Evidence for active microbial nitrogen transformations in sea ice (Gulf of Bothnia, Baltic Sea) in midwinter

Nutrient concentrations, chlorophyll-a, bacterial biomass and relative activity of denitrifying organisms were investigated from ice-core, brine and underlying water samples in February 1998 in the Gulf of Bothnia, Baltic Sea. Examined sea ice was typical for the Baltic Sea; ice bulk salinity varied from 0.1 to 1.6 psu, and in underlying water salinity was from 4.2 to 4.7 psu. In 2- to 3-months-old sea ice (thickness 0.4–0.6 m), sea-ice communities were at the winter stage; chl-a concentrations were generally below 1 mg m⁻³ and heterotrophic organisms composed 7–20% of organism assemblage. In 1-month-old ice (thickness 0.2–0.25 m), an ice spring bloom was already developing and chl-a concentrations were up to 5.6 mg m⁻³. In relation to low salinity, high concentrations of NH⁺ ₄, NO⁻ ₂, PO³⁺ ₄ and SiOH₄ were found in the ice column. The results suggest that the upper part of ice accumulates atmospheric nutrient load during the ice season, and nutrients in the upper 10–20 cm of ice are mainly of atmospheric origin. The most important biological processes controlling the sea-ice nutrient status are nutrient regeneration, nutrient uptake and nitrogen transformations. Nutrient regeneration is specially active in the middle parts of the 50- to 60-cm-thick ice and subsequent accumulation of nutrients probably enhances the ice spring bloom. Nitrite accumulation and denitrifying activity were located in the same ice layers with nutrient regeneration, which together with the observed significant correlation between the concentrations of nitrogenous nutrients points to active nitrogen transformations occurring in the interior layers of sea ice in the Baltic Sea. Accepted: 12 June 2000     

Successional and physical controls on the retention of nitrogen in an undisturbed boreal forest ecosystem

Floristic succession in the boreal forest can have a dramatic influence on ecosystem nutrient cycling. We predicted that a decrease in plant and microbial demand for nitrogen (N) during the transition from mid- to late-succession forests would induce an increase in the leaching of dissolved inorganic nitrogen (DIN), relative to dissolved organic nitrogen (DON). To test this, we examined the chemistry of the soil solution collected from within and below the main rooting zones of mid- and late-succession forests, located along the Tanana River in interior Alaska. We also used a combination of hydrological and chemical analyses to investigate a key assumption of our methodology: that patterns of soil water movement did not change during this transition. Between stands, there was no difference in the proportion of DIN below the rooting zone. 84–98% of DIN at both depths consisted of nitrate, which was significantly higher in the deeper mineral soil than at the soil surface (0.46±0.12 mg NO⁻ ₃ –N l⁻¹ vs 0.17±0.12 mg NO⁻ ₃ –N l⁻¹, respectively), and 79–92% of the total dissolved N consisted of DON. Contrary to our original assumption that nutrients were primarily leached downward, out of the rooting zone, we found much evidence to suggest that the glacially-fed Tanana River (>200 m from these stands) was contributing to the influx of water and nutrients into the soil active layer of both stands. Soil water potentials were positively correlated with river discharge; and ionic and isotopic (δ¹⁸O of H₂O) values of the soil solution closely matched those of river water. Thus, our ability to elucidate biological control over ecosystem N retention was confounded by riverine nutrient inputs. Climatic warming is likely to extend the season of glacial melt and increase riverine nutrient inputs to forests along glacially-fed rivers.     

Biofiltering efficiency, uptake and assimilation rates of Ulva clathrata (Roth) J. Agardh (Clorophyceae) cultivated in shrimp aquaculture waste water

The growth, biofiltering efficiency and uptake rates of Ulva clathrata were studied in a series of outdoor tanks, receiving waste water directly from a shrimp (Litopenaeus vannamei) aquaculture pond, under constant aeration and two different water regimes: (1) continuous flow, with 1 volume exchange a day (VE day^-1) and (2) static regime, with 1 VE after 4 days. Water temperature, salinity, pH, dissolved inorganic nitrogen (DIN), phosphate (PO₄), chlorophyll-a (chl-a), total suspended solids (TSS), macroalgal biomass (fresh weight) and tissue nutrient assimilation were monitored over 12 days. Ulva clathrata was highly efficient in removing the main inorganic nutrients from effluent water, stripping 70–82% of the total ammonium nitrogen (TAN) and 50% PO₄ within 15 h. Reductions in control tanks were much lower (Tukey HSD, P < 0.05). After 3 days, the mean uptake rates by the seaweed biomass under continuous flow were 3.09 mg DIN g DW day⁻¹ (383 mg DIN m⁻² day⁻¹) and 0.13 mg PO₄ g DW day⁻¹ (99 mg PO₄ m⁻² day⁻¹), being significantly higher than in the static regime (Tukey HSD, P < 0.05). The chl-a decreased in seaweed tanks, suggesting that U. clathrata inhibited phytoplankton growth. Correlations between the cumulative values of DIN removed from the water and total nitrogen assimilated into the seaweed biomass (r = 0.7 and 0.8, P < 0.05), suggest that nutrient removal by U. clathrata dominated over other processes such as phytoplankton and bacterial assimilation, ammonia volatilization and nutrient precipitation.     

