Plankton Photosynthesis Research Articles

Linking Biogeochemical and Hydrodynamic Processes to Model Methane Fluxes in Shallow, Tropical Floodplain Lakes

AbstractFloodplains lakes are abundant in the Amazon basin and are important methane sources to the atmosphere. Existing biogeochemical models require modifications and inclusion of hydrodynamic processes operative in shallow, warm waters to be applied to these aquatic ecosystems. We modified a 1‐dimensional process‐based, lake biogeochemical model and combined a 3‐dimensional hydrodynamic model to suit Amazon floodplains. We evaluated the combined model's performance simulating methane concentrations and fluxes and several related processes in the open lake and an embayment of a well‐studied Amazon lake. Parameters for calibration were selected through sensitivity tests using a machine learning‐based algorithm, classification, and regression trees. Comparison between simulated and measured fluxes indicate generally good agreement in seasonal patterns and magnitudes. Comparisons of near‐surface concentrations varied with no clear patterns. Simulations of methane concentrations at near‐surface and near‐bottom, and diffusive emissions are most sensitive to carbon mineralization rate, Q10 factors for methanogenesis and oxidation, and methane oxidation potential. Modeled rates of planktonic photosynthesis were generally lower than measurements, though simulated planktonic respiration was often similar to measurements. Simulated rates of methane oxidation were considerably lower, with a few exceptions, than measurements of methane oxidation in oxic water of the lake. Improvements of results of the linked hydrodynamic‐biogeochemical model will result from inclusion of advective transport, use of parameter values appropriate for tropical waters, especially for methane oxidation and photosynthesis, and addition of changes in hydrostatic pressure to model of ebullition.

Respiration, phytoplankton size and the metabolic balance in the Atlantic gyres

The balance between plankton photosynthesis (GPP) and community respiration (CR) in the euphotic zone (net community production, NCP) is an essential driver of the biological carbon pump. Deficient datasets and a lack of knowledge of the mechanisms regulating CR cause poor empirical models and oversimplified parameterisations that maintain NCP as one of the most important unknowns for projections of the carbon pump. One important unresolved issue is the unexpected lack of empirical relationships between CR and the biomass or size-structure of the phytoplankton, which undermines the use of remotely sensed observations to predict net community metabolism. Here we analyse the spatial variation of plankton metabolism, chlorophyll a concentration (Chla), pico- and nanophytoplankton abundance and size-fractionated primary production (14CPP) along a latitudinal (49°N–46°S) transect of 73 stations across the Atlantic Ocean (AMT-22 cruise). The use of depth-weighted rates (rates integrated to the depth of 0.1% PAR, divided by the regionally varying depth of integration) markedly improved the depiction of latitudinal patterns and the significance of relationships, over volumetric or integrated rates. Depth-weighted CR showed clear and consistent latitudinal patterns with relevance for the distribution of NCP. Depth-weighted Chla and CR exhibited a significant relationship (CRZ=1.42ChlaZ-0.21, r2 = 0.69, N=37, p<0.001) with potential for the difficult prediction of CR. A general ratio of 1.42 mmolO2 mgChla-1 d-1 and a threshold Chla for net heterotrophy of ca. 0.25 mgChla m-3 can be tentatively proposed for the Atlantic, although further analyses of spatial and seasonal variation are necessary. We observed unusually positive NCP rates in the central part of the N gyre, due to a marked decrease of CR in a patch of high Synechococcus spp. abundance and high 14CPP by large phytoplankton. However, no relationship was observed between size-fractionated 14CPP and CR or the GPP : CR ratio during the cruise, contradicting the hypothesis that food web functioning is determined by the phytoplankton size structure. Such independence, together with the persistence of distinct GPP : CR and 14CPP : NCP relationships in distinct biogeographic provinces suggest a resilience of trophic dynamics and the existence of alternative ecosystem states, whose implications for projections of the metabolic state of the ocean are discussed.

