Photosynthetic Activity Of Phytoplankton Research Articles

Stability analysis for the phytoplankton-zooplankton model with depletion of dissolved oxygen and strong Allee effects

The photosynthetic activity of phytoplankton in the seas is responsible for an estimated 50–80 % of the world's oxygen generation. Both phytoplankton and zooplankton require some of this synthesized oxygen for cellular respiration. This study aims to better understand how the oxygen-phytoplankton dynamics are altered due to the Allee effect in phytoplankton development, particularly when considering the time-dependent oxygen generation rate. The dynamic analysis of the model is dedicated to finding the possible equilibrium points. The analysis reveals that three equilibrium points can be obtained. The stability study demonstrates that one of the equilibrium points is always stable. The remaining equilibrium points are stable under specific conditions. We also identify bifurcations originating from these equilibrium points, including transcritical, pitchfork, and Hopf bifurcation. We derive conditions for stable limit cycles (supercritical Hopf bifurcation) and, in some cases, establish the non-existence of solutions. Numerical simulations are performed to validate our theoretical findings. Furthermore, it is noted that the Allee threshold for the phytoplankton population (k0) significantly influences the overall dynamics of the system. When k0≤0.001, the population of plankton is at risk of extinction. On the other hand, when 0.001<k0≤0.01, the population of zooplankton is at risk of extinction. When 0.01<k0≤2, the solution reaches a stable condition of coexistence. Conversely, when k0≥2.1, the solution exhibits periodic attractor behaviour.

Chlorophyll Content and Photosynthetic Activity of Phytoplankton in Reservoirs of the Volga River (Russia)

Using the fluorescent method in the modification of Krasnoyarsk State University, we studied the development (as chlorophyll content, ∑CHL) and photosynthetic activity of phytoplankton in seven large flat reservoirs of the Volga River cascade (Russia) in August 2015–2017. In the period of the maximal warming of water, average ∑CHL varied in limits of 19.4–33.7 μg L−1 in the Upper Volga, 8.5–27.8 μg L−1 in the Middle Volga, and 5.2–11.3 μg L−1 in the Lower Volga. The photosynthetic activity coefficient (PhAC) varied mostly in limits of 0.12–0.59, with an average of 0.22–0.38 and only in 2017 decreased to minimal &lt; 0.10 and average &lt; 0.20. The average PhAC values show the normal physiological state of the phytoplankton of the Upper Volga during all periods of observation, with an occasional decrease in PhAC in the Middle Volga and low photosynthetic activity in the Lower Volga. A decrease in the average ∑Chl and PhAC in 2017 was under cyclonic windy weather with a large amount of precipitation, low solar radiation, and large volume of flow. A trend towards a decrease in ∑CHL, like a decrease in PhAC from the Upper Volga to the Lower Volga, is explained by an increase in the flow rate and volume of runoff downstream of the Volga River.

Two Approaches to Modeling Phytoplankton Biomass Dynamics Based on the Droop Model

This work continues the study of the Droop model based on the concept of cell quota. Description of the photosynthetic processes in phytoplankton includes in the model structure. The concept of chlorophyll quota is used. It is the proportion of photosynthetic substances in plant cells. In addition to the chlorophyll quota, the photosynthetic activity of phytoplankton is determined by external conditions, primarily by the level of photosynthetically active radiation (PAR). The model is based on separating the dependence of phytoplankton reproduction on external conditions according to the stages of photosynthesis. The light stage is largely determined by the PAR, and the dark stage is limited by the nutrient resource under the controlling influence of the temperature of the aquatic environment. In order to develop the model, the storage of energy in the light stage of photosynthesis is described in detail. Energy is stored in the form of energy-intensive substances in macroergic molecules (macroergs). The most common cell macroerg is adenosine triphosphate (ATP). The proportion of ATP in phytoplankton varies depending on the light regime and on the energy amount stored in the dark stage. The model includes the Droop kinetics and equations for the dynamics of the chlorophyll quota and the ATP pool. The conditions for the existence and stability of equilibrium solutions are compared for the same values of parameters common to both models. The greatest influence on the dynamic modes of the minimum value of the cell quota has been established. The proportion of biomass associated with the light period of photosynthesis is also significant. For the first model that is the biomass produced during daylight hours. And in terms of the second model, it is the biomass formed due to the energy of ATP stored in the light phase. The influence of the structure of dynamic models on the daily and annual dynamics of phytoplankton was revealed. Scenarios of behavior of models under various lighting conditions, including constant and periodically changing lighting, have been studied.

