Dust mediated transfer of phosphorus to alpine lake ecosystems of the Wind River Range,Wyoming, USA

Alpine lakes receive a large fraction of their nutrients from atmospheric sources and are consequently sensitive to variations in both the amount and chemistry of atmospheric deposition. In this study we explored the spatial changes in lake water chemistry and biology along a gradient of dust deposition in the Wind River Range, Wyoming. Regional differences were explored using the variation in bulk deposition, lake water, sediment, and bedrock geochemistry and catchment characteristics. Dust deposition rates in the Southwestern region averaged 3.34 g m^?2 year^?1, approximately three times higher than deposition rates in the Northwestern region (average 1.06 g m^?2 year^?1). Dust-P deposition rates ranged from 87 µg P m² day^?1 in the Northwestern region to 276 µg P m² day^?1 in the Southwestern region. Subalpine and alpine lakes in the Southwestern region had greater total phosphorus (TP) concentrations (5–13 µg L^?1) and greater sediment phosphorus (SP) concentrations (2–5 mg g^?1) than similar lakes elsewhere in the region (1–8 µg L^?1 TP, 0.5–2 mg g^?1 SP). Lake phosphorus concentrations were related to dissolved organic carbon (DOC) across vegetation gradients, but related to the percent of bare rock, catchment area to lake area, and catchment steepness across dust deposition gradients. Modern phytoplankton and zooplankton biomasses were two orders of magnitude greater in the Southwest than in the Northwest, and alpine lakes in the Southwest had a unique diatom species assemblage with relatively higher concentrations of Asterionella formosa, Pseudostaurosira pseudoconstruens, and Pseudostaurosira brevistriata. These results suggests that catchment controls on P export to lakes (i.e. DOC) are overridden in dominantly bare rock basins where poor soils cannot effectively retain dust deposited P.     

Remote sensing of phytoplankton-macrophyte coexistence in shallow hypereutrophic fluvial lakes

We investigated with remote sensing (APEX images) the coexistence of phytoplankton and macrophytes in three interconnected shallow and hypereutrophic fluvial lakes (Mantua Lakes, Northern Italy). High concentrations of chlorophyll-a, up to 60 mg m^?3, were determined in the open water between well-developed stands of floating-leaved, submerged, and emergent macrophytes. Our data suggest a general inhibition of phytoplankton by macrophytes, evidenced by decreasing chlorophyll-a concentrations in proximity of macrophyte stands. Chlorophyll-a concentrations halved in the proximity of emergent stands (~6 mg m^?3 within 21 m from the stand border) when compared to the outer zones (~13 mg m^?3). Contrasting trends were observed for submerged stands, where concentrations decreased inwards from ~8 to ~3 mg m^?3. Floating leaved stands had a neutral effect, chlorophyll-a being nearly constant in both inner and outer zones. Overall, remotely-sensed data allow evaluation of quantitative and spatially defined interactions of macrophytes and phytoplankton at the whole ecosystem scale.     

Gross primary productivity of phytoplankton and planktonic respiration in inland floodplain wetlands of southeast Australia: habitat-dependent patterns and regulating processes

