Light limitation of phytoplankton development in an oligotrophic lake - Loch Ness, Scotland

• 1 The underwater light climate in Loch Ness is described in terms of mixing depth (Z_m) and depth of the euphoric zone (Z_eu). During periods of complete mixing, Z_m equates with the mean depth of the loch (132 m), but even during summer stratification the morphometry of the loch and the strong prevailing winds produce a deep thermocline and an epilimnetic mixed layer of about 30 m or greater. Hence, throughout the year the quotient Z_m/Z_eu is exceptionally high and the underwater light climate particularly unfavourable for phytoplankton production and growth.
• 2 Phytoplankton biomass expressed as chlorophyll a is very low in Loch Ness, with a late summer maximum of less than 1.5 mg chlorophyll a m^-3 in the upper 30 m of the water column. This low biomass and the resulting very low photosynthetic carbon fixation within the water column are evidence that a severe restraint is imposed on the rate at which phytoplankton can grow in the loch.
• 3 The chlorophyll a content per unit of phytoplankton biovolume and the maximum, light-saturated specific rate of photosynthesis are both parameters which might be influenced by the light climate under which the phytoplankton have grown. However, values obtained from Loch Ness for both chlorophyll a content (mean 0.0045 mg mm^-3) and maximum photosynthetic rate (1–4 mg C mg Chla^-1 h^-1) are within the range reported from other lakes.
• 4 Laboratory bioassays with the natural phytoplankton community from Loch Ness on two occasions in late summer when the light climate in the loch is at its most favourable, suggest that even then limitation of phytoplankton growth is finely balanced between light and phosphorus limitation. Hence, for most of the year, when the light climate is less favourable, phytoplankton growth will be light limited.
• 5 Quotients relating mean annual algal biomass as chlorophyll a (c. 0.5 mg Chla m^-3) and the probable annual specific areal loading of total phosphorus (0.4–1.7 g TP m^-2 yr^-1) suggest that the efficiency with which phytoplankton is produced in Loch Ness per unit of TP loading is extremely low when compared with values from other Scottish lochs for which such an index has been calculated. This apparent inefficiency can be attributed to suppression of photosynthetic productivity in the water column due to the unfavourable underwater light climate.
• 6 These several independent sources of evidence lead to the conclusion that phytoplankton development in Loch Ness is constrained by light rather than by nutrients. Loch Ness thus appears to provide an exception to the generally accepted paradigm that phytoplankton development in lakes of an oligotrophic character is constrained by nutrient availability.

     

Protozooplankton and bacterioplankton in a large oligotrophic lake--Loch Ness, Scotland

The seasonal changes in the abundance of protozoan and bacterialplankton in a large, coloured, oligotrophic lake. Loch Ness(Scotland), were investigated between August 1991 and January1993. The coloured water supported only low concentrations ofchlorophyll a (<1.6 µgl^–1). with the highestvalues occurring in summer. Mean bacterial abundance rangedbetween 2.3 x 10⁸ and 7.1 x 10⁸ l^–1 in the 100 m watercolumn. Maximum abundance did not correlate with maximum chlorophylla concentrations, but appeared to be related to the input ofallochthonous carbon from the catchment, which in turn was influencedby rainfall levels. Consequently, the highest bacterioplanktonconcentrations occurred in autumn and winter. The pattern ofheterotrophic nanoflagellate abundance tended to follow thatfor bacteria, with mean concentrations in the top 100 m of thewater column of between 12 x 10³ and 273 x 10³ l^–1. Ciliateabundance showed no seasonal trends over the study period andprobably mirrored the fluctuating availability of various foodresources. Oligotrichs, particularly mixotrophic taxa, werea prominent element of the community throughout the year. Aggregatesof detrital material were a regular feature in the plankton.When these occurred, they formed foci for bacteria and nanoflagellates.The evidence suggests that the dynamics of the microbial planktonin Loch Ness may be driven by allochthonous carbon inputs ratherthan by the more usual dominance of carbon fixed within thesystem. ¹Present address: School of Zoology, La Trobe University, Bundoora,Melbourne, Victoria 3083, Australia ²Present address: Loch Ness & Morar Project, Loch Ness Centre,Drumnadrochit, Invernesshire, UK     