Insights into Phytoplankton Dynamics and Water Quality Monitoring with the BIOFISH at the Elbe River, Germany

Understanding the key factors influencing the water quality of large river systems forms an important basis for the assessment and protection of cross-regional ecosystems and the implementation of adapted water management concepts. However, identifying these factors requires in-depth comprehension of the unique environmental systems, which can only be achieved by detailed water quality monitoring. Within the scope of the joint science and sports event “Elbschwimmstaffel” (swimming relay on the river Elbe) in June/July 2017 organized by the German Ministry of Education and Research, water quality data were acquired along a 550 km long stretch of the Elbe River in Germany. During the survey, eight physiochemical water quality parameters were recorded in high spatial and temporal resolution with the BIOFISH multisensor system. Multivariate statistical methods were applied to identify and delineate processes influencing the water quality. The BIOFISH dataset revealed that phytoplankton activity has a major impact on the water quality of the Elbe River in the summer months. The results suggest that phytoplankton biomass constitutes a substantial proportion of the suspended particles and that photosynthetic activity of phytoplankton is closely related to significant temporal changes in pH and oxygen saturation. An evaluation of the BIOFISH data based on the combination of statistical analysis with weather and discharge data shows that the hydrological and meteorological history of the sampled water body was the main driver of phytoplankton dynamics. This study demonstrates the capacity of longitudinal river surveys with the BIOFISH or similar systems for water quality assessment, the identification of pollution sources and their utilization for online in situ monitoring of rivers.

Dynamics of Oxygen-Plankton Model with Variable Zooplankton Search Rate in Deterministic and Fluctuating Environments

It is estimated by scientists that 50–80% of the oxygen production on the planet comes from the oceans due to the photosynthetic activity of phytoplankton. Some of this production is consumed by both phytoplankton and zooplankton for cellular respiration. In this article, we have analyzed the dynamics of the oxygen-plankton model with a modified Holling type II functional response, based on the premise that zooplankton has a variable search rate, rather than constant, which is ecologically meaningful. The positivity and uniform boundedness of the studied system prove that the model is well-behaved. The feasibility conditions and stability criteria of each equilibrium point are discussed. Next, the occurrence of local bifurcations are exhibited taking each of the vital system parameters as a bifurcation parameter. Numerical simulations are illustrated to verify the analytical outcomes. Our findings show that (i) the system dynamics change abruptly for a low oxygen production rate, resulting in depletion of oxygen and plankton extinction; (ii) the proposed system has oscillatory behavior in an intermediate range of oxygen production rates; (iii) it always has a stable coexistence steady state for a high oxygen production rate, which is dissimilar to the outcome of the model of a coupled oxygen-plankton dynamics where zooplankton consumes phytoplankton with classical Holling type II functional response. Lastly, the effect of environmental stochasticity is studied numerically, corresponding to our proposed system.

Assessment of phytoplankton photosynthetic activity in the Upper Volga reservoirs via fluorescence diagnosis

Based on the materials of field observations in 2015–2018, the first data on the photosynthetic activity of phytoplankton in the Upper Volga reservoirs are estimated using fluorescence diagnostics. During the summer maximum of phytoplankton, the coefficient of photosynthetic activity (CPhA) varied from 0,18 to 0,84 (standard error &lt;0,005), averaging 0,49±0,003, 0,52±0,002, and 0,51±0,004 in Ivankovo, Uglich, and Rybinsk reservoirs. Spatial and interannual changes of CPhA are considered. In interannual dynamics, higher CPhA values were noted in 2015 and 2018 (average 0,53), lower values in 2016 and 2017 (0,47–0,50). A decrease in CPhA with an increase in the concentration of chlorophyll (r = -0.30 – -0.56) was revealed.

МОДЕЛЬНЫЕ ОЦЕНКИ ДОЛИ ХЛОРОФИЛЛА В ФИТОПЛАНКТОНЕ

The article proposes and examines a dynamic model of phytoplankton biomass. The amount of chlo-rophyll content in phytoplankton changes in response to environmental situation. The model takes into account the dependence of biomass growth on environmental conditions, as well as the parameters phytoplankton photosynthetic activity.