Spatial variability in Florida Bay particulate organic matter composition: combining flow cytometry with stable isotope analyses

Long-term management plans for restoration of natural flow conditions through the Everglades increase the importance of understanding potential nutrient impacts of increased freshwater delivery on Florida Bay biogeochemistry. Planktonic communities respond quickly to changes in water quality, thus spatial variability in community composition and relationships to nutrient parameters must be understood in order to evaluate future downstream impacts of modifications to Everglades hydrology. Here we present initial results combining flow cytometry analyses of phytoplankton and bacterial populations (0.1–50 μm size fraction) with measurements of δ¹³C and δ¹⁵N composition and dissolved inorganic nutrient concentrations to explore proxies for planktonic species assemblage compositions and nutrient cycling. Particulate organic material in the 0.1–50 μm size fraction was collected from five stations in Northeastern and Western Florida Bay to characterize spatial variability in species assemblage and stable isotopic composition. A dense bloom of the picocyanobacterium, Synechococcus elongatus, was observed at Western Florida Bay sites. Smaller Synechococcus sp. were present at Northeast sites in much lower abundance. Bacteria and detrital particles were also more abundant at Western Florida Bay stations than in the northeast region. The highest abundance of detritus occurred at Trout Creek, which receives freshwater discharge from the Everglades through Taylor Slough. In terms of nutrient availability and stable isotopic values, the S. elongatus population in the Western bay corresponded to low DIN (0.5 μM NH ₄ ⁺ ; 0.2 μM NO ₃ ⁻ ) concentrations and depleted δ¹⁵N signatures ranging from +0.3 to +0.8‰, suggesting that the bloom supported high productivity levels through N₂-fixation. δ¹⁵N values from the Northeast bay were more enriched (+2.0 to +3.0‰), characteristic of N-recycling. δ¹³C values were similar for all marine Florida Bay stations, ranging from −17.6 to −14.4‰, however were more depleted at the mangrove ecotone station (−25.5 to −22.3‰). The difference in the isotopic values reflects differences in carbon sources. These findings imply that variations in resource availability and nutrient sources exert significant control over planktonic community composition, which is reflected by stable isotopic signatures.     

Phytoplankton dynamics in the stratified water column of Lake Bonney,Antarctica

Phytoplankton populations in perennially ice-covered Lake Bonney, Antarctica grow in a unique non-turbulent environment. The absence of turbulence generated by winds or major streams, combined with strong vertical gradients in temperature and nutrients, create vertically stratified environmental conditions that support three discrete phytoplankton populations in the east lobe of this lake. Phytoplankton biomass and photosynthesis were measured in the east lobe of Lake Bonney during the winter-spring transicion (September) to mid-summer (January). During this period, irradiance beneath the ice increased from 0.03 to 1.9 mol quanta m⁻² d⁻¹. Chlorophylla concentrations ranged from 0.03 to 3.8 μl⁻¹ within the trophogenic zone (just beneath the permanent ice cover to 20 m) and photosynthesis ranged from below detection to 3.2 μg Cl⁻¹ d⁻¹. Our results indicate: (1) phytoplankton photosynthesis began in late winter (before 9 September, our earliest sampling date); (2) maxima for phytoplankton biomass and production developed sequentially in time from the top to the bottom of the trophogenic zone, following the seasoral increase in irradiance; and (3) the highest photosynthetic efficiencies occurred in early spring, then decreased over the remainder of the phytoplankton growth season. The spring decrease in photosynthetic rates for shallower phytoplankton appeared to be related to nutrient availability, while photosynthesis in the deeper populations was solely lightdependent.     

