Longitudinal and temporal patterns of benthic coarse particulate organic matter in the Agüera stream (northern Spain)

Benthic coarse particulate organic matter (CPOM) was studied between November 92 and December 93 at four sites along the longitudinal gradient of the Agüera stream system (Northern Spain). CPOM was sorted in four main categories: leaves (several species), fruits and seeds, twigs and debris. Headwater site showed higher densities of total CPOM, leaves and twigs than downstream reaches, but no regular longitudinal pattern of change was noticed. The ranges of mean CPOM standing stock at the sampling sites were 20.5–74.1 g AFDW m^–2 (site B), 9.9–47.7 g AFDW m^–2 (site 5), 4.3–21.4g AFDW m^–2 (site 7) and 9.8–37.9 g AFDW m^–2 (site 9). The particulate matter at downstream sites was in a more advanced stage of breakdown probably as a result of processing and transport from upstream reaches. Leaves species composition of benthic CPOM clearly reflected the type of riparian vegetation at each site. The timing of inputs and the hydrologic regime appeared to act together influencing temporal dynamics of benthic CPOM. A gradual temporal change in species composition of benthic leaf litter was observed under natural mature deciduous forest: first alder, later chestnut and finally oak.     

Effect of plant richness on the dynamics of coarse particulate organic matter in a Brazilian Savannah stream

The high plant richness in riparian zones of tropical forest streams and the relationship with an input of organic matter in these streams are not well understood. In this study, we assessed (i) the annual dynamics of inputs of coarse particulate organic matter (CPOM) in a tropical stream; and (ii) the relationship of species richness on riparian vegetation biomass. The fluxes and stock of CPOM inputs (vertical-VI = 512, horizontal-HI = 1912, and terrestrial-TI = 383 g/m²/year) and the benthic stock (BS = 67 g/m²/month) were separated into reproductive parts, vegetative parts and unidentified material. Leaves that entered the stream were identified and found to constitute 64 morphospecies. A positive relationship between species richness and litterfall was detected. The dynamics of CPOM were strongly influenced by rainfall and seasonal events, such as strong winds at the end of the dry season. Leaves contributed most to CPOM dynamics; leaf input was more intense at the end of the dry season (hydric stress) and the start of the rainy season (mechanical removal). Our study show an increase of litter input of CPOM by plant diversity throughout the year. Each riparian plant species contributes uniquely to the availability of energy resources, thus highlighting the importance of plant conservation for maintaining tropical streams functioning.     

Periphyton communities in Amazonian black- and whitewater habitats: Community structure, biomass and productivity

Chlorophyll a-concentrations, AFWD (ash-free-dry-weight) and photosynthesis rates were estimated for periphyton assemblages in Amazonian black-and white-water habitats over 14 months. Cellulose-acetate strips were incubated in situ and showed few major differences in periphyton quality as compared to natural substrata. The only exceptions were submersed Igapó forest leaves, which exhibited higher proportions of green algae and cyanobacteria though not producing differences in total periphyton biomass. Enclosure experiments showed a considerable nutrient release by inundated non-senescent Igapó forest leaves. Periphyton biomass and productivity were found to be highest in black-and white water mixing zones, where biomass peaked at 41.6 mg Chla/m² and 19.8 g/m² AFDW. Production was estimated to be 380 gC/m²·a. Maximum biomass of periphyton in floating meadows was 46 mg Chla/m² and 10.6 g/m² AFDW, with an annual production of 170 gC/m²·a. Solimões main channel periphyton values were low: maximum Chla was 7.1 mg/m², AFDW 0.8 g/m² and annual production was estimated to be 30 gC/m². Blackwater periphyton values were lower compared to whitewater and mixed water values but an enlarged trophogenic zone has to be taken into account. Highest Chla content reached 30.9 mg/m², AFDW 1.43 g/m². Estimated annual production was 110 gC/m². Observed mean periphyton productivity of Amazonian blackwater habitats approximately corresponded to mesotrophic attached algae productivity in temperate zones, whereas productivity of whitewater periphyton approached those of temperate eutrophic lakes. The role of periphyton in the Amazon food web is discussed.     