Effects of temperature and salinity on respiratory losses and the ratio of photosynthesis to respiration in representative Antarctic phytoplankton species.

The Southern Ocean (SO) is a net sink for atmospheric CO2 whereby the photosynthetic activity of phytoplankton and sequestration of organic carbon (biological pump) plays an important role. Global climate change will tremendously influence the dynamics of environmental conditions for the phytoplankton community, and the phytoplankton will have to acclimate to a combination of changes of e.g. water temperature, salinity, pH, and nutrient supply. The efficiency of the biological pump is not only determined by the photosynthetic activity but also by the extent of respiratory carbon losses of phytoplankton cells. Thus, the present study investigated the effect of different temperature and salinity combinations on the ratio of gross photosynthesis to respiration (rGP/R) in two representative phytoplankton species of the SO. In the comparison of phytoplankton grown at 1 and 4°C the rGP/R decreased from 11.5 to 7.7 in Chaetoceros sp., from 9.1 to 3.2 in Phaeocystis antarctica strain 109, and from 12.4 to 7.0 in P. antarctica strain 764, respectively. The decrease of rGP/R was primarily dependent on temperature whereas salinity was only of minor importance. Moreover, the different rGP/R at 1 and 4°C were caused by changes of temperature-dependent respiration rates but were independent of changes of photosynthetic rates. For further interpretation, net primary production (NPP) was calculated for different seasonal conditions in the SO with specific combinations of irradiance, temperature, and salinity. Whereas, maximum photosynthetic rates significantly correlated with calculated NPP under experimental ‘Spring’, ‘Summer’, and ‘Autumn’ conditions, there was no correlation between rGP/R and the respective values of NPP. The study revealed species-specific differences in the acclimation to temperature and salinity changes that could be linked to their different original habitats.

CONDITIONS FOR PRIMARY PRODUCTION OF PHYTOPLANKTON IN THE VOSTOK BAY (JAPAN SEA) IN SPRING 2016

The Vostok Bay was surveyed on March 16–18, 2016 with measuring of water properties profiles by oceanographic sondes Sea-Bird SBE-19plus V2 and Rinko Profiler ASTD-102 with sensors of pressure, temperature, conductivity, turbidity, chlorophyll fluorescence, dissolved oxygen, and photosynthetically active radiation (PAR) and collecting of water samples by SBE-32 carousel sampler with 10 liter bottles for further measuring of nutrients (P, Si and N in forms of nitrate and ammonium) and chlorophyll a concentration and phyto- and zooplankton abundance and species composition. Assimilation number (Pb ) of phytoplankton was determined using the optical sensor of dissolved oxygen mounted on logger Rinko AR01-USB and primary production was calculated from the measured values of Pb , Chl a and PAR. Values of primary production ranged from 200 to 2100 mgC/(m2.day). The highest phytoplankton growth was detected at the depth of 8–10 m in the northern Vostok Bay and 10–16 m in its southern part. The total daily production of phytoplankton within the Bay was estimated as 12.5 tC. Species composition of phytoplankton was formed mainly by diatoms (Bacillariophyta) and dinophytes (Dinophyta). The highest biomass of raw phytoplankton was registered at the sea surface, whereas the highest values of chlorophyll concentration occurred mainly at the bottom of the bay. Species composition of zooplankton was typical for spring season, with domination of copepods presented mainly by neritic species; its biomass was in 12 times lower than the phytoplankton biomass, on average. There was concluded that photosynthetic activity of phytoplankton was limited by nitrate availability, therefore it was intensified by penetration of relatively cold, nitrogen-rich waters from the deep-water sea to the Vostok Bay.

Seasonal dependence and functional implications of macrophyte–phytoplankton allelopathic interactions