Gross primary productivity (GPP) of phytoplankton and planktonic respiration (PR) (i.e., planktonic metabolism) are critical pathways for carbon transformation in many aquatic ecosystems. In inland floodplain wetlands with variable inundation regimes, quantitative measurements of GPP and PR are rare and their relationships with wetland environmental conditions are largely unknown. We measured PR and the GPP of phytoplankton using light and dark biological oxygen demand bottles in open waters of channel and non-channel floodplain habitats of inland floodplain wetlands of southeast Australia that had been inundated by environmental water. Overall, GPP varied from 3.7 to 405.5 mg C m^?3 h^?1 (mean ± standard error: 89.4 ± 9.2 mg C m^?3 h^?1, n = 81), PR from 1.5 to 251.6 mg C m^?3 h^?1 (43.2 ± 5.6 mg C m^?3 h^?1, n = 81), and GPP/PR from 0.2 to 15.6 (3.0 ± 0.3, n = 81). In terms of wetland environmental conditions, total nitrogen (TN) ranged from 682.0 to 14,700.0 mg m^?3 (mean ± standard error: 2,643.0 ± 241.6 mg m^?3, n = 81), total phosphorus (TP) from 48.0 to 1,405.0 mg m^?3 (316.8 ± 31.4 mg m^?3, n = 81), and dissolved organic carbon (DOC) from 1.9 to 46.3 g m^?3 (22.0 ± 1.6 g m^?3, n = 81). Using ordinary least-squares multiple regression analyses, the rates of GPP and PR, and their ratio (GPP/PR) were modeled as a function of TN, TP, and DOC that had been measured concomitantly. The “best” models predicted GPP and GPP/PR ratio in channel habitats as a function of DOC; and GPP, PR, and GPP/PR in non-channel floodplain habitats as a function of TN and/or TP. The models explained between 46 and 74 % of the variance in channel habitats and between 17 and 87 % of the variance in non-channel floodplain habitats. Net autotrophy (mean GPP/PR 3.0) of planktonic metabolism in our work supports the prevailing view that wetlands are a net sink for carbon dioxide. We propose a nutrient-DOC framework, combined with hydrological and geomorphological delineations, to better predict and understand the planktonic metabolism in inland floodplain wetlands.     

Biomass and terpenoids produced by mutant strains of <Emphasis Type="Italic">Arthrospira</Emphasis> under low temperature and light conditions

The filamentous Cyanobacterium Arthrospira is commercially produced and is a functional, high-value, health food. We identified 5 low temperature and low light intensity tolerant strains of Arthrospira sp. (GMPA1, GMPA7, GMPB1, GMPC1, and GMPC3) using ethyl methanesulfonate mutagenesis and low temperature screening. The 5 Arthrospira strains grew rapidly below 14?°C, 43.75 μmol photons m^?2 s^?1 and performed breed conservation at 2.5?°C, 8.75 μmol photons m^?2 s^?1. We used morphological identification and molecular genetic analysis to identify GMPA1, GMPA7, GMPB1 and GMPC1 as Arthrospira platensis, while GMPC3 was identified as Arthrospira maxima. Growth at different culture temperatures was determined at regular intervals using dry biomass. At 16?°C and 43.75 μmol photons m^?2 s^?1, the maximum dry biomass production and the mean dry biomass productivity of GMPA1, GMPB1, and GMPC1 were 2057?±?80 mg l^?1, 68.7?±?2.5 mg l^?1 day^?1, 1839?±?44 mg l^?1, 60.6?±?1.8 mg l^?1 day^?1, and 2113?±?64 mg l^?1, 77.7?±?2.5 mg l^?1 day^?1 respectively. GMPB1 was chosen for additional low temperature tolerance studies and growth temperature preference. In winter, GMPB1 grew well at mean temperatures <10?°C, achieving 3258 mg dry biomass from a starting 68 mg. In summer, GMPB1 grew rapidly at mean temperatures more than 28?°C, achieving 1140 mg l^?1 dry biomass from a starting 240 mg. Phytonutrient analysis of GMPB1 showed high levels of C-phycocyanin and carotenoids. Arthrospira metabolism relates to terpenoids, and the methyl-d-erythritol 4-phosphate pathway is the only terpenoid biosynthetic pathway in Cyanobacteria. The 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR) gene from GMPB1 was cloned and phylogenetic analysis showed that GMPB1 is closest to the Cyanobacterium Oscillatoria nigro-viridis PCC711. Low temperature tolerant Arthrospira strains could broaden the areas suitable for cultivation, extend the seasonal cultivation time, and lower production costs.     