Seasonal variations in phytoplankton biomass and primary production in the Ethiopian Rift Valley lakes Ziway, Awassa and Chamo – The basis for fish production

Seasonal variations in the biomass (Chl a) and primary production (¹⁴C-method) of phytoplankton were studied during 12 months of 2005 in the three Ethiopian Rift Valley Lakes (ERVL) Ziway, Awassa and Chamo. Chl a showed an average value of 40, 20, and 30 mg m⁻³ for the three lakes, respectively. Integrated areal primary production for the total phytoplankton (g C m⁻² d⁻¹) varied 2-fold in the three lakes but on different levels, from 0.67–1.8 in L. Ziway, 1.8–4.6 in L. Awassa, and 1.0–2.6 in L. Chamo. The overall photosynthetic efficiency of utilizing photosynthetically active radiation by the phytoplankton on molar basis (mmol C mol of photons⁻¹) resulted in an average value of 1.4 for L. Ziway, 3.5 for L. Awassa and 1.6 for L. Chamo. Among the different factors regulating phytoplankton primary productivity, light penetration and nutrients were the most important in the three lakes. The seasonal variations of incident radiation (most values between 5 and 7 E m⁻² h⁻¹) and water temperature (most values between 22 and 24 °C) were small and unlikely to result in the marked differences in phytoplankton primary production. Although relative increase in nutrient concentrations occurred following the rainy periods, the major algal nutrients were either consistently low (nitrate and/or silicate) or high (phosphate and/or ammonium) and remained within a narrow range for most of the study period in all the three lakes. Consequently, phytoplankton biomass and primary production seem to be maintained more by nutrient regeneration or turnover (facilitated by high temperature) than by allochthonous nutrient input. This would be coupled with wind-induced mixing that would play an important role in determining hydrographic characteristics (water column structure) and the associated redistribution of nutrients and phytoplankton, the availability of light and subsequently the spatial (vertical) and temporal patterns of phytoplankton production in these three ERVL. Phytoplankton production (PP) is regarded as a good predictor of fish yield in lakes and seasonal measurements of PP is a prerequisite for good such estimates.     

Substrate influence and temporal changes on periphytic biomass accrual and metabolism in a tropical humic lagoon

We performed a field experiment in a tropical humic coastal lagoon to evaluate periphyton biomass accrual and metabolism on three different substrates (1) plastic ribbons, (2) green and (3) senescent leaves of the emergent macrophyte Typha domingensis) over 30 days. The contribution of autotrophic biomass decreased as total biomass increased over the time. Mean periphytic ash free dry weight ranged from 0.8 to 5.6 mg cm⁻², but periphyton chlorophyll a concentrations presented shorter amplitudes, which oscillated from 0.12 to 0.44 μg cm⁻² throughout the experiment. Periphyton metabolism was overall heterotrophic on all substrates, especially on senescent leaves. Our data show that substrate type influenced both biomass accrual and periphyton net productivity and respiration rates throughout periphyton development and highlighted the dominance of heterotrophic metabolism. The periphyton respiration may be subsidized by both water- and substrate-derived allochthonous energy pathways, shedding light on the role of periphytic assemblages to the carbon cycling, as a source of CO₂ to the system.     

Light climate as the key factor controlling the spring dynamics of phytoplankton in Lake Zürich

