light soil erosion levels, whereas relatively few areas exhibited intensive to severe soil erosion. Soil erosion in the MLYB displayed a decreasing trend from 2001 to 2014; over 80% of the region displayed a decreasing soil erosion intensity change rate, indicating that soil conservation in most of the MLYB has improved over the past 14 years. However, 12.8% of the area presented an increasing change rate, and the region with the maximum increasing change rate was located mainly in the lower Yangtze basin. Furthermore, spatial heterogeneities were found in the soil erosion intensities throughout the MLYB: soil erosion improved in the upper and middle regions of the MLYB, whereas soil erosion worsened in the lower regions of the MLYB. Among the sub-basins of the MLYB, obvious soil erosion occurred most frequently in the Hanjiang basin and least frequently in the Taihu basin. A driving force analysis showed that the influence of precipitation on soil erosion is more evident than that of human activities in all sub-basins except the Dongting basin. A correlation analysis between soil erosion and water turbidity/water transparency showed that 45.9% of the decreasing water turbidity is correlated with decreasing soil erosion and that 42.5% of the increasing water turbidity might be influenced by increasing soil erosion. Decreased soil erosion might be responsible for the improved water transparency for 50% of the lakes, whereas increased soil erosion is correlated with a decrease in water transparency for over 50% of the lakes.

Linking fisheries to land use: How anthropogenic inputs from the watershed shape fish habitat quality

Aquatic ecosystems are increasingly threatened by anthropogenic stressors, both at local and larger scales. For instance, runoff from intensively cultivated areas leads to higher nutrient and sediment concentrations deteriorating water quality, which potentially trigger trophic state changes. Unfortunately, we have a poor understanding of the complex relationships linking water quality degradation and different ecosystem components. Here we analyze the long-term cascading effects of several anthropogenic stressors on both submerged aquatic vegetation (SAV) and the key traits of an exploited yellow perch (Perca flavescens, YP) population from the watershed of Lake Saint-Pierre — the largest fluvial lake of the St. Lawrence River (Québec, Canada). Lake Saint-Pierre drains one of the most impacted watersheds in Eastern Canada and had sustained a YP fishery (worth up to 10 M$ CAN/year) until the population collapsed in the mid-1990s. SAV abundance has declined since the 1980s, partially overlapping with the YP collapse. Within a structural equation modeling framework, we tested the links between changes in both SAV abundance and the YP fishery with abiotic stressors acting at both local and larger scales. Our results show that both SAV and YP declines are causally associated with anthropogenic nutrient and sediment loadings from the watershed. The decline of YP landings is also explained by a reduction in SAV abundance and YP juvenile growth, mainly caused by a sharp decrease in water transparency over the last decades. These results suggest a causal association between environmental degradation due to nutrients and sediments and different components of the trophic aquatic network. Such an integrative approach is crucial for the development of management strategies that consider cultivated lands and aquatic systems as a continuum rather than separate compartments. SAV restoration is thus a critical feature contributing to water depuration and promoting the recovery of fish populations threatened by habitat degradation.

Water Level in a Shallow Highly Eutrophic Lake: Development Factor by Macrophyte or Phytoplankton Type: Case Study of Lake Nero, Yaroslavl Oblast

Long-term dynamics of water level in Lake Nero (1930–2011) has been analyzed. Pronounced trends, determined by the climate and anthropogenic components have been identified. Up to the 1970s, at a drop in lake water level, the ecosystem still showed the main features of a shallow water body overgrown with macrophytes. Water level rise caused a more intense development of phytoplankton, an increase in biogenic element concentrations, a decrease in water transparency and the contribution of cyanobacteria of S1 codon; pondweed disappeared almost completely. Step-by-step regression analysis of phytoplankton development indices and hydrological characteristics showed that water transparency in Lake Nero depends by 80% directly on water exchange and inversely on the concentration of chlorophyll a in seston. The main hypothesis of the study was confirmed: the key trigger in phytoplankton–macrophytes competition is water level rise.

Browning-related oxygen depletion in an oligotrophic lake

ABSTRACTIn recent decades, terrestrial dissolved organic matter (DOM) has increased in many northeastern North American and European lakes and is contributing to long-term browning. We used a long-term dataset (1988–2014) to study the consequences of browning-related decreased water transparency on dissolved oxygen dynamics in 2 small temperate lakes in Pennsylvania, USA, that differ in their dissolved organic carbon concentrations. The oligotrophic (“clearer”) lake has low productivity and historically oxygenated deep waters. The mesotrophic–slightly dystrophic (“browner”) lake also has relatively low productivity but historically anoxic deep waters. We examined whether browning coincided with changes in summer dissolved oxygen dynamics, with a focus on deep-water oxygen depletion. In the clearer lake, we found that minimum oxygen concentrations decreased by ∼4.4 mg L−1 over the 27-year period, and these changes were strongly associated with both decreased water transparency and increased water column stability. We also found a shallowing of the maximum dissolved oxygen depth by ∼4.5 m and anoxic conditions established in more recent years. In the browner lake, the metrics we used did not detect any significant changes in dissolved oxygen, supporting the prediction that vertical temperature and oxygen patterns in clearer lakes may be more sensitive to increasing DOM than darker lakes. Anoxia is traditionally considered to be a consequence of anthropogenic nutrient loading and, more recently, a warming climate. We show that browning is another type of environmental change that may similarly result in anoxia in oligotrophic lakes.