Analysis of long-term variation in phytoplankton biovolume in the northern basin of Lake Biwa

The long-term variation in phytoplankton biovolume in the northern basin of Lake Biwa was analyzed using periodic phytoplankton census data from January 1979 to December 2009. Population densities obtained from census data were transformed into biovolumes, and phytoplankton species were categorized into three size fractions: net phytoplankton (≥4,000 μm³ cell^?1, ≥ca. 20 μm in diameter), large nanophytoplankton (100–4,000 μm³ cell^?1, ca. 6–20 μm in diameter), and small nanophytoplankton (<100 μm³ cell^?1, <ca. 6 μm in diameter). Although the annual total biovolume gradually decreased over time, the total biovolumes in winter and spring were found to increase. Furthermore, a decrease in the biovolume of net phytoplankton and an increase in that of small nanophytoplankton were observed. Because of succession in the phytoplankton community, the average cell volume of the phytoplankton community decreased from 269 μm³ cell^?1 in the 1980s to 56 μm³ cell^?1 in the 2000s. Lake warming accompanied with the intensification of thermal stratification and the augmentation of wind speed were observed at Lake Biwa over the study period. Serial analysis correcting for autocorrelation revealed that oligotrophication in the epilimnion, induced by lake warming and limitation of light available for phytoplankton growth by wind-induced water mixing, was a potential factor in the succession of the phytoplankton community.     

Estimation of reference conditions for phytoplankton in a naturally eutrophic shallow lake

Classification of waters using biological quality elements and determination of the degree of deviation from reference levels is a key issue in the Water Framework Directive of EU. Lakes in reference conditions with sufficient biological data are available for several boreal lake types with the exception of naturally eutrophic lakes. An empirical approach is one alternative for estimating the reference conditions of such lakes. We used the water transparency of the naturally eutrophic Lake Tuusulanjärvi recorded in August in the early 1910s to estimate reference values for phytoplankton biomass and chlorophyll a concentrations. Three phytoplankton samples during August 2000–2001 corresponded to the estimated reference values for total biomass (<5.6 mg l^?1) and chlorophyll a (<28 μg l^?1), as did the simultaneous Secchi depths. The phytoplankton assemblage in these samples with 24 eutrophy indicators (17% of the total taxa number) corresponded in general the species list from the early 1900s, which as such could be regarded as reference assemblage. Furthermore, in August 2000, 3 years after intensive fish removal a prominent decrease in cyanobacterial biomass and toxin concentration was observed. The costs of the measures and studies in Lake Tuusulanjärvi during 1989–2003 have been approximately 2.5 million euros.     

A comparison of zooplankton communities in saline lakewater with variable anion composition

Although salinity and aquatic biodiversity are inversely related in lake water, the relationship between types of salts and zooplankton communities is poorly understood. In this study, zooplankton species were related to environmental variables from 12 lakes: three saline lakes with water where the dominant anions were SO₄ and CO₃, four saline lakes with Cl-dominated water, and five dilute, subsaline (0.5–3 gl^?1 total dissolved solids) lakes of variable anion composition. Although this study comprised only 12 lakes, distinct differences in zooplankton communities were observed among the two groups of chemically defined saline lakes. Canonical correspondence analysis identified total alkalinity, sulphate, chloride, calcium, sodium, potassium, and total phosphorus as all contributing to the first two ordination axes (λ₁ = 0.97 and λ₂ = 0.62, P<0.05). The rotifer Brachionus plicatilis and the harpactacoid copepod Cletocamptus sp. prevailed lakes with Cl-dominated water. In contrast, the calanoid copepods Leptodiaptomus sicilis and Diaptomus nevadensis were dominant in the SO₄/CO₃-dominated lake water with elevated potassium (79–128 mg l^?1) and total phosphorus concentrations (1322-2915 μg l^?1). The contrasting zooplankton species distribution among these two saline lake types is likely explained by variable selective pressure on zooplankton and their predators from differing physiological tolerances to salt stress and specific ions. While inland saline lakes with Cl as the dominant anion are relatively rare in Canada and SO₄/CO₃ are the common features, our study provided an opportunity to compare zooplankton communities across the two groups of lakes.     