During the spring seasons of 1983, 1986 and 1987 the development of phytoplankton in Lake Zürich was investigated (from February to May) using samples taken at short term intervals. The aim was to describe the effects of the short term dynamics of environmental factors on the algal growth. The results could then be used to discuss the existing theories to assess the start of phytoplankton growth pulses in spring. Only 7 to 10 days without wind driven vertical mixing were required in spring to start the first growth pulse, despite of a still very unstable water column (sometimes inverse thermal stratification). Mainly flagellates andStephanodiscus hantzschii increased their biomass and achieved net growth rates of 0.1 and up to 0.65 d⁻¹ respectively. During such a phase the mixing depth was always smaller than the euphotic depth. Later on, at the start of the spring bloom (=last growth pulse in spring before the clear water stage), the intensity of vertical mixing as well as the mixing depth were markedly reduced due to an increase in heat input and low wind. Then flagellates dominated (contribution up to 75.5% of the areal biomass reaching 60 g fresh weight m⁻²) and the growth rate rose to a maximum of 0.65 d⁻¹. Standard models of critical depth considers that there is only a biomass increase if the mixing depth is smaller than the depth of a water layer positive balanced between production and respiration. This model for determining the beginning of a phytoplankton growth pulse in spring takes no account of the favorable light conditions for phytoplankton cells at calm and sunny days in February and March. The newly developed threshold value model takes these situations into account: It assumes that the phytoplankton biomass increases when the calculated effective light climate is equal or greater than a previously fixed threshold. The calculations are based on the mean light intensity within the mixed layer at windy days or within the euphotic depth (z _eu) at calm days. In Lake Zürich a minimum of 0.2 10⁶ J m⁻²d⁻¹ (=0.9 mol quanta m⁻²d⁻¹) has to be reached or surpassed in at least 3 days before an exponential increase of algal biomass can occur. The value does not depend on short term fluctuations in neither radiation nor mixing depth. It seems that this value is rather low comparing with those of investigations in other water bodies (up to 0.8 10⁶ J m⁻² d⁻¹) but high related to values from algal cultures (0.02 10⁶ J m⁻²d⁻¹). As the weather can only be forecasted a few days ahead with any certainty the period for a more or less accurate prediction of an algal bloom is restricted to about 1 to 5 days.     

草型湖泊与藻型湖泊大型底栖动物生产力的比较

较系统地比较了典型草型湖泊(扁担塘)与典型藻型湖泊(后湖)底栖动物优势种类、主要类群及整个群落的周年生产量.结果表明,典型草型湖泊扁担塘底栖动物群落的周年生产量(湿重或带壳湿重)38.926g/穴m2·a雪比典型藻型湖泊后湖47.505g/穴m2·a雪为低.各类群具体情况为,后湖(藻型湖泊)的营养水平较高,更适合苏氏尾鳃蚓等耐污性种类的生长,因此寡毛类生产量较扁担塘的为高;藻型湖泊比草型湖泊更适合铜锈环棱螺等腐生螺类的生长,螺类的生产量也比草型湖泊略高;藻型湖泊摇蚊的生产量较高,原因在于藻型湖泊更适合具有较多耐污种类的摇蚊生长.但从大型底栖动物群落生产量的组成看,摇蚊、软体动物、寡毛类等三大类群所占份额相对较稳定,分别为14%-15%、77%-79%、7%-8%.     

Variations in light absorption properties during a phytoplankton bloom in the Pearl River estuary

From 15 to 28 August in 2007, a Chaetoceros socialis bloom was detected in the Pearl River Estuary water with chlorophyll a concentration (Chl a) up to 30 mg m⁻³ and cell density up to 10⁶ cells L⁻¹. Time series of bio-optical measurements was obtained at a single site (114.29°E, 22.06°N) with the mooring of marine optical buoy. Light absorption properties of seawater experienced large variability throughout the algal bloom. Absorption by colored dissolved organic matter (CDOM) was one of the dominant optical components of the light absorption (30–70%) especially for pre- and post-bloom waters, and it tended to decrease with Chl a during the algal bloom. Absorption by phytoplankton was another dominant optical component (18–50%) and increased rapidly with Chl a. Phytoplankton and accompanying material played dominant roles in light absorption as indicated by the relationship between absorption coefficient and Chl a. At high pigment concentrations, water samples showed significantly lower specific phytoplankton absorption, compared with pre- and post-bloom conditions, with the specific phytoplankton concentration at 443 nm varied between 0.011 and 0.022 m² mg⁻¹ and that at 676 nm between 0.007 and 0.018 m² mg⁻¹; small values of blue-to-red ratio of phytoplankton were also observed. These lower values were associated with variations in phytoplankton size structure. Spectral variability of phytoplankton absorption and total absorption (not including the fixed background absorption by pure water itself) could be expressed as simple linear functions linking absorption at one wavelength to the absorption at the other wavelengths, with the slope of the relationship changing with wavelength. The absorption coefficients by non-algal particles and CDOM follow the general exponential functions with remarkably limited variability in the exponent with means of 0.0105 and 0.0166 nm⁻¹, respectively. These spectral dependencies of absorption coefficients provide useful information for retrieving inherent optical properties from reflectance data in a remote-sensing context.     

