Phytoplankton Photosynthesis Research Articles

Scaling of phytoplankton photosynthesis and cell size in the ocean

We have determined the scaling relationship between photosynthesis rate and cell size in natural phytoplankton assemblages of contrasting marine environments. We found that phytoplankton photosynthesis in the ocean does not scale as the L‐power of cell size, but scales approximately isometrically with cell size, indicating that a single model cannot predict the metabolism‐size relationship in all photosynthetic organisms. The scaling relationship between cellular chlorophyll a content and cell size is also isometric. Taxonomical changes along the size spectrum may explain the deviation of phytoplankton photosynthesis from the general allometric rule. The size scaling exponent for photosynthesis is significantly higher (1.14) in coastal productive waters than in the oligotrophic open ocean (0.96), which provides a physiological basis to explain the dominance of larger cells in nutrient‐rich environments. The size scaling exponent for phytoplankton abundance is significantly less negative in coastal productive waters (–0.90) than in the oligotrophic open ocean (–1.25). The observed size scaling relationships imply that carbon fixation per unit volume decreases with cell size in oligotrophic waters, whereas the opposite occurs in productive ones. By controlling the metabolism‐size scaling relationship, nutrient supply plays a major role in determining community size structure and the energy flow through the pelagic ecosystem.

Calcium carbonate supersaturation and precipitation in Chinese mitten crab (Eriocheir japonica sinensis) larval ponds in China: Mass mortality, crystal form analysis, and safety saturation index

In recent years, mass mortalities of Chinese mitten crab (Eriocheir japonica sinensis) larvae have been reported in several crab culture regions in China and have resulted in great economic losses. The mass mortality usually occurs in seawater ponds in the afternoon of sunny days when water color suddenly changes from dark green or brown to cloudy white or light yellow, and mineral precipitate occurs. Moribund and dead larvae are typically covered with debris. A series of experiments was conducted to examine the cause of precipitation and mass mortality of crab larvae. Analysis of the precipitate from ponds with mass mortalities showed that CaCO3 was the main component of the precipitate, accounting for 70.8% of the total weight. In an animal experiment, mortalities were observed within 48 h in all groups with CaCO3 precipitate, and generally increased as pH and CaCO3 saturation rates increased. Mortalities in groups with CaCO3 precipitate were greater than those in groups without CaCO3 precipitate (P≤0.05). All larvae in groups with CaCO3 precipitate sank to the bottom of the beaker within 48 h. The animal experiment also showed that increased pH (up to 9.4) alone without CaCO3 precipitate did not result in larval mortality within 48 h. Results from the present investigation indicate that mass mortalities of the crab larvae are caused by CaCO3 supersaturation and rapid precipitation possibly triggered by increased pH as a result of intensive photosynthesis of phytoplankton in the seawater ponds. By comparing mean Ca2+ and CO32− ionic product obtained at the equilibrium with solubility product constants for various CaCO3 crystal forms, it appears that CaCO3·H2O is the main CaCO3 form. Based on the CaCO3·H2O solubility product constant and our field investigations, we suggest that CaCO3·H2O saturation indices of 2.5 and below be considered safe and 3.4 as warning signal for CaCO3 precipitation. Since several factors may cause CaCO3 supersaturation and precipitation in crab larval ponds, preventive measures should be implemented to stabilize pH and reduce the CaCO3 saturation rate.