Primary production of aquatic macrophytes and their epiphytes in two shallow lakes (Peipsi and Võrtsjärv) in Estonia

In shallow lakes with large littoral zones, epiphytes and submerged macrophytes can make an important contribution to the total annual primary production. We investigated the primary production (PP) of phytoplankton, submerged macrophytes, and their epiphytes, from June to August 2005, in two large shallow lakes. The production of pelagic and littoral phytoplankton and of the dominant submerged macrophytes in the littoral zone (Potamogeton perfoliatus in Lake Peipsi and P. perfoliatus and Myriopyllum spicatum in Lake Võrtsjärv) and of their epiphytes was measured using a modified ¹⁴C method. The total PP of the submerged macrophyte area was similar in both lakes: 12.4 g C m^?2 day^?1 in Peipsi and 12.0 g C m^?2 day^?1 in Võrtsjärv. In Peipsi, 84.2% of this production was accounted for by macrophytes, while the shares of phytoplankton and epiphytes were low (15.6 and 0.16%, respectively). In Võrtsjärv, macrophytes contributed 58%, phytoplankton 41.9% and epiphytes 0.1% of the PP in the submerged macrophyte area. Epiphyte production in both lakes was very low in comparison with that of phytoplankton and macrophytes: 0.01, 5.04, and 6.97 g C m^?2 day^?1, respectively, in Võrtsjärv, and 0.02, 1.93, and 10.5 g C m^?2 day^?1, respectively, in Peipsi. The PP of the littoral area contributed 10% of the total summer PP of Lake Peipsi sensu stricto and 35.5% of the total summer PP of Lake Võrtsjärv.     

Spatial and temporal dynamics of diffusive methane emissions in the Okavango Delta,northern Botswana,Africa

Global warming is associated with the continued increase in the atmospheric concentrations of greenhouse gases; carbon dioxide, methane (CH₄) and nitrous oxide. Wetlands constitute the largest single natural source of atmospheric CH₄ in the world contributing between 100 and 231 Tg year^?1 to the total budget of 503–610 Tg year^?1, approximately 60 % of which is emitted from tropical wetlands. We conducted diffusive CH₄ emission measurements using static chambers in river channels, floodplains and lagoons in permanent and seasonal swamps in the Okavango Delta, Botswana. Diffusive CH₄ emission rates varied between 0.24 and 293 mg CH₄ m^?2 h^?1, with a mean (±SE) emission of 23.2 ± 2.2 mg CH₄ m^?2 h^?1 or 558 ± 53 mg CH₄ m^?2 day^?1. These emission rates lie within the range reported for other tropical wetlands. The emission rates were significantly higher (P < 0.007) in permanent than in seasonal swamps. River channels exhibited the highest average fluxes at 31.3 ± 5.4 mg CH₄ m^?2 h^?1 than in floodplains (20.4 ± 2.5 mg CH₄ m^?2 h^?1) and lagoons (16.9 ± 2.6 mg CH₄ m^?2 h^?1). Diffusive CH₄ emissions in the Delta were probably regulated by temperature since emissions were highest (20–300 mg CH₄ m^?2 h^?1) and lowest (0.2–3.0 mg m^?2 h^?1) during the warmer-rainy and cooler winter seasons, respectively. Surface water temperatures between December 2010 and January 2012 varied from 15.3 °C in winter to 33 °C in summer. Assuming mean inundation of 9,000 km², the Delta’s annual diffusive emission was estimated at 1.8 ± 0.2 Tg, accounting for 2.8 ± 0.3 % of the total CH₄ emission from global tropical wetlands.     