Variation in diet across an elevational gradient in the larvae of two Hydropsyche species (Trichoptera)

While specialized species are linked to a particular resource, omnivorous species may switch between food items according to the availability and the quality of resources. Here we use larvae of the omnivorous caddisfly genus Hydropsyche (Trichoptera) to analyse changes in diet composition across an elevational gradient. Periphyton and Hydropsyche larvae were sampled from 22 populations at stream orders from 2 to 5 on the German part of the Bohemian Forest. Elevations of sampling sites ranged between 300 m and 900 m a.s.l.. Diet composition was estimated by the analyses of the gut content of larvae as well as by stable nitrogen isotopes (δ¹⁵N). The δ¹⁵N values of the periphyton decreased and the C/N ratio of periphyton increased with increasing environmental harshness (decreasing water pH, temperature and conductivity with increasing elevation) indicating a decrease of periphyton food quality. Across individuals, the proportion of animals in the gut of Hydropsyche larvae was positively related to the difference of δ¹⁵N values between larvae and periphyton. The proportion of animals within the gut and (baseline corrected) δ¹⁵N values of Hydropsyche populations increased with increasing environmental harshness. We suggest that the (i) low primary production caused by shading, low temperatures and low nutrient levels, (ii) the low nutrient quality of periphyton and (iii) the availability of animal prey due to the input of allochthonous resources in headwaters caused the shift in the diet of Hydropsyche larvae along the river continuum.     

Phytoplankton biomass and primary production responses to physico-chemical forcing across the northeastern New Zealand continental shelf

Phytoplankton biomass and primary production were monitored in the Hauraki Gulf and on the northeastern continental shelf, New Zealand - using ship surveys, moored instruments and satellite observations (1998-2001) - capturing variability across a range of space and time scales. A depth-integrated primary production model (DIM) was used to predict integrated productivity from surface parameters, enabling regional-specific estimates from satellite data. The shelf site was dominated by pico-phytoplankton, with low chlorophyll-a (<1 mg m⁻³) and annual production (136 g C m⁻² yr⁻¹). In contrast, the gulf contained a micro/nano-phytoplankton-dominated community, with relatively high chlorophyll-a (>1 mg m⁻³) and annual production (178 g C m⁻² yr⁻¹). Biomass and productivity responded to physico-chemical factors; a combination of light, critical mixing depths and/or nutrient limitation—particularly new nitrate-N. Relatively low biomass and production was observed during 1999. This coincided with inter-annual variability in the timing and extent of upwelling- and downwelling-favourable along-shelf wind-stress, influencing the fluxes of new nitrate-N to the shelf and gulf. Relationships with the Southern Oscillation Index are also discussed. Our multi-scaled sampling highlighted details associated with stratification and de-stratification events, and deep sub-surface chlorophyll-a not visible to satellite sensors. This study demonstrates the importance of multi-scaled sampling in gaining estimates of regional production and its responses to physico-chemical forcing.     

Reduction of nutrient loading, planktivore removal and piscivore stocking as tools in water quality management: The feldberger haussee biomanipulation project

The Feldberger Haussee, a highly eutrophic stratified hard-water lake located in the eastern part of Germany's Baltic lake district, was selected for a restoration programme combining external nutrient loading reduction and long-term biomanipulation. In 1980 the external phosphorus loading (1.8 g TP m⁻² a⁻¹) decreased by 90%, but water quality did not improve significantly within the following 6 years. In 1985 biomanipulation was initiated, with manual removal of cyprinid fish coupled with piscivore introductions as the principal measures. The expected changes in the pelagic community and improvements of water quality occurred after a delay of several years. Despite intensive manual removal of cyprinids and stocking of piscivores, standing stocks of cyprinid fish remained relatively high (130-260 kg ww ha⁻¹) after some years of decline. Compared to the pre-biomanipulation period, mean seasonal (May-September) Daphnia spp. biomass roughly doubled (0.037 g C m⁻³vs. 0.084 g C m⁻³). However, the predominance of small (<1 mm) D. cucullata throughout the whole investigation period indicated that planktivory was still substantial. Paired observations between edible phytoplankton biomass and Daphnia spp. indicated that a significant decline in algal stocks would only occur if herbivorous biomass was above a certain threshold (0.2 g C m⁻³). Reduced external and internal loading in concert with pelagic calcite precipitation were most likely responsible for the decline in lake phosphorus concentrations, thereby substantially improving the water quality of Feldberger Haussee. Although this is not supported by quantitative evidence, we hypothesize that resource-related water quality improvements were caused by changes in the structure of the pelagic community leading to increased calcite precipitation. In agreement with the results of other investigations, we conclude that because stabilising mechanisms such as macrophyte growth were lacking in Feldberger Haussee, biomanipulation in stratified lakes may not be as successful as has been observed in shallow lakes. However, in hard-water lakes, calcite precipitation may act as another stabilising resource-related mechanism. Phosphorus associated with sedimenting calcite particles is insensitive to redox-conditions and may therefore not be re-mobilised from lake sediments even if hypolimnetic oxygen is depleted.     