Short-term covariation of dissolved oxygen and phytoplankton photosynthesis in a coastal fish aquaculture site

The influence of phytoplankton photosynthesis (PP) on dissolved oxygen (DO) dynamics was evaluated in a red sea bream ( Pagrus major) aquaculture site. The surface PP was monitored continuously by a natural fluorescence sensor for 2–3 weeks during all seasons, except spring. In summer, when the daily PP was low during cloudy weather, DO at the surface markedly decreased below the critical level (5.7 mg O 2 L −1), which is needed for normal fish growth. However, on the subsequent clear day, active PP restored the DO back to above the critical level. In fall, under cloudy skies, the surface DO also decreased below the critical level and remained there, even when the subsequent days were clear. In winter, DO remained at high levels throughout the water column due to an active supply from the air and vertical mixing. Besides seasonal changes, surface DO showed a diel cycle with a minimum early in the morning and maximum in the evening. This diel cycle was also regulated by planktonic photosynthesis. This was most obvious and its amplitude was largest in summer, as PP was very high and thermal stratification prevented diffusion of DO from the surface to deeper water. The present study is the first detailed report on the close coupling of DO with PP in coastal waters, even under non-bloom conditions. Cloudy weather and early mornings were found to be the most critical periods in oxygen supply to cultured fish and other pelagic organisms in a fish culture site and other eutrophic coastal area.

Variability of UVR Effects on Photosynthesis of Summer Phytoplankton Assemblages from a Tropical Coastal Area of the South China Sea†

From June to September 2005, we carried out experiments to determine the ultraviolet radiation (UVR) -induced photoinhibition of summer phytoplankton assemblages from a coastal site of the South China Sea. Variability in taxonomic composition was determined throughout the summer, with a peak chlorophyll a (chl a approximately 20 microg chl a L(-1)) dominated by the diatom Skeletonema costatum that was detected early in the study period; the rest of the time samples were characterized by monads and flagellates, with low chl a values (1-5 chl a microg L(-1)). Surface water samples were placed in quartz tubes, inoculated with radiocarbon and exposed to solar radiation for 2-3 h to determine photosynthetic rates under three quality radiation treatments (i.e. PAB, 280-700 nm; PA, 320-700 nm and P, 400-700 nm) using different filters and under seven levels of ambient irradiance using neutral density screens (P vs E curves). UVR inhibition of samples exposed to maximum irradiance (i.e. at the surface) varied from -12.2% to 50%, while the daytime-integrated UVR-related photoinhibition in surface seawater varied from -62% to 7%. The effects of UVR on the photosynthetic parameters P(B)(max) and E(k) were also variable, but UV-B accounted for most of the observed variability. During sunny days, photosynthesis of microplankton (>20 microm) and piconanoplankton (<20 microm) were significantly inhibited by UVR (mostly by UV-B). However, during cloudy days, while piconanoplankton cells were still inhibited by UVR, microplankton cells used UVR (mostly UV-A) as the source of energy for photosynthesis, resulting in higher carbon fixation in samples exposed to UVR than the ones exposed only to photosynthetically active radiation (PAR). Our results indicate that size structure and cloudiness clearly condition the overall impact of UVR on phytoplankton photosynthesis in this tropical site of South China. In addition, model predictions for this area considering only PAR for primary production might have underestimated carbon fixation due to UVR contribution.