Depositional fluxes and sources of particulate carbon and nitrogen in natural lakes and a young boreal reservoir in Northern Québec

We investigated the depositional trends of total particles, carbon and nitrogen in a newly created, 600-km² hydroelectric reservoir in Northern Québec, and compared the results with those observed in lakes of the surrounding region. We show that particulate fluxes exhibit a large degree of spatial heterogeneity in both the reservoir (68–548 mg POC m^?2 d^?1 and 5–33 mg PN m^?2 d^?1) and the natural lakes (30–150 mg POC m^?2 d^?1 and 3–12 mg PN m^?2 d^?1) and that on average, settling fluxes of the reservoir (211 ± 46 mg POC m^?2 d^?1 and 14 ± 3 mg PN m^?2 d^?1) exceeded lake deposition (79 ± 13 mg POC m^?2 d^?1 and 7 ± 1 mg PN m^?2 d^?1) by approximately two-fold. Our results also show that the nature of the organic matter reaching the sediments was significantly different between lakes and the reservoir, which can have consequences for benthic metabolism and the long-term storage. We found that sinking fluxes in the reservoir were mostly regulated by local morphological and hydrological conditions, with higher fluxes along or in the vicinity of the old riverbed (average 400 ± 73 mg POC m^?2 d^?1 and 24 ± 5 mg PN m^?2 d^?1) and lower fluxes in calmer zones such as side bays (average 106 ± 10 mg POC m^?2 d^?1 and 8 ± 1 mg PN m^?2 d^?1). In lakes, where settling fluxes were not linked to the trophy, or dissolved organic carbon, the actual nature of the sedimenting organic material was influenced by lake morphometry and the relative contribution of algal versus terrestrial sources. We conclude that re-suspension and erosion play a major role in shaping the reservoir sinking fluxes which explain both, the higher reservoir deposition and also some of the qualitative differences between the two systems. Despite all these differences, sinking particulate organic carbon fluxes were small and surprisingly similar relative to the surface carbon dioxide emissions in both the reservoir and lakes, representing approximately 16–17 % of the carbon efflux estimated for these same systems in 2008.     

Effects of glaciers on nutrient concentrations and phytoplankton in lakes within the Northern Cascades Mountains (USA)

We compared nitrate concentrations, phytoplankton biomass, and phytoplankton community structure in lakes fed by glacier melt and snowmelt (GSF lakes) and by snowmelt only (SF lakes) within North Cascades National Park (NOCA) in Washington State, USA. In the U.S. Rocky Mountains, glacier melting has greatly increased nitrate concentrations in GSF lakes (52–236 µg NO₃–N L^?1) relative to SF lakes (1–14 µg NO₃–N L^?1) and thereby stimulated phytoplankton changes in GSF lakes. Considering NOCA contains approximately one-third of the glaciers in the continental U.S., and many mountain lakes that receive glacier meltwater inputs, we hypothesized that NOCA GSF lakes would have greater nitrate concentrations, greater phytoplankton biomass, and greater abundance of nitrogen-sensitive diatom species than NOCA SF lakes. However, at NOCA nitrate concentrations were much lower and differences between lake types were small compared to the Rockies. At NOCA, nitrate concentrations averaged 13 and 5 µg NO₃–N L^?1 in GSF and SF lakes, respectively, and a nitrate difference was not detectable in several individual years. There also was no difference in phytoplankton biomass or abundance of nitrogen-sensitive diatoms between lake types at NOCA. In contrast to the Rockies, there also was not a significant positive relationship between watershed percent glacier area and lake nitrate at NOCA. Results demonstrate that biogeochemical responses to global change in Western U.S. mountain lake watersheds may vary regionally. Regional differences may be affected by differing nitrogen deposition, climate, geology, or microbial processes within glacier environments, and merit further investigation.     