Riparian alder fens — source or sink for nutrients and dissolved organic carbon? — 2. Major sources and sinks

Natural riparian forest wetlands are known to be effective in their ability to remove nitrate by denitrification and sediments with attached phosphorus via sedimentation. On the other hand, litter input and decomposition is a process of crucial importance in cycling of nitrogen and phosphorus in a forest ecosystem.In this study we investigated the amount of nitrogen and phosphorus entering the alder fen ecosystem through leaf litter and its decomposition and the removal capacity of nitrogen and phosphorus by measuring denitrification and sedimentation in the alder fen.We found an average input of leaf litter during fall 1998 of 226 g m⁻² yr⁻¹ DW with nutrient concentration of 0.17% P and 1.6% N. This means a yearly input of 0.4 g m⁻² yr⁻¹ P and 3.6 g m⁻² yr⁻¹ N. The decomposition of leaf litter using litter bags with small and large mesh size resulted in bags with macroinvertebrates (large mesh size) and without macroinvertebrates (small mesh size). After 57 days the litter bags with macroinvertebrates had a decomposition rate of 79%.Denitrification was measured in May and June of 1997 using the acetylene inhibition technique on intact soil cores and slurry-experiments. The average annual denitrification rate was 0.2 g m⁻² yr⁻¹ N using data from the core experiments. The denitrification rate was higher after addition of nitrate, indicating that denitrification in the riparian alder fen is mainly controlled by nitrate supply.The sedimentation rate in the investigated alder fen ranged from 0.47 kg m⁻² yr⁻¹ DW to 4.46 kg m⁻² yr⁻¹ DW in 1998 depending on the study site and method we used. Sedimentation rates were lower in newly designed plate traps than in cylinder traps. The alder fen also showed lower rates than the adjacent creek Briese. Average phosphorus removal rate was 0.33 g m⁻² yr⁻¹ P.Input sources for the surface water of the alder fen are sediment mineralization and decomposition of leaf litter; output sources are sedimentation and denitrification. This study showed that a nutrient input of 24.58 kg ha⁻¹ yr⁻¹ N, 8.8 kg ha⁻¹ yr⁻¹ P and 419 kg ha⁻¹ yr⁻¹ DOC into the surface water of the alder fen is possible. Alder fens cannot improve water quality of an adjacent river system. This is only true for a nearly pristine alder fen with the hydrology of 10 months flooded conditions and 2 months non-flooding conditions a year.     

Pelagic trophic transfer efficiency in an oligotrophic, dimictic deep lake (Lake Stechlin, Germany) and its relation to fisheries yield

We investigated trophic transfer efficiency in the pelagic food chain of deep, oligotrophic Lake Stechlin (Germany) by analyses of the primary, secondary, and fish production. Primary production between April and November 2000 was estimated at 78 g C m⁻², pelagic secondary production at 14 g C m⁻², and production of the main planktivorous fish species [European cisco, Coregonus albula (L.)] at 0.77 g C m⁻². Thus, trophic transfer efficiency between primary and pelagic secondary production was around 18%, whereas between pelagic and fish production around 6%. The high efficiency at the first step of the chain is discussed to be due to the high food quality in oligotrophic lakes due to the dominance of Bacillariophyceae and Chlorophyceae rich in essential fatty acids. In turn, the relatively low trophic transfer efficiency between the secondary and the fish production is mainly explained by the avoidance of calanoid copepods as food source by the ciscoes. Concerning the trophic transfer efficiency, results from this study support the general assumption of a 10% transfer between neighbouring trophic levels within ecosystems.     