Relationship between bacteria, phytoplankton and heterotrophic nanoflagellates along the trophic gradient

Bacterial and heterotrophic nanoflagellates (HNF) abundance, as well as bacterial production and chlorophylla levels, were measured at five sites extending from the coastal zone toward the open Adriatic in the period from March to October 1995. The investigated areas were grouped into trophic categories according to concentrations of chlorophylla. All the biotic-para-meters increased along the trophic gradient, leading to eutrophy, but they did not increase at the same rate. The bacterial biomass: phytoplankton biomass (BB: chla) ratio decreased from about 10 in the very oligotrophic area to 0.8 at the eutrophic site. In contrast, the bacterial abundance: HNF abundance ratio (B: HNF) increased from 1000 bacteria per 1 flagellate in the oligotrophic system to 1700 bacteria flagellate⁴ in the eutrophic area. Decreasing BB: chla and increasing B: HNF ratios along the trophic gradient might reflect the different structures of the microbial food web. Relationships between bacterial abundance and production, and chla and HNF showed that bacterial abundance along the trophic gradient was regulated by the interplay between nutrient supply and grazing pressure. But in the oligotrophic system, bacterial abundance was more closely related to bacterial production and chla than in the eutrophic system, suggesting stronger control of bacterial abundance by substrate supply. On the other hand, the coupling between bacteria and HNF, and uncoupling between bacterial abundance and production in the eutrophic system, showed that the importance of bacteriovory increased in richer systems.     

OPTICAL PROPERTIES OF DENSE ALGAL CULTURES OUTDOORS AND THEIR APPLICATION TO REMOTE ESTIMATION OF BIOMASS AND PIGMENT CONCENTRATION IN SPIRULINA PLATENSIS (CYANOBACTERIA)1

Reflectance and vertical attenuation coefficient spectra from 400 to 1100 nm were investigated in detail on dense algal cultures of Spirulina in order to create algorithms for remote estimation of pigment and biomass concentration. Reflectance and the vertical attenuation coefficients were compared with biomass and pigment concentration in outdoor algal cultures. For assessing biomass concentration, the sum of reflectance above the base line from 670 to 950 nm was used. This allows the estimation of biomass with an error of less than 0.06 g·L^?1 For chlorophyll a and phycocyanin estimation, vertical attenuation coefficients at the wavelengths 440 nm (or 676 nm) and 624 nm, respectively, were employed. The developed algorithms were tested by using independent data sets in the range of chlorophyll a from 0.2 to 20mg·L^?1 and biomass from 0.15 to 1.1 g·L^?1. An error of pigment estimation of less than 0.80 mg·L^?1 was achieved. The potential use of the algorithms in algal biotechnology is further discussed.     

Impacts of acidification on brown trout Salmo trutta populations and the contribution of stocking to population recovery and genetic diversity

Anthropogenic acidification in SW-Scotland, from the early 19th Century onwards, led to the extinction of several loch (lake) brown trout (Salmo trutta) populations and substantial reductions in numbers in many others. Higher altitude populations with no stocking influence, which are isolated above natural and artificial barriers and subjected to the greatest effect of acidification, exhibited the least intrapopulation genetic diversity (34% of the allelic richness of the populations accessible to anadromous S. trutta). These, however, were characterised by the greatest interpopulation divergence (highest pairwise D_EST 0.61 and F_ST 0.53 in contemporary samples) based on 16 microsatellite loci and are among the most differentiated S. trutta populations in NW-Europe. Five lochs above impassable waterfalls, where S. trutta were thought to be extinct, are documented as having been stocked in the late 1980s or 1990s. All five lochs now support self-sustaining S. trutta populations; three as a direct result of restoration stocking and two adjoining lochs largely arising from a small remnant wild population in one, but with some stocking input. The genetically unique Loch Grannoch S. trutta, which has been shown to have a heritable increased tolerance to acid conditions, was successfully used as a donor stock to restore populations in two acidic lochs. Loch Fleet S. trutta, which were re-established from four separate donor sources in the late 1980s, showed differential contribution from these ancestors and a higher genetic diversity than all 17 natural loch populations examined in the area. Genetically distinct inlet and outlet spawning S. trutta populations were found in this loch. Three genetically distinct sympatric populations of S. trutta were identified in Loch Grannoch, most likely representing recruitment from the three main spawning rivers. A distinct genetic signature of Loch Leven S. trutta, the progenitor of many Scottish farm strains, facilitated detection of stocking with these strains. One artificially created loch was shown to have a population genetically very similar to Loch Leven S. trutta. In spite of recorded historical supplemental stocking with Loch Leven derived farm strains, much of the indigenous S. trutta genetic diversity in the area remains intact, aside from the effects of acidification induced bottlenecks. Overall genetic diversity and extant populations have been increased by allochthonous stocking.     

