Phytoplankton Photosynthesis Research Articles

Physio-ecological responses of Patagonian coastal marine phytoplankton in a scenario of global change: Role of acidification, nutrients and solar UVR

The aim of our study was to experimentally determine the future combined effects of solar UVR (280–400nm), nutrient enrichment and acidification on a natural phytoplankton community (late phase of the bloom) of the Chubut River estuary (Patagonia, Argentina). We exposed the community to two radiation conditions (i.e., with and without UVR) under a future scenario of global change as compared to present conditions, using a cluster environmental approach. We combined short- (hours) and mid-term (days) incubations under solar radiation of the two clusters (present vs. future) to focus on changes in phytoplankton photosynthesis, growth and biodiversity. Our results indicate that the future conditions of increased nutrient availability and acidification, together with solar radiation, would shape the coastal phytoplankton community of Patagonia. The observed change for future conditions was towards a community dominated by relatively large diatoms, with high growth rates, little or no UVR-inhibition of photosynthesis, and better light-utilization efficiency. Given the species-specificity in responses, our results should not be generalized, especially considering that open waters of Patagonia are characterized by recurrent blooms of coccolithophorids that could be severely affected by acidification process. This future global change scenario in coastal waters, however, will produce significant increases in primary production, with phytoplankton communities that may sequester significant carbon amounts and thus they might sustain important secondary production (including fisheries) if their nutritional quality is not compromised.

Chlorophyll variability in the oligotrophic gyres: mechanisms, seasonality and trends

A 16-year (1998-2013) analysis of trends and seasonal patterns was conducted for the five subtropical ocean gyres using satellite data: chlorophyll-a (Chl-a) retrievals from ocean color, sea surface temperature (SST), and sea-level anomaly (SLA). Trend analysis was also performed on mixed-layer data derived from ocean model gridded temperature and salinity profiles (1998-2010). The Chl-a monthly composites were constructed from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate-resolution Imaging Spectroradiometer (MODIS) on Aqua using two different algorithms: the standard algorithm (STD) that has been in use since the start of the SeaWiFS mission in 1997, and a more recently developed Ocean Color Index (OCI) algorithm with improved accuracy in low Chl-a waters. Trends were obtained for all gyres using both STD and OCI algorithms, which demonstrated generally consistent results. The North Pacific, Indian Ocean, North Atlantic and South Atlantic gyres showed significant downward trends in Chl-a, while the South Pacific gyre has a much weaker upward trend with no statistical significance. Time series of satellite-derived net primary production (NPP) showed downward trends for all the gyres, while all five gyres exhibited positive trends in SST and SLA. The seasonal variability of Chl-a in each gyre is tightly coupled to the variability in mixed layer depth (MLD) with peak values in winter in both hemispheres when vertical mixing is more vigorous, reaching depths approaching the nutricline. On a seasonal basis, Chl-a concentrations increase when the MLD approaches or is deeper than the nutricline depth, in agreement with the concept that vertical mixing is the major driving mechanism for phytoplankton photosynthesis in the interior of the gyres. The combination of surface warming trends and biomass reduction over the 16-year period has the potential to reduce atmospheric CO2 uptake by the gyres and therefore influence the global carbon cycle.

Evaluation of the sinks and sources of atmospheric CO2 by artificial upwelling

Artificial upwelling is considered a promising way to reduce the accumulation of anthropogenic carbon dioxide in the atmosphere. This practice could transport nutrient-rich deep water to the euphotic zone, enhance phytoplankton growth and consequently increase organic carbon exportation to the deep ocean via the biological pump. However, only a few studies quantitatively assess changes in oceanic CO2 uptake resulting from artificial upwelling. This article uses a simulation to examine the effect of hypothetical artificial upwelling-induced variations of CO2 fugacity in seawater (fCO2) using observed carbon and nutrient data from 14 stations, ranging from 21 to 43°N, in the West Philippine Sea (WPS), the East China Sea (ECS) and the Sea of Japan. Calculations are based on two basic assumptions: First, a near-field mixing of a nutrient-rich deep-ocean water plume in a stratified ocean environment is assumed to form given the presence of an artificial upwelling devise with appropriate technical parameters. Second, it is assumed that photosynthesis of marine phytoplankton could deplete all available nutrients following the stoichiometry of the modified Redfield ratio C/H/O/N/S/P=103.1/181.7/93.4/11.7/2.1/1. Results suggest artificial upwelling has significant effects on regional changes in sea–air differences (ΔfCO2sea–air) and the carbon sequestration potential (ΔfCO2mixed‐amb). Large variations of ΔfCO2sea–air and ΔfCO2mixed‐amb are shown to be associated with different regions, seasons and technical parameters of the artificial upwelling device. With proper design, it is possible to reverse the contribution of artificial upwelling from a strong CO2 source to sink. Thus, artificial upwelling has the potential to succeed as a geoengineering technique to sequester anthropogenic CO2, with appropriate technical parameters in the right region and season.