Sedimentary inputs and oxygen uptake by the sediment in Lake Geneva (Switzerland)

Sediment cores and sediment traps were collected twice a month in two 35 m deep stations of Lake Geneva (Switzerland). The organic input sedimenting to the bottom is equal to 157 g C m⁻²y⁻¹ in station 1, to 214 g C in station 2. In spite of this difference, the oxygen uptake by the sediment (OUS) is similar in both locations (46–47 g C m⁻²y⁻¹). The oxygen uptake by the matter sedimenting to the bottom (OUSM) is respectively 45 g C m⁻²y⁻¹ and 41 g C in stations 1 and 2. The equivalence between OUS and OUSM implies that most of the sedimented matter arriving to the bottom is directly oxidized at the sediment surface. In station 1, OUS is positively correlated to OUSM, and OUSM is positively correlated to chlorophyll-a concentrations in the water column (0–20 m) one week before sediment sampling. In location 2, OUS is positively correlated to the percentage of organic carbon and nitrogen in the sedimented matter, negatively to its C:N ratio. Increasing allochthonous inputs have a negative influence on benthic respiration. At both sites, OUS is not directly related to macrobenthic biomass or to temperature of bottom water.     

The contribution of various types of settling particles to the flux of organic carbon in the Gulf of St. Lawrence

The contents of 31 samples from free-drifting sediment traps deployed in the Gulf of St. Lawrence (GSL) were analyzed for the individual contribution of the different types of particles encountered to the total particulate organic carbon (POC) flux. Two trap models were used in 1993-1994: small traps at 50 m depth and large traps at 50 and 150 m. Total POC fluxes averaged 42 mg C m⁻² d⁻¹ for the more reliable large trap and 149 mg C m⁻² d⁻¹ for the small trap. The POC fluxes were attributed to different classes of particles based upon microscopically determined particle dimensions and carbon/volume algorithms available in the literature. Fecal pellets, followed by phytoplankton, were the major attributable components, with important contributions by microzooplankton, particularly during the summer of 1994. The mean fluxes for pellets (6 and 60 mg  C m⁻² d⁻¹, for the large and small traps, respectively) and phytoplankton (3.2 and 42.9 mg C m⁻² d⁻¹) were in the range of those encountered in other areas of moderate primary productivity. Mean zooplankton carbon fluxes (1.8 and 8.5 mg C m⁻² d⁻¹, respectively), however, reflect higher than average zooplankton abundances in the GSL. The C fluxes of specific algal groups confirmed the existence of three trophic regimes previously identified from water column studies and numeric cell fluxes: (1) a period when diatoms were dominant during the spring, (2) a longer interval, which was dominated by dinoflagellates at most others times of the year, and (3) a period of transition during summer. Carbon of animal origin dominated the attributable flux, including an important fraction associated with heterotrophic dinoflagellates. The contribution of marine snow to the total flux (estimated as the difference between the total POC flux and the sum of the attributed components) frequently amounted to more than 60%. The true importance of marine snow remains uncertain, however, because the errors associated with each of the measured components accumulate to produce large uncertainties. The methodological problems involved are discussed.     

A 14?500 year record of the accumulation of atmospheric mercury in peat: volcanic signals, anthropogenic influences and a correlation to bromine accumulation