Spatiotemporal variability in pH and carbonate parameters on the Canadian Atlantic continental shelf between 2014 and 2022

Abstract. The Atlantic Zone Monitoring Program (AZMP) was established by Fisheries and Oceans Canada (DFO) in 1998 with the aim of monitoring physical and biological ocean conditions in Atlantic Canada in support of fisheries management. Since 2014, at least two of the carbonate parameters (pH; total alkalinity, TA; and dissolved inorganic carbon, DIC) have also been systematically measured as part of the AZMP, enabling the calculation of derived parameters (e.g., carbonate saturation states, Ω, and partial pressure of CO2, pCO2). The present study gives an overview of the spatiotemporal variability in these parameters between 2014 and 2022. Results show that the variability in the carbonate system reflects changes in both physical (e.g., temperature and salinity) and biological (e.g., plankton photosynthesis and respiration) parameters. For example, most of the region undergoes a seasonal warming and freshening. While the former will tend to increase Ω, the latter will decrease both TA and Ω. Spring and summer plankton blooms decrease DIC near the surface and then remineralize and increase DIC at depth in the fall. The lowest pCO2 values (down to ∼ 200 µatm) are located in the cold coastal Labrador Current, whereas the highest values (>1500 µatm) are found in the fresh waters of the Gulf of St. Lawrence and the St. Lawrence Estuary. The latter is also host to the lowest pH values of the zone (7.48 in the fall of 2022). Finally, most of the bottom waters of the Gulf of St. Lawrence (>90 %) are undersaturated with respect to aragonite (Ωarg<1). In addition to providing a baseline of carbonate parameters for the Atlantic Zone as a whole, this comprehensive overview is a necessary and useful contribution for the modelling community and for more in-depth studies. The full dataset of measured and derived parameters is available from the Federated Research Data Repository: https://doi.org/10.20383/102.0673 (Cyr et al., 2022a).

Satellite Remote Sensing of Water Quality Variation in a Semi-Enclosed Bay (Yueqing Bay) under Strong Anthropogenic Impact

The semi-enclosed bays impacted by heavy anthropogenic activities have weak water exchange and purification capacities. Most of the sea bays have suffered severe eutrophication, water quality deterioration, ecosystem degradation and other problems. Although many countries and local governments have carried out corresponding environmental protection actions, the evaluation of their effectiveness still requires monitoring technology and data support for long-term water environment change. In this study, we take Yueqing Bay, the fourth largest bay in China, as a case to study the satellite-based water quality monitoring and variation analysis. We established a nutrient retrieval model for Yueqing Bay to produce a long-term series of nutrient concentration products in Yueqing Bay from 2013 to 2020, based on Landsat remote sensing images and long-term observation data, combined with support vector machine learning and water temperature and satellite spectra as input parameters, and then we analyzed its spatiotemporal variations and driving factors. In general, nutrient concentrations in the western part of the bay were higher than those in the eastern part. Levels of dissolved inorganic nitrogen (DIN) were lower in summer than in spring and winter, and reactive phosphate (PO4-P) levels were lower in summer and higher in autumn. In terms of natural factors, physical effects (e.g., seasonal variations in flow field) and biological effects (e.g., seasonal differences in the intensity of plankton photosynthesis) were the main causes of seasonal differences in nutrient concentration in Yueqing Bay. Nutrient concentration generally increased from 2013 to 2015 but decreased slightly after 2015. Over the past decade, the economy and industry of Yueqing Bay basin have developed rapidly. Wastewater resulting from anthropogenic production and consumption was transported via streams into Yueqing Bay, leading to the continuous increase in nutrient concentrations (the variation rates: aDIN>0, aPO4−P>0), which directly or indirectly caused high nutrient concentrations in some areas of the bay (e.g., Southwest Shoal at the mouth of Yueqing Bay). After 2015, the various ecological remediation policies adopted by cities around Yueqing Bay have mitigated, to some extent, the increasing nutrient concentration trends (the variation rates: aDIN<0, aPO4−P<0), but not significantly (P > 0.1). The environmental restoration of Yueqing Bay also requires continuous and long-term ecological protection and restoration work to be effective. This research can provide a reference for ecological environment monitoring and remote sensing data application for similar semi-enclosed bays, and support the sustainable development of the bay.

