Limnological Measurements Research Articles

What's good for fish is good for fishes: Interspecific consistency of growth, condition, and abundance among glacial lakes

AbstractObjectiveFish abundance and growth are regulated by a combination of bottom‐up and top‐down forces, but many management techniques depend more heavily on the latter. Here, we evaluated whether intersystem variation in nutrient loading and bottom‐up forces play more dominant roles in the control of abundance and growth of species across similar lakes than intra‐ and interspecific compensatory effects. We aimed to assess whether patterns of abundance and growth are consistent among fish species across lakes.MethodsWe tested this by evaluating pairwise comparisons of catch‐per‐unit‐effort, condition, and length‐at‐age data for 11 common warmwater fish species from 184 mesotrophic and eutrophic glacial lakes in Indiana, United States. We characterized the environmental conditions of each lake using limnological measurements (e.g., chlorophyll‐a concentration and surface temperature), lake morphology descriptions (e.g., depth and size), catchment characteristics (e.g., percent agricultural land cover), and nutrient load modeling using the Long‐Term Hydrologic Impact Analysis model. Taking a meta‐analysis approach, we used effect size calculation from pairwise correlations among lakes to identify environmental and community impacts on species abundance, condition, and length at age.ResultWe demonstrated that there were positive associations among most species comparisons (i.e., multiple species experiencing relatively fast growth in the same lake). Evaluations of environmental conditions among systems suggested that differences in estimated phosphorus input and the limnological measurements of total phosphorus, Secchi depth, and chlorophyll‐a concentration were good predictors of length at age and catch per unit effort for fish.ConclusionOur results indicated that there is a strong and consistent influence of environmental conditions and bottom‐up processes in determining species abundance and growth. This suggests that bottom‐up forces and environmental conditions linked to nutrient loading likely determine the upper boundary of fish abundance and growth in these lakes.

Deep and Frozen: High‐Mountain Lakes as Sentinels of Regional Limnology and Global Environmental Changes

AbstractHigh‐mountain lakes possess tremendous ecological significance and contribute to the well‐being of local mountain population and those living in adjacent lowland areas. Over the years, the structure and function of these ecosystems have experienced significant variations particularly due to the synergistic effects of climate stressors and human perturbations as well as reformed water consumption patterns. However, research on hydrology of mountain lakes remains focused on particular regions and related scientific perspectives on lake ecological shifts are still insufficient. Furthermore, the lack of strong nexus between scientists, stakeholders, and local community also promotes data gaps in mountain paleolimnology. Hence, it should be recognized that investigating the nexus between different stakeholders and global climate shifts as well as investigating the anthropogenic forcing factors in high‐mountain lakes is fundamental for future hydrological studies. New proxy‐based assessments and continuous, high‐frequency limnological measurements will help improve spatiotemporal resolution and address existing data gaps in high‐mountain limnology and paleolimnology.

Temporal changes in the remote sensing reflectance at Lake Vänern

The Aerosol Robotic Network - Ocean Color (AERONET-OC) instrument located at the Pålgrunden site in Lake Vänern provides values of remote sensing spectral reflectance RRS(λ) since 2008. These in situ RRS(λ) indicated a temporal increase from 2015 at center-wavelengths in the green and red spectral regions. To investigate the environmental and climate processes responsible for this increase, water color trends in Lake Vänern were analyzed considering in situ limnological measurements, meteo-climatic quantities and additionally satellite-derived data products from the Moderate Resolution Imaging Spectroradiometer on board the Aqua platform (MODIS-A). Satellite ocean color RRS(λ) data assessed against in situ RRS(λ) from the Pålgrunden site showed satisfactory agreement at a number of spectral bands. Relying on these validation results, comprehensive statistical analysis were performed using MODIS-A RRS(λ). These indicated periodical changes between 2002 and 2021 with clear minima occurring between 2010 and 2013. The complementary analyses of temporal changes characterizing limological and meteo-climatic quantities, and also relationships between these quantities and RRS(λ), indicated the existence of complex and concurrent bio-geochemical processes influencing water color in Lake Vänern. In particular, significant correlations were observed between RRS(λ) and turbidity, and also between RRS(λ) and total biovolume. Additionally, an early warming of Lake Vänern surface waters was identified since spring 2014. This occurrence could potentially affect the vertical mixing and water exchange between turbid coastal and pelagic waters with implications for phytoplankton phenology.