Sampling biases in MODIS and SeaWiFS ocean chlorophyll data

Although modern ocean color sensors, such as MODIS and SeaWiFS, are often considered global missions, in reality it takes many days, even months, to sample the ocean surface enough to provide complete global coverage. The irregular temporal sampling of ocean color sensors can produce biases in monthly and annual mean chlorophyll estimates. We quantified the biases due to sampling using data assimilation to create a “truth field”, which we then sub-sampled using the observational patterns of MODIS and SeaWiFS. Monthly and annual mean chlorophyll estimates from these sub-sampled, incomplete daily fields were constructed and compared to monthly and annual means from the complete daily fields of the assimilation model, at a spatial resolution of 1.25° longitude by 0.67° latitude. The results showed that global annual mean biases were positive, reaching nearly 8% (MODIS) and > 5% (SeaWiFS). For perspective the maximum interannual variability in the SeaWiFS chlorophyll record was about 3%. Annual mean sampling biases were low (< 3%) in the mid-latitudes (between − 40° and 40°). Low interannual variability in the global annual mean sampling biases suggested that global scale trend analyses were valid. High latitude biases were much higher than the global annual means, up to 20% as a basin annual mean, and over 80% in some months. This was the result of the high solar zenith angle exclusion in the processing algorithms. Only data where the solar angle is < 75° are permitted, in contrast to the assimilation which samples regularly over the entire area and month. High solar zenith angles do not facilitate phytoplankton photosynthesis and low chlorophyll concentrations occurring here are missed by the data sets. Ocean color sensors selectively sample in locations and times of favorable phytoplankton growth, producing overestimates of chlorophyll. The biases derived from lack of sampling in the high latitudes varied monthly, leading to artifacts in the apparent seasonal cycle from ocean color sensors. A false secondary peak in chlorophyll occurred in May–August, which resulted from lack of sampling in the Antarctic. Persistent clouds, characteristic in the North Pacific, also produced overestimates, again by selectively sampling only the high growth periods. In contrast, areas characterized by thick aerosols showed chlorophyll underestimates to nearly − 30% in basin monthly means. This was the result of selective sampling in lower aerosol thickness periods, which corresponded with lower phytoplankton growth periods. A combination of MODIS and SeaWiFS sampling was most effective at reducing mid-latitude biases due to inter-orbit gaps, sun glint, and sensor tilt changes. But these biases were low using a single sensor, suggesting multiple sensors had little effect in reducing global and regional monthly and annual mean biases. Ocean color data are an invaluable source of information about global biological processes. However, these results suggest that sampling errors need to be considered in applications involving global and regional mean chlorophyll biomasses as well as seasonal variability and regional trend analysis.

Solar UV radiation drives CO2 fixation in marine phytoplankton: a double-edged sword.

Photosynthesis by phytoplankton cells in aquatic environments contributes to more than 40% of the global primary production (Behrenfeld et al., 2006). Within the euphotic zone (down to 1% of surface photosynthetically active radiation [PAR]), cells are exposed not only to PAR (400-700 nm) but also to UV radiation (UVR; 280-400 nm) that can penetrate to considerable depths (Hargreaves, 2003). In contrast to PAR, which is energizing to photosynthesis, UVR is usually regarded as a stressor (Hader, 2003) and suggested to affect CO2-concentrating mechanisms in phytoplankton (Beardall et al., 2002). Solar UVR is known to reduce photosynthetic rates (Steemann Nielsen, 1964; Helbling et al., 2003), and damage cellular components such as D1 proteins (Sass et al., 1997) and DNA molecules (Buma et al., 2003). It can also decrease the growth (Villafane et al., 2003) and alter the rate of nutrient uptake (Fauchot et al., 2000) and the fatty acid composition (Goes et al., 1994) of phytoplankton. Recently, it has been found that natural levels of UVR can alter the morphology of the cyanobacterium Arthrospira (Spirulina) platensis (Wu et al., 2005b). On the other hand, positive effects of UVR, especially of UV- A (315-400 nm), have also been reported. UV- A enhances carbon fixation of phytoplankton under reduced (Nilawati et al., 1997; Barbieri et al., 2002) or fast-fluctuating (Helbling et al., 2003) solar irradiance and allows photorepair of UV- B-induced DNA damage (Buma et al., 2003). Furthermore, the presence of UV-A resulted in higher biomass production of A. platensis as compared to that under PAR alone (Wu et al., 2005a). Energy of UVR absorbed by the diatom Pseudo-nitzschia multiseries was found to cause fluorescence (Orellana et al., 2004). In addition, fluorescent pigments in corals and their algal symbiont are known to absorb UVR and play positive roles for the symbiotic photosynthesis and photoprotection (Schlichter et al., 1986; Salih et al., 2000). However, despite the positive effects that solar UVR may have on aquatic photosynthetic organisms, there is no direct evidence to what extent and howUVR per se is utilized by phytoplankton. In addition, estimations of aquatic biological production have been carried out in incubations considering only PAR (i. e. using UV-opaque vials made of glass or polycarbonate; Donk et al., 2001) without UVR being considered (Hein and Sand-Jensen, 1997; Schippers and Lurling, 2004). Here, we have found that UVR can act as an additional source of energy for photosynthesis in tropical marine phytoplankton, though it occasionally causes photoinhibition at high PAR levels. While UVR is usually thought of as damaging, our results indicate that UVR can enhance primary production of phytoplankton. Therefore, oceanic carbon fixation estimates may be underestimated by a large percentage if UVR is not taken into account.

