Carbon Uptake Rates Research Articles

Comparison of size‐dependent carbon, nitrate, and silicic acid uptake rates in high‐ and low‐iron waters

We compare the relative contribution of large phytoplankton (>5 and >10 µm) to uptake rates of carbon (C), nitrate (NO3−), and dissolved silicon (Si) and uptake ratios of Si :NO and Si :C in Monterey Bay, California (a high‐iron region) with three low‐iron regions in the eastern tropical Pacific: the Costa Rica upwelling dome, Humboldt current, and Peru upwelling. We also demonstrate the effect of iron enrichment on the above parameters in the latter three regions. In Monterey Bay waters, the >5‐µm size fraction accounted on average for the majority of particulate organic C and was responsible for 92% of total C uptake, 81% of total NO3− uptake, and 98% of total 23 Si uptake. In contrast, in low‐iron eastern tropical Pacific waters, the >5‐µm size fraction accounted for less than half of the particulate organic C and was responsible for a substantially smaller proportion of total C and NO3− uptake: 27—43% for C and 34—59% for NO3−. Iron enrichment experiments in the eastern tropical Pacific resulted in much higher NO3−, C, and Si uptake rates, although increases were restricted to cells in the >5‐µm size fraction. Si:NO and Si:C uptake ratios decreased after iron addition at most locations, and decreases were a direct result of lower Si:NO3− and Si:C uptake ratios in the >5‐µm size fraction. Our results suggest that iron availability is a major factor regulating primary production, new production, Si uptake, and Si:NO3− and Si :C uptake ratios in the larger phytoplankton size classes in high‐nitrate, low‐chlorophyll (HNLC) regions in the eastern tropical Pacific.

Short- and long-term effects of solar ultraviolet radiation on the red algae Porphyridium cruentum (S. F. Gray) Nägeli

During spring 2002 and fall 2003 we carried out experiment in tropical southern China to determine the short- and long-term effects of solar ultraviolet radiation (UVR, 280-400 nm) on photosynthesis and growth in the unicellular red alga Porphyridium cruentum. During the experimentation, cells of P. cruentum were exposed to three radiation treatments: (a) samples exposed to PAR (400-700 nm) + UV-A (315-400 nm) + UV-B (280-315 nm)(PAB treatment); (b) samples exposed to PAR + UV-A (PA treatment) and, (c) samples exposed only to PAR (P treatment). To assess the short-term impact of UVR as a function of irradiance, we determined photosynthesis versus irradiance (Pvs.E) curves. From these curves the maximum carbon uptake rate (P(max)) and the light saturation parameter (E(k)) were obtained, with values of approximately 12.8-14.4 microg C (microg chl a)(-1) h(-1), and approximately 250 micromol m(-2) s(-1), respectively. A significant UVR effect on assimilation numbers was observed when samples were exposed at irradiances higher than E(k), with samples exposed to full solar radiation having significant less carbon fixation than those exposed only to PAR. Biological weighting functions of P. cruentum were used to evaluate the UVR impact per unit energy received by the cells; the data indicate that the species is as sensitive as natural phytoplankton from the southern China Sea; however, it is much more resistant than Antarctic assemblages. When evaluating the combined effects of mixing speed and UVR, it was seen that samples rotating fast within the upper mixed layer were less inhibited by UVR as compared to those under slow mixing or in fixed samples. Growth of P. cruentum over a week-long experiment was not affected by neither UVR nor UV-A; additionally, low photoinhibition was found at the end as compared to that at the beginning of this experiment. Our results thus indicate that, although on short-term basis P. cruentum is affected by solar UVR, it can acclimate to minimize UVR-induced effects when given enough time.

Effects of season and low-level air pollution on physiology and element content of lichens from the U.S. Pacific Northwest