Fe availability drives phytoplankton photosynthesis rates during spring bloom in the Amundsen Sea Polynya, Antarctica

Abstract To evaluate what drives phytoplankton photosynthesis rates in the Amundsen Sea Polynya (ASP), Antarctica, during the spring bloom, we studied phytoplankton biomass, photosynthesis rates, and water column productivity during a bloom of Phaeocystis antarctica (Haptophyceae) and tested effects of iron (Fe) and light availability on these parameters in bioassay experiments in deck incubators. Phytoplankton biomass and productivity were highest (20 µg chlorophyll a L−1 and 6.5 g C m−2 d−1) in the central ASP where sea ice melt water and surface warming enhanced stratification, reducing mixed layer depth and increasing light availability. In contrast, maximum photosynthesis rate (P*max), initial light-limited slope of the photosynthesis–irradiance curve (α*), and maximum photochemical efficiency of photosystem II (Fv/Fm) were highest in the southern ASP near the potential Fe sources of the Dotson and Getz ice shelves. In the central ASP, P*max, α*, and Fv/Fm were all lower. Fe addition increased phytoplankton growth rates in three of twelve incubations, and at a significant level when all experiments were analyzed together, indicating Fe availability may be rate-limiting for phytoplankton growth in several regions of the ASP early in the season during build-up of the spring bloom. Moreover, Fe addition increased P*max, α*, and Fv/Fm in almost all experiments when compared to unamended controls. Incubation under high light also increased P*max, but decreased Fv/Fm and α* when compared to low light incubation. These results indicate that the lower values for P*max, α*, and Fv/Fm in the central ASP, compared to regions close to the ice shelves, are constrained by lower Fe availability rather than light availability. Our study suggests that higher Fe availability (e.g., from higher melt rates of ice shelves) would increase photosynthesis rates in the central ASP and potentially increase water column productivity 1.7-fold, making the ASP even more productive than it is today.

Mathematical Modelling of Spatiotemporal Dynamics of Oxygen in a Plankton System

Oxygen production due to phytoplankton photosynthesis is a crucial phenomenon underlying the dynamics of marine ecosystems. However, most of the existing literature focus on other aspects of the plankton community functioning, thus leaving the issue of the coupled oxygen-plankton dynamics understudied. In this paper, we consider a generic model of the oxygen-phytoplankton-zooplankton dynamics to make an insight into the basic properties of the plankton-oxygen interactions. The model is analyzed both analytically and numerically. We first consider the nonspatial model and show that it predicts possible oxygen depletion under certain environmental conditions. We then consider the spatially explicit model and show that it exhibits a rich variety of spatiotemporal patterns including travelling fronts of oxygen depletion, dynamical stabilization of unstable equilibrium and spatiotemporal chaos.

ВЛИЯНИЕ РАЗРУШЕНИЯ БЕРЕГОВ И ОБЩЕГО ЗАГРЯЗНЕНИЯ НА ИЗМЕНЕНИЕ ПРОЗРАЧНОСТИ ВОДЫ И СОКРАЩЕНИЯ БИОМАССЫ БЕНТОСНОГО СООБЩЕСТВА В ЗАПАДНОЙ ЧАСТИ ЮЖНОГО КАСПИЯ

Aim . As a result of fluctuations in water level, waves, circulation, rise and fall of the water levels there are constant partial erosion and leaching of soil banks in water bodies, with intensive ongoing pollution of the waters of the Caspian Sea, which we have seen in recent decades, and in the long run may dangerously affect the transparency of the water and reduction of biomass of the benthic community in the western part of the South Caspian Sea. The paper presents the results of the last 60 years of monitoring studies to determine the nature of transparency, depth of the euphotic layer, the primary production of phytoplankton photosynthesis, phytoand zoobentos biomass and destruction of organic matter in the coastal western shelf of the South Caspian. Methods. In the course of research, modern methods of quantitative and qualitative analysis were used, regulated by normative documents, duly approved for environmental monitoring of water bodies. Results. Studies have shown that since the second half of the 90’s, the mainland coast strip is strongly deformed, starting from the village of Shihva to Kura area, further from the south of the village of Narimanabada to Astara. Dozens of artificial coves, bays, islands were created with clay-soil mounds. Conclusions. A huge mass of clay and soil goes to the coastal waters of the western shelf as a result of the destruction of the structure of the coasts due to waves and wind, resulting in clastic particles in the water increased by 2.5 3 times, the transparency of the water in the area of 5-10 m depth contours is reduced by more than 2 fold, increasing the degradation 1.6 times. During this period, because of the intensity of sedimentation and the overall pollution in the study area, phytobenthos has almost disappeared and flowering has been reduced by 7580%. Zoobenthos has been badly damaged and biomass of bento detritivorous organisms has also been reduced.