Vertical mixing within the epilimnion modulates UVR‐induced photoinhibition in tropical freshwater phytoplankton from southern China

Summary1. The importance of vertical mixing in modulating the impact of UVR on phytoplankton photosynthesis was assessed in a tropical, shallow lake in southern China from late winter to mid‐spring of 2005.2. Daily cycles of fluorescence measurements (i.e. photosynthetic quantum yield, Y) were performed on both ‘static’ and in situ samples. Static samples were of surface water incubated at the surface of the lake under three radiation treatments – PAB (PAR + UVR, 280–700 nm), PA (PAR + UV‐A, 320–700 nm) and P (PAR, 400–700 nm). In situ samples were collected every hour at three different depths – 0, 0.5 and 1 m.3. The general daily pattern was of a significant decrease in Y from early morning towards noon, with partial recovery in the afternoon. Samples incubated under static conditions always had lower Y than those under in situ conditions at the same time of the day.4. Under stratified conditions, no overall impact of UVR impact could be detected in situ when compared with the static samples. Further rapid vertical mixing not only counteracted the impact of UVR but also stimulated photosynthetic efficiency.5. Based on these measurements of fluorescence, the mixing speed of cells moving within the epilimnion was estimated to range between 0.53 and 6.5 cm min−1.6. These data show that mixing is very important in modulating the photosynthetic response of phytoplankton exposed to natural radiation and, hence, strongly conditions the overall impact of UVR on aquatic ecosystems.

High Variability of Phytoplankton Photosynthesis in Response to Environmental Forcing in Oligotrophic Lake Malawi/Nyasa

In southern Lake Malawi, seasonal pelagic chlorophyll means were 1.0 ± 0.3 μg L −1 in the deep mixing season (DMS) (May–August), 0.8 ± 0.3 μg L −1 in the dry stratified season (DSS) (September to November) and 0.7 ± 0.3 μg L −1 in the wet stratified season (WSS) (December to April). Despite the low variability in chlorophyll, there was a wide range in chlorophyll specific photosynthetic activity. The photosynthetic parameters, P b m (the light saturated rate) and α b (the light limited slope), varied significantly among seasons and were highly positively correlated, with lowest values in the DSS and highest values in WSS. During deep mixing, P b m did not covary with α b; and the light saturation index, E k (=P b m/α b), varied in response to changes in α b rather than in P b m. Phytoplankton appeared to be nutrient deficient at all times but less deficient during deep vertical mixing in the DMS. Average daily rates of integrated phytoplankton primary productivity were lowest in the DSS (337 mg C m −2 d −1) and highest in the WSS (629 mg C m −2 d −1) despite nearly identical mean chlorophyll concentrations. Along a near shore transect off the Linthipe River, chlorophyll concentrations were higher and more variable (1.4 ± 1.3 μg L −1), phytoplankton were not strongly nutrient deficient and chlorophyll specific photosynthetic activity was as high or higher than at the offshore station. Estimates of phytoplankton productivity in this tropical great lake must account for spatial and temporal variability in photosynthetic parameters imposed by seasonal changes in mixing dynamics.

