Showing papers on "Secchi disk published in 1996"

Long‐term trends in Secchi depth in the Baltic Sea

Abstract: dNutrient loads to the Baltic Sea have increased during the last century, and primary production has probably also risen. However, the evidence of such a rise is circumstantial, and most of the available findings concerning primary production cover only brief time periods. A more appropriate type of data in this area is Secchi disk measurements. We present the results of trend tests applied to Secchi depth values recorded during two discrete time periods: 19 19-l 939 and 1969-l 99 1. We performed a step trend test to compare the data from the two periods and applied a monotonic trend test to the later series. Both tests showed that Secchi depth decreased by -0.05 m yr-’ during both periods. Calculations of changes in chlorophyll concentrations suggest a yearly increase of - 1%. Extending the calculations to represent primary production indicates an increase of slightly < 1% yr- l. These calculations, however, are unreliable due to substantial uncertainty regarding the relationships between Secchi depth and chlorophyll concentration and chlorophyll and primary produclion.

Historical changes in Connecticut lakes over a 55-year period

Abstract: Changes in the chemical and physical conditions of 42 Connecticut lakes are compared between three time periods, the late 1930s, the mid- to late 1970s and the early 1990s On average, lakes have decreased in Secchi disk depth by 12 m and doubled in total phosphorus concentration, many in a unidirectional manner As a result, the suite of lakes can be characterized as having shifted from an oligo-mesotrophic condition (1930s) to a late mesotrophic condition (1990s) Since the 1970s, lakes have increased in base cation concentrations an average of 70 μeq/L, many as the result of an increase in sodium Increases in sodium were generally coupled with increases in chloride ions Many of the lakes positioned in watersheds that have become more residential since the 1930s and/or 1970s have also increased in alkalinity Despite the overall increase in base cations, chloride, and alkalinity, about 25% of the waterbodies that have remained situated in primarily forested watersheds in crystalline rock regions have decreased in total cation concentrations ; about half of these lakes have also significantly decreased in alkalinity since the 1930s The changes are discussed in relation to the degree of urbanization of the watersheds over the same time period

Temperature, Water Chemistry, and Optical Properties of Crater Lake

Abstract: Water temperature, water chemistry, and optical properties of Crater Lake were studied from 1983 to 1991. In winter and spring, wind energy and convection mixed the water column to a depth of 200 to 250 m. The lake was thermally stratified in summer and early fall; however, the epilimnion was only 5 to 20 m thick, and most of the 589 m deep water column was a cold hypolimnion. The lake was slightly basic, with moderate alkalinity and conductivity. The water column was oxygenated, although slight decreases in dissolved oxygen concentration were noticed near the lake bottom in late summer and early fall. Phosphorus and nitrogen concentrations were low. Orthophosphorus-P concentrations increased slightly with increased lake depth, whereas nitrate-N was below detection limits in the upper 200 m of the water column and then increased with increased lake depth. Secchi disk clarity typically varied from the high-20-m to low-30-m range. The depth of 1 % surface incident light (425–655 nm) in July and Aug...

General description of Lake Peipsi-Pihkva

Abstract: Lake Peipsi-Pihkva (3555 km2, mean depth 8.3 m), consisting of three parts, (L. Peipsi, L. Pihkva, L. Lammijarv) is located on the border of Estonia and Russia. L. Peipsi belongs to unstratified eutrophic lakes with mesotrophic features, L. Lammijarv has some dyseutrophic features, while L. Pihkva is strongly eutrophic. The total annual nutrient load is 15.57 tons N km−2 and 327 kgP km−2 with 74% of N and 39% of P originating from agriculture. The mean concentrations of total N and P in the lake are 876 mg m−3 and 46 mg m−3, respectively, both being the highest in L. Pihkva and the lowest in the northern part of L. Peipsi. Average pH is 8.14 and Secchi disk transparency 1.63 m. Diatoms and blue-green algae prevail in phytoplankton biomass. The blue-greens Gloeotrichia echinulata and Aphanizomenon flos-aquae dominate in summer causing the water-blooms. The concentration of Chla was the lowest in the northern part of L. Peipsi (mean 14.7 mg m−3) and the highest in the southern part of L. Pihkva (mean 47.9 mg m−3, median 16.3 mg m−3). An increase of Chla and decrease of Secchi depth could be noticed in 1983–1988, while in 1988–1994 the tendency was opposite. The long-term average primary production is 0.8 g C m−2 d−1. Zooplankton is remarkably rich in species, the average biomass in the vegetative period being 2–3 g m−3 and production 22 g C m−2. The role of rotifers in production is 53% followed by that of cladocerans (30%), copepods (16%) and Dreissena polymorpha larvae (1%). The total count of bacteria is 1–9 million cells per ml. Chironomus plumosus and Potamothrix hammoniensis are dominating in the profundal. The average abundance of macrozoobenthos (without big molluscs) 2617 ind. m−2, and their biomass 12.34 g m−2 are considered to be the highest among the large lakes of North Europe. Macroflora occupies a small percentage of the total lake area but is rich in species. Taxa forming communities are Potamogeton perfoliatus, Phragmites australis, Schoenoplectus laccustris, Potamogeton lucens, Eleocharis palustris, and Polygonum amphibium. Submerged vegetation occupies the first place amongst different growth forms, followed by emergent plants. The main commercial fishes are lake smelt, perch, ruff, roach, bream, pike, vendace and pikeperch. The stock of vendace has sharply decreased in the last years, while the amount of pikeperch has increased. Considering annual fish catches (9000–12000 tons or 25–34 kg ha−1), L. Peipsi-Pihkva exceeds all large lakes in North Europe.