Lichens were collected from three low-elevation sites in the western Cascade Range: HJ Andrews, OR (clean) and Bull Run, OR, and Pack Forest, WA (moderately enhanced nitrogen and sulfur deposition). The latter sites were within 50 km of Portland and Seattle/Centralia urban-industrial areas, respectively. Tissue concentrations of sulfur, nitrogen, and other macronutrients; rates of net carbon uptake; concentrations of photosynthetic pigments; and thallus density were correlated with season and seasonal changes in Platismatia glauca. Ion concentrations in precipitation and total wet deposition were measured from natural settings. Concentrations of depositional ions in precipitation, including NO 3 − and NH 4 +, were generally highest at Bull Run and Pack Forest; SO 4 2− concentrations and acidity were highest at Pack Forest. Total wet deposition was higher in the winter rainy season than the dry summer season at all three sites. Lichens adapted physiologically and morphologically to the higher light intensity and the warm, dry climate of summer through decreased optimal water content for CO 2 uptake, increased concentrations of carotenoids and increased thallus density. Compared to the clean site, the sites with enhanced deposition were associated in P. glauca with year-round higher tissue concentrations of N, S, K, and Na; higher concentrations of total chlorophyll and carotenoids; higher OD 435/415 ratios; higher CO 2 uptake and lower thallus density in summer; and a general absence of other sensitive lichens. These results indicate that moderate levels of fertilizing air pollutants can stimulate carbon uptake and provide protection against chlorophyll degradation in air pollution-tolerant lichens of the Pacific Northwest, especially during the dry summer season.

Species-specific ingestion of organic carbon by deep-sea benthic foraminifera and meiobenthos: In situ tracer experiments

We measured organic carbon uptake rates by deep-sea benthic foraminifera and studied differences among species, living depth, and seasons to investigate how these protists contribute to carbon consumption on the deep-sea floor. In situ feeding experiments using

Regulation of the Si and C uptake and of the soluble free-silicon pool in a synchronised culture of Cylindrotheca fusiformis (Bacillariophyceae): effects on the Si/C ratio

Silicon and carbon uptake rates were studied over a 24 h light/dark cycle in a synchronised culture of the marine diatom Cylindrotheca fusiformis (Reimann et Lewin) using 32Si and 14C. The silicic acid uptake rate per cell (ρcSi) varied between 1.2 and 20.0 fmol Si cell−1 h−1 and was closely correlated to the G2+M phase of the cell cycle. A linear and significant relationship was determined between the percentage of cells present in G2+M and ρcSi. Evolution of the soluble free-silicon pool was studied simultaneously. The concentration of the total soluble free pool of silicon (QPSi) varied from 1% to 7% of the total silicon content. A significant difference of 1.5 fmol Si cell−1 between QPSi and the labelled free pool (QnpSi) was measured, indicating the presence of an unlabelled fraction of the pool. The concentration of QnpSi was around 1.0 fmol Si cell−1 prior to cell division and did not change as a function of ρcSi, which indicated a feedback mechanism coupling uptake into the free pool and incorporation into the frustule. In parallel, 14C uptake variation (ρcC) was measured during the division of the population. The value of ρcC varied between 0.44 and 0.78 pmol C cell−1 h−1 and appeared to be maximal when cells were in the G1 phase. This variation of ρcC marginally affected the total carbon content of the cells (QTC) in comparison with the light/dark cycle. The variations in the Si/C ratio, from 0.021 to 0.046, demonstrated the different control mechanisms of Si and C metabolisms during the course of the cell- and photocycle.

Survival and photosynthetic activity of different Dinophysis acuminata populations in the northern Baltic Sea

This study deals with a recently found phenomenon in the northern Baltic Sea: the occurrence of the dinoflagellate Dinophysis acuminata in the deep water below the thermocline. This was first observed in July 2001 at the station BY 15 in the Gotland Deep, where a sharp and intensive chlorophyll fluorescence signal was encountered at 77 m depth. The fluorescence peak was due to a dinoflagellate community dominated by Dinophysis acuminata (approximately 18 000 cells l −1). The survival of this community was followed in laboratory incubations in low light (20 μE m −2 s −1) and low temperature (+5 °C). After 5 weeks incubation, 67–84% of the initial cell abundance was lost, while few D. acuminata cells survived up to 24 weeks in the original sample. During the incubation, the fluorescence signal of the cells became fainter and the chloroplasts smaller and aggregated. On two occasions a D. acuminata cell was found attached to a smaller cell by a thin cytoplasm strand, possibly indicating mixotrophic behavior. During the following summer (2002), the photosynthetic efficiency of D. acuminata collected from thermocline layers of few stations and from the nitracline (75–80 m) at one station was studied in photosynthesis irradiance ( P–E) incubations. Photosynthetic activity occurred in all populations, with differences in their photosynthetic carbon uptake rates. Photosynthesis of D. acuminata populations was saturated between 250 and 500 μE m −2 s −1; maximum cell-specific carbon uptake rates ( P m) ranged from 160–925 pg C cell −1 h −1. The P m-rates in populations originating below the thermocline and in an artificially darkened population were markedly lower than in populations from upper water layers. The varying maximum photosynthetic rates of these populations may reflect their history, e.g. time spent in different light environments.