Spectral intensity in some Scottish freshwater lochs

In the course of investigations on the ecology of submerged aquatic macrophytes a narrow-bandwidth spectroradiometer has been used to measure underwater spectral intensity (390–750 nm) in a series of lochs approaching the extremes in optical and chemical properties of Scottish fresh waters. Cosine response and immersion-effect properties of the collector were determined in the laboratory with a colUmated hght source. Diffuse attenuation coefficients, Ee, ranged from 0·55 in Loch Croispol, a calcareous loch, to 2·9 in Loch Leven, a lowland eutrophic loch. Underwater spectral intensity (1 m) relative to subsurface values show a proportional increase in short-wave radiation in the blue-green water of Loch Croispol, while the converse is true of the peaty brown water of Loch Uanagan. Attenuation coefficients were derived over 25 nm wavebands in Lochs Croispol, Leven and Uanagan. The water in the latter two lochs is optically similar although Leven is rich in phytopiankton and has an extinction peak at 675 nm. Uanagan represents the brown-water type of loch most common in Scotland. Loch Croispol has attenuation coefficients for shorter wave-lengths at least ten times lower than the other two lochs and only approaches their values at 750 nm. Croispol resembles Crater Lake, Oregon. The colour range at the maximum colonizable depths by rooted macrophytes of brown (Uanagan 4 m) to blue-green (Croispol 6 m) water is equivalent to 196·0 and 230·6 kJ/Einstein (48·2 and 55·6 kcal/Einstein). Red/far-red ratios even in plankton-rich water (Leven) were at least three times the value for sunlight (1·3) and within the total photic zone reached three-figure proportions. Possible implications for light-sensitive seeds of aquatic species and for morphogenesis and zonation are discussed.     

南亚热带贫营养水库春季浮游植物群落结构与动态

2005年1~6月,通过每两周一次的高频率采样,对南亚热带贫营养水库——梅溪水库的水文、营养盐和浮游植物进行了调查,并计算水体浮游植物生物量。主要结果如下:梅溪水库浮游植物具有物种少,生物量低,以飞燕角甲藻(Ceratium hirundinella)和多甲藻(Peridinium sp.)为优势藻的特征。12次采样24个样品共检测到浮游植物42种。浮游植物在早春(1~3月)和晚春(4~6月)有显著的差别,其中每次采样浮游植物早春平均13种,晚春平均21种。浮游植物总的细胞丰度为31~273 cells·ml^-1,总生物量为0.176~2.024 mg·L^-1之间。晚春浮游植物平均生物量明显高于早春。低营养盐和弱酸性水体有利于能够垂直迁移获得营养的鞭毛藻类和其它藻类之间竞争,而使其成为整个春季优势类群。在晚春,随着水温显著增加,浮游植物丰度和生物量也明显增加,但是降雨的增加降低了水体的透明度,大大减缓了由水温上升导致生物量增长的趋势。水温是梅溪水库浮游植物变化的主要限制因子,但是降雨有明显的干扰作用。     

Sulfate and sulfite translocation via the phosphate translocator of the inner envelope membrane of chloroplasts