Biogeochemical responses associated with the passage of a cyclonic eddy based on shipboard observations in the western North Pacific

A shipboard high-resolution hydrographic survey in the subtropical region of the western North Pacific conducted from October to November 2008 detected part of a cyclonic eddy around 30°N, 145°E. This eddy had propagated westward in the region south of the Kuroshio extension for at least 6 months as a wavelike disturbance. Within this eddy, isopycnals shallowed between a depth of 600 m and just below the surface mixed layer. In addition, maximal dissolved oxygen concentrations were observed in the subsurface layer between depths of 50 and 100 m. Nitrate was depleted within this subsurface maximal oxygen layer. These results suggest that nutrients in the deeper layers were supplied into the euphotic layer as a result of the uplift of isopycnals in the eddy, fueling the photosynthesis of phytoplankton in the subsurface and emitting an excess of oxygen due to new production. Compared with the outside of the eddy, the enhancement of oxygen and the decrease of nitrate in the center of the eddy were estimated to be 2.7 mol O2 m−2 and 0.22 mol N m−2, respectively. The primary productivity calculated using the eddy transition speed of 5.1 km day−1 was 548 mg C m−2 day−1 at the center of the eddy. The enhanced primary productivity due to the passage of the eddy is likely to have an important role in the ecosystem and on material cycling in the subtropical region.

Adaptive critic design and Hopfield neural network based simulation of time delayed photosynthetic production and prey–predator model

This paper presents a neural networks simulation of two complex adaptive ecological systems with a discrete time delay. The first system is related to the photosynthetic production of phytoplankton, the second with a time delayed prey–predator system incorporating a prey refuge. The iterative adaptive critic design is developed to simulate the maximal photosynthetic production of the mechanistic model of phytoplankton photosynthesis with discrete time delays. The prey–predator system is simulated using back-propagation learning of infinite-dimensional dynamical systems. The proposed simulation method is based on the time-dependent recurrent learning of continuous-time Hopfield neural network with a discrete time delay with the prey–predator system as a teacher signal. Furthermore, numerical calculations are included to demonstrate the proposed simulation algorithms.

Distribution of dissolved oxygen and causes of maximum concentration in the Bering Sea in July 2010

According to data obtained in the Bering Sea during the 4th Chinese National Arctic Research Expedition, the distribution of dissolved oxygen (DO) was studied, causes of its maximum concentration were discussed, and the relationships between DO and other parameters, such as salinity, temperature, and chlorophyll a were analyzed. The results showed DO concentration ranged from 0.53 to 12.05 mg/L in the Bering Sea basin. The upper waters contained high concentrations and the maximum occurred at the depth range from 20 to 50 m. The DO concentration decreased rapidly when the depth was deeper than 200 m and reached the minimum at the depth range from 500 to 1 000 m, and then increased slowly with the depth increasing but still kept at a low level. On the shelf, the DO concentration ranged from 6.53 to 16.63 mg/L with a mean value of 10.75 mg/L, and showed a characteristic of decreasing from north to south. The DO concentration was higher in the area between the Bering Sea and Lawrence Island and was lower in the southeast and southwest of Lawrence Island at the latitude of 62°N. The formation of maximum DO concentration was concerned with phytoplankton photosynthesis and formation of the themocline. To the south of Sta. B07 in the Bering Sea basin, the oxygen produced by photosynthesis permeated to the deeper water and the themocline made it difficult to exchange vertically, and to the north of Sta. B07, the maximum DO concentration occurred above the themocline due to phytoplankton activities. On the shelf, the oxygen produced by phytoplankton photosynthesis gathered at the bottom of the thermocline and formed the DO maximum concentration. In the Bering Sea basin, the DO and salinity showed a weak negative correlation (r=0.40) when the salinity was lower than 33.1, a significant negative correlation (r=0.92) when the salinity ranged from 33.1 to 33.7, and an irregular reversed parabola (r=0.95) when the salinity was greater than 33.7.