Functional diversity of photosynthesis during drought in a model tropical rainforest – the contributions of leaf area, photosynthetic electron transport and stomatal conductance to reduction in net ecosystem carbon exchange

ABSTRACTThe tropical rainforest mesocosm within the Biosphere 2 Laboratory, a model system of some 110 species developed over 12 years under controlled environmental conditions, has been subjected to a series of comparable drought experiments during 2000–2002. In each study, the mesocosm was subjected to a 4–6 week drought, with well‐defined rainfall events before and after the treatment. Ecosystem CO2 uptake rate (Aeco) declined 32% in response to the drought, with changes occurring within days and being reversible within weeks, even though the deeper soil layers did not become significantly drier and leaf‐level water status of most large trees was not greatly affected. The reduced Aeco during the drought reflected both morphological and physiological responses. It is estimated that the drought‐induced 32% reduction of Aeco has three principal components: (1) leaf fall increased two‐fold whereas leaf expansion growth of some canopy dominants declined to 60%, leading to a 10% decrease in foliage coverage of the canopy. This might be the main reason for the persistent reduction of Aeco after rewatering. (2) The maximum photosynthetic electron transport rate at high light intensities in remaining leaves was reduced to 71% for three of the four species measured, even though no chronic photo‐inhibition occurred. (3) Stomata closed, leading to a reduced ecosystem water conductance to water vapour (33% of pre‐drought values), which not only reduced ecosystem carbon uptake rate, but may also have implications for water and energy budgets of tropical ecosystems. Additionally, individual rainforest trees responded differently, expressing different levels of stress and stress avoiding mechanisms. This functional diversity renders the individual response heterogeneous and has fundamental implications to scale leaf level responses to ecosystem dynamics.

Physiological regulation of productivity and water use in Eucalyptus: a review

Eucalyptus species in Australia occur naturally across a wide range of environmental conditions. In native forests, the leaves are typically thick, tough and long-lived with low specific leaf area, and low leaf concentrations of nitrogen and phosphorus. Potential productivity is high but maximum rates are rarely achieved because of limitations due to drought and nutrient availability. In contrast, when Eucalyptus species are grown in conditions with adequate supplies of water and nutrients, or in managed plantations at favourable sites, light-use efficiency and productivity are high. We review the physiological basis for the observed rates of growth and water use in Eucalyptus forests. Our aim is to demonstrate that a process-based approach provides a robust framework to predict productivity and water use by incorporating differences between species, the effects of climate and options for silvicultural management. Concern about decreases in water yield from catchments where Eucalyptus plantations have been established has prompted interest in estimating transpiration rates using a range of techniques. The consequence of the marked sensitivity of decreasing stomatal conductance to increasing air saturation deficit, D, is that, in well-watered conditions, transpiration from Eucalyptus forests can be explained largely by leaf area index and D. Measurements of photosynthesis for many Eucalyptus species over a wide range of conditions have confirmed the potential for high rates of carbon uptake. Measurements of maximum rates of photosynthesis, A max, maximum rate of carboxylation, V cmax, and the maximum rate of electron transport, J max, for Eucalyptus trees are high in relation to other broad-leaved tree species, but actual rates of photosynthesis are often much lower because of water and nutrient limitations. This results in a wide variation in light-use efficiency ranging from 0.7 to 2.7 g (dry matter) MJ −1 (intercepted photosynthetically active radiation between 400 and 700 nm). Several mechanisms for drought avoidance are identified, including low values and large seasonal dynamic changes in leaf area index, near-vertical arrangement of leaves, high stomatal sensitivity to air saturation deficit, deep rooting ability and osmotic manipulation to maintain turgor in leaves. Further evidence from measurements of carbon isotope fractionation at sites along rainfall gradients and estimation of the relationships between leaf area, sapwood cross-sectional area and conductance demonstrate homeostatic adjustment of hydraulic properties in relation to growing conditions. Validation of models for native and managed Eucalyptus stands confirm their usefulness as a management tool to predict productivity and water use in relation to species, climate and silvicultural options.