The permeability of the inner envelope membranes of spinach (Spinacia oleracea) chloroplasts to sulfite and sulfate was investigated in vitro, using the technique of silicone oil centrifugal filtration. The results show that there is a permeability towards both ions, resulting in rates of uptake of about 1.0 (SO ₃ ^2- ) and 0.7 (SO ₄ ^2- ) mol mg chlorophyll^-1 h^-1 respectively (external concentration 2 mmol l^-1). The rates depend on the external concentration of the anions. Anion exchange experiments with ³⁵S-preloaded chloroplasts indicate that sulfite and sulfate are exchanged for inorganic phosphate, phosphoglyceric acid, and dihydroxyacetone phosphate with rates up to 14 nmol mg chlorophyll^-1 min^-1. There is no exchange for glucose-6-phosphate and malate. Because of the similarities to the transport of inorganic phosphate and triose phosphates the results give evidence that the phosphate translocator of the inner envelope membrane of chloroplasts is also involved in sulfite and sulfate transport — at least in part.Abbreviations DHAP dihydroxyacetone phosphate - PGA 3-phosphoglycerate - P_i inorganic phosphate - S_i sultite, sulfate     

The ostracod fauna (Crustacea,Ostracoda) of the profundal benthos of Loch Ness

We report the results of quantitative and qualitative investigations of the ostracod fauna of the profundal benthos of Loch Ness, an oligotrophic lake in Scotland, UK. Six ostracod species were recorded from profundal samples: Candona angulata, C. candida, Cryptocandona reducta, Cypria ophthalmica, Cyclocypris ovum and Potamocypris smaragdina. In addition, Psychrodromus robertsoni was found in fish gut contents. The mean density of profundal ostracods was 262 individuals m², with an average Brillouin's Diversity of 0.7. A weak inverse relationship between fish body weight and the number of ostracods eaten is reported for Arctic charr, Salvelinus alpinus.     

Ecological effects of low-level phosphorus additions on two plant communities in a neotropical freshwater wetland ecosystem

We conducted a low-level phosphorus (P) enrichment study in two oligotrophic freshwater wetland communities (wet prairies [WP] and sawgrass marsh [SAW]) of the neotropical Florida Everglades. The experiment included three P addition levels (0, 3.33, and 33.3 mg P m^–2 month^–1), added over 2 years, and used in situ mesocosms located in northeastern Everglades National Park, Fla., USA. The calcareous periphyton mat in both communities degraded quickly and was replaced by green algae. In the WP community, we observed significant increases in net aboveground primary production (NAPP) and belowground biomass. Aboveground live standing crop (ALSC) did not show a treatment effect, though, because stem turnover rates of Eleocharis spp., the dominant emergent macrophyte in this community, increased significantly. Eleocharis spp. leaf tissue P content decreased with P additions, causing higher C:P and N:P ratios in enriched versus unenriched plots. In the SAW community, NAPP, ALSC, and belowground biomass all increased significantly in response to P additions. Cladium jamaicense leaf turnover rates and tissue nutrient content did not show treatment effects. The two oligotrophic communities responded differentially to P enrichment. Periphyton which was more abundant in the WP community, appeared to act as a P buffer that delayed the response of other ecosystem components until after the periphyton mat had disappeared. Periphyton played a smaller role in controlling ecosystem dynamics and community structure in the SAW community. Our data suggested a reduced reliance on internal stores of P by emergent macrophytes in the WP that were exposed to P enrichment. Eleocharis spp. rapidly recycled P through more rapid aboveground turnover. In contrast, C. jamaicense stored added P by initially investing in belowground biomass, then shifting growth allocation to aboveground tissue without increasing leaf turnover rates. Our results suggest that calcareous wetland systems throughout the Caribbean, and oligotrophic ecosystems in general, respond rapidly to low-level additions of their limiting nutrient.     