Effects of long-term exposure to industrial wastewater on photosynthetic performance of Euglena gracilis measured through chlorophyll fluorescence

The pollution of aquatic environments by industrial wastewater is one of the major environmental problems in the world. Due to factors like simplicity in application, cost-effective, rapid and sensitive response to pollutants, algae can be used as early indicator of pollution in aquatic environments. Euglena gracilis has been widely used as a model organism in assessment of various water pollutants with its different parameters as endpoints. Wastewater has been frequently reported to adversely affect aquatic organisms, but its effect on photosynthesis of phytoplankton, particularly in long-term exposure, has been addressed rarely. In the present study, long-term effects of wastewater samples from ten industries on photosynthesis of E. gracilis were assessed using a pulse-amplitude-modulated (PAM) fluorometer. Euglena cultures were exposed to different concentrations of wastewaters, and the effect on quantum yield of photosystem II (F v/F m), and relative electron transport rate (rETR) was assayed after 7 days of exposure. Five samples out of the total ten were found to adversely affect photosynthesis in Euglena. One sample was found to have a stimulatory effect on photosynthesis in Euglena. Unfiltered solutions were observed to affect rETR more severely than filtered ones of the same sample at the same concentration. By comparing the results with previous reports of short-term effects of wastewater on photosynthesis, it is concluded that wastewater can be a risk to photosynthesis in aquatic organisms in long-term exposure.

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.

Photosynthesis and respiration in marine phytoplankton: Relationship with cell size, taxonomic affiliation, and growth phase

We determined the rates of photosynthesis and respiration in batch cultures of 22 marine phytoplankton species from five phyla covering a range of 7 orders of magnitude in cell size. Rates were determined during the exponential growth phase and also during the stationary phase, when cell growth was limited by nitrogen availability. We observed, in all growth phases, a curvature in the size scaling of carbon fixation, such that the relationship between carbon-specific photosynthesis and cell size was unimodal, with the highest rates being measured in intermediate-size species. The log–log relationship between individual metabolic rates and cell size showed an overall lineal pattern with a slope equal or near 1, irrespective of whether volume or carbon is used as a metric for cell size. Thus, our results demonstrate that when small species (<50μm3 cell diameter) are considered together with intermediate- and large-sized species phytoplankton metabolism does not follow Kleiber's 3/4-power rule. Considering all species together, respiration losses represented on average 9% and 22% of total carbon fixation during the exponential growth and stationary phases, respectively. Carbon-specific respiration was largely independent of cell size and growth phase, but tended to take higher values in the dinoflagellates. During the stationary growth phase, and contrary to other groups, most diatoms were able to maintain carbon fixation rates similar to those measured during exponential growth. Our results highlight the ability of intermediate-to-large size species to sustain high metabolic rates in spite of their cell size, which helps to explain why they dominate phytoplankton blooms in the ocean.

A fiber-optic fluorometer for measuring phytoplankton photosynthesis parameters

A fluorometer model designed at the Institute of Automatics and Control Processes and intended for measuring phytoplankton photosynthesis parameters and chlorophyll-a concentration in sea water in vivo in a wide range of depths, is described. The feature of the fluorometer is the use of fiber-optic sensors for measuring fluorescence, temperatures, and depths. The structure and design of the fluorometer is described. The measurement results of photosynthesis parameters, obtained in a series of expeditions in the Sea of Japan in autumn 2012, are discussed.

Biochemical characterization of the δ-carbonic anhydrase from the marine diatom Thalassiosira weissflogii, TweCA

Diatom genome sequences clearly reveal the presence of different systems for HCO3− uptake. Carbon-concentrating mechanisms (CCM) based on HCO3− transport and a plastid-localized carbonic anhydrase (CA, EC 4.2.1.1) appear to be more probable than the others because CAs have been identified in the genome of many diatoms. CAs are key enzymes involved in the acquisition of inorganic carbon for photosynthesis in phytoplankton, as they catalyze efficiently the interconversion between carbon dioxide and bicarbonate. Five genetically distinct classes of CAs exist, α-, β-, γ-, δ- and ζ and all of them are metalloenzymes. Recently we investigated for the first time the catalytic activity and inhibition of the δ-class CA from the marine diatom Thalassiosira weissflogii, named TweCA. This enzyme is an efficient catalyst for the CO2 hydration and its inhibition profile with sulfonamide/sulfamate and anions have also been investigated. Here, we report the detailed biochemical characterization and chemico-physical properties of the δ-CA of T. weissflogii. The δ-CA encoding gene was cloned and expressed in Artic Express cells and the recombinant protein purified to homogeneity. Interesting to note that TweCA has no intrinsic esterase activity with 4-nitrophenyl acetate (pNpA) as substrate although the phylogenetic analysis showed that δ-CAs are closer to the α-CAs than to the other classes of such enzymes.