Seasonal variation in primary production of a large high altitude tropical lake (Lake Tana,Ethiopia): effects of nutrient availability and water transparency

Primary production rates, chlorophyll and phytoplankton biovolume were measured monthly from April 2003 to November 2004 in Lake Tana, a large tropical lake in the highlands of Ethiopia. The lake is characterised by low nutrient concentrations, and a low water transparency due to high silt load of the inflowing rivers during the rainy seasons (May–November) and daily resuspension of sediments in the inshore zone. The mean chlorophyll-a concentrations varied seasonally and ranged from 2.6 mg m⁻³ to 8.5 mg m⁻³ (mean: 4.5 mg m⁻³) in the offshore zone. Primary production was measured using the light–dark bottles technique. We incubated only at three depths, i.e. 0.6, 1.2 and 1.8 m. Therefore, we may have missed a substantial part of the depth production profile and probably also frequently missed P _max. Gross primary production in the openwater averaged 2.43 g O₂ m⁻² d⁻¹ and ranged between 0.03 g O₂ m⁻² d⁻¹ and 10.2 g O₂ m⁻² d⁻¹; production was significantly higher in the inshore zone. The highest production rates were observed in the post-rainy season (Oct–Nov), which coincided with a bloom of Microcystis and higher chlorophyll levels. This seasonal high production is probably caused by a relatively high nutrient availability in combination with favourable light conditions. The gross primary production rates of L. Tana are among the lowest compared with other tropical lakes. This will be partly the result of our underestimation of gross primary production by often missing P _max. Another cause is the oligotrophic nature of the lake in combination with its relatively low water transparency. The gross primary production per unit chlorophyll in the openwater zone was in the same range as in 30 other tropical lakes and reservoirs. The higher primary production in the inshore zone is probably the result of the daily water column mixing (Z _mix ≥ Z _t) in this area, enhancing nutrient recycling. A large proportion of the annual primary production is realised in one of the four seasons only. This productive post-rainy season is relatively short (2 months) and therefore efficiency of transfer of matter between the first and second trophic level of the Lake ecosystem will be poor.     

Hypereutrophication events in the Ca’Pisani lagoons associated with intensive aquaculture

Parameters of ecosystem structure and functioning were analyzed in three hypereutrophic lagoons of Ca’Pisani during the season of 2001. Lagoons are situated at wetlands of the NW Adriatic in the vicinity of the Porto Viro, Po River delta. They are associated with intensive fish culture enterprise and accept its wastewater. In June, the lagoons were found overloaded with the biomass of nitrophylic algae. At the end of July, a bloom of potentially toxic dinoflagellate Alexandrium tamarense occurred. Soon, it was supplanted by the picocyanobacterial assemblage, which arrived into the lagoons from the coastal Adriatic via the Marine channel. Wet biomass of this new picocyanobacterial bloom arrived in September attained 30–60 g m⁻³. Decrease of white disk water transparency down to 30–40 cm resulted in a gross mortality of macrophytes accompanied by spreading of floating saprobic alga Enteromorpha. Phytoplankton was dominated in June to July by small mixotrophic phytoflagellates with a wet biomass of 200–1300 mg m⁻³. Number of bacterioplankton ranged between 4 and 7 × 10⁶ ml⁻¹ and its wet biomass between 1.4 and 2.1 g m⁻¹. Its maximum of 18 × 10⁶ ml⁻¹ was observed in late August, when the mortality of macrophytes had occurred. Zooplankton and zoobenthos were found depleted in the lagoons especially during the blooms. Diel fluctuations of dissolved oxygen in the lagoons in June to July reached 150–200% of saturation. Photosynthetic oxygen production ranged between 15 and 30 g O₂ m⁻² d⁻¹. Water column deoxygenation rate was 1–1.5 mg O₂ l⁻¹ h⁻¹. Total photosynthesis production reached 3–8 g C m⁻² d⁻¹ by the share of phytoplankton 5–15%. Hyper-accumulation of total phosphorus in the water column and of toxic labile sulfides in the bottom sediments was documented. Content of inorganic phosphorus in water remained unusually high even by its intensive uptake by microplankton. The PO₄P uptake rate measured with ³²P-label ranged during the bloom of picocyanobacteria between 10 and 50 nM l⁻¹ min⁻¹, and the residence time of PO₄P between 15 and 50 min. The data were generalized via the calculation of energy balance and the deduction of the energy flow scheme in the ecosystem. Their analysis demonstrates the invalidation of ecosystems in hypereutrophic lagoons due to their overload with organic matter, with nutrients and with labile sulfides. After having depleted their animal food web, they are unable to decompose local plus external organic loading. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Nutrients in precipitation and the phytoplankton responses to enrichment in surface waters of the Albemarle Peninsula,NC, USA after the establishment of a large-scale chicken egg farm