Regional and seasonal variability in planktonic photosynthesis and planktonic community respiration in Amazon floodplain lakes

Results from two regional surveys and multi-lake seasonal studies were used to investigate the variability of phytoplankton photosynthesis and planktonic community respiration in central Amazon floodplain lakes. Hypothesized effects of optical and chemical variables on planktonic photosynthesis and respiration were examined statistically. Changes in dissolved oxygen in light and dark bottles distributed along light-gradients in a shipboard incubator or in situ were used to calculate volumetric community respiration (Rc), volumetric gross photosynthesis (P), daily integral gross photosynthesis (Π), and daily integral community respiration rates (Λ). Π varied significantly among all lakes with source-water river stage and source-water river type. Λ also varied significantly with source-water river stage. Variation in maximum depth linked to source-water river stage was a key factor controlling seasonal variations in Π through its influence on total suspended solids and total phosphorus concentrations which affected light extinction and light-saturated photosynthesis, respectively. The predominance of sub-saturated dissolved O2 in the pelagic surface waters of Amazon floodplain lakes was attributed to high integral Rc:P ratios, indicating the existence of large sustained inputs of non-phytoplankton organic carbon to these environments.

Stable isotopes measurements reveal dual carbon pools contributing to organic matter enrichment in marine aerosol.

Stable carbon isotope ratios in marine aerosol collected over the Southern Indian Ocean revealed δ13C values ranging from −20.0‰ to −28.2‰. The isotope ratios exhibited a strong correlation with the fractional organic matter (OM) enrichment in sea spray aerosol. The base-level isotope ratio of −20.0‰ is characteristic of an aged Dissolved Organic Matter (DOM) pool contributing a relatively homogeneous background level of DOM to oceanic waters. The range of isotope ratios, extending down to −28.2‰, is characteristic of more variable, stronger, and fresher Particulate Organic Matter (POM) pool driven by trophic level interactions. We present a conceptual dual-pool POM-DOM model which comprises a ‘young’ and variable POM pool which dominates enrichment in sea-spray and an ‘aged’ but invariant DOM pool which is, ultimately, an aged end-product of processed ‘fresh’ POM. This model is harmonious with the preferential enrichment of fresh colloidal and nano-gel lipid-like particulate matter in sea spray particles and the observed depleted δ13C ratio resulting from isotope equilibrium fractionation coupled with enhanced plankton photosynthesis in cold water (−2 °C to +8 °C). These results re-assert the hypothesis that OM enrichment in sea-spray is directly linked to primary production and, consequently, can have implications for climate-aerosol-cloud feedback systems.

Component elements of the carbon cycle in the middle and lower Yenisei River

An integrated study of the middle and lower Yenisei River was performed in the summer of 2012. It involved monitoring the key elements and ecological processes associated with the carbon cycle of the river ecosystem (the study area is more than 1800 km long). Measurements of the production and destruction processes have shown the failure of the “neutral pipe” hypothesis claiming that any river is a simple drain of carbon from terrestrial ecosystems to the ocean. The Yenisei River is not a purely heterotrophic ecosystem. It also has autotrophic areas, where the primary production of planktonic photosynthesis is higher than respiration (above the Angara River and near the Bolshaya and Malaya Heta rivers). According to the data, the respiration rate of the plankton community in the river depends mostly on the water temperature and the content of inorganic phosphorous that can restrict the amount of organic matter consumed by bacterioplankton.