Monitoring 13 years of drastic catchment change and the hydroecological responses of a drained thermokarst lake

Catastrophic drainage of thermokarst lakes transform portions of former lakebed to terrestrial settings, which have largely unknown consequences for the remaining aquatic habitat. Old Crow Flats, northern Yukon (Canada), is a lake-rich area that has recently experienced a climate-driven increase in lake drainage frequency. A notable example occurred during June 2007 when Zelma Lake (originally 12 km2) lost over 80% of its volume. Here we integrate remote sensing techniques with in-situ hydrological and limnological measurements over 13 years following drainage to 1) monitor water surface area and terrestrial land cover change and 2) identify associated effects on aquatic conditions. An airborne drone system was used to provide training data for land cover classification of AVIRIS-NG data, which indicated that tall willow shrubs covered 30.8% of the former lake area by 2017. Lake water isotope-derived deuterium-excess increased during the 13-year record indicating that hydrological input increased with greater snowpack accumulation within encroaching vegetation. Limnological conditions were highly variable and eutrophic during the first few years following drainage but became more stable as vegetation colonized the former lakebed. This long-term study provides insight of aquatic responses to thermokarst lake drainage and shrub vegetation proliferation, which are increasing in Arctic and subarctic regions.

Evaluating the multidecadal response of historic seawater incursion events and salinity-induced meromixis at Laytons Lake, Nova Scotia, Canada

McGuire HE, Dunnington DW, Loder AL, Spooner IS, Mallory ML, McLellan NR, Su, C-C. 2021. Evaluating the multidecadal response of historic seawater incursion events and salinity-induced meromixis at Laytons Lake, Nova Scotia, Canada. Lake Reserv Manage. 37:378–390. Long-term monitoring data from freshwater lakes can be challenging to retrieve yet are valuable for assessing lakes at risk of saltwater intrusion. We demonstrate the use of an applied paleolimnological method to identify the response of a freshwater lake to seawater incursion and salinity-induced meromixis by examining the geochemistry of lake sediments deposited over the past century. Laytons Lake is located 1.5 km from the Bay of Fundy coastline, and experienced documented seawater incursion events in 1944 and 1949 leading to a period of extended meromixis (>40 yr). Our limnological measurements indicate that meromixis no longer exists at Laytons Lake, and hypolimnetic phosphorus (P) concentrations and conductivity values have declined since 1982. Meromictic conditions, the establishment of the chemocline, and high sedimentation rates collectively preserve a geochemical record of seawater incursion, which was identified as a short-duration, positive chloride (Cl-) excursion followed by the precipitation of iron sulfide (FeS2). A decline in Cl- concentration after the incursion events in the paleorecord may represent the deterioration of the chemocline, and the reestablishment of thermal stratification and mixing. We suspect that seawater incursions can have a multidecadal impact on aquatic systems, and that bulk geochemical analysis of paleolimnological records is a useful, low-cost tool that can provide a long-term perspective on the limnological consequences of such events. These long-term data can inform the assessment of aquatic systems at risk of saltwater incursion due to sea-level rise.

Seasonal variation in light penetration and subsurface chlorophyll-α in southern Lake Michigan observed by a glider

The Cooperative Institute for Great Lakes Research (CIGLR) in collaboration with the Great Lakes Observing System and National Oceanic and Atmospheric Administration, Great Lakes Environmental Research Laboratory (NOAA GLERL) deployed an autonomous underwater glider in southern Lake Michigan several times per year between 2012 and 2019 to collect offshore (>30 m depth) limnological measurements, including temperature, photosynthetically active radiation (beginning during 2015), and chlorophyll-α fluorescence. From these data, we calculated mixed layer depth, several measures of light penetration (diffuse attenuation coefficient, first optical depth, euphotic zone depth), and depth of the subsurface chlorophyll-α maxima. During summer, mean offshore mixed layer depth was typically 10–15 m, Kd for PAR was 0.1–0.17 m−1, first optical depth was 6–9 m, euphotic zone depth was 35–40 m, and depth of subsurface chlorophyll-α maxima was 30–35 m. We also observed substantial spatial and temporal variation in these values across the basin and within and among seasons. Glider-based observations provide a wider horizontal and vertical perspective than other methods (e.g., ship- and satellite-based observations, buoys, and fixed moorings), and are therefore a valuable, complementary tool for Great Lakes limnology. The set of observations reported here provide seasonal and basin-scale information that may help to identify anomalies useful for future glider-assisted investigation into the role of biophysical processes in Great Lakes limnology and ecology.