Kinetics study of pyridine biodegradation by a novel bacterial strain,Rhizobium sp. NJUST18

Biodegradation of pyridine by a novel bacterial strain, Rhizobium sp. NJUST18, was studied in batch experiments over a wide concentration range (from 100 to 1,000 mg l^?1). Pyridine inhibited both growth of Rhizobium sp. NJUST18 and biodegradation of pyridine. The Haldane model could be fitted to the growth kinetics data well with the kinetic constants μ* = 0.1473 h^?1, K _s = 793.97 mg l^?1, K _i = 268.60 mg l^?1 and S _m = 461.80 mg l^?1. The true μ _max, calculated from μ*, was found to be 0.0332 h^?1. Yield coefficient Y _X/S depended on S _i and reached a maximum of 0.51 g g^?1 at S _i of 600 mg l^?1. V _max was calculated by fitting the pyridine consumption data with the Gompertz model. V _max increased with initial pyridine concentration up to 14.809 mg l^?1 h^?1. The q _S values, calculated from $V_{ \hbox{max} }$ , were fitted with the Haldane equation, yielding q _Smax = 0.1212 g g^?1 h^?1 and q* = 0.3874 g g^?1 h^?1 at S _m′ = 507.83 mg l^?1, K _s′ = 558.03 mg l^?1, and K _i′ = 462.15 mg l^?1. Inhibition constants for growth and degradation rate value were in the same range. Compared with other pyridine degraders, μ _max and S _m obtained for Rhizobium sp. NJUST18 were relatively high. High K _i and K _i′ values and extremely high K _s and K _s′ values indicated that NJUST18 was able to grow on pyridine within a wide concentration range, especially at relatively high concentrations.     

Enhancement of electricity production in a mediatorless air–cathode microbial fuel cell using <Emphasis Type="Italic">Klebsiella</Emphasis> sp. IR21

A novel dissimilatory iron-reducing bacteria, Klebsiella sp. IR21, was isolated from the anode biofilm of an MFC reactor. Klebsiella sp. IR21 reduced 27.8 % of ferric iron to ferrous iron demonstrating that Klebsiella sp. IR21 has electron transfer ability. Additionally, Klebsiella sp. IR21 generated electricity forming a biofilm on the anode surface. When a pure culture of Klebsiella sp. IR21 was supplied into a single chamber, air–cathode MFC fed with a mixture of glucose and acetate (500 mg L^?1 COD), 40–60 mV of voltage (17–26 mA m^?2 of current density) was produced. Klebsiella sp. IR21 was also utilized as a biocatalyst to improve the electrical performance of a conventional MFC reactor. A single chamber, air–cathode MFC was fed with reject wastewater (10,000 mg L^?1 COD) from a H₂ fermentation reactor. The average voltage, current density, and power density were 142.9 ± 25.74 mV, 60.5 ± 11.61 mA m^?2, and 8.9 ± 3.65 mW m^?2, respectively, in the MFC without inoculation of Klebsiella sp. IR21. However, these electrical performances of the MFC were significantly increased to 204.7 ± 40.24 mV, 87.5 ± 17.20 mA m^?2, and 18.6 ± 7.23 mW m^?2, respectively, with inoculation of Klebsiella sp. IR21. The results indicate that Klebsiella sp. IR21 can be utilized as a biocatalyst for enhancement of electrical performance in MFC systems.     

The Absorption of Light in Lakes: Negative Impact of Dissolved Organic Carbon on Primary Productivity

Colored dissolved organic matter (CDOM) absorbs a substantial fraction of photosynthetically active radiation (PAR) in boreal lakes. However, few studies have systematically estimated how this light absorption influences pelagic primary productivity. In this study, 75 boreal lakes spanning wide and orthogonal gradients in dissolved organic carbon (DOC) and total phosphorus (TP) were sampled during a synoptic survey. We measured absorption spectra of phytoplankton pigments, CDOM, and non-algal particles to quantify the vertical fate of photons in the PAR region. Area-specific rates of gross primary productivity (PP_A) were estimated using a bio-optical approach based on phytoplankton in vivo light absorption and the light-dependent quantum yield of photochemistry in PSII measured by a PAM fluorometer. Subsequently, we calculated the effects of CDOM, DOC, and TP concentration on PP_A. CDOM absorbed the largest fraction of PAR in the majority of lakes (mean 56.3%, range 36.9–76.2%), phytoplankton pigments captured a comparatively minor fraction (mean 6.6%, range 2.2–28.2%). PP_A estimates spanned from 45 to 993 mg C m^?2 day^?1 (median 286 mg C m^?2 day^?1). We found contrasting effects of CDOM (negative) and TP (positive) on PP_A. The use of DOC or CDOM as predictors gave very similar results and the negative effect of these variables on PP_A can probably be attributed to shading. A future scenario of increased DOC, which is highly correlated with CDOM in these lakes, might impose negative effects on areal primary productivity in boreal lakes.     