Effects of ultraviolet radiation B (UV-B) on photosynthesis of natural phytoplankton assemblages in a marine bay in Southern China

Solar ultraviolet radiation B (UVB) is known to have inhibitive effects on phytoplankton photosynthesis. UVB light decreases rapidly with increasing depth in the water column and exerts different degrees of UVB inhibitive effects on phytoplankton photosynthesis. In this study, the objectives were to quantify effects of UVB on phytoplankton photosynthesis and quantum yield, and to examine UVB effects on phytoplankton photosynthesis when light varies. The in-situ experiments were conducted in Da Ya Bay, which is a semi-enclosed area in the subtropical South China. The results showed a significant reduction of photosynthetic rates and quantum yield in the presence of UVB. Maximum photosynthetic rates (P-max) and maximum quantum yield (Phi(max)) were 11%-22% and 17%-49% less under solar radiation with UVB than without UVB. A simplified model was developed to describe the UVB biologically effective fluence rate (E*(inh)) as an exponential decay function of depth. Light-shift experiments, in which water samples from the surface and at depth of 4 m were divided into several subsamples, and each subsamples were then incubated at different depths with and without UVB in the water column, showed that phytoplankton from the deeper water (4 m) had more inhibitive rates by UVB than that from the surface when exposed to the same light condition.

Effects of diurnal variations in phytoplankton photosynthesis obtained from natural fluorescence

Effects of diurnal variation in phytoplankton photosynthesis on estimating daily primary production (DPP) were examined using field data from Sagami Bay, Japan. DPP at 5 m depth was calculated from the continuous data of chlorophyll a (Chl a) and light intensity monitored by a natural fluorescence sensor with and without considering time-dependent changes in the photosynthesis–irradiance (P–E) relationship. Chl a could be estimated from natural fluorescence examining the variations in the quantum yield of fluorescence (φf) and Chl a-specific light absorption coefficient (a*ph), and relating them to Chl a. The P–E relationship was determined by water sampling three times daily. A distinct diurnal pattern was observed for the maximum photosynthetic rate (P*max), being maximal at noon, while periodicity of the maximum light utilization coefficient (α*) was less obvious. The actual DPP was calculated by interpolating the P–E parameters from those obtained at dawn, noon, and dusk. For comparison, DPP was calculated by fixing the P–E parameters as the constants measured at dawn, noon or dusk for a day. The difference from the actual DPP was small when the P–E parameters measured at dawn (3% on average) and noon (5%) were used as the constants for a day. The difference was largest when the values at dusk were used (−43%). The medium values of P*max at dawn, its low values at dusk, and the fact that a major part of the DPP was produced around noon were responsible for these results. The present study demonstrates that measurement of the P–E parameters at dawn or noon can give a good estimation of DPP from natural fluorescence.

Partitioning of river metabolism identifies phytoplankton as a major contributor in the regulated Murray River (Australia)

Summary1. River metabolism was measured over an annual cycle at three sites distributed along a 1000 km length of the lowland Murray River, Australia.2. Whole system metabolism was measured using water column changes in dissolved oxygen concentrations while planktonic and benthic metabolism were partitioned using light‐dark bottles and benthic chambers.3. Annual gross primary production (GPP) ranged from 775 to 1126 g O2 m−2 year−1 which in comparison with rivers of similar physical characteristics is moderately productive.4. Community respiration (CR) ranged from 872 to 1284 g O2 m−2 year−1 so that annual net ecosystem production (NEP) was near zero, suggesting photosynthesis and respiration were balanced and that allochthonous organic carbon played a minor role in fuelling metabolism.5. Planktonic rates of gross photosynthesis and respiration were similar to those of the total channel, indicating that plankton were responsible for much of the observed metabolism.6. Respiration rates correlated with phytoplankton standing crop (estimated as the sum of GPP plus the chlorophyll concentration in carbon units), yielding a specific respiration rate of −1.1 g O2 g C−1 day−1. The respiration rate was equivalent to 19% of the maximum rate of phytoplankton photosynthesis, which is typical of diatoms.7. The daily GPP per unit phytoplankton biomass correlated with the mean irradiance of the water column giving a constant carbon specific photon fixation rate of 0.35 gO2 g Chl a−1 day−1 per μmole photons m−2 s−1 (ca. 0.08 per mole photons m−2 on a carbon basis) indicating that light availability determined daily primary production.8. Annual phytoplankton net production (NP) estimates at two sites indicated 25 and 36 g C m−2 year−1 were available to support riverine food webs, equivalent to 6% and 11% of annual GPP.9. Metabolised organic carbon was predominantly derived from phytoplankton and was fully utilised, suggesting that food‐web production was restricted by the energy supply.