Climate impact from peat utilisation in Sweden

The climate impact from the useof peat for energy production in Sweden hasbeen evaluated in terms of contribution toatmospheric radiative forcing. This wasdone by attempting to answer the question`What will be the climate impact if onewould use 1 m2 of mire for peatextraction during 20 years?'. Two differentmethods of after-treatment were studied:afforestation and restoration of wetland.The climate impact from a peatland –wetland scenario and a peatland –forestation – bioenergy scenario wascompared to the climate impact from coal,natural gas and forest residues.Sensitivity analyses were performed toevaluate which parameters that areimportant to take into consideration inorder to minimize the climate impact frompeat utilisation. In a `multiple generationscenario' we investigate the climate impactif 1 Mega Joule (MJ) of energy is produced every yearfor 300 years from peat compared to otherenergy sources.The main conclusions from the study are:•The accumulated radiative forcing from the peatland – forestation – bioenergy scenario over a long time perspective (300 years) is estimated to be 1.35 mJ/m2/m2 extraction area assuming a medium-high forest growth rate and medium original methane emissions from the virgin mire. This is below the corresponding values for coal 3.13 mJ/ m2/ m2 extraction area and natural gas, 1.71 mJ/ m2/ m2 extraction area, but higher than the value for forest residues, 0.42 mJ/ m2/ m2 extraction area. A `best-best-case' scenario, i.e. with high forest growth rate combined with high `avoided' methane (CH4) emissions, will generate accumulated radiative forcing comparable to using forest residues for energy production. A `worst-worst-case' scenario, with low growth rate and low `avoided' CH4 emissions, will generate radiative forcing somewhere in between natural gas and coal.•The accumulated radiative forcing from the peatland – wetland scenario over a 300-year perspective is estimated to be 0.73 –1.80 mJ/ m2/ m2 extraction area depending on the assumed carbon (C) uptake rates for the wetland and assuming a medium-high methane emissions from a restored wetland. The corresponding values for coal is 1.88 mJ/ m2/ m2 extraction area, for natural gas 1.06 mJ/ m2/ m2 extraction area and for forest residues 0.10 mJ/ m2/ m2 extraction area. A `best-best-case' scenario (i.e. with high carbon dioxide CO2-uptake combined with high `avoided' CH4 emissions and low methane emissions from the restored wetland) will generate accumulated radiative forcing that decreases and reaches zero after 240 years. A `worst-worst-case' (i.e. with low CO2-uptake combined with low `avoided' CH4 emissions and high methane emissions from the restored wetland) will generate radiative forcing higher than coal over the entire time period.•The accumulated radiative forcing in the `multiple generations' – scenarios over a 300-year perspective producing 1 MJ/year is estimated to be 0.089 mJ/ m2 for the scenario `Peat forestation – bioenergy', 0.097 mJ/ m2 for the scenario `Peat wetland with high CO2-uptake' and 0.140 mJ/ m2 for the scenario `Peat wetland with low CO2-uptake'. Corresponding values for coal is 0.160 mJ/ m2, for natural gas 0.083 mJ/ m2 and for forest residues 0.015 mJ/ m2. Using a longer time perspective than 300 years will result in lower accumulated radiative forcing from the scenario `Peat wetland with high CO2-uptake'. This is due to the negative instantaneous forcing that occurs after 200 years for each added generation.•It is important to consider CH4 emissions from the virgin mire when choosing mires for utilization. Low original methane emissions give significantly higher total climate impact than high original emissions do.•Afforestation on areas previously used for peat extraction should be performed in a way that gives a high forest growth rate, both for the extraction area and the surrounding area. A high forest growth rate gives lower climate impact than a low forest growth rate.•There are great uncertainties related to the data used for emissions and uptake of greenhouse gases in restored wetlands. The mechanisms affecting these emissions and uptake should be studied further.

Temporal Evolution of the North Pacific CO2Uptake Rate

The recent changes in the North Pacific uptake rate of carbon have been estimated using a number of different techniques over the past decade. Recently, there has been a marked increase in the number of estimates being submitted for publication. Most of these estimates can be grouped into one of five basic techniques: carbon time-series, non-carbon tracers, carbon tracers, empirical relationships, and inverse calculations. Examples of each of these techniques as they have been applied in the North Pacific are given and the estimates summarized. The results are divided into three categories: integrated water column uptake rate estimates, mixed layer increases, and surface pCO2 increases. Most of the published values fall under the water column integrated uptake rate category. All of the estimates varied by region and depth range of integration, but generally showed consistent patterns of increased uptake from the tropics to the subtropics. The most disagreement between the methods was in the sub-arctic Pacific. Column integrated uptake rates ranged from 0.25 to 1.3 mol m−2yr−1. The mixed layer uptake estimates were much more consistent, with values of 1.0–1.3 μmol kg−1yr−1 based on direct observations and multiple linear regression approaches. Surface pCO2 changes showed the most obvious regional variability (0.5–2.5 μatm yr−1) reflecting the sensitivity of these measurements to differences in the physical and biological forcing. The different techniques used to evaluate the changes in North Pacific carbon distributions do not completely agree on the exact magnitude or spatial and temporal patterns of carbon uptake rate. Additional research is necessary to resolve these issues and better constrain the role of the North Pacific in the global carbon cycle.