Nitrogen (N) and phosphorus (P) released in waste from animal feeding operations can stimulate phytoplankton biomass production in local receiving waters. Changes in weekly wet atmospheric N and P were measured from 2005 to 2008 at monitoring stations located 0.8, 7.9, and 10.3 km downwind from a new chicken egg production facility on the Albemarle Peninsula, North Carolina. After this farm began operating, there was a significant doubling in mean wet NH₄ ⁺ concentrations (465–1,022 μg l⁻¹) at 0.8 km with no change at the other sites. To measure the phytoplankton responses to nutrient enrichment, we conducted seasonal N and P enrichment bioassays from 2006 to 2008 on nearby Phelps Lake and Alligator River. These low-nutrient waters responded to nutrient additions, with the highest increases in phytoplankton primary productivity (¹⁴C uptake) and biomass (chlorophyll a) occurring in the combined N and P treatments suggesting co-limitation of N and P. Although we did not find an increased nutrient signal in precipitation at sites >0.8 km from the farm, there is the potential for atmospheric deposition of N to these and other waters located N/NE of the farm given prevailing winds and distance that NH₄ ⁺ aerosols can travel. Furthermore, surface runoff from the farm may impact receiving waters downstream (e.g., Pungo and Pamlico Rivers). In order to prevent excessive phytoplankton productivity and biomass both N and P inputs should be carefully assessed and potentially controlled in these nutrient-sensitive waters.     

Short-term response of nutrients,carbon and planktonic microbial communities to floodplain wetland inundation

Environmental flows were released to the Macquarie Marshes (~210,000 ha) in north-west NSW of Australia between October and December 2005, inundating an estimated 24,600 ha of floodplain area. According to the flood pulse concept, the marsh floodplains would have stored large amounts of nutrients and carbon during dry antecedent conditions, which would be released into the overlaying flood water. Field studies were conducted in mid-December 2005 at two sites, one on open floodplain woodland with a sparse canopy of River Red Gum and ground cover dominated by saltbushes and the other on open floodplain with black roly-poly. At each site, nutrients, dissolved organic carbon (DOC), planktonic bacteria and phytoplankton were monitored daily for a 6-day period from the overlaying water of a floodplain inundated by the environmental water release. Those in mesocosms deployed in situ, containing marsh floodplain sediments that had been inundated artificially, were also monitored. The mesocosm results from both the sites showed that release of nitrogen was rapid, attaining mean concentrations of total nitrogen of 3.7–14.8 mg l⁻¹, followed by more gradual increases in total phosphorus (mean concentrations 0.6–0.8 mg l⁻¹) and DOC (26.1–50.2 mg l⁻¹) within the 6-day experiment; planktonic microbial communities developed concomitantly with the increasing concentrations of nutrients and DOC, attaining mean densities of (6.0–6.9) × 10⁶ cells ml⁻¹ of planktonic bacteria and (80.7–81.4) × 10³ cells ml⁻¹ of phytoplankton; and for each site the overall measured condition of the mesocosm tended to approach that of the Marshes, over the course of the 6-day experiment. The present study (both observational and experimental) demonstrates that the floodplain sediments in the Marshes, which have been exposed to dry antecedent conditions, release nutrients and carbon to the overlaying flood water following inundation. These resources are thought to have been stored during the dry antecedent phase in accord with the flood pulse concept. Based on the mesocosm experiment, the released nutrients and carbon are in turn most likely to be used by microbial components, such as bacteria and algae, which develop within days of inundation of the floodplain sediments. Thus, environmental flow release provides potential for floodplains to attain a series of ecological responses including initial release of inorganic nutrients and dissolved organic matter and increase in planktonic bacteria and phytoplankton.     

Effects of the halocline on water quality and phytoplankton composition in a shallow brackish lake (Lake Obuchi, Japan)

The relationships of the halocline to both water quality and phytoplankton composition in Lake Obuchi, a shallow brackish lake in northern Japan, were investigated from April 2001 to December 2004. The halocline in this lake became stronger in summer (July–September, mean maximum density gradient 4.3–5.8 ρ_tm⁻¹) but weaker in spring, fall, and winter (1.9–3.3 ρ_tm⁻¹). Although the difference in water quality between the upper and lower layers separated by the halocline was high in summer, nutrients (PO₄³⁻-P and NH₄⁺-N) were eluted from the bottom sediment as levels of dissolved oxygen decreased in the bottom layer because of the strong stratification caused by the halocline formed over the long term. Moreover, phytoplankton taxa composition also differed between the upper and lower layers in summer, but was similar in other seasons. The dominant phytoplankton taxa in the upper layer in summer were Skeletonema costatum and Cyclotella spp., whereas in the lower layer, Gymnodinium spp. (Dinophyceae) and Chlorophyceae, which prefer eutrophic and low dissolved oxygen conditions, dominated. This suggests that the halocline was related to differentiations in both water quality and ecosystem components between the upper and lower layers in the brackish lake water.     