AbstractInvasive plant species such as Ludwigia hexapetala might have a competitive advantage if they produce allelopathically active compounds against primary producers. Both phytoplankton and plant community structure may be affected due to different, species‐specific sensitivity to allelochemicals. Moreover, such allelopathic interactions could vary over the year depending on (i) the plant's phenological stage and (ii) the abilities of the native macrophytes to suppress—or the non‐native macrophytes to stimulate—the non‐native macrophyte population. We tested the allelopathic effects of aqueous leaf extracts of L. hexapetala on the photosynthetic activity of three target phytoplankton strains (Scenedesmus communis, a toxic Microcystis aeruginosa strain and a non‐toxic Microcystis aeruginosa strain) over three seasons of development (spring, summer and autumn). We also tested seasonal allelopathic effects of aqueous leaf extracts of both L. hexapetala (i.e. the non‐native invasive species) and the native Mentha aquatica on L. hexapetala seed germination. Finally, we identified three main secondary compounds present in the aqueous leaf extracts of L. hexapetala and we tested each individual compound on the phytoplankton's photosynthetic activity and on L. hexapetala seed germination. We observed marked seasonal and species‐specific patterns of L. hexapetala allelopathy on phytoplankton. The photosynthetic activities of S. communis and the toxic M. aeruginosa strain were stimulated by L. hexapetala aqueous leaf extracts in autumn and spring, respectively, whereas the non‐toxic M. aeruginosa strain was strongly inhibited in these two seasons. In summer, photosynthesis of all phytoplankton strains was inhibited. The germination rate of L. hexapetala seeds was stimulated by both L. hexapetala and M. aquatica aqueous leaf extracts, especially in summer, concomitant with the strong negative effects observed on the three phytoplankton strains. Three flavonoid glycosides (myricitrin, prunin and quercitrin) were identified as the main secondary compounds present in the L. hexapetala aqueous leaf extracts. The photosynthetic activity of S. communis was slightly stimulated by the three compounds. The photosynthetic activity of the toxic M. aeruginosa strain was stimulated by myricitrin and quercitrin, whereas that of the non‐toxic M. aeruginosa strain was inhibited by prunin. Finally, the germination rate and the germination velocity of L. hexapetala seeds were stimulated by myricitrin and prunin. These findings suggest that L. hexapetala could favour the photosynthetic activity of toxic cyanobacteria in spring and reduce their photosynthetic activity in summer, potentially leading to drastic changes in the phytoplankton communities and therewith ecological functioning of invaded ponds. Moreover, the stimulation of its seed germination could give a strong competitive advantage to L. hexapetala, thus promoting its invasiveness.

Rapid light curves (RLC) or non-sequential steady-state light curves (N-SSLC): which fluorescence-based light response curve methodology robustly characterizes phytoplankton photosynthetic activity and acclimation status?

This study is the first in situ work comparing rapid light curves (RLC) and non-sequential steady-state light curves (N-SSLC) in their efficiency to characterize phytoplankton photosynthetic activity and acclimation status. Measurements were carried out at two time scales (daily and annual) using the Pulse Amplitude Modulated fluorometry on samples taken in the coastal waters of a macrotidal ecosystem (the Strait of Dover, eastern English Channel). RLC and N-SSLC were compared under a wide range of environmental conditions and phytoplankton composition in order to define the best methodology to accurately capture short and long-term adjustments in the functioning of the photosynthetic apparatus. The relationships between the photosynthetic parameters extracted from RLC and N-SSLC were also studied to evaluate the possibility to use RLC to predict N-SSLC photosynthetic parameters and thus obtaining the acclimation status at steady state. At daily scale, the maximum electron transport rate and light saturation coefficient resulting from RLC (respectively, ETRm_RLC and E k_RLC) were found to follow more closely short-term environmental light variations than ETRm and E k resulting from N-SSLC (ETRm_N-SSLC and E k_N-SSLC) did. RLC were thus able to detect rapid changes in photosynthetic activity that would have been overlooked with N-SSLC measurements. At annual scale, few differences were found between RLC and N-SSLC. Variations of ETRm and α derived from RLC and N-SSLC were very similar but absolute values were lower for RLC measurements. Because, at daily scale, RLC better capture the short-term changes in photosynthetic activity than N-SSLC do, using RLC to predict N-SSLC photosynthetic parameters and getting information about steady-state acclimation status is not possible at this time scale. However, this can be done at seasonal scale.

The identification of IsiA proteins binding chlorophyll d in the cyanobacterium Acaryochloris marina.