Factors affecting the distribution of the rate of carbon uptake by forests in South Korea.

The biomass of forests in South Korea has significantly increased during the last 30 years because of a national reforestation project and forest management. Despite the high potential of this biomass for sequestering atmospheric carbon dioxide, little effort has been devoted to identifying the factors affecting the rate of carbon uptake by forests in Korea. Recently, we reported that Korean forests have a higher carbon uptake rate (1.5 Mg C ha(-1) year(-1)) than those of North America, Europe, and China. In this study, as a follow-up to that work, we examine the distributions of total forest carbon, carbon density, and carbon uptake rate at the province and subprovince levels in Korea and elucidate the relationships between forest and climatic variables within these distributions. The provinces can be classified into three distinct groups according to their carbon uptake rate and forest age class: group A (Gyeonggi, Chungbuk, Chungnam), group B (Gyeongbuk, Gyeongnam, Jeonbuk, Jeonnam), and group C (Gangwon, Jeju). When all forest and climatic variables are considered, the provinces Gangwon and Jeju in group C are found to belong to distinct groups. The rate of carbon uptake in each province is not significantly correlated to most forest and climatic variables but is highly correlated to forest age class. A multivariate statistical analysis also supports our conclusion that forest age class is the major factor affecting the current distribution of the rate of carbon uptake in Korea. We conclude that for several decades Korean forests will have a high capacity for sequestering carbon dioxide.

Oceanic Suess effect of δ13C in subpolar region: The North Pacific

We propose a simple approach for estimating the recent oceanic Suess effect of δ13C in the subpolar region of the North Pacific, an area which experiences a large seasonal variability of carbonate species, due to the biological activity and vertical water mixing. This approach is based simply on time‐series data of δ13C from July 1997 to July 2001 and an equation of the Fourier sine expansion. Applying our approach to the fixed sampling location in the western North Pacific, station ‘KNOT’, we estimated values of −0.012‰‐δ13C yr−1 for the Suess effect of δ13C with an amplitude of 1.3‰‐δ13C and 1.0 μmol‐C kg−1 yr−1 for the increase rate of dissolved inorganic carbon with an amplitude of 134 μmol‐C kg−1 in surface water. The Suess effect of δ13C in this region was a similar value to that estimated in the Southern Ocean producing the deep water, while it was half as much as values estimated in the subtropical regions which experience a small seasonal variability. The ratio of Suess effect to the increase rate of dissolved inorganic carbon was −0.012‰ (μmol kg−1)−1, which was different from the global mean values calculated by recent modeling study. Therefore, we conclude that the Suess effect of δ13C in the polar and subpolar regions is generally smaller than found in other regions, suggesting that it is necessary to reevaluate the uptake rate of anthropogenic carbon which has been based on the assumption that the ratio is uniform over the entire ocean.

The effect of potassium deficiency on growth and N2‐fixation in Trifolium repens

The effects of potassium (K) deficiency on growth, N2‐fixation and photosynthesis in white clover (Trifolium repens L.) were investigated using natural occurring gas fluxes on the nodules in real time of plants under three contrasting relative addition rates of K causing mild K deficiency, or following abrupt withdrawal of the K supply causing strong K deficiency of less than 0.65% in dry matter. A steady‐state below‐optimum K supply rate led to an increase in CO2‐fixation per unit leaf surface area as well as per plant leaf surface. However, nitrogenase activity per unit root weight and per unit nodule weight was maintained, as was the efficiency with which electrons were allocated to the reduction of N2 in the nodules. Abrupt K removals stimulated nodule growth strongly without delay, but as K concentrations decreased in the plant tissue a significant decline in nitrogenase activity per unit root weight as well as per unit nodule mass occurred. Further, the rate of photosynthesis per unit leaf area was unaffected, while the CO2 acquisition for the plant as a whole increased due to an expansion of total leaf area whereas the leaf area per unit leaf weight was unaffected. The ratio between CO2‐fixation and N2‐fixation increased, although not statistically significant, under short‐term K deprivation as well as under long‐term low K supply indicating a downregulation of nodule activity following morphological and growth adjustments. This downregulation took place despite a partly substitution of the K by Na. It is concluded that N2‐fixation does not limit the growth of K‐deprived clover plants. K deprivation induces changes in the relative growth of roots, nodules, and shoots rather than changes in N and/or carbon uptake rates per unit mass or area of these organs.