Variability of nutrients and phytoplankton biomass in a shallow brackish water ecosystem (Chilika Lagoon, India)

Seasonal and spatial variations in water quality parameters, such as nutrients [NH₄ ⁺–N, NO₂⁻–N, NO₃⁻–N, PO₄³⁻–P, total nitrogen (TN) and total phosphorus (TP)], Secchi disc depth, salinity, dissolved oxygen, chlorophyll a, primary productivity and phytoplankton standing stock, were studied in Chilika Lagoon (from 27 sampling locations) during 2001–2003 to assess the present ecological status. The study was undertaken after a major hydrological intervention in September 2000, which connected the lagoon body and the Bay of Bengal via a manmade opening (new mouth). Current and old data on water quality were also compared to establish the changes that had occurred after the hydrological intervention. Multivariate techniques and gridding methods were used to investigate the spatial and seasonal variability of the data and to characterize the trophic evolution of the basin. Results of principal component analysis (PCA) indicated that the 27 stations can be classified into five groups based on similarities in the temporal variation of nutrients, chlorophyll a concentration, salinity, and other physicochemical parameters. The tributaries and the exchange of lagoon water with the Bay of Bengal most probably determine the water quality and the dynamics of the ecosystem. Hydrodynamics of the lagoon, weed coverage, input of urban sewage through tributaries and agricultural runoff are probably the key factors controlling the trophic conditions of the lagoon. An increase in salinity and total phosphorus was noted after the new mouth was opened, while the total suspended sediment load, the water column depth, and nitrogenous nutrients decreased. The new mouth opening also brought changes in the phytoplankton species composition.     

Solute dynamics in a North Brazilian mangrove: the influence of sediment permeability and freshwater input

Although water in mangrove sediments influences nutrient cycling in both, mangrove forest and estuary, little information exists on seasonal and vertical distribution of dissolved organic and inorganic compounds in the sediment column. We studied the influence of sediment texture and chemistry, permeability (K), tides, and rainfall on dissolved organic carbon (DOC) and nitrogen (DON), dissolved inorganic phosphate (DIP) and salinity in creek and sediment waters of a mangrove in Pará, Brazil. Water samples were taken from boreholes and piezometers in the mangrove forest and from an adjacent tidal creek at neap and spring tides, during the dry and rainy season. Forest sediment was analysed for carbon (C), nitrogen (N), salinity and permeability. Clay, C and N decreased with depth. Sediment permeability (K) was lowest (<0.1 m day⁻¹) in the upper, clay-rich and crab-burrow-free mud layer. In the deeper, fine sand strata, K ranged from 0.7 to 1.8 m day⁻¹. Tidal range in the creek was 3.5 and 5.5 m for neap and spring tides, respectively. Salinity, DOC, DON and DIP in creek water were inversely related to tidal height. Piezometer data revealed significant water level changes in deeper, sandy sediment layer, which followed, time-lagged, the tidal fluctuations. In contrast, tide did not affect the water level in the upper sediment due to low permeability. Compared with creek water, sediment water was enriched in DOC, DON and DIP because of organic matter input and mineralization. In deeper layers, solute concentration was most likely affected by sorption processes (DOC and DIP) and reduction reactions (DIP). During the rainy season, DOC and DON in creek and sediment water were higher than in the dry season. DIP appeared invariant to seasonal changes. In the rainy season, salt flushing from surface sediments resulted in higher salinities at intermediate sediment depths, while in the deeper layers salinity was lower due to exchange with water from the tidal creek.     

Benthic nutrient fluxes in Cadiz Bay (SW Spain)

During summer and autumn 1988, benthic fluxes of nutrients and oxygen were measured in the Bay of Cadiz. The study was carried out using benthic chambers and in addition by determining gradients of nutrient concentration in interstitial water. Fluxes ranged between 13.5–24.3, 3.4–7.8, 6.1–28.4 and (− 99.4)−(− 188.5) mmol m^{− 2} d⁻¹ for NH₄ ⁺ , o-P, SiO₂ and O₂ respectively. These values are far higher than those reported by other authors for locations at similar latitudes. The stoichiometry of O, N and P transference suggest that benthic degradation of principally allochthonous organic matter takes place mainly through anaerobic pathways.     