The bioavailable iron in many aquatic ecosystems is extremely low, and limits the growth and photosynthetic activity of phytoplankton. In response to iron limitation, a group of chlorophyll-binding proteins known as iron stress-induced proteins are induced and serve as accessory light-harvesting components for photosystems under iron limitation. In the present study, we investigated physiological features of Acaryochloris marina in response to iron-deficient conditions. The growth doubling time under iron-deficient conditions was prolonged to ~3.4 days compared with 1.9 days under normal culture conditions, accompanied with dramatically decreased chlorophyll content. The isolation of chlorophyll-binding protein complexes using sucrose density gradient centrifugation shows six main green bands and three main fluorescence components of 712, 728, and 748nm from the iron-deficient culture. The fluorescence components of 712 and 728nm co-exist in the samples collected from iron-deficient and iron-replete cultures and are attributed to Chl d-binding accessory chlorophyll-binding antenna proteins and also from photosystem II. A new chlorophyll-binding protein complex with its main fluorescence peak at 748nm was observed and enriched in the heaviest fraction from the samples collected from the iron-deficient culture only. Combining western blotting analysis using antibodies of CP47 (PSII), PsaC (PSI) and IsiA and proteomic analysis on an excised protein band at ~37kDa, the heaviest fraction (-F6) isolated from iron-deficient culture contained Chl d-bound PSI-IsiA supercomplexes. The PSII-antenna supercomplexes isolated from iron-replete conditions showed two fluorescence peaks of 712 and 728nm, which can be assigned as 6-transmembrane helix chlorophyll-binding antenna and photosystem II fluorescence, respectively, which is supported by protein analysis of the fractions (F5 and F6).

Influence of environemntal factors on phytoplankton photosynthetic activity in the Volga River reservoirs

The relation of the photosynthetic activity of phytoplankton biomass in the Volga River reservoirs to environmental factors has been analyzed. It is revealed that values of A/B and P/B coefficients depend to the largest extent on the content of total nitrogen, as well as on the temperature and illumination, the factors integrating the effect of geographical zonality. Among the proper phytoplankton parameters, its production capabilities are determined by the taxonomic composition, cenotic diversity and cell sizes, biomass, and physiological state of populations. Maximal values of A/B and P/B coefficients are characteristic of the algal communities with prevailing small-celled green, dinophytic, and cryptophytic algae.

Effects of sediment and turbulence on alkaline phosphatase activity and photosynthetic activity of phytoplankton in the shallow hyper-eutrophic Lake Taihu, China.

Sediments play important roles, as nutrient reservoir, especially in shallow lake ecosystem. The water column of large shallow lakes is often stable but also disturbed by turbulence causing resuspension of sediments. While considerable research has been carried out to investigate the influence of sediment resuspension on nutrient release, fewer studies have been done to understand the contribution of alkaline phosphatase activity (APA) in water as a response to the two conditions (turbulence and stability). Also, effects of the two lake conditions on photosynthetic efficiency of phytoplankton are still poorly understood. This study will evaluate the effect of these two conditions on photosynthetic efficiency and APA. Sediments used in the indoor experiments were collected from Zhushan Bay in Lake Taihu. Turbulence was generated by rotors to simulate the strong wind-induced disturbance in Lake Taihu. Results of the experiments showed that TN and TP in the stable and episodically turbulent conditions were not significantly different, with TN ranging from 1.34 to 1.90mg/L and TP from 0.08 to 0.18mg/L. Whereas, the soluble reactive phosphorus in the episodically turbulent condition was significantly higher than in the stable condition. Episodic turbulence could enhance P cycling by resuspending sediment-associated P, which alleviated algal P limitation. In stable conditions, P deficiency induced the production of high APA, which enhanced the availability of P. Although episodic turbulence could also cause increased algal biomass, photosynthetic efficiency of the algae was also affected not only by the nutrients but also by many other factors, especially light availability. Our results suggest that episodic turbulence is an important driver of biogeochemical cycling in large shallow hypertrophic lake ecosystem.

In situ study on photosynthetic characteristics of phytoplankton in the Yellow Sea and East China Sea in summer 2013