Photoheterotrophy and light‐dependent uptake of organic and organic nitrogenous compounds by Planktothrix rubescens under low irradiance

Summary1. Planktothrix rubescens is the dominant photoautotrophic organism in Lake Zürich, a prealpine, deep, mesotrophic freshwater lake with an oxic hypolimnion. Over long periods of the year, P. rubescens accumulates at the metalimnion and growth occurs in situ at irradiance near the photosynthesis compensation point. Experiments were conducted to evaluate the contribution of photoheterotrophy, heterotrophy and light‐dependent uptake of nitrogenous organic compounds to the carbon and nitrogen budget of this cyanobacterium under conditions of restricted availability of light quanta.2. We used both purified natural populations of P. rubescens from the depth of 9 m and an axenic culture grown under low irradiance at 11 μmol m−2 s−1 on a light : dark cycle (10 : 14 h) to determine the uptake rates of various amino acids, urea, glucose, fructose, acetate and inorganic carbon. The components were added to artificial lake water in low amounts that simulated the naturally occurring potential concentrations.3. The uptake rates of acetate and amino acids (glycine, serine, glutamate and aspartate) were strongly enhanced at low irradiance as compared with the dark. However, no difference was observed in the uptake of arginine, which was taken up at high rates under both treatments. The uptake rates of glucose, fructose and urea were very low under all conditions. Similar results were obtained for both axenic P. rubescens and for purified natural populations of P. rubescens that were separated from bacterioplankton and other phytoplankton.4. Metalimnetic P. rubescens that was stratified at low irradiance for weeks exhibited much higher uptake rates than filaments that were entrained in the deepening surface mixed layer and experienced higher irradiance. The added organic compounds contributed up to 62% to the total carbon uptake of metalimnetic P. rubescens. On the basis of a molar C : N ratio of 4.9, the nitrogen uptake as organic compounds satisfied up to 84% of the nitrogen demand.5. The experiments indicate that photoheterotrophy and light‐dependent uptake of nitrogenous organic compounds may contribute significantly to the carbon and nitrogen budget of filaments at low irradiance typical for growth of P. rubescens in the metalimnion and at the bottom of the surface mixed layer.

Siliceous plankton dominate primary and new productivity during the onset of El Niño conditions in the Santa Barbara Basin, California

The potential for carbon export and the role of siliceous plankton in the cycling of C and N was assessed in natural plankton assemblages in the Santa Barbara Basin, California, by examining uptake rates of inorganic carbon, nitrate and silicic acid. In April–August 1997, the concentrations of chlorophyll a, particulate organic carbon, particulate organic nitrogen and biogenic silica were measured twice monthly, and results revealed the occurrence of at least three blooms, the largest in June. Particulate elemental ratios of C, N and Si were similar to ratios of nutrient-replete diatoms, suggesting that they dominated this bloom. Mean integrated rates of carbon, nitrate and silicon uptake during the 4-month study period are similar to other productive coastal and upwelling regions (103, 8.3 and 13 mmol m −2 day −1, respectively). New production rates were twice as high as previously reported in this region and indicate that high rates of new production along eastern boundary currents are not confined to the major coastal upwelling regions. C/NO 3 −, Si/NO 3 − and Si/C uptake ratios varied widely, and mean integrated ratios were 14±5.4, 1.6±1.0 and 0.12±0.07 (S.D.), respectively. That mean C/NO 3 − uptake ratio corresponds to an f-ratio of about 0.5 indicating a large potential for particulate export. Based on the average Si/NO 3 − and Si/C uptake ratios, diatoms could perform all of the primary production and nitrate uptake that occurred during the study; these rates also suggest that export is controlled by diatoms in this system. The mean Si/C biomass ratio was lower than the mean Si/C uptake ratio, consistent with the preferential export of Si relative to C observed in sediment traps in the basin. The study took place during a period of surface-water warming, with nitrate and silicic acid concentrations decreasing throughout the onset of the 1997–1998 El Niño conditions. Although diatoms contributed less to particulate biomass during the low nutrient conditions, high f-ratios (0.33–0.66) were maintained.