Mountain lakes: Eyes on global environmental change

Abstract Mountain lakes are often situated in protected natural areas, a feature that leads to their role as sentinels of global environmental change. Despite variations in latitude, mountain lakes share many features, including their location in catchments with steep topographic gradients, cold temperatures, high incident solar and ultraviolet radiation (UVR), and prolonged ice and snow cover. These characteristics, in turn, affect mountain lake ecosystem structure, diversity, and productivity. The lakes themselves are mostly small, and up until recently, have been characterized as oligotrophic. This paper provides a review and update of the growing body of research that shows that sediments in remote mountain lakes archive regional and global environmental changes, including those linked to climate change, altered biogeochemical cycles, and changes in dust composition and deposition, atmospheric fertilization, and biological manipulations. These archives provide an important record of global environmental change that pre-dates typical monitoring windows. Paleolimnological research at strategically selected lakes has increased our knowledge of interactions among multiple stressors and their synergistic effects on lake systems. Lakes from transects across steep climate (i.e., temperature and effective moisture) gradients in mountain regions show how environmental change alters lakes in close proximity, but at differing climate starting points. Such research in particular highlights the impacts of melting glaciers on mountain lakes. The addition of new proxies, including DNA-based techniques and advanced stable isotopic analyses, provides a gateway to addressing novel research questions about global environmental change. Recent advances in remote sensing and continuous, high-frequency, limnological measurements will improve spatial and temporal resolution and help to add records to spatial gaps including tropical and southern latitudes. Mountain lake records provide a unique opportunity for global scale assessments that provide knowledge necessary to protect the Earth system.

Seasonality of pCO2 in a hard‐water lake of the northern Great Plains: The legacy effects of climate and limnological conditions over 36 years

AbstractBiogeochemical processes are active year‐round in ice‐covered lakes, such that processes in one season can affect limnological conditions in subsequent seasons. However, the extent and nature of these legacy effects are poorly understood, particularly for the CO2 content of lakes and when considering gas exchange with the atmosphere. Here, we used a unique 36‐yr dataset of weekly limnological measurements of Buffalo Pound Lake in the northern Great Plains to assess seasonal changes in CO2 concentration and flux and determine how dependent lake pCO2 is on limnological conditions of previous seasons. We found that the lake was a net source of CO2 to the atmosphere (mean 18.5 ± 7.4 mol CO2 m−2 yr−1), with spring potentially accounting for the majority (~ 64% ± 20%) of CO2 efflux, assuming ice in spring was permeable to gas exchange (32.9% ± 19.8% if not). Analysis with generalized additive models (GAMs) demonstrated that current and antecedent seasonal conditions combined to explain 72.6% of deviance in spring pCO2, but that the strength of model predictions and the importance of antecedent conditions diminished in GAMs of summer (43.6%) and fall (23.3%) CO2 levels. This research suggests that pCO2 is regulated by a combination of coeval and historical environmental conditions and shows that quantification of seasonal and annual fluxes requires a mechanistic understanding of the legacy effects of preceding time intervals.

Kolme kertomusta järviemme tilasta

Finland is officially marketed as a land of a thousand clean lakes, but people’s concerns on negative trends is common. We used questionnaires and interviews to gather information on people’s experiences on how lakes have been changed, and compared the experiences with limnological research results and administrative lake classifications. Both human observations and limnological evidence detected negative changes in lake ecosystem quality during the past 30-50 years, whereas the administrative lake classification considered lakes to be mainly in good/excellent quality. In addition, small lakes with severe ecosystem changes reported by people, were seldom covered by classification. People’s observations and limnological measurements revealed independent and identical information on the trends in water quality, whereas much of the official lake classification is based on inadequate data and lack of observations on lake littoral areas. Thus, we suggest that people’s everyday experiences should be collected and analyzed more systematically in lake quality assessment processes.