The Status and Characteristics of Eutrophication in the Yangtze River (Changjiang) Estuary and the Adjacent East China Sea, China

Eutrophication has become increasingly serious and noxious algal blooms have been of more frequent occurrence in the Yangtze River Estuary and in the adjacent East China Sea. In 2003 and 2004, four cruises were undertaken in three zones in the estuary and in the adjacent sea to investigate nitrate (NO₃–N), ammonium (NH₄–N), nitrite (NO₂–N), soluble reactive phosphorus (SRP), dissolved reactive silica (DRSi), dissolved oxygen (DO), phytoplankton chlorophyll a (Chl a) and suspended particulate matter (SPM). The highest concentrations of DIN (NO₃–N+NH₄–N+NO₂–N), SRP and DRSi were 131.6, 1.2 and 155.6 μM, respectively. The maximum Chl a concentration was 19.5 mg m⁻³ in spring. An analysis of historical and recent data revealed that in the last 40 years, nitrate and SRP concentrations increased from 11 to 97 μM and from 0.4 to 0.95 μM, respectively. From 1963 to 2004, N:P ratios also increased from 30–40 up to 150. In parallel with the N and P enrichment, a significant increase of Chl a was detected, Chl a maximum being 20 mg m⁻³, nearly four times higher than in the 1980s. In 2004, the mean DO concentration in bottom waters was 4.35 mg l⁻¹, much lower than in the 1980s. In comparison with other estuaries, the Yangtze River Estuary was characterized by high DIN and DRSi concentrations, with low SRP concentrations. Despite the higher nutrient concentrations, Chl a concentrations were lower in the inner estuary (Zones 1 and 2) than in the adjacent sea (Zone 3). Based on nutrient availability, SPM and hydrodynamics, we assumed that in Zones 1 and 2 phytoplankton growth was suppressed by high turbidity, large tidal amplitude and short residence time. Furthermore, in Zone 3 water stratification was also an important factor that resulted in a greater phytoplankton biomass and lower DO concentrations. Due to hydrodynamics and turbidity, the open sea was unexpectedly more sensitive to nutrient enrichment and related eutrophication processes.     

Nitrogen and Phosphorous Excretion Rates by Tubificids from the Prahova River (Romania)

Nitrogen and phosphorous exchange at the water–sediment interface is controlled both by complex physico-chemical factors and biological processes. Zoobenthos excretion is one of the most important processes in the mineralization of sedimented organic mater. In polluted freshwaters, tubificid worms are among the dominant components of the benthic community. Rates of ammonium and inorganic phosphate excretion by tubificids were experimentally assessed. They were related to the tubificid abundance in a stream ecosystem polluted with municipal and industrial wastewater. The relationship between these rates and temperature were investigated within the range of 4–23 °C. Relatively constant excretion rates were obtained for both nutrients in the first 8 h of excretion, ranging between 0.076 and 0.226 μg N mg d.w.⁻¹ h⁻¹ and 0.0065–0.01 μg P mg d.w.⁻¹ h⁻¹_, respectively. Q₁₀ values of 2.52 for ammonium and 1.31 for phosphate were calculated. If we presume that all excreta eventually enters the water column, then we can calculate that these invertebrates potentially add 39.17 mg N m⁻² day⁻¹ and 0.49 mg P m⁻² day⁻¹. These values accounts for 17.16 and 7.56% of the nutrient load in the river water, respectively.     

Seasonal patterns of pore-water nutrients, benthic chlorophyll a and sedimentary AVS in a macrobenthos-rich tidal flat

Monthly field investigations were carried out at controlled low-tide in an estuarine intertidal sandflat of the Seto Inland Sea (Japan) between January 1995 and April 1996. We assessed the spatial and temporal distribution of pore-water nutrient [NH ₄ ⁺ -N, (NO ₃ ⁻ + NO ₂ ⁻ )-N, PO ₄ ³⁻ -P and Si(OH)₄-Si], chlorophyll a (chl a) and acid-volatile sulphide (AVS) concentrations in the uppermost 0–10 cm sediment horizon, and evaluated their relationships with macrobenthic assemblages. Monthly hydrological data, nutrient and chl a concentrations in low-tide creek water adjacent to the flat were used as a complementary environmental characterisation of the study area. All different pore-water nutrients showed a 10 to > 30-fold variability between different layers and periods. NH ₄ ⁺ -N, PO ₄ ³⁻ -P and Si(OH)₄-Si concentrations were lowest in winter, progressively increased throughout spring and summer, and were highest between September and October, with a major increase at intermediate (4–8 cm) layers. In contrast, (NO ₃ ⁻ + NO ₂ ⁻ )-N concentrations, correlated positively with chl a, peaked in winter and sharply decreased with depth in all different seasons and in most occasions. Depth-integrated NH ₄ ⁺ -N, PO ₄ ³⁻ -P and Si(OH)₄-Si pools were correlated positively with biogenic nutrient flux calculated from the nutrient excretion rates of dominant bivalves Ruditapes philippinarum and Musculista senhousia. These relationships indicated rapid removal pathways of P, Si and N (i.e., every 0.1, 0.24 and 0.34 day, respectively), implying highly dynamic sediments. Based on the mass balance P = F + I, biogenic production (P) vs. diffusive flux (F) estimates suggested a predominant contribution of animal bioirrigation (I) to the upward flux of nutrients. AVS concentrations, correlated negatively with both (NO ₃ ⁻ + NO ₂ ⁻ )-N and chl a, showed a progressive increase from late summer throughout winter, subsequent to that of NH ₄ ⁺ -N, PO ₄ ³⁻ -P and Si(OH)₄-Si. The results indicate that in our study area the spatial and temporal distribution of pore-water nutrients, chl a and AVS in sediments are strongly interlinked and suggest that the metabolic processes (e.g., excretion, bio-deposition) and behavioural activities (e.g., particle mixing, bioirrigation) of abundant macrobenthos play an important role in the year-round biogeochemical processes occurring on this flat.     