Effects of benthivorous and planktivorous fish on phosphorus cycling, phytoplankton biomass and water transparency of a tropical shallow lake

We investigated the roles of a benthivorous (Prochilodus brevis, Steindachner 1875) and a planktivorous (Oreochromis niloticus, Linnaeus, 1758) fish in translocating phosphorus from the benthic to the pelagic habitat of a tropical eutrophic shallow lake and its impact on phytoplankton biomass and water transparency. We performed two field experiments in 20 mesocosms (6 m3) with a 2 × 2 factorial design. Fish presence/absence was manipulated in combination with the presence/absence of a fish cage (4 m3) that prevented fish accessing the sediment. Benthivorous fish increased total phosphorus and chlorophyll a concentrations and decreased water transparency, but only when they had access to the sediment. Planktivorous fish increased the concentration of chlorophyll a without changing total phosphorus concentrations, whether or not they had access to the sediment. Results suggest that only the benthivorous fish increased phytoplankton biomass by translocating phosphorus from benthic to pelagic habitats. However, the planktivorous fish increased phytoplankton biomass by removing zooplankton and recycling nutrients within the pelagic zone. We conclude that removal of either fish species can improve the water quality of eutrophic shallow lakes in tropical regions, but only the removal of benthivorous fish will reduce the internal loading of phosphorus.

Spatial and temporal limnological changes of an aquaculture area in a neotropical reservoir

This study evaluated the spatial and temporal limnological characteristics of an aquaculture area in the Itaipu reservoir, Brazil. During a one-year period, water quality parameters were evaluated in seven collection sites using a fish culture system as the reference point. A regression analysis was performed on water transparency versus chlorophyll a and total phosphorus versus chlorophyll a. The trophic state index (TSI) was calculated using data from water transparency, chlorophyll a, and total phosphorus. In addition, a study of the carrying capacity was conducted in situations of high and low flow and rainfall based on 725 m3 s−1 and 298 mm and 34.5 m3 s−1 and 11 mm, respectively. No difference was observed in the parameters of water quality and the TSI in relation to the collection sites; these parameters only varied significantly throughout the seasons. The regression analysis showed that increased total phosphorus in the environment leads to increase in chlorophyll a concentrations and consequently decreased water transparency. Evaluation of the carrying capacity demonstrated that under conditions of high precipitation and flow it is possible to produce 623.7 tons of fish per year, which is above the total cultivated in fish farms at the study site.

Factors related to Secchi depths and their stability over time as determined from a probability sample of US lakes.

A probabilistic sample of lakes in the 48 coterminous US lakes was made by the United States Environmental Protection Agency in the 2007 National Lakes Assessment. Because of the statistical design, the results of our analyses of Secchi depths (SD) apply to a population of 45,265 lakes. We found statistically significant differences in mean Secchi depths between natural (1.57m) and man-made lakes (1.18m). The most important variable correlated with SD was turbidity, an optical measure related to suspended particles in the water column. For most lakes, chlorophyll a was highly correlated with both turbidity and SD, but several lakes had more turbidity and lower SD than expected based on chlorophyll a alone, indicating that non-algal suspended solids were an important factor. On an ecoregion basis, the non-algal suspended solids in the lake waters were related to the average levels of suspended solids in streams located in that ecoregion, and the non-algal suspended solids were more important in man-made than natural lakes. Phosphorus and nitrogen were directly correlated with chlorophyll a and turbidity and inversely correlated with SD. Based on diatom-inferred Secchi depths for the tops and bottoms of sediment cores from lakes in Ecoregions VIII and VII (excluding lakes in Minnesota) representing 40% of the natural lakes in the US, there has been no decrease in water transparency in that population of lakes in the past 70 or more years when the US population increased by 134%. We do not have information to determine if the other 60% of lakes have or have not changed.

A call to action: strong long-term limnological changes in the two largest Ethiopian Rift Valley lakes, Abaya and Chamo

The 2 largest Ethiopian Rift Valley lakes, Abaya and Chamo, are dominant socioecological systems with important potential for ecotourism because of their attractive setting with Nechisar National Park. We report on changes in water quality in these lakes during recent decades. We integrated data on key limnological variables during the last 55 years by supplementing historical literature data with our own field measurements. Our analyses provide strong evidence for a steady increase in nutrient concentrations and decrease in water transparency. Total phosphorus and total nitrogen concentrations in both lakes over the study period increased 5- and 7-fold, respectively. Similarly, water transparency decreased, especially in Lake Chamo, which used to have clear water but is now becoming a sediment-loaded lake similar to Lake Abaya. We reconstructed fertilizer use and land use changes in the region during the past decades. The combined data suggest that the dramatic eutrophication we observed in the lakes is likely associated with increased fertilizer use or other activities related to intensified agricultural practices, and the decline in water transparency is probably associated with low tree cover due to deforestation. The profound changes in the ecology of both lakes may jeopardize the delivery of ecosystem services in the region, including water supply, fisheries, and ecotourism. Our results stress the urgent need for measures that prevent further environmental deterioration of the unique heritage provided by the Ethiopian Rift Valley lakes.