Tracking climate signals in varved lake sediments: research strategy and key sites for comprehensive process studies in the Masurian Lakeland

Abstract Lake sediments are excellent archives of environmental and climate change. Especially important are varved sediments which can provide high-resolution (annual) records of those changes. Process studies including limnological measurements, particle flux monitoring and analyses of sediment structures give an opportunity to explain relationships between meteorological conditions, in-lake processes and varve formation. In our study, three lakes were selected in the Masurian Lakeland: Lake Żabińskie, Łazduny and Rzęśniki. These relatively small and deep lakes contain well preserved biogenic varves. The lakes are influenced by the same meteorological conditions but differ in terms of their catchment size, land use, hydrology, lake basin morphology and trophic status. To explore the relationships between different parameters and preservation/transformation of climate signals in the sediments we started systematic limnological measurements in the water column of these lakes, water sampling for hydrochemical analyses, monitoring of modern sedimentation using sediment traps and analysis of topmost varves from short sediment cores. With this comprehensive and high-resolution monitoring program scheduled for at least four years we are going to verify the potential of varves to track short-term meteorological phenomena in lake sediments.

Avian-Driven Modification of Seasonal Carbon Cycling at a Tundra Pond in the Hudson Bay Lowlands (Northern Manitoba, Canada)

The past ∼40 years have seen a geometric increase (5–7% per year) in the size of the lesser snow goose (LSG; Chen caerulescens caerulescens) population and marked spatial expansion of the area they inhabit within the coastal fen ecotype of Wapusk National Park (Hudson Bay Lowlands, northern Canada), raising concerns and uncertainty about the environmental effects of their activities (grubbing of vegetation, soil disturbance, deposition of feces) on the abundant shallow tundra ponds. In this study, we use conventional limnological measurements as well as water and carbon (C) isotope tracers to explore similarities and differences in seasonal patterns of hydrological, limnological, and biogeochemical conditions of 15 shallow coastal fen ponds that currently have minimal (if any) disturbance from the LSG population with one pond (WAP 20) that is subject to substantial LSG activity. Carbon isotope measurements reveal that C cycling at WAP 20 (LSG-disturbed site) is markedly different compared to the other ponds, whereas only small differences were observed in hydrological conditions and concentrations of major nutrients and chlorophyll a of pond water. A mid-summer decrease in C isotope composition of dissolved inorganic carbon (DIC) occurred at WAP 20, likely as a consequence of high pond-water pH and intense C demand by aquatic productivity. These conditions appear to have promoted “chemically enhanced CO2 invasion,” which causes strong kinetic C isotope fractionation. High C demand at WAP 20 is also suggested by mid-summer 13C enrichment in particulate organic matter. In contrast, the ponds with little to no LSG activity exhibited expected seasonal C isotope behavior (i.e., 13C enrichment of DIC) under conditions of increasing productivity when C is in relatively low demand. Small differences in nutrient concentrations may be due to rapid uptake by the benthic mat at WAP 20. Data from the low disturbance ponds also provide baseline information for future studies assessing potential effects of LSG.

Análise da dinâmica espaço-temporal de macrófitas submersas mapeadas com técnicas hidroacústicas no reservatório de Taquaruçu, rio Paranapanema

O objetivo deste trabalho foi avaliar o desenvolvimento da vegetação submersa, em termos da altura dos dosséis, considerando as dimensões espaço e tempo, usando técnicas de hidroacústica. Foram realizados dez levantamentos de campo no período de outubro de 2009 a dezembro de 2010, para aquisição de pontos georreferenciados de altura dos dosséis, frequência de ocorrência de vegetação, bem como de profundidade. Medidas limnológicas também foram feitas, a fim de verificar se suas variações poderiam explicar a distribuição espacial das macrófitas. Os dados de vegetação foram analisados por levantamento e por profundidade; além disso, compuseram um banco de dados implementado em um Sistema de Informação Geográfica. Foram então interpolados e das superfícies resultantes foram geradas cartas, que indicam a distribuição espacial do crescimento ou decaimento da vegetação. Modelos em três dimensões dos dosséis foram produzidos, para representar a ocupação volumétrica das macrófitas submersas. Os resultados mostraram que houve significativa redução da infestação de um ano para outro. Observou-se, ainda, que os maiores dosséis concentram-se em uma profundidade de 2 a 4 m. O mapeamento identificou tanto áreas de crescimento quanto de decaimento, distribuídas de modo heterogêneo. Não foi possível observar relação direta das medidas limnológicas com a dinâmica da vegetação, pois não apresentaram variação espaço-temporal significativa. Foi possível estimar o volume ocupado pelas macrófitas submersas, e a tendência observada é de que o aumento de volume é precedido por uma aparente homogeneização dos dosséis.