A Total Synthesis of (±)-Pachyrrhizone

We examined whether developing cotyledons of soybean seed had photosynthetic activities. The cotyledons evolved oxygen under illumination and the activity was inhibited by 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea. The rate of oxygen evolution decreased during the development of seeds; about 30μ miol O₂ · mg chlorophyll^-1 · hr^-1 at the early developing stage and about 10μ miol O₂-mg chlorophyll^-1 · hr^-1 at the late developing stage. The rate of oxygen uptake remained at an almost constant level of 40^mol 0₂-mg chlorophyll^-1 · hr^-1 throughout the development. Photosynthetic ¹⁴CO₂-fixation by the cotyledon was observed. Ribulose bisphosphate carboxylase was immunochemically detected in the developing cotyledons. These results show that functional photosynthetic apparatus is present in the developing cotyledons of soybean seeds.     

Catabolic diversity of periphyton and detritus microbial communities in a subtropical wetland

The catabolic diversity of wetland microbial communities may be a sensitive indicator of nutrient loading or changes in environmental conditions. The objectives of this study were to assess the response of periphyton and microbial communities in water conservation area-2a (WCA-2a) of the Everglades to additions of C-substrates and inorganic nutrients. Carbon dioxide and CH₄ production rates were measured using 14 days incubation for periphyton, which typifies oligotrophic areas, and detritus, which is prevalent at P-impacted areas of WCA-2a. The wetland was characterized by decreasing P levels from peripheral to interior, oligotrophic areas. Microbial biomass and N mineralization rates were higher for oligotrophic periphyton than detritus. Methane production rates were also higher for unamended periphyton (80 mg CH₄-C kg⁻¹ d⁻¹) than detritus (22 mg CH₄-C kg⁻¹ d⁻¹), even though the organic matter content was higher for detritus (80%) than periphyton (69%). Carbon dioxide production for unamended periphyton (222 mg CO₂-C kg⁻¹ d⁻¹) was significantly greater than unamended detritus (84 mg CO₂-C kg⁻¹ d⁻¹). The response of the heterotrophic microbial community to added C-substrates was related to the nutrient status of the wetland, as substrate-induced respiration (SIR) was higher for detritus than periphyton. Amides and polysaccharides stimulated SIR more than other C-substrates, and methanogenesis was greater contributor to SIR for periphyton than detritus. Inorganic P addition stimulated CO₂ and CH₄ production for periphyton but not detritus, indicating a P limitation in the interior areas of WCA-2a. Continued nutrient loading into oligotrophic areas of WCA-2a or enhanced internal nutrient cycling may stimulate organic matter decomposition and further contribute to undesirable changes to the Everglades ecosystem caused by nutrient enrichment.     

Phosphorus budgets in Everglades wetland ecosystems: the effects of hydrology and nutrient enrichment

The Florida Everglades is a naturally oligotrophic hydroscape that has experienced large changes in ecosystem structure and function as the result of increased anthropogenic phosphorus (P) loading and hydrologic changes. We present whole-ecosystem models of P cycling for Everglades wetlands with differing hydrology and P enrichment with the goal of synthesizing existing information into ecosystem P budgets. Budgets were developed for deeper water oligotrophic wet prairie/slough (‘Slough’), shallower water oligotrophic Cladium jamaicense (‘Cladium’), partially enriched C. jamaicense/Typha spp. mixture (‘Cladium/Typha’), and enriched Typha spp. (‘Typha’) marshes. The majority of ecosystem P was stored in the soil in all four ecosystem types, with the flocculent detrital organic matter (floc) layer at the bottom of the water column storing the next largest proportion of ecosystem P pools. However, most P cycling involved ecosystem components in the water column (periphyton, floc, and consumers) in deeper water, oligotrophic Slough marsh. Fluxes of P associated with macrophytes were more important in the shallower water, oligotrophic Cladium marsh. The two oligotrophic ecosystem types had similar total ecosystem P stocks and cycling rates, and low rates of P cycling associated with soils. Phosphorus flux rates cannot be estimated for ecosystem components residing in the water column in Cladium/Typha or Typha marshes due to insufficient data. Enrichment caused a large increase in the importance of macrophytes to P cycling in Everglades wetlands. The flux of P from soil to the water column, via roots to live aboveground tissues to macrophyte detritus, increased from 0.03 and 0.2 g P m⁻² yr⁻¹ in oligotrophic Slough and Cladium marsh, respectively, to 1.1 g P m⁻² yr⁻¹ in partially enriched Cladium/Typha, and 1.6 g P m⁻² yr⁻¹ in enriched Typha marsh. This macrophyte translocation P flux represents a large source of internal eutrophication to surface waters in P-enriched areas of the Everglades.     