Leaf gas exchange, chlorophyll fluorescence and growth responses of Genipa americana seedlings to soil flooding

Effects of soil flooding on photosynthesis and growth of Genipa americana L. seedlings, a neotropical fruit-tree species used in gallery forest restoration programs, were studied under glasshouse conditions. Despite the high survival rate and wide distribution in flood-prone habitats of the neotropics, previous studies demonstrated that growth of G. americana is reduced under soil flooding. Using leaf gas exchange and chlorophyll fluorescence measurements, we tested the hypothesis that stomatal limitation of photosynthesis is the main factor that reduces carbon uptake and growth rates of G. americana seedlings. Throughout a 63-day flooding period, the survival rates were 100%. The maximum values of the net photosynthetic rate ( A) and stomatal conductance to water vapor ( g s) of control seedlings were 9.86 μmol CO 2 m −2 s −1 and 0.525 mol H 2O m −2 s −1, respectively. The earliest effects of flooding were significant decreases in g s and A, development of hypertrophied lenticels and decrease in the dry weight of roots. A strong effect of the leaf-to-air vapor pressure deficit (LAVPD) on g s and A were observed that was enhanced under flooded conditions. Between 14 and 63 days after flooding, significant reductions in g s (31.7% of control) and A (52.9% of control) were observed followed by significant increments in non-photochemical quenching ( q N) (187.5% of control). During the same period, there were no differences among treatments for the ratio between variable to initial fluorescence ( F v/ F 0), the maximum quantum efficiency of the photosystem II ( F v/ F m) and photochemical quenching ( q P), indicating that there was no damage to the photosynthetic apparatus. Based on the results, we conclude that decreases in stomatal opening and stomatal limitation of photosynthesis, followed by decrease in individual leaf area are the main causes of reductions in carbon uptake and whole plant biomass of flooded seedlings.

The Gaia Hypothesis: Conjectures and Refutations

The uncertainties surrounding global climate change provide ample evidence, if any were necessary, of the need for a whole-system view of the Earth. Arguably the most visible - and controversial - attempt to understand Earth as a system has been Lovelock's Gaia theory. Gaia has been a fruitful hypothesis generator, and has prompted many intriguing conjectures about how biological processes might contribute to planetary-scale regulation of atmospheric chemistry and climate. In many important cases, however, these conjectures are refuted by the available data. For example, Gaia theory predicts that the composition of the atmosphere should be tightly regulated by biological processes, but rates of carbon uptake into the biosphere have accelerated by only about 2% in response to the 35% rise in atmospheric CO2 since pre-industrial times. Gaia theory would predict that atmospheric CO2 should be more sensitively regulated by terrestrial ecosystem uptake (which is biologically mediated) than by ocean uptake (which is primarily abiotic), but both processes are about equally insensitive to atmospheric CO2 levels. Gaia theory predicts that biological feedbacks should make the Earth system less sensitive to perturbation, but the best available data suggest that the net effect of biologically mediated feedbacks will be to amplify, not reduce, the Earth system's sensitivity to anthropogenic climate change. Gaia theory predicts that biological by-products in the atmosphere should act as planetary climate regulators, but the Vostok ice core indicates that CO2, CH4, and dimethyl sulfide - all biological by-products - function to make the Earth warmer when it is warm, and colder when it is cold. Gaia theory predicts that biological feedbacks should regulate Earth's climate over the long term, but peaks in paleotemperature correspond to peaks in paleo-CO2 in records stretching back to the Permian; thus if CO2 is biologically regulated as part of a global thermostat, that thermostat has been hooked up backwards for at least the past 300 million years. Gaia theory predicts that organisms alter their environment to their own benefit, but throughout most of the surface ocean (comprising more than half of the globe), nutrient depletion by plankton has almost created a biological desert, and is kept in check only by the nutrient starvation of the plankton themselves. Lastly, where organisms enhance their environment for themselves, they create positive feedback; thus Gaia theory's two central principles - first, that organisms stabilize their environment, and second, that organisms alter their environment in ways that benefit them - are mutually incon- sistent with one another. These examples suggest that the further development of Gaia theory will require more deliberate comparison of theory and data.