Retrospective analysis of spatial and temporal variability of chlorophyll-a in the Curonian Lagoon

The Curonian Lagoon, the largest in Europe (total surface area 1584 km2), is located in the southern part of the Baltic Sea, from which it is separated by a narrow sand bar and to which it is connected by the Klaipeda Straits. It is characterised by shallow eutrophic waters (mean depth 3.8 m) with average low salinity (<5‰) due to the nutrient-rich freshwaters discharged by the river Nemunas. Cyanobacteria blooms, including species producing toxic metabolites, have been a frequent phenomenon in summer. In this study a series of MERIS Full Resolution (FR) satellite images acquired between 2004 and 2009 during summer periods were used to assess the temporal evolution and spatial variability of the lagoon water quality in terms of chlorophyll-a. The models/algorithms were calibrated/validated with field data collected in 2009, based on in situ radiometric (remote sensing reflectance -Rrs-) and limnological (chlorophyll-a concentrations -chl-a-) measurements. The chl-a concentrations were estimated by developing a semi-empirical band ratio (Rrs(708)/Rrs(664)) equation applied to MERIS images after correction for atmospheric effects with the 6S code. Results from this study suggest elevated spatial and temporal variability of chl-a. Spatial variability probably arises from the mosaic of situations within the Curonian lagoon in terms of bottom sediment features, nutrient availability in the water column, water depth and hydrodynamics. Temporal variability is probably linked to complex and interplaying metereological and environmental constraints such as the length of the ice cover period, average water temperatures, wind-induced sediment re-suspension, the pattern of precipitations and river-associated nutrient transport to the lagoon. This study stresses the importance of remote sensing as a valid tool for long-term whole ecosystem studies. The preliminary results need more through analyses and intersection with other environmental data in order to better comprehend algal bloom determinants in complex and extremely dynamic systems such as coastal lagoons.

Discharge pulses of hydroelectric dams and their effects in the downstream limnological conditions: a case study in a large tropical river (SE Brazil)

AbstractTo address daily fluctuations in electricity demands, the quantities of water passing through the turbines of hydropower plants can vary significantly (up to fourfold) during a 24‐h cycle. This study evaluates the effects of hourly variations in water discharges on the limnological conditions observed in two below‐dam river stretches. The study reservoirs, Capivara and Taquaruçu, are the 9th and 10th reservoirs in a cascade of dams in the Paranapanema River in south‐east Brazil. The reservoirs exhibit different trophic conditions, water retention times, thermal regimes and spillway positions. Capivara Reservoir is deeper, meso‐eutrophic, with a high water retention time and hypolimnetic discharges (32 m) varying between 500 and 1400 m3 s−1. In contrast, Taquaruçu Reservoir is relatively shallow, oligo‐mesotrophic, and has a low retention time, with water discharges varying between 500 and 2000 m3 s−1. Its turbine water intake zone also is more superficial (7 m). For two periods of the year, winter and summer, profiles of limnological measurements were developed in the lacustrine (above‐dam site) zones of the reservoirs, as well as in the downstream river stretches (below‐dam site). In both cases, the sampling was carried out at 4‐h intervals over a complete nictemeral cycle. The results demonstrated that the reservoir operating regime (water discharge variations) promoted significant differences in the conditions of the river below the dams, especially for water velocity, turbidity, and nutrient and suspended solids concentrations. The reservoir physical characteristics, including depth, thermal stratification and outlet structure, are also key factors influencing the limnology and water quality at the below‐dam sampling sites. In the case of Capivara Reservoir, for example, the low dissolved oxygen concentration (&lt;5.0 mg L−1) in its bottom water layer was transferred to the downstream river stretches during the summer. These study results demonstrated that it is important to continue such investigations as a means of verifying whether or not these high‐amplitude/low‐frequency variations could negatively affect the downstream river biota.