A 14?500 calendar year record of mercury accumulation rates has been obtained from an ombrotrophic peat bog in the Swiss Jura mountains. The range of natural (pre-industrial) mercury accumulation rates varied from 0.3 to 8.0 μg m⁻² yr⁻¹. During the Late Glacial and Holocene, climatic and volcanic signals were evident in the mercury record. Mercury accumulation rates increased by a factor of ca 5 during the Younger Dryas cold period. Short-term spikes in mercury accumulation rates, which correspond in time to known volcanic eruptions, occur during the late Boreal and Older Atlantic periods, when volcanic influences on mercury deposition appear to have been intensified due to increased atmospheric humidity. A correlation of mercury to bromine accumulation is observed throughout the pre-anthropogenic period. During the Holocene, mercury accumulation only exceeded the range of this correlation for a few short periods of elevated mercury deposition which correspond to known volcanic eruptions during the Older Atlantic. During historical times, mercury accumulation rates have exceeded the range of the correlation of mercury to bromine continuously since ca 1330 AD. This excess in mercury accumulation is interpreted as an indication of pollution. During the industrial period, mercury accumulation rates reached 107.6 μg m⁻² yr⁻¹, of which 84% was mercury that exceeded the correlation range. Mercury accumulation rates peaked again during the 1970s, with 78.8 μg m⁻² yr⁻¹. Early 20th century pollution appears to have been dominated by non-Swiss emissions from coal burning, whereas Swiss mercury emissions appear to have been the dominant pollution source during the mid and late 20th century. Current mercury accumulation rates at the site are similar to those ca 10 years ago, with modern deposition rates being ca 15 times their prehistorical average. Anthropogenic emissions of reactive brominated compounds could be contributing to increased atmospheric deposition rates of mercury.     

Dry deposition and particle-size distribution of phosphorus in the marine atmosphere over the northeastern coast of Taiwan

This study is the first to measure the particulate phosphorus, including total inorganic phosphorus (TIP) and organic phosphorus (OP), in size-fractionated atmospheric particles. The results indicate that continental and marine sources are the key controls on the particle-size distribution of phosphorus species. For continental and local anthropogenic sources, both TIP and OP are associated with fine-mode aerosols during the winter and spring, and both are also associated with coarse particles during the summer and autumn. The coarse/fine ratios are low during periods with a non-oceanic source but high at other times, probably because of the biological growing season in the surface waters of the study area. The calculated annual fluxes based on estimates of dual-mode particles are 532±185, 435±172, and 96.8±48.8 μmol m⁻² yr⁻¹ for TP, TIP, and OP, respectively. Based on previously published solubility data for particulate phosphorus (34%), we calculated an annual flux of 180±63 μmol m⁻² yr⁻¹ for readily soluble particulate phosphorus.     

Recent carbon and nitrogen uptake rates of phytoplankton in Bering Strait and the Chukchi Sea

Cruises to Bering Strait and the Chukchi Sea in US waters from late June in 2002 to early September in 2004 and the Russian–American Long-term Census of the Arctic (RUSALCA) research cruise in 2004 covered all major water masses and contributed to a better understanding of the regional physics, nutrient dynamics, and biological systems. The integrated concentration of the high nitrate pool in the central Chukchi Sea was greater in this study than in previous studies, although the highest nitrate concentration (∼22 μM) in the Anadyr Water mass passing through the western side of Bering Strait was consistent with prior observations. The chlorophyll-a concentrations near the western side of the Diomede Islands ranged from 200 to 400 mg chl-a m⁻² and the range in the central Chukchi Sea was 200–500 mg chl-a m⁻² for the 2002–2004 Alpha Helix (HX) cruises. Chlorophyll-a concentrations for the 2004 RUSALCA cruise were lower than those from previous studies. The mean annual primary production of phytoplankton from this study, using a ¹³C–¹⁵N dual-isotope technique, was 55 g C m⁻² for the whole Chukchi Sea and 145 g C m⁻² for the plume of Anadyr–Bering Shelf Water in the central Chukchi Sea. In contrast, the averages of annual total nitrogen production were 13.9 g N m⁻² (S.D.=±16.2 g N m⁻²) and 33.8 g N m⁻² (S.D.=±14.1 g N m⁻²) for the Chukchi Sea and the plume, respectively. These carbon and nitrogen production rates of phytoplankton were consistently two-or three-fold lower than those from previous studies. We suggest that the lower rates in this study, and consequently more unused nitrate in the water column, were caused by lower phytoplankton biomass in the Bering Strait and the Chukchi Sea. However, we do not know if the lower rate of production from this study is a general decreasing trend or simply temporal variations in the Chukchi Sea, since temporal and geographical variations are substantially large and presently unpredictable.