Biological mechanisms driving the seasonal changes in the internal loading of phosphorus in shallow lakes

Because of the obvious importance of P as a nutrient that often accelerates growth of phytoplankton (including toxic cyanobacteria) and therefore worsens water quality, much interest has been devoted to P exchange across the sediment-water interface. Generally, the release mode of P from the sediment differed greatly between shallow and deep lakes, and much of the effort has been focused on iron and oxygen, and also on the relevant environmental factors, for example, turbulence and decomposition, but a large part of the P variation in shallow lakes remains unexplained. This paper reviews experimental and field studies on the mechanisms of P release from the sediment in the shallow temperate (in Europe) and subtropical (in the middle and lower reaches of the Yangtze River in China) lakes, and it is suggested that pH rather than DO might be more important in driving the seasonal dynamics of internal P loading in these shallow lakes, i.e., intense photosynthesis of phytoplankton increases pH of the lake water and thus may increase pH of the surface sediment, leading to enhanced release of P (especially iron-bound P) from the sediment. Based on the selective pump of P (but not N) from the sediment by algal blooms, it is concluded that photosynthesis which is closely related to eutrophication level is the driving force for the seasonal variation of internal P loading in shallow lakes. This is a new finding. Additionally, the selective pump of P from the sediment by algal blooms not only explains satisfactorily why both TP and PO4-P in the hypereutrophic Lake Donghu declined significantly since the mid-1980s when heavy cyanobacterial blooms were eliminated by the nontraditional biomanipulation (massive stocking of the filter-feeding silver and bighead carps), but also explains why TP in European lakes decreased remarkably in the spring clear-water phase with less phytoplankton during the seasonal succession of aquatic communities or when phytoplankton biomass was decreased by traditional biomanipulation. Compared with deep lakes, wax and wane of phytoplankton due to alternations in the ecosystem structure is also able to exert significant influences on the P exchange at the sediment-water interface in shallow lakes. In other words, biological activities are also able to drive P release from sediments, and such a static P release process is especially more prominent in eutrophic shallow lakes with dense phytoplankton.

A numerical simulation of the role of zooplankton in C, N and P cycling in Lake Kinneret, Israel

We quantify the role of zooplankton in nutrient cycles in Lake Kinneret, Israel, using field data and a numerical model. A coupled ecological and hydrodynamic model (Dynamic Reservoir Model (DYRESM)–Computational Aquatic Ecosystem Dynamics Model (CAEDYM)) was validated with an extensive field data set to simulate the seasonal dynamics of nutrients, three phytoplankton groups and three zooplankton groups. Parameterization of the model was conducted using field, experimental and literature studies. Sensitivity of simulated output was tested over the full parameter space and established that the most sensitive parameters were related to zooplankton grazing rates, temperature responses and food limitation. The simulated results predict that, on average, 51% of the carbon from phytoplankton photosynthesis is consumed by zooplankton. Excretion of dissolved nutrients by zooplankton accounts for 3–46 and 5–58% of phytoplankton uptake of phosphorus and nitrogen, respectively. Comparison of nutrient fluxes attributable to zooplankton with nutrient loads from inflows and release from bottom sediments shows that the relative contribution by zooplankton to inorganic nutrients in the photic zone varies seasonally in response to the annual hydrodynamic cycle of stratification and mixing. As a percent of total dissolved organic sources relative contributions by zooplankton excretion are highest (62%) during periods of stratification and when inflow nutrient loads are low, and lowest (2%) during the breakdown of stratification and when inflow loads are high. The results illustrate the potential of a lake ecosystem model to extract useful process information to complement field data collection and address questions related to the role of zooplankton in nutrient cycles.