A heuristic approach to fed-batch optimisation of streptokinase fermentation

Previous studies have shown that the rate of formation of streptokinase, a secondary metabolite, in batch fermentation is proportional to the specific growth rate of the biomass, which in turn is inhibited by its substrate and the primary product (lactic acid). These kinetics suggest the suitability of fed-batch operation to increase the yield of streptokinase. A near-optimal feed policy has been calculated by the chemotaxis algorithm, and it shows a substrate feed rate decreasing nonlinearly and vanishing after 11 hours. This is followed by batch fermentation for a further 8 hours, at the end of which 12% more streptokinase is generated than by purely batch fermentation. Further improvements in productivity are possible.List of Symbols k _dh^–1 decay constant for active cells - k _ph^–1 decay constant for streptokinase - K _Igl^–1 inhibition constant for lactic acid - K_S gl^–1 inhibition constant for substrate - M gl^–1 lactic acid concentration - P gl^–1 streptokinase concentration - Q 1h^–1 substrate feed rate - S gl^–1 substrate concentration - S _ingl^–1 inlet concentration of substrate - t h time - t _bh end-point of batch fermentation - t _fh end-point of fed-batch fermentation - V l volume of broth in fermenter - V ₀ l initial value of V (at t=0) - V _ml maximum value of V - X gl^–1 total biomass concentration - X _agl^–1 concentration of active biomass - Y _MX yield coefficient for lactic acid from biomass - Y _PX yield coefficient for streptokinase from biomass - Y _XS yield coefficient for biomass from substrate Greek Letters h^–1 specific growth rate of biomass - _mh^–1 maximum specific growth rate     

Leaf litter breakdown budgets in streams of various trophic status: effects of dissolved inorganic nutrients on microorganisms and invertebrates

1. We investigated the effect of trophic status on the organic matter budget in freshwater ecosystems. During leaf litter breakdown, the relative contribution of the functional groups and the quantity/quality of organic matter available to higher trophic levels are expected to be modified by the anthropogenic release of nutrients. 2. Carbon budgets were established during the breakdown of alder leaves enclosed in coarse mesh bags and submerged in six streams: two oligotrophic, one mesotrophic, two eutrophic and one hypertrophic streams. Nitrate concentrations were 4.5–6.7 mg L⁻¹ and the trophic status of each stream was defined by the soluble reactive phosphorus concentration ranging from 3.4 (oligotrophic) to 89 μg L⁻¹ (hypertrophic). An ammonium gradient paralleled the phosphate gradient with mean concentrations ranging from 1.4 to 560 μg L⁻¹ NH₄‐N. The corresponding unionised ammonia concentrations ranged from 0.08 to 19 μg L⁻¹ NH₃‐N over the six streams. 3. The dominant shredder taxa were different in the oligo‐, meso‐ and eutrophic streams. No shredders were observed in the hypertrophic stream. These changes may be accounted for by the gradual increase in the concentration of ammonia over the six streams. The shredder biomass dramatically decreased in eu‐ and hypertrophic streams compared with oligo‐ and mesotrophic. 4. Fungal biomass increased threefold from the most oligotrophic to the less eutrophic stream and decreased in the most eutrophic and the hypertrophic. Bacterial biomass increased twofold from the most oligotrophic to the hypertrophic stream. Along the trophic gradient, the microbial CO₂ production followed that of microbial biomass whereas the microbial fine particulate organic matter and net dissolved organic carbon (DOC) did not consistently vary. These results indicate that the microorganisms utilised the substrate and the DOC differently in streams of various trophic statuses. 5. In streams receiving various anthropogenic inputs, the relative contribution of the functional groups to leaf mass loss varied extensively as a result of stimulation and the deleterious effects of dissolved inorganic compounds. The quality/quantity of the organic matter produced by microorganisms slightly varied, as they use DOC from stream water instead of the substrate they decompose in streams of higher trophic status.     