Response of a complex ecosystem model of the northern Adriatic Sea to a regional climate change scenario

This paper investigates the impact of a regional climate change scenario on an ecosys- tem model of the northern Adriatic Sea. The study was performed by applying a biogeochemical bio- mass-based ecosystem model (ERSEM: European Regional Seas Ecosystem Model) coupled with a 1-dimensional version of a commonly used hydrodynamical model (POM: Princeton Ocean Model). The model response was studied by performing 2 simulations: one with atmospheric forcing functions for present-day climate conditions ('Control', 1970-1999), and one for the future climate after a dou- bling of the CO2 concentration ('Scenario', 2060-2089). Time-series of meteorological forcing func- tions were obtained from a greenhouse gas experiment with the high-resolution version of the ECHAM4 atmospheric general circulation model with a horizontal resolution of about 100 km. Pres- ent-day simulation results were compared to the available historical observations and showed a sat- isfactory agreement with the main seasonal cycles of hydrodynamical variables and nutrients in the limits of the 1-dimensional representation. Under the Scenario conditions, the model predicted an overall enhancement of the water-column stratification on an annual basis, with stronger intensifica- tion during the summer periods. The diffusion of oxygen and nutrients between surface and bottom layers is reduced, and the transfer of organic matter through the food web is shifted towards the smaller components of the microbial web. The model gives indications of an increase in the specific phytoplankton uptake rates of inorganic carbon in the Scenario, but the photosynthesized carbon ends up in the DOC pool because of the reduced supply of nutrients. The mean annual concentration of DOC is thus higher in the Scenario, with possible negative consequences on the water quality. Additional experiments show that the response of primary producers is not directly linked to the increase in ambient temperature. The structure of the food web modulates the interaction of phyto- plankton with the abiotic conditions. The results give indications of a possible non-linear response of the biogeochemical cycles to the climate change warming scenario and confirm the feasibility of using the downscaling technique to couple large-scale climate models with comprehensive regional ecosystem models.

THE INVOLVEMENT OF A PLASMA MEMBRANE H+‐ATPase IN THE BLUE‐LIGHT ENHANCEMENT OF PHOTOSYNTHESIS IN LAMINARIA DIGITATA (PHAEOPHYTA)1

Many brown algae, including the kelp Laminaria digitata (Huds.) Lamour., exhibit enhanced photosynthesis when they are given a small amount of blue‐light in addition to a background of saturating red light. This blue light effect is correlated with an increased uptake of carbon. In the present study, we tested the hypothesis that blue light acts by increasing the activity of a plasma membrane H+‐ATPase, thereby promoting an active carbon uptake across the plasma membrane. Photosynthetic carbon uptake was studied in pH‐drift experiments under illumination with red and blue light and using different inhibitors. Vanadate, an inhibitor of plasma membrane H+‐ATPases, had a minor inhibitory effect on carbon uptake rates under saturating red light conditions, but inhibited the blue‐light enhancement by approximately 60%. An inhibitor of external carbonic anhydrase, acetazolamide, decreased the carbon uptake in both red light and in red plus blue light by 48% and 68%, respectively. These results suggest that photosynthetic carbon uptake depends on an external carbonic anhydrase under both red and red plus blue light conditions, and that blue light induces an increased activity of a P‐type H+‐ATPase in the plasma membrane. The proton buffer Tris, which has a buffering capacity similar to vanadate in seawater, had no inhibitory effect on carbon uptake rates neither in red light nor in red plus blue light, showing that the inhibitory effect of vanadate is not caused by its effect as a buffer. The blue‐light enhancement was also abolished by a protein kinase inhibitor (H‐7), suggesting that the transduction of the blue‐light signal involves a protein kinase, which activates the plasma membrane H+‐ATPase by phosphorylation.

Revision of the global carbon budget due to changing air‐sea oxygen fluxes

Carbon budgets inferred from measurements of the atmospheric oxygen to nitrogen ratio (O2/N2) are revised considering sea‐to‐air fluxes of O2 and N2 in response to global warming and volcanic eruptions. Observational estimates of changes in ocean heat content are combined with a model‐derived relationship between changes in atmospheric O2/N2 due to oceanic outgassing and heat fluxes to estimate ocean O2 outgassing. The inferred terrestrial carbon sink for the 1990s is reduced by a factor of two compared with the most recent estimate by the Intergovernmental Panel on Climate Change (IPCC). This also improves the agreement between calculated ocean carbon uptake rates and estimates from global carbon cycle models, which indicate a higher ocean carbon uptake during the 1990s than the 1980s. The simulated decrease in oceanic O2 concentrations is in qualitative agreement with observed trends in oceanic O2 concentrations.