Planktonic processes contribute significantly to the organic carbon budget of a coastal fish-culturing area

We assessed the role of planktonic processes, in comparison to allochthonous input from fish cages and sedimentary loss, in the organic carbon (OC) budget of the water column in a semi-enclosed fish-culturing area (culturing red sea bream Pagrus major and yellow tail Seriola quinqueradiata). The sedimentation rate of particulate organic carbon (POC) at the fish-cage sta- tion was an average of 1.5 times that at non-cage stations. There was no significant difference in photosynthesis or respiration rates between fish-cage and non-cage stations. Annual allochtho- nous OC input in the form of leftover feed and fish feces was estimated to be 5 or 10 times that of autochthonous OC input by planktonic photosynthesis. In contrast, POC derived from phytoplank- ton accounted for a significant part (8 to 61%) of total POC sedimentation. As to sinks of OC in the water column, annual planktonic respiration was twice as high as sedimentary loss at the fish- cage station. The plankton community tended to act as a source of OC in spring and summer and as an OC sink in fall and winter. The present study shows that a significant part of allochthonous and autochthonous OC input is respired by plankton and that the remaining OC input is deposited on the seafloor of fish-culturing areas.

Plankton dynamics and photosynthesis responses in a eutrophic lake in Patagonia (Argentina): influence of grazer abundance and UVR

Se estudio a lo largo del ano una comunidad planctonica natural de la laguna eutrofica Cacique Chiquichano de Patagonia-Argentina, para evaluar los cambios en parametros fotosinteticos como resultados de la exposicion a radiacion ultravioleta (RUV, 280-400 nm), abundancia de herbivoros y composicion taxonomica de la comunidad de fitoplancton. Tanto las variables fisicas (temperatura, radiacion solar) como biologicas (herbivoros, composicion taxonomica, parametros fotosinteticos) variaron a lo largo del estudio. Los crustaceos zooplanctonicos mostraron dominancia alternada entre un cladocero (Daphnia spinulata) y un copepodo (Metacyclops mendocinus). La comunidad de fitoplancton sufrio cambios concomitantes durante el ano, con cianobacterias y diatomeas alternando su dominancia. Sumado a esto, si bien la abundancia de copepodos no estuvo significativamente relacionada con cambios en el fitoplancton, la presencia de D. spinulata fue significativa en periodos de aguas mas transparentes, durante los cuales las diatomeas fueron dominantes. Por otra parte, durante los periodos de menor penetracion de la radiacion solar en la columna de agua, las cianobacterias dominaron la comunidad fitoplanctonica. La inhibicion fotosintetica debido a RUV disminuyo durante los periodos dominados por diatomeas. En cambio, la inhibicion aumento al incrementarse la proporcion de cianobacterias, probablemente como resultado de una aclimatacion a la baja irradiancia durante la fase de menor transparencia de la laguna. Mas aun, la presencia de D. spinulata estuvo asociada a una mayor penetracion de la radiacion en la columna de agua, por lo cual aumento indirectamente la inhibicion de la fotosintesis en cianobacterias. Los resultados sugieren que tanto la radiacion solar como la abundancia de herbivoros tienen una fuerte influencia en la dinamica y actividad fotosintetica del fitoplancton en la Laguna Cacique Chiquichano.

Photosynthesis, respiration, and stable isotopes of oxygen in a large oligotrophic lake (Lake Erie, USA–Canada)

Dissolved oxygen (DO) concentration, isotopic composition (18:16O), and respiratory fractionation (αr) were measured in the east basin of Lake Erie to determine the patterns of planktonic photosynthesis (P) and respiration (R) and to assess a dual isotope, steady-state model for P/R ratios. Values of αr varied significantly between months but averaged close to a widely accepted norm for planktonic respiration (0.982). Isotope dynamics in situ predicted that 33%–53% of hypolimnetic oxygen consumption was by the sediments, consistent with previous knowledge. Except during fall turnover, DO was generally above, and 18:16O below, atmospheric equilibrium in the surface mixed layer (SML), suggesting P/R is usually >1 and agreeing with independent estimates by incubation experiments. P/R ratios calculated from the oxygen isotope model were often inconsistent with the DO and 18:16O data, likely because of uncertainties in αr and nonsteady-state conditions in the SML. The autumnal transition to P/R < 1 was captured better in the DO and 18:16O data than in the incubation experiments. Despite problems with the steady-state model, the isotope data were able to reveal spatial and temporal variations in lake metabolism and indicated a strong autochthonous basis for community metabolism in the SML.