Trophic‐level dependent effects on CO2 emissions from experimental stream ecosystems

Concern over accelerating rates of species invasions and losses have initiated investigations into how local and global changes to predator abundance mediate trophic cascades that influence CO₂ fluxes of aquatic ecosystems. However, to date, no studies have investigated how species additions or losses at other consumer trophic levels influence the CO₂ flux of aquatic ecosystems. In this study, we added a large predatory stonefly, detritivorous stonefly, or grazer tadpole to experimental stream food webs and over a 70‐day period quantified their effects on community composition, leaf litter decomposition, chlorophyll‐a concentrations, and stream CO₂ emissions. In general, streams where the large grazer or large detritivore were added showed no change in total invertebrate biomass, leaf litter loss, chlorophyll‐a concentrations, or stream CO₂ emissions compared with controls; although we did observe a spike in CO₂ emissions in the large grazer treatment following a substantial reduction in chlorophyll‐a concentrations on day 28. However, the large grazer and large detritivore altered the community composition of streams by reducing the densities of other grazer and detritivore taxa, respectively, compared with controls. Conversely, the addition of the large predator created trophic cascades that reduced total invertebrate biomass and increased primary producer biomass. The cascading effects of the predator additions on the food web ultimately led to decreased CO₂ emissions from stream channels by up to 95%. Our results suggest that stream ecosystem processes were more influenced by changes in large predator abundance than large grazer or detritivore abundance, because of a lack of functionally similar large predators. Our study demonstrates that the presence/absence of species with unique functional roles may have consequences for the exchange of CO₂ between the ecosystem and the atmosphere.     

Continuous and biomass recycle fermentations of Clostridium acetobutylicum

The availability and demand of biosynthetic energy (ATP) is an important factor in the regulation of solvent production in steady state continuous cultures of Clostridium acetobutylicum. The effect of biomass recycle at a variety of dilution rates and recycle ratios under both glucose and non-glucose limited conditions on product yields and selectivities has been investigated. Under conditions of non-glucose limitation, when the ATP supply is not growth-limiting, a lower growth rate imposed by biomass recycle leads to a reduced demand for ATP and substantially higher acetone and butanol yields. When the culture is glucose limited, however, biomass recycle results in lower solvent yields and higher acid yields.List of Symbols A ₆₀₀ absorbance at 600 nm - ATP adenosine triphosphate - C _imol/dm³ concentration of componenti in the fermentor - C _i ⁰ mol/dm³ concentration of componenti in the feed - D h^–1 dilution rate - F dm³/h feed flow rate - FdH₂ ferredoxin, reduced form - NAD nicotinamide adenine dinucleotide, oxidized form - NADH nicotinamide adenine dinucleotide, reduced form - NfF mmol/g/h NADH produced from oxidation of FdH₂ per unit biomass per unit time - P dm³/h filtrate flow during biomass recycle operation - PCRP C-mole carbon per C-mole glucose utilized percent of (substrate) carbon recovered in products - R recycle ratio,P/F - SPR mmol/g/h specific production rate - X _imol product/100 mol glucose utilized product yield - Y _ATP g biomass/mol ATP biomass yield on ATP - Y _GLU g biomass/mol glucose biomass yield on glucose - Y _ig biomass/mol biomass yield on nutrienti - h^–1 specific growth rate     

Organic enrichment of sediments reduces arbuscular mycorrhizal fungi in oligotrophic lake plants