Imbalance between phytoplankton production and bacterial carbon demand in relation to mucilage formation in the Northern Adriatic Sea

Spatial and temporal changes in phytoplankton production and bacterial C demand were investigated at four stations in the Northern Adriatic Sea over 3 years. The effect of the Po River plume was observed at the western stations; in particular, the northernmost one (B06) showed the highest values of primary production, both as hourly peaks (up to 14 mg C m − 3 h − 1 ) and daily water column integrated values (up to 740 mg C m − 2 day − 1 ), the southern station (C04) was only sporadically influenced and did not differ significantly from the easternmost ones (C12 and B13), where the lowest TPP values were recorded (around 1 mg C m − 3 h − 1 ). In this study the first in situ data are reported on short-term phytoplankton C extra cellular release in the Northern Adriatic Sea. At every station a considerable percentage of primary production (PER > 20% as an average, with peaks of up to 70%) was released as dissolved organic carbon. In particular, an association of fairly high PER (> 10%) and specific production ( P b > 10 mg C mg chl − 1 h − 1 ) was observed from spring to summer, when the mucilage phenomenon usually starts. This result might suggest the presence of an uncoupling between photosynthesis and growth, probably related with nutrient availability, which would be responsible for a high production of extra cellular organic carbon. Phytoplankton primary production and bacterial carbon production were closely related and bacterial C production accounted, on average, for a higher percentage of primary production than the values typically reported in the literature on aquatic environments. The flow of organic matter from phytoplankton to bacteria seems to satisfy the bacterial carbon demand during most of the spring and summer, at least in the upper water layers. However, during the summer, there is evidence that BCD sometimes exceeds the amount of C produced by phytoplankton. Neither phytoplankton nor bacterial production showed significant differences over the relevant years, and their absolute values did not change when comparing periods with or without mucilage. However, there were indications of an uncoupling between phytoplankton photosynthesis and growth and of a shift from an autotrophic to a heterotrophic metabolism, especially during the spring and summer period when mucilage might occur.

Coupling between ocean biota and atmospheric aerosols: Dust, dimethylsulphide, or artifact?

Two hypotheses that postulate interactions between ocean biota and aerosols in the atmosphere have generated substantial research into marine systems. The stimulation of phytoplankton photosynthesis by the provision of iron, a micronutrient contained in deposited aeolian dust (the Iron Hypothesis), and the contribution of dimethylsulphide (DMS) produced by marine ecosystems to the atmospheric burden of aerosols (the CLAW Hypothesis) have been the focus of much research. Satellite sensors, such as the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) now provide moderate‐resolution time series of measurements of the optical properties of the oceans and atmosphere over most of the Earth's surface. These data provide an unprecedented opportunity to investigate the ubiquity of biotic linkages between the ocean and atmosphere at the global scale. We analyzed 5 years of SeaWiFS 8‐day fields of two variables, chlorophyll concentration and aerosol optical depth, for the global oceans. This first global, multiyear approach does not yet allow unequivocal conclusions, as satellite measurements of chlorophyll can be influenced by aerosol properties of the atmosphere and several variables we do not yet examine are likely to play a role. We find correlation between optical properties of the ocean and atmosphere over much of the globe, in particular the midlatitudes. While some regional analyses indicate that SeaWiFS chlorophyll retrievals are biased by dust in the atmosphere, our results do not support the existence of widespread bias in the SeaWiFS products, but are consistent with global‐scale couplings posited by the Iron and CLAW hypotheses.