In situ photochemical activity of the phytobenthic communities in two Antarctic lakes

Photochemical activity of phytobenthic communities in two freshwater lakes in East Antarctica was estimated using a submersible pulse-amplitude modulation (PAM) chlorophyll fluorometer, to answer the following questions: (1) Are the communities under bright summer photosynthetically active radiation (PAR) photosynthetically active? (2) If active, which community shows the most active signals? (3) Where is the most productive part (or depth) in the lake? Our limnological measurements indicated the two lakes were ultra-oligotrophic. Diving observations revealed that the phytobenthos of the lakes was moss-dominated which had different life-forms (moss shoots in shallow depths of both lakes, moss-pillars in the shallow lake, pinnacle moss-microbial complex community in the deeper lake). In addition, various mat-forming microbial communities inhabited the lake beds. In situ measurements of photochemical parameters indicated that shoots of mosses living just below the littoral slope, and the apical part of the moss pillars, had the highest photosynthetic activity in open water summer conditions, but mat-forming microbial communities and the other moss-microbial complex communities, showed rather lower activity. Most of the mat-forming phytobenthos surface also showed positive photosynthetic activity, but there were some cases of negligible signals in the shallow depth. This suggests that the photosynthetic activities of mat-forming communities in the shallow water were suppressed by strong ambient light in summer.

Performance Evaluation of Honeywell Silicon Piezoresistive Pressure Transducers for Oceanographic and Limnological Measurements*

Abstract Simultaneous calibrations of three temperature-compensated piezoresistive ruggedized precision “absolute” pressure transducers (Honeywell model PPTR0040AP5VB-BD), which have been designed specially for long-term coastal oceanographic and limnological measurements, have been carried out at four differing temperatures (10°, 20°, 30°, and 40°C) to evaluate their suitability for such applications. The full-scale pressure range of these shallow water absolute pressure sensors is ≈ 2800 hPa (equivalent to water depth of ≈ 18 m). Measurement results have been used to examine the transducers’ performance indicators, such as zero-point offset, accuracy, linearity, hysteresis, temperature sensitivity, and slope. Differing piezoresistive ruggedized precision absolute pressure transducers (PPTRs) exhibited differing zero-point offset values, ranging from 2 to −79 hPa. Temperature sensitivity of zero-point offset was ≈0.3 hPa over the temperature range 10°–40°C. The mean hysteresis over the full-scale absolute pressure range (≈2800 hPa) varied from approximately 2 to 8 hPa over the temperature range 10°–40°C. The slope of the least squares–fitted linear graph (taking the mean of ascending and descending pressures) was close to the ideal value of unity (deviation from 1 over the temperature range 10°–40°C was in the range of −0.001 to +0.005). Linearity was excellent, its mean over the entire pressure range being between ≈ −0.006% and 0.008% of full-scale (FS) over the above temperature range. The worst performance was exhibited at input pressures below ≈1500 hPa. Zero-point offset has played a significant role in deteriorating the accuracy of the PPTR, the mean accuracy (within ≈0.1% and −5%) having been exhibited by those transducers having offsets of 2 and −79 hPa, respectively. The mean accuracy exhibited temperature sensitivity of ≈1% in the range 10°–20°C and negligible sensitivity beyond 20°C. Use of a calibration equation significantly improved the mean static accuracy obtainable from the PPTR, to between −0.04% and 0.01% of FS. Evaluation results have indicated that a suitably calibrated temperature-compensated Honeywell PPTR provides an alternate cost-effective means for pressure measurements for coastal oceanographic and limnological studies.

Degassing Lakes Nyos and Monoun: Defusing certain disaster

Since the catastrophic releases of CO(2) in the 1980s, Lakes Nyos and Monoun in Cameroon experienced CO(2) recharge at alarming rates of up to 80 mol/m(2) per yr. Total gas pressures reached 8.3 and 15.6 bar in Monoun (2003) and Nyos (2001), respectively, resulting in gas saturation levels up to 97%. These natural hazards are distinguished by the potential for mitigation to prevent future disasters. Controlled degassing was initiated at Nyos (2001) and Monoun (2003) amid speculation it could inadvertently destabilize the lakes and trigger another gas burst. Our measurements indicate that water column structure has not been compromised by the degassing and local stability is increasing in the zones of degassing. Furthermore, gas content has been reduced in the lakes approximately 12-14%. However, as gas is removed, the pressure at pipe inlets is reduced, and the removal rate will decrease over time. Based on 12 years of limnological measurements we developed a model of future removal rates and gas inventory, which predicts that in Monoun the current pipe will remove approximately 30% of the gas remaining before the natural gas recharge balances the removal rate. In Nyos the single pipe will remove approximately 25% of the gas remaining by 2015; this slow removal extends the present risk to local populations. More pipes and continued vigilance are required to reduce the risk of repeat disasters. Our model indicates that 75-99% of the gas remaining would be removed by 2010 with two pipes in Monoun and five pipes in Nyos, substantially reducing the risks.