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.

Ammonium accumulation during a silicate-limited diatom bloom indicates the potential for ammonia emission events

Ammonium (NH 4 +) concentration was measured at 15 stations in the NE Atlantic during the declining phase of the spring diatom bloom as part of the FISHES 2001 cruise. The NE Atlantic temperate spring bloom is one of the largest seasonal events in the oceanic cycle of primary productivity and represents the conversion of large amounts of oxidized nitrogen to organic (reduced) nitrogen by planktonic photosynthesis, much of which is recycled by heterotrophs as reduced N (DON and ammonium). We might expect recycled ammonium to be rapidly re-used by photosynthesizers. However, during the FISHES cruise the decline of the bloom was accompanied by a substantial buildup of dissolved ammonium (from ∼ 100 nM to > 500 nM near the sea surface) caused by a temporary imbalance in production and consumption processes. Calculations of air–sea ammonia (NH 3) flux under assumed ‘normal’ ‘peak bloom’ and ‘post-diatom-bloom’ conditions are presented and these demonstrate that over a wide range of temperatures, post-bloom accumulation of ammonium will lead to changes in the magnitude and direction of air–sea ammonia flux. If our results are generally applicable then the seasonal succession of plankton communities may be accompanied by periods of anomalously high ammonium concentration and hence be disproportionately important for air–sea ammonia flux. Once in the atmosphere, ammonia neutralizes acidity and in doing so plays a potentially significant role in climate forcing via aerosol formation.

Mesoscale variability in the metabolic balance of the Sargasso Sea

Net community production (NCP) experiments based on in vitro changes in dissolved oxygen were carried out in three mesoscale eddies investigated in the Sargasso Sea in the summer of 2004. NCP estimates ranged from negative to positive values, and the age and type of the sampled eddy were important factors responsible for this variability. Positive rates occurred in younger cyclones and in areas of eddy‐eddy interactions, whereas negative NCP rates were observed in anticyclones and older cyclone features that were decaying. This information was combined with a nine‐year data set from the BATS program and satellite altimeter data to assess the role of mesoscale eddies in the metabolic balance of the Sargasso Sea. Indirect NCP estimates (NCPe) were obtained by calculating the difference between 14C phytoplankton production and bacterial carbon demand rates. The results showed highly variable NCPe rates that spanned the range of direct NCP observations.

Local production does not control the balance between plankton photosynthesis and respiration in the open Atlantic Ocean

Abstract Recent experimental data from the subtropical NE Atlantic appear to support the prevalence of net heterotrophy in unproductive pelagic ecosystems. However, the proximity of these studies to the NW African upwelling does not exclude the possibility that remote areas of the oceanic gyres are in metabolic balance. Here we present measurements of plankton gross photosynthesis (GPP) and community respiration (CR) made during a latitudinal transect (Atlantic Meridional Transect (AMT) cruise 11, 49°N–33°S) that traversed five biogeochemical provinces of the Atlantic Ocean, including both the subtropical NE Atlantic and the central part of the South Atlantic subtropical gyre. In these oligotrophic provinces, the euphotic zone average chlorophyll a concentration (0.15±0.01 and 0.16±0.02 mg m−3, respectively), the relative contribution of picoplankton (%Chla n = 3 ), while the net metabolism of the plankton community in the central S Atlantic gyre was autotrophic (0.20±0.02 mmol O2 m−3 d−1, n = 5 ). These results therefore suggest the existence of more than one type of unproductive open-ocean situation, that may be characterised by differences in the relative importance of local vs. allochthonous sources of organic matter.