Grazing effects of a freshwater bivalve (Corbicula leana Prime) and large zooplankton on phytoplankton communities in two Korean lakes

This study examined the effects of a freshwater filter feeding bivalve (Corbicula leana Prime) and large zooplankton (>200 μm, mostly cladocerans and copepods) on the phytoplankton communities in two lakes with contrasting trophic conditions. A controlled experiment was conducted with four treatments (control, zooplankton addition, mussel addition, and both zooplankton and mussel addition), and each established in duplicate 10-l chambers. In both lakes there were significant effects of mussel grazing on phytoplankton density and biomass. The effects were greater in mesotrophic Lake Soyang than in hypertrophic Lake Ilgam. Effects of zooplankton grazing did not differ between these lakes, and zooplankton effects on phytoplankton were much less than the effects of mussels. Although mussels exerted a varying effect on phytoplankton according to their size, mussels reduced densities of almost all phytoplankton taxa. Total mean filtering rate (FR) of mussels in Lake Soyang was significantly greater than that in Lake Ilgam (p=0.002, n=5). Carbon fluxes from phytoplankton to mussels (977–2,379 μgC l^?1d^?1) and to zooplankton (76–264 μgC l^?1 d^?1) were always greater in Lake Ilgam due to the greater phytoplankton biomass (p<0.01, n=6). Based on the C-flux to biomass ratios, the mussels consumed 170–754% (avg. 412%) of phytoplankton standing stock in Lake Soyang, and 38–164% (avg. 106%) in Lake Ilgam per day. The C-flux to biomass ratio for mussels within each lake was much greater than for large zooplankton. Mussels reduced total phosphorus concentration by 5–34%, while increasing phosphate by 30–55% relative to the control. Total nitrogen also was reduced (by 9–25%), but there was no noticeable change in nitrate among treatments. The high consumption rate of phytoplankton by Corbicula leana even in a very eutrophic lake suggests that this mussel could affect planktonic and benthic food web structure and function by preferential feeding on small seston and by nutrient recycling. Control of mussel biomass therefore might be an effective tool for management of water quality in shallow eutrophic lakes and reservoirs in Korea.     

Seasonal and Interannual changes of chemical characteristics and phytoplankton in a mountain lake of the Eastern Italian Alps (Lake Calaita,Trentino)

Measurements at Lake, Calaita, a small mountain lake located at 1605 m. a.s .l. in a metamorphic catchment area in the south of the Trentino Region (Northern Italy), have revealed great seasonal variations in the chemical characteristics and phytoplankton community during the ice free period in 1992 and 1993. The acidity present in wet precipitations (H⁺, NH₄⁺) was neutralised within the drainage basin by mineral dissolution which led to an increase of basic cations and alkalinity in the runoff. The dilution during periods of higher discharge, e.g. in spring and autumn, resulted in low values of alkalinity (up to 60 μeq 1⁻¹), pH (mostly <6.7) and conductivity (<8 μS cm⁻¹, 20°C). In summer, the decrease in runoff caused higher alkalinity (>10 μeq 1⁻¹), pH (6.9–7.4) and conductivity values (up to 30 μS cm⁻¹). The phytoplankton showed a major development in summer (with biovolume values of up to 7000 mm³ m⁻³), two different taxa being dominant in 1992 (Oocystis cf. lacustris) and 1993 (Synedra sp.). Unpredictable climatic conditions have a strong influence on the physical stability of the lake, which makes it more difficult to explain the evolution of the phytoplankton community as opposed to deeper lakes.     