1. Arbuscular mycorrhizal fungi (AMF) commonly colonise isoetid species inhabiting oxygenated sediments in oligotrophic lakes but are usually absent in other submerged plants. We hypothesised that organic enrichment of oligotrophic lake sediments reduces AMF colonisation and hyphal growth because of sediment O₂ depletion and low carbon supply from stressed host plants. 2. We added organic matter to sediments inhabited by isoetids and measured pore‐water chemistry (dissolved O₂, inorganic carbon, Fe²⁺ and ), colonisation intensity of roots and hyphal density after 135 days of exposure. 3. Addition of organic matter reduced AMF colonisation of roots of both Lobelia dortmanna and Littorella uniflora, and high additions stressed the plants. Even small additions of organic matter almost stopped AMF colonisation of initially un‐colonised L. uniflora, though without reducing plant growth. Mean hyphal density in sediments was high (6 and 15 m cm^?3) and comparable with that in terrestrial soils (2–40 m cm^?3). Hyphal density was low in the upper 1 cm of isoetid sediments, high in the main root zone between 1 and 8 cm and positively related to root density. Hyphal surface area exceeded root surface area by 1.7–3.2 times. 4. We conclude that AMF efficiently colonise isoetids in oligotrophic sediments and form extensive hyphal networks. Small additions of organic matter to sediments induce sediment anoxia and reduce AMF colonisation of roots but cause no apparent plant stress. High organic addition induces night‐time anoxia in both the sediment and the plant tissue. Tissue anoxia reduces root growth and AMF colonisation, probably because of restricted translocation of nutrient ions and organic solutes between roots and leaves. Isoetids should rely on AMF for P uptake on nutrient‐poor mineral sediments but are capable of growing without AMF on organic sediments.     

Role of planktonic bacteria in productivity and cycling of organic matter in the Eastern Pacific Ocean

Total number, biomass, production, and respiration of bacterioplankton were measured in oligotrophic, mesotrophic and eutrophic waters of the Eastern Pacific. Total number of bacteria in the upper mixed layer and in the upper thermocline boundary layers varied from 30–60.10³ ml^-1 in oligotrophic waters to 100–400.10³ ml^-1 in mesotrophic waters of fronts and divergences, and to 1–2,5.10⁶ ml^-1 in eutrophic waters of coastal upwellings. Wet biomass varied from 5–10 mg l^-1 in oligotrophic waters, to 50–200 mg l^-1 in mesotrophic waters, and to 1–2 g m^-3 in eutrophic waters. Below the layer of maximum temperature gradient i.e. below 35–50 m, bacterioplankton density decreased 5–10 times. P/B coefficients per day were highest in the oligotrophic surface water ( 1), and lowest in the eutrophic ones (0.2–0.4). In mesotrophic waters they were intermediate (0.4–1.0). the stock of labile organic matter (LOM) accessible to microbial action varied from 0.3 to 1.6 mg Cl^-1. Its highest value occurred in the upwelling area. The stock of LOM does not noticeably decrease from the euphotic zone to a depth of 2 000 m. Its turnover time varied from 5 to 45 days in surface waters, and 30–50 years in deep oceanic waters. The role of bacterioplankton in productivity and in cycling of organic matter in surface — and deep oceanic waters is discussed.     

Predator-induced bottom-up effects in oligotrophic systems

Five treatments (replication n=2) were applied to mesocosms in an oligotrophic lake (TP=6–10 µg 1^-1) to assess the effects of fish on planktonic communities. The treatments were: (1) high fish (30 kg ha^–1 Lepomis auritus, Linnaeus), (2) low fish (10 kg ha^–1), (3) high removal of zooplankton, (4) low removal of zooplankton and (5) control. Total phosphorus, chlorophyll a, zooplankton biomass, and species richness decreased from high fish > low fish > control > low removal > high removal treatments. The fish treatments were dominated by crustacean zooplankton, while rotifers outnumbered the other zooplankters in the removal treatments. Calculations of zooplankton grazing rates suggested that clearance rates seldom exceeded 2% of the enclosure volume d^–1 and were unlikely to have had much influence on phytoplankton biomass. Calculations from a phosphorus bioenergetics model revealed that when fish were present, their excretion rates were higher than the rates ascribed to zooplankton. Diet analysis showed that the fish derived most of their energy from the benthos and periphyton, and that fish excretion and egestion made significant contributions to the very oligotrophic pelagic phosphorus pool. In the absence of fish, zooplankton excretion was highest in the control treatments and lowest in the zooplankton removal treatments. Our results suggest that in oligotrophic systems, planktivorous fish can be significant sources of phosphorus and that fish and zooplankton induced nutrient cycling have significant impacts on planktonic community structure.