Variation in temporal [14C]plankton photosynthesis among warm monomictic lakes of coastal British Columbia

Seasonal patterns of [ 14 C]phytoplankton photosynthesis (PP) were examined in six warm monomictic lakes of coastal British Columbia. Four of our study lakes followed typical lake patterns with maximum PP occurring in the spring and minimal rates occurring during the winter. However, the spring maximum occurred several weeks earlier than lakes in other climatic regions. In addition, maximum rates of daily photosynthesis were observed to occur during the winter months in Maxwell Lake, rather than during the standard growing season. All study lakes except Maxwell Lake had large Daphnia in the plankton community. Maxwell was dominated by small crustacean zooplankton implying the importance of trophic structure in mediating seasonal patterns of productivity. The four oligotrophic lakes in our study also exhibited P-deficiency during winter, as indicated by P-debt bioassays and rapid 32 PO 3 - turnover rates. Our data suggest that these coastal oligotrophic lakes were co-limited by nutrients and light during winter. The importance of winter (November-March) photosynthetic production to the total annual carbon budget in the six lakes studied here is greater than that typically reported for other temperate zone lakes. If plankton community respiration decreases more than photosynthetic production with wintertime temperatures, then >50% of annual net pelagic carbon fixation could occur in winter in some coastal lakes.

Latitudinal variation of the balance between plankton photosynthesis and respiration in the eastern Atlantic Ocean

A knowledge of the balance between plankton gross primary production (GPP) and community respiration (CR) in the open ocean is vital to the accurate determination of the global carbon cycle, yet the paucity of open ocean measurements severely limits our understanding. This study measured GPP, net community production, dark CR, and size‐fractionated primary production in the upper 200 m of a 12,100 km latitudinal (32°S‐48°N) transect in the Eastern Atlantic Ocean during May and June 1998. This comprehensive data set, which spans five contrasting plankton regimes, including two open ocean oligotrophic provinces, is used to derive a GPP:CR relationship, which suggests that net heterotrophy (GPP , CR) prevails in the eastern Atlantic when primary production falls below ~100 mmol O2 m−2 d−1. The predictive capability of this relationship is compared with that of the only other published relationship based on similar methodologies and is found to give a more representative description of the autotrophic (GPP > CR) to heterotrophic seasonal cycle in the Bay of Biscay. This improved predictive power is attributed to the increased representativeness of the current data set. Specifically, the interpretation suggests that the influence of community structure on net ecosystem metabolism implies that prediction of GPP:CR balances in pelagic ecosystems can be best achieved by use of a data set that covers a wide range of community structure and not only a wide range in the magnitude of primary production.

The relative importance of the planktonic food web in the carbon cycle of an oligotrophic mountain lake in a poorly vegetated catchment (Redó, Pyrenees)

The biological activity of the planktonic community of lake Redo, expressed in terms of carbon fluxes, was measured and compared to the changes in DIC, DOC and POC in the water column. Planktonic photosynthesis ranged between c. 0.01 - 0.3 μg C m-2 h-1. Release of EOC phytoplankton was highly variable, between 5 and 80% of total fixation. Bacterial uptake of EOC ranged between 1-20% of total fixation. Bacterial activities were, in absolute numbers, very low: 0.005±0.003 μg C m-2 h-1, in contrast with the higher grazing rates on bacteria of 0.036±0.021 μg C m-2 h-1. Respiration and diffusion of CO2 to the atmosphere seem to be the main processes controlling DIC concentration. DOC and POC concentrations were highly correlated, and their fluxes presented large fluctuations. These changes in DOC are larger than those due to the processes we have measured. Other processes that might affect DOC include diffusion from sediments, inputs from the catchment, uptake by mixotrophic algae and zooplankton, bacterial respiration, UV photoxidation, and flocculation. Lake Redo seems to act in general terms as an heterotrophic system: respiration is higher than photosynthesis, and the budget is balanced by the import of DOC and, to a lesser extent, POC. Most of the carbon seems to be ultimately released to atmosphere, since little is accumulated in sediments. The estimates of diffusive fluxes agreed with this hypothesis. At this stage, the comparison of biogeochemical budgets with biological activity measurements only serves as a rough approximation of the main pathways in the C cycling in the lake, and to point the issues that need further research in order to calculate the C budget in the lake with accuracy.