Ambient levels of UV-B in Hawaii combined with nutrient deficiency decrease photosynthesis in near-isogenic maize lines varying in leaf flavonoids: Flavonoids decrease photoinhibition in plants exposed to UV-B

Near-isogenic lines of maize varying in their genes for flavonoid biosynthesis were utilized to examine the effects of foliar flavonoids and nutrient deficiency on maximum net photosynthetic rate (P _N) and chlorophyll (Chl) fluorescence (F_v/F_m) in response to ultraviolet-B (UV-B) radiation. Plants with deficient (30 to 70 % lower N, K, Mn, Fe, and Zn) and sufficient nutrients were exposed to four irradiation regimes: (1) no UV-B with solar photosynthetically active radiation (PAR), (2) two day shift to ambient artificial UV-B, 8.2–9.5 kJ m⁻² d⁻¹ (21–25 mmol m⁻² d⁻¹); (3) continuous ambient artificial UV-B; (4) continuous solar UV-B in Hawaii 12–18 kJ m⁻² d⁻¹ (32–47 mmol m⁻² d⁻¹). The natural ratio of UVB: PAR (0.25–0.40) was maintained in the UV-B treatments. In the adequately fertilized plants, lines b and lc had higher contents of flavonoids and anthocyanins than did lines hi27 and dta. UV-B induced the accumulation of foliar flavonoids in lines hi27 and b, but not in the low flavonoid line dta or in the high flavonoid line lc. In plants grown on deficient relative to adequate nutrients, flavonoid and anthocyanin contents decreased by 30–40 and 40–50 %, respectively, and Chl a and Chl b contents decreased by 30 and 70 %, respectively. The UV-B treatments did not significantly affect P _N and F_v/F_m in plants grown on sufficient nutrients, except in the low flavonoid lines dta and hi27 in which P _N and F_v/F_m decreased by ∼15 %. P _N, F_v/F_m, and stomatal conductance decreased markedly (20–30 %) in all lines exposed to UV-B when grown on low nutrients. The decrease in F_v/F_m was 10 % less in higher flavonoid lines b and lc. The photosynthetic apparatus of maize readily tolerated ambient UV-B in the tropics when plants were adequately fertilized. In contrast, ambient UV-B combined with nutrient deficiency significantly reduced photosynthesis in this C₄ plant. Nutrient deficiency increased the susceptibility of maize to UV-B-induced photoinhibition in part by decreasing the contents of photoprotective compounds.     

Spatial and seasonal patterns of periphyton biomass and productivity in the northern Everglades,Florida, U.S.A.