The influence of the North Atlantic Oscillation on the physical, chemical and biological characteristics of four lakes in the English Lake District

Summary1. Thirty‐six years of winter meteorological and limnological measurements from four lakes in the English Lake District are analysed and related to variations in the North Atlantic Oscillation (NAO). Winter weather conditions were strongly influenced by the NAO with mild, wet winters being associated with strongly positive values of the NAO index (NAOI).2. Lake surface and bottom temperatures were strongly positively correlated with the NAOI, with the highest correlations being recorded in the shallower lakes.3. Variations in the NAOI also had a significant effect on the winter concentration of nitrate. In all the lakes, there was a significant negative correlation between the NAOI and the detrended winter concentration of nitrate. The key driving variable was the local air temperature, which appeared to limit the quantity of nitrate reaching the lake by increasing the amount assimilated in the surrounding catchment in mild winters.4. Dissolved reactive phosphorus (DRP) concentrations were not significantly correlated with the NAOI in the two larger basins but significant positive correlations were recorded in the two smaller lakes. The key driving variable was the local rainfall with higher DRP concentrations being recorded after heavy rain in the lakes with a short retention time.5. The NAOI‐related changes in rainfall also influenced the phytoplankton. In wet winters the concentration of chlorophyll in the two smaller lakes with the shortest retention time was lower and the spring growth of Asterionella formosa was delayed in the smallest lake.6. These differential responses demonstrate how the large‐scale effects associated with the NAO can be ‘filtered’ by the physical characteristics of a particular site.

Great Lakes monitoring results--comparison of probability based and deterministic sampling grids.

The Great Lakes may be viewed as a coastal environment, affected by the same meteorological and physical forces as the coastal ocean. The U.S. EPA, Great Lakes National Program Office (GLNPO) has monitored the open waters of the lakes, annually, since 1983. As part of the U.S. EPA Environmental Monitoring and Assessment Program (EMAP), a pilot study was performed in Lake Michigan to compare the existing GLNPO deterministic sampling grid with the EMAP probabilistic grid. Results of chemical analyses of trophic status indicators (total phosphorus and chlorophyll a) as well as nutrients and conventional limnological measurements, from spring and summer surveys in 1992 indicate little difference between the grids in the offshore region of the lake. The few statistically significant differences may be due to station distribution throughout the lake, or simple chance. This might be expected due to the well mixed nature of the open waters of Lake Michigan. The detection of a long-term trend for total phosphorus in Lake Michigan benefits from an annual program: viewing cumulative frequency distributions based on a four year EMAP interval does not convey information on the decrease in phosphorus in the lake. If the EMAP sampling grid were to be used in the Great Lakes, pilots in each of the lakes would be necessary for utilization of the existing long-term record as a basis for trend detection.

Retrieval of water quality from airborne imaging spectrometry of various lake types in different seasons

The suitability of the AISA airborne imaging spectrometer for monitoring lake water quality was tested in four surveys carried out in southern Finland in 1996–1998. Altogether, 11 lakes were surveyed and the total number of stations with concurrent remote sensing and limnological measurements was 127. The ranges of the water quality variables were: the sum of chlorophyll a and phaeophytin a 1–100 μg l −1, turbidity 0.4–26 FNU, total suspended solids 0.7–32 mg l −1, absorption coefficient of aquatic humus at 400 nm 1.2–14 m −1 and secchi disc transparency 0.4–7 m. For the retrieval analyses, 24 AISA channels in the 450–786 nm range with a channel width of 6–14 nm were used. The agreement between estimated and observed water quality variables was generally good and R 2 for the best algorithms was in the range of 0.72–0.90 over the whole dataset. The channels used for May were, in most cases, the same as those for August, but the empirical parameters of the algorithms were different. After seasonal grouping, R 2 varied from 0.84 to 0.95. The use of apparent reflectance instead of radiance improved the estimation of water quality in the case of total suspended solids and turbidity. In the most humic lake, the empirical algorithms tested were suitable only for the interpretation of total suspended solids and turbidity.