Optical investigations of Estonian and Finnish lakes

Abstract: Sixteen lakes in Estonia and five lakes in Finland were investigated during 1992–95, using mainly optical methods. A total of 14 expeditions were undertaken, but the number was different for each lake. Vertical profiles of solar irradiance (spectral and integral), temperature and dissolved oxygen in the water column were measured, and Secchi disk depths were determined. Chlorophyll a and suspended matter concentrations were determined in the laboratory from water samples. Spectrophotometrical processing of the water samples (unfiltered and filtered water) was carried out to describe the beam attenuation coefficient spectra and to determine the spectral influence of yellow substance in the water. Passive optical remote sensing measurements were made from on board a boat. The data obtained show a rather high variability in water characteristics, although the lakes investigated are situated in the same climatic region. Several lakes receive human impact, and this has led to a decrease in water transparency and an increase in eutrophication. Despite rather low values for attenuation depth in lakes, water quality can be estimated using optical remote sensing data.

Water quality in the Knysna estuary

Abstract: Measurements of water temperature, salinity, pH, dissolved oxygen, secchi disk depth, turbidity and total suspended solids were taken monthly in the Knysna estuary between 1991 and 1994. Measurements of turbidity and total suspended solids of waters entering the Knysna estuary via rivers and man-made inlets were also taken on an ad hoc basis. These results are described and compared to published data on past water quality conditions. No clear long-term changes in water quality in the estuary were evident. High inputs of sediments from minor catchments indicate the necessity for remedial actions.

Indagine limnologica pluriennale (dal 1991 al 1993) sul lagoartificiale di Sos Canales (Sardegna settentrionale)

Abstract: From January 1991 to November 1993 a limnological survey was carried out on Lake Sos Canales (Sardinia), a reservoir that gives water for drinking use. The study started just after a dry period which was responsible for a strong decrease of the lake volume. This fact affected the dynamics of some parameters. During the three years studied the annual mean values of nitrate decreased (from 948 mg N m -3 in 1991 to 204 mg N m -3 in 1993) whereas the total phosphorus was more stable, variing from 34 mg P m -3 to 27 mg P m -3 in the three years. Phytoplankton increased, as indicated by the annual mean values in the photic zone of chlophyll a , cellular density and biomass; on the contrary, mean values of transparency decreased, as indicaed by the Secchi disk. Cyanophyceae were the most important group for the density amount and Dinophyceae for the biomass amount; in 1993 the Cyanophyceae biomass was important, too. The trophic status (O.E.C.D., 1982) was dubious and the lake was classified as eutrophic for the phytoplankton biomass and composition, clorophyll a , transparency and oxygen dynamics and as mesotrophic on the basis of the mean values of total phosphorus and the pH dynamics. The experimental chlorophyll and phosphorous concentrations were higher than theorethical values calculated on the basis of Vollenweider's model (O.E.C.D., 1982) using theoretic loading from watershed.

Water Quality Changes, from 1987 too 1991, in Broken Bow Lake, Oklahoma

Abstract: Water quality data were collected at five sampling sites on Broken Bow Lake from May through August in 1987 and from June through August in 1991 in order to examine temporal trends. Parameters selected for analysis were total phosphorus, chlorophyll a, and Secchi disk transparency. Data from all sampling locations were identified by year (1987 and 1991) and by lake sampling location (upper lake vs. lower lake). Using a one-way analysis of variance, results revealed significant water quality changes occurred between 1987 and 1991. Mean total phosphorus concentrations were significantly different between years and between locations as were mean Secchi disk transparencies. For chlorophyll a, a significant difference occurred between locations, but not between years. Trophic state values indicate the lake has gone from oligotrophic to mesotrophic, and some areas in the upper portion are nearing eutrophy.

Nutrient Limitation in Lake Tenkiller, Oklahoma

Abstract: Nutrient limitation in an Oklahoma reservoir was assessed by standard bottle test assays, and water quality parameters were correlated to indigenous phytoplankton community structure. Bottle tests indicated phosphorus limitation. Canonical correspondence analysis correlated phytoplankton community structure with Secchi disk transparencies. The relationship between dominant algal taxa and bottle test results indicated that phytoplankton communities were regulated primarily by nutrient availability rather than some other factor.

Michael Faraday and his “Secchi disks”

Abstract: Michael Faraday (1791-1867) is best known as a brilliant scientist and experimenter in chemistry, electricity and magnetism; however, his biography and bibliography show that he published and lectured on a wide array of subjects (Thomas, 1991). It is apparently unknown in optical limnology and oceanography, that in 1855 Faraday experimented with a kind of Secchi disk "avant la lettre" to measure the turbidity of the River Thames. This is not mentioned in reviews and textbooks (Boguslawski, 1884; Forel, 1901; Krummel, 1923; Hutchinson, 1957; Holmes, 1957; Jerlov, 1976). Faraday's experiments were done 10 years before Secchi used a white disk and the depth of its disappearance from sight for transparency measurements in the Mediterranean in 1865. Such disks later became known as Secchi disks.