In situ studies on photosynthetic characteristics of phytoplankton were important for the analysis of changes in community structure and for the prediction and control of algal blooms, but such studies of phytoplankton in offshore China were few. In this study, the detailed distribution of photosynthetic characteristics of phytoplankton in the summer of 2013 in the Yellow Sea and East China Sea was measured using Phyto-PAM (Pulse Amplitude Modulation). The phytoplankton community structure and the environmental parameters were also investigated to estimate the relationship between the distribution of the photochemical competence of phytoplankton and ecological factors. The total average Fv/Fm (the potential maximum quantum yield) value of phytoplankton in the Yellow Sea and East China Sea in summer 2013 was less than 0.5, reflecting that the photosynthetic activity of phytoplankton was relatively low. Fv/Fm of phytoplankton in summer was significantly positively associated with nitrate content (NO2−), which reflects relationship between metabolism and photosynthesis of phytoplankton: accompanied by NO2− metabolism, photosynthesis and photosynthetic capacity may be enhanced simultaneously, so the Fv/Fm value would increase with the NO2− released by phytoplankton. Through the in situ study on photosynthetic characteristics of phytoplankton in the Yellow Sea and East China Sea, we come to the conclusion that photosynthetic characteristics and activity of phytoplankton are influenced by its biological characteristics and surrounding ecological factors, such as irradiance, nutrients and phytoplankton community. Meanwhile, the thermally stratified structure and the movement of water masses, such as the Yangtze River diluted water, the Yellow Sea cold water mass and other different water system, also have an important impact on phytoplankton photosynthetic activity and characteristics. Greater understanding of the detailed photosynthetic characteristics of phytoplankton coupled with their community structure and the environmental parameters will encourage new researches and developments in studies of marine ecosystems.

Carbon Dynamics in a Hardwater Lake: Effect of Charophyte Biomass on Carbonate Deposition

ABSTRACT: The photosynthetic activity of phytoplankton (in pelagial) and macrophytes (in littoral) is considered to be one of the main factors affecting the carbon cycle in lakes. This concerns in particular hardwater ecosystems, where most of the carbon is available in the form of Ca- or Mg-bicarbonates. In such ecosystems charophytes (macroscopic green algae, forming dense meadows) are regarded as the most effective carbonate producer due to the HCO3 — utilization and the formation of thick CaCO3 encrustations. Calcium carbonate and biomass production of charophytes were studied in a small and shallow charophyte-dominated Lake Jasne (Western Poland). Fresh and dry weight of plants, percentage contribution of calcium carbonate and production of CaCO3 per 1 m2 were studied at three depths (1, 3 and 5 m) in three sample sites (each sampled area — 0.04 m2). Additionally, physical-chemical parameters of water samples were studied. It was found that the dry weight of charophytes and the values of calcium carb...

Variation in photosynthetic activity of phytoplankton during the spring algal blooms in the adjacent area of Changjiang River estuary

Nine cruises were undertaken in the adjacent area of Changjiang River estuary from late March to May in 2011 to investigate the variation in photosynthetic activity (Fv/Fm) of surface water during algal blooms. The time series covered the period from Skeletonema sp. bloom to Prorocentrum donghaiense bloom. The results showed that Fv/Fm ranged from 0.17 to 0.73. Temperature, turbidity and nutrients were the primary influencing factors on Fv/Fm and Fv/Fm showed a strong diel periodicity. Additionally, the change in Fv/Fm during Skeletonema sp. and P. donghaiense bloom was similar. Fv/Fm increased to more than 0.60 before bloom initiation, and then chlorophyll a (Chl a) concentration rocketed and the bloom broke out. Fv/Fm was above 0.6 during bloom development and declined to below 0.6 when bloom dissipated. Fv/Fm increasing to higher level occurred several days prior to significant increases in Chl a and outbreak of bloom, indicating Fv/Fm can be used to predict onset of algal blooms.

Phytoplankton photosynthetic activity dynamics in a temperate macrotidal ecosystem (the Strait of Dover, eastern English Channel): Time scales of variability and environmental control