Inherent optical properties of suspended particulates in four temperate lakes: application of in situ spectroscopy

Instrumentation measuring hyperspectral particle attenuation and absorption was used to assess particle concentration and size, chlorophyll, and spectral characteristics as a function of depth in four temperate lakes of different trophy. Partitioning the absorption coefficient permitted us to analyze properties of phytoplankton absorption as a function of ambient illumination and hydrographic conditions. Stratification was found to be a controlling factor in the size distribution and concentration of particles. Bloom cycles (chlorophyll > 10 mg m^?3) were observed to evolve over several weeks but on occasion did change rapidly. Total chlorophyll concentration revealed the majority of the lakes did not follow the typical seasonal succession of biomass associated with temperate waters. Particle and chlorophyll concentration maxima did not always coincide, cautioning the use of chlorophyll a as a surrogate for algal biomass. Phytoplankton near the base of the euphotic zone, including a deep chlorophyll maximum in an oligotrophic system, were found to exhibit significant chromatic adaptation. Unique absorption peaks identified the ubiquitous presence of cyanobacteria in all four lakes. Finally, particle resuspension and possible nepheloid layers were observed in the two smallest lakes.     

Reference conditions and WFD compliant class boundaries for phytoplankton biomass and chlorophyll-a in Alpine lakes

The intercalibration (IC) exercise is a key element in the implementation of the Water Framework Directive (WFD) in Europe. Its focus lies on the harmonization of national classification methods to guarantee a common understanding of ‘Good Ecological Status’ in surface waters. This article defines reference conditions and sets class boundaries for deep (mean depth >15 m, IC lake type L-AL3) and moderately deep (mean depth 3–15 m, IC lake type L-AL4) Alpine lakes >0.5 km². Data were collated from each of the five EU member states included in the Alpine Geographical Intercalibration Group (Alpine GIG: Austria, France, Germany, Italy and Slovenia). Hydro-morphological, chemical and biological data from 161 sites (sampling stations) in 144 Alpine lakes over a period of seven decades were collated in a database. Based on a set of reference criteria, 18 L-AL3 and 13 L-AL4 reference sites were selected. Reference conditions were defined using a combined approach, based on historical, paleolimnological and monitoring data in conjunction with trophic modelling and expert judgement. Reference values and class boundaries were set for annual mean total biomass (biovolume), and then derived for annual mean chlorophyll-a using a regression between the two parameters. In order to allow for geographical differences within the Alpine GIG and to facilitate the inclusion of the broadly defined common IC types and national lake types, ranges were defined for each reference value. Range of reference values are 0.2–0.3 mg l^?1 (L-AL3) and 0.5–0.7 mg l^?1 (L-AL4) for total biovolume and 1.5–1.9 μg l^?1 (L-AL3) and 2.7–3.3 μg l^?1 (L-AL4) for chlorophyll-a. Depending on lake type and variable, the ecological quality ratios (EQR) for setting the class boundaries lie between 0.60 and 0.75 for the high/good class boundary and between 0.25 and 0.41 for the good/moderate class boundary. The response of sensitive phytoplankton taxa along a nutrient gradient and the occurrence of ‘undesirable conditions and secondary effects’ as defined in the WFD was used to validate the class boundary values, which are thus considered to be compliant with the requirements of the WFD.     

Lake Muzahi,Rwanda: limnological features and phytoplankton production

Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y⁻¹ in the fifties vs 30 T y⁻¹ in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 μS cm⁻¹ at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NH _inf4 ^sup+ -N can be high. Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m⁻² in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O₂ m⁻² d⁻¹ with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d⁻¹ for a mixed layer of 4 m) In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha⁻¹ y⁻¹, the yield estimated from primary production ranges between 46 and 64 kg ha⁻¹ y⁻¹. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.