A mixotrophic ciliate as a major contributor to plankton photosynthesis in Australian lakes

A large mixotrophic ciliate (∼200 µm long) of the genus Stentor is a common constituent of the protozooplankton of Australian lakes. We investigated the photosynthetic rates of populations of this ciliate from two lakes, one in the Australian Capital Territory and the other on the New South Wales/Victorian border, in relation to photosynthesis by the whole phytoplankton community. The concentration of the ciliate varied between 192 and 4,267 cells liter−1 during the study period of May–January (the austral winter, spring, and autumn) and it contributed between 4.3 and 69.3% of total plankton photosynthesis. Individual photosynthetic rates ranged between 1.03 ± 0.8 and 3.98 ± 0.6 ng C cell−1 h−1 and individual Chl a content between 925 ± 62 to 1,461 ± 63 pg cell−1, giving assimilation numbers of 1.00–2.74. Light‐response curves indicated that the ciliate achieved its highest rates of photosynthesis at high photon fluxes, typical of the surface waters. Vertical distribution patterns of Stentor in the water column of one of the lakes supported these physiological data. Southern hemisphere lakes seem to have a protozooplankton that may contain substantial numbers of a large ciliate capable of contributing a significant portion of carbon fixation in the plankton.

Roles of Nutrients in Controlling Growth of Epilithon in Oligotrophic Lakes of Low Alkalinity

The ability of nutrients to control photosynthesis was compared in epilithon (the association on rock surfaces in the littoral zone) and phytoplankton of 13 low alkalinity lakes of the Experimental Lakes Area of northwestern Ontario. The study included (1) surveys of lakes varying in nutrient concentrations; (2) experimental additions to lakes of carbon and nitrogen (N), with or without phosphorus (P); and (3) experimental additions to lakes of sulfuric and nitric acids. Nutrient controls of planktonic and epilithic algal photosynthesis differed consistently. Phosphorus limited planktonic algal photosynthesis. In contrast, dissolved inorganic carbon (DIC) limited epilithic photosynthesis in both perturbed and unperturbed lakes because diffusive resistance kept the effective supply of DIC below the level needed for optimal growth. Epilithic photosynthesis was lowered when lake disturbances (e.g., acidification) reduced epilimnetic concentrations of DIC. Expected increases in atmospheric carbon dioxide can, therefore, differentially affect the littoral and pelagic food webs in low DIC lakes. Epilithic photosynthesis in all study lakes was unrelated to N or P availability despite apparent N and P deficiencies, based upon particulate nutrient ratios. Rates of epilithic respiration were, however, correlated with epilimnetic concentrations of inorganic N.

Sensitivity of planktonic photosynthesis to various toxicants in the river rhine

AbstractNatural populations of phytoplankton from the lower River Rhine were exposed to selected pollutants in order to estimate their sensitivity to chemical pollution. The photosynthesis, measured as 14C incorporation, was inhibited by 50% when circa 100 μg · L−1 copper or atrazine was added. Potassium dichromate (K2Cr2O7) and an anionic detergent (tetrapropylbenzenesulphonate) affected the photosynthesis of Rhine phytoplankton only at concentrations of about 3 and 40 mg L−1, respectively. A tenfold concentrate of the organic compounds from river water, harvested on XAD resin, inhibited the photosynthesis by 50%. Comparison of the results of this study with literature data indicate that the algal populations in the River Rhine are as sensitive and not more tolerant than several algae from less polluted habitats. These observations on the Rhine plankton also indicate that the pollution level in the River Rhine is likely to affect the photosynthesis of phytoplankton. This supports the request for a further reduction in toxicant input in the River Rhine. © 1993 John Wiley & Sons, Inc.