The temporal variability in phytoplankton photosynthetic activity was studied in the coastal waters of the Strait of Dover and related to environmental conditions. Phytoplankton photosynthetic activity data collected from October 2008 and August 2010 at different time scales (hourly to interannual) using a nested sampling design are presented. Photosynthetic parameters including the maximum quantum yield (Fv/Fm), maximal light utilization efficiency (α), maximum electron transport rate (ETRm) and light saturation coefficient (Ek) were measured using Pulse Amplitude Modulation (PAM) fluorometry. Information on the taxonomic composition of phytoplankton assemblages and physicochemical parameters were also collected. Marked changes in photosynthetic parameters were observed at the different time scales investigated. The variability of photosynthetic parameters at sub-seasonal scale (hourly to monthly) could be of the same order of magnitude as at seasonal scale. At short time scale (hour to week), there was no consistent pattern of diel periodicity in photosynthetic parameters. There was a decline of ETRm and α during the day as often as there was a peak at the maximum of irradiance. Light conditions were the main influencing factor on photosynthetic parameters at these time scales. The optimal use of incident irradiance seemed focused on between-day rather than within-day variability. At longer time scale (seasonal to interannual), close interplays between shifts in community taxonomic composition and changes in the physicochemical characteristics of the environment controlled the variability in photosynthetic parameters. Whatever the time scale and period of year considered, variability in photosynthetic parameters was “Ek-independent”. In well-mixed macrotidal ecosystems, such as the Strait of Dover, a nested sampling design allowing to characterize the short- (intraday and daily) as well as long-term variability (within months, monthly, seasonal, annual and interannual) in photosynthetic parameters is required to get a suitable picture of phytoplankton photosynthesis and avoid potential problems of aliasing.

Spatio-temporal variability of phytoplankton photosynthetic activity in a macrotidal ecosystem (the Strait of Dover, eastern English Channel)

Photosynthetic parameters, phytoplankton assemblages and physicochemical parameters were analysed in the Strait of Dover along an inshore–offshore transect between September 2008 and December 2009. The samples were collected weekly at 9 stations only at the surface or at both surface and depth. The photosynthetic activity was obtained by measuring Rapid Light Curves (RLC) using Pulse Amplitude Modulated (PAM) fluorometry. Temporal variability of the maximal light utilization efficiency (α), maximum electron transport rate (ETRm) and light saturation coefficient (Ek) were greater than spatial variability (in horizontal and vertical dimensions) whereas temporal variability of the maximum quantum yield (Fv/Fm) was of the same order of magnitude as its spatial variability within the water column (in the vertical dimension). ETRm and α were positively correlated and the slope of the relation varied significantly between the different depths and seasons. The highest values of ETRm and α were obtained in summer (June) and late autumn – early winter (September–October & December) and the lowest in spring (March–April). The spatial variability of these parameters was greater vertically than horizontally. Differences between depths were particularly pronounced at the station nearest to the coast where photosynthetic response seems to be controlled by the light history of the cells. Seasonal variability in photosynthetic activity was mostly related to phytoplankton assemblages, light and temperature whereas the light history of the cells within the water column could be the most important factor in depth variability.

Short-term variability and control of phytoplankton photosynthetic activity in a macrotidal ecosystem (the Strait of Dover, eastern English Channel)

Short-term changes in phytoplankton photosynthetic activity were studied during different periods of the years 2009 and 2010 in the coastal waters of a macrotidal ecosystem (the Strait of Dover, eastern English Channel). During each sampling period, samples were taken every 1.45 h., from sunrise to sunset, during at least 5 days distributed along a complete spring–neap tide cycle. The photosynthetic parameters were obtained by measuring rapid light curves using pulse amplitude modulated fluorometry and were related to environmental conditions and phytoplankton taxonomic composition. The maximum quantum yield (Fv/Fm) showed clear light-dependent changes and could vary from physiological maxima (0.68–0.60) to values close to 0.30 during the course of 1 day, suggesting the operation of photoprotective mechanisms. The maximum electron transport rate (ETRm) and maximal light utilization efficiency (α) were generally positively correlated and showed large diel variability. These parameters fluctuated significantly from hour to hour within each day and the intraday pattern of variation changed significantly among days of each sampling period. Stepwise multiple linear regressions analyses indicated that light fluctuations explained a part of this variability but a great part of variability stayed unexplained. Fv/Fm, ETRm and α were not only dependent on the light conditions of the sampling day but also on those of the previous days. A time lag of 3 days in the effect of light on ETRm and α variation was highlighted. At these time scales, changes in phytoplankton community structure seemed to have a low importance in the variability in photosynthetic parameters. The photoacclimation index Ek showed a lower variability and was generally different from the incident irradiance, indicating a limited acclimation capacity with a poor optimization of light harvesting during the day. However, in well-mixed systems such as the Strait of Dover, the short-term photoacclimation is disrupted by the high level of variability in environmental conditions. Also, the variability observed in the present study can be associated with a particular kind of photosynthetic response: the “Ek-independent” variability. The physiological basis of this photosynthetic response is largely unresolved and further researches on this subject are still required to better explain the dynamics of phytoplankton activity in the Strait of Dover.