Phytoplankton production modelling in three marine ecosystems—static versus dynamic approach

Phytoplankton productivity is usually determined from water samples incubated at a number of irradiance levels during several hours. The resultant productivity-irradiance ( P– E) curves are then used to estimate local and/or global phytoplankton production. However, there is growing evidence that these curves, referred as static, underestimate phytoplankton photosynthesis to a great deal, by assuming a stable response to light over the incubation period. One of the drawbacks of static P– E curves is the overestimation of photoinhibition. In this work, three one-dimensional vertically resolved models were developed as simply as possible, to investigate differences between static and dynamic phytoplankton productivity in three marine ecosystems: a turbid estuary, a coastal area and an open ocean ecosystem. The results show that, when photoinhibition development time is considered (dynamic model), the primary production estimates are always higher than when calculated with the static model. The quantitative importance of these differences varies with the type of ecosystem and it appears to be more important in coastal areas and estuaries (from 21 to 72%) than in oceanic waters (10%). Thus, these results suggest that primary production estimates, obtained under the assumption of a static behaviour response to light, may underestimate the real values of global phytoplankton primary production. Calculations suggest that the quantitative importance of this underestimation may be larger than the global missing carbon sink.

Size dependence of coastal phytoplankton photosynthesis under vertical mixing conditions

We have determined the relationship between carbon-specific photosynthesis and phytoplankton cell size in a coastal ecosystem. The normalized size spectra of carbon (C) biomass and photosynthesis allow to determine both biomass and photosynthesis within any size class along the community size spectrum. By dividing the size spectra of photosynthesis and biomass, the size spectrum of C-specific photosynthesis is derived. Our results indicated a high variability in the slope of the C-specific photosynthesis size spectrum. Under favourable conditions for growth, in the upper euphotic layer, the slope was positive, indicating that larger phytoplankton attained higher C-specific photosynthesis rates than the smaller cells. This pattern represents a significant departure from the expected, literature value of � 0.25 for the size-scaling of biomass-specific metabolism. We suggest that this change in the slope may be caused by the changes in the taxonomic composition along the community size spectrum. Towards the bottom of the euphotic layer, we observed a decrease in the slope of the C-specific photosynthesis size spectrum, which could be associated with an enhanced package effect in larger cells under light-limited conditions. These results question the applicability of single and overall exponents to describe the size scaling of photosynthesis in natural phytoplankton assemblages.

On the variation of alkalinity during phytoplankton photosynthesis

The alkalinity of the organic constituents of marine phytoplankton and their participation in the total alkalinity (TA) change of seawater during photosynthesis are carefully assessed. Quantification of the contribution of phytoplankton chlorophyll, proteins and phosphorus compounds to the hydrogen ion balance of seawater in terms of total inorganic nitrogen (∆[NT] = ∆NH4 +] + ∆[N2] + ∆[NO2–] + ∆[NO3–]) and total inorganic phosphorus (∆[PT]) changes during photosynthesis yielded that the organic components of marine phytoplankton are alkaline by –0.06 × ∆[NT] – 0.49 × ∆[PT], and that the potential total alkalinity (TAP) during photosynthesis is TAP = TA – [NH4– + 0.93 × [NO2–] + [NO3–] + 0.08 × [NT] + 0.23 × [PT] for unfiltered seawater samples and TAP = TA – [NH4–] + 0.93 × [NO2–] + [NO3–] + 0.02 × [NT] + 0.26 × [PT] for filtered seawater samples. These equations correct the traditionally used expression TAP = TA + [NO3–]. The TAP anomalies are produced, in order of increasing importance, by N2 fixation, DMSP production and CaCO3 fixation.

Comparative assessment of phytoplankton photosynthesis using conventional 14C-determination and Fast Repetition Rate Fluorometry in freshwaters

Click to increase image sizeClick to decrease image sizeKey words: phytoplankton photosynthesislakesFast Repetition Rate Fluorometry

Combined Influence of Some Pollutants and Ultraviolet Radiation on Phytoplankton Photosynthesis

Combined Influence of Some Pollutants and Ultraviolet Radiation on Phytoplankton Photosynthesis