We sampled periphyton in dominant habitats at oligotrophic and eutrophic sites in the northern Everglades during the wet and the dryseasons to determine the effects of nutrient enrichment on periphytonbiomass, taxonomic composition, productivity, and phosphorus storage. Arealbiomass was high (100–1600 g ash-free dry mass [AFDM]m⁻²) in oligotrophic sloughs and in stands of the emergentmacrophyte Eleocharis cellulosa, but was low in adjacent stands of sawgrass,Cladium jamaicense (7–52 g AFDM m⁻²). Epipelon biomasswas high throughout the year at oligotrophic sites whereas epiphyton andmetaphyton biomass varied seasonally and peaked during the wet season.Periphyton biomass was low (3–68 g AFDM m⁻²) and limitedto epiphyton and metaphyton in open-water habitats at eutrophic sites andwas undetectable in cattail stands (Typha domingensis) that covered morethan 90% of the marsh in these areas. Oligotrophic periphytonassemblages exhibited strong seasonal shifts in species composition and weredominated by cyanobacteria (e.g., Chroococcus turgidus, Scytonema hofmannii)during the wet season and diatoms (e.g. Amphora lineolata, Mastogloiasmithii) during the dry season. Eutrophic assemblages were dominated byCyanobacteria (e.g., Oscillatoria princeps) and green algae (e.g., Spirogyraspp.) and exhibited comparatively little seasonality. Biomass-specific grossprimary productivity (GPP) of periphyton assemblages in eutrophic openwaters was higher than for comparable slough assemblages, but areal GPP wassimilar in these eutrophic (0.9–9.1 g C m⁻²d⁻¹) and oligotrophic (1.75–11.49 g C m⁻²d⁻¹) habitats. On a habitat-weighted basis, areal periphytonGPP was 6- to 30-fold lower in eutrophic areas of the marsh due to extensiveTypha stands that were devoid of periphyton. Periphyton at eutrophic siteshad higher P content and uptake rates than the oligotrophic assemblage, butstored only 5% as much P because of the lower areal biomass.Eutrophication in the Everglades has resulted in a decrease in periphytonbiomass and its contribution to marsh primary productivity. These changesmay have important implications for efforts to manage this wetland in asustainable manner. This revised version was published online in July 2006 with corrections to the Cover Date.     

Changes in the Above- and Below-ground Biomass and Nutrient Pools of Ground Vegetation After Clear-cutting of a Mixed Boreal Forest

Ground vegetation may act as a sink for nutrients after clear-cutting and thus decrease leaching losses. Biomass and nutrient (N, P, K, Ca) pools of ground vegetation (mosses, roots and above-ground parts of field layer) were determined one year before and five years after clear-cutting of a Norway spruce (Picea abies (L.) H. Karst.) dominated boreal mixed forest stand in eastern Finland (63°51′ N, 28°58′ E). Before clear-cutting the average biomass of ground vegetation was 5307 kg ha⁻¹, with nutrient contents of 46.9 kg N ha⁻¹1, 4.1 kg P ha⁻¹1, 16.2 kg K ha⁻¹1 and 13.9 kg Ca ha⁻¹1. The biomass and nutrient pools decreased after clear-cutting being lowest in the second year, the biomass decreasing by 46–65% in the cut plots. The nutrient pools decreased as follows: N 54–72%, P 36–68%, K 51–71% and Ca 57–74%. The decrease in ground vegetation nutrient uptake, and the observed reduced depth of rooting may decrease nutrient retention after clear-cutting and decomposing dead ground vegetation is a potential source of leached nutrients. These negative effects of clear-cutting on the nutrient binding capacity of ground vegetation was short-lived since the total biomass and nutrient pools returned to pre-cutting levels or were even greater by the end of the 5-year study period.     

Coarse woody debris mass and nutrients in forest ecosystems of Korea

Coarse woody debris (CWD) is an essential component of forests. However, quantification of both the mass and nutrient content of CWD within a given environment tends to be a fairly labor-intensive proposition that requires long-term studies to be conducted for viable data to be obtained. As a result, various aspects of CWD in forest ecosystems remain somewhat poorly understood. In this review, we have compiled all available estimates of CWD mass and nutrients from both coniferous and deciduous forests in Korea. The CWD mass data varied substantially by forest type, age, location, and sampling time, ranging from 1.5 to 24.5 Mg ha⁻¹, and for the amount (kg ha⁻¹) of nutrients in the CWD, ranging from 3.5 to 23.6 for nitrogen (N), 0.8 to 4.7 for phosphorus (P), 3.9 to 13.3 for potassium (K), 25.9 to 30.9 for calcium (Ca), 1.4 to 4.2 for magnesium (Mg), and 0.1 to 0.6 for sodium (Na). The mass of CWD transferred from live trees to the forest floor ranged between 0.1 and 4.9 Mg ha⁻¹ year⁻¹, and these values were roughly equivalent to 26–42% of the annual litterfall inputs (2.5–10.8 Mg ha⁻¹ year⁻¹) for mixed Quercus spp. forests within the relevant region. Annual nutrients inputs (kg ha⁻¹ year⁻¹) through CWD decomposition were 0.7–1.6 for N, 0.04–0.3 for P, 0.3–1.0 for K, 1.7–3.1 for Ca, and 0.1–0.3 for Mg. Consequently, these results revealed that the ecological value of CWD for C and nutrient cycling was relatively insignificant. However, only a limited number of studies have been conducted on CWD in different coniferous or mixed deciduous forests in the region. As a direct result of this paucity of data, further long-term studies on CWD mass and nutrients in a variety of forest types are required in order to be able to evaluate accurately the ecological value of CWD on biodiversity and physical properties in Korean forest ecosystems.