Light utilization efficiency of phytoplankton in the Western Subarctic Gyre of the North Pacific during summer

We investigated the water-column light utilization efficiency (Ψ) of phytoplankton photosynthesis in the Western Subarctic Gyre (WSG) of the North Pacific during summer 2008. The Ψ values (0.64–1.86 g C [g Chl a]−1 [mol photon]−1 m2) obtained were observed to increase significantly with decreasing daily photosynthetic available radiation (PAR) and were generally higher than those of previous studies, not only from the subarctic Pacific but also from the world’s oceans. To examine the effect of iron availability on Ψ in the WSG, Ψ values were estimated from the data of two in situ iron fertilization experiments: the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study I (SEEDS-I) and II (SEEDS-II). We found that iron availability did not affect Ψ values. Overall, this study revealed that Ψ values changed remarkably in the WSG during the summer, and that higher values were found at the stations where moderate PAR levels (ca. 10–30 mol photons m−2 day−1) were observed and where autotrophic flagellates predominated in the phytoplankton assemblages.

Response of dissolved oxygen and related marine ecological parameters to a tropical cyclone in the South China Sea

It is well known that tropical cyclones can cause upwelling, decrease of sea surface temperature, increase of chlorophyll-a (Chl-a) concentration and enhancement of primary production. But little is known about the response of dissolved oxygen (DO) concentration to a typhoon in the open ocean. This paper investigates the impact of a typhoon on DO concentration and related ecological parameters using in situ and remote sensing data. The in situ data were collected 1week after the passage of the super-typhoon Nanmadol in the northern South China Sea in 2011. An increase in DO concentration, accompanied by a decrease in water temperature and an increase in salinity and Chl-a concentration, was measured at sampling stations close to the typhoon track. At these stations, maximum DO concentration was found at a depth of around 5m and maximum Chl-a concentration at depths between 50 and 75m. The layer of high DO concentration extends from the surface to a depth of 35m and the concentrations stay almost constant down to this depth. Due to the passage of the typhoon, also a large sea level anomaly (21.6cm) and a high value of Ekman pumping velocity (4.0×10−4ms−1) are observed, indicating upwelling phenomenon. At the same time, also intrusion of Kuroshio waters in the form of a loop current into the South China Sea (SCS) was observed. We attribute the increase of DO concentration after the passage of the typhoon to three effects: (1) entrainment of oxygen from the air into the upper water layer and strong vertical mixing of the water body due to the typhoon winds, (2) upwelling of cold nutrient-rich water which stimulates photosynthesis of phytoplankton and thus the generation of oxygen, which also increases the DO concentration due to cold water since the solubility of oxygen increase with decreasing water temperature, and, possibly, (3) transport of DO enriched waters from the Western Pacific to the SCS via the intrusion of Kuroshio waters.

Variations in phytoplankton photo-physiology and productivity in a dynamic eutrophic ecosystem: a fast repetition rate fluorometer-based study

We investigated the photoacclimation state and photosynthetic rate of cells in the highly eutrophic upper Gulf of Thailand (UGOT), using fast repetition rate (FRR) fluorometry. Observations revealed differences in photosystem II (PSII) parameters: the maximum photochemical quantum efficiency of PSII (Fv/Fm), the functional absorption cross-section of PSII (σPSII) and the rate of reoxidation of the primary PSII electron acceptor (1/τQa), which were dependent on the hydrographic structure of the water column. FRR-derived parameters were used to estimate the chlorophyll a-normalized, photosynthetic rate (PB); the photosynthesis vs. irradiance (P–E) curves were derived from in situ PB profiles. The P–E parameters differed markedly between the upper stratified waters and the mixed water column; that is, the maximum photosynthetic rate |$P_{{\rm max}}^{\rm B} $| and the light saturation parameter (Ek) were higher in the upper stratified waters, while the initial slope of the P–E curve (α) was higher in the mixed waters. This indicates that cells acclimated to relatively bright light dominated the upper stratified waters, while cells acclimated to lower light dominated the mixed waters. A significant, positive relationship was found between the average PB in the euphotic zone and surface photosynthetically available radiation, from all profiles at both sampling sites, which suggests that phytoplankton photosynthesis in the UGOT was controlled primarily by irradiance. Furthermore, as stress from nutrient-limitation is unlikely in the UGOT, cells might realize their photosynthetic potential by means of photoacclimation, even under the different light regimes present in the UGOT.

Strong seasonal effect of moderate experimental warming on plankton respiration in a temperate estuarine plankton community

Climate change projections forecast a 1.1–6.4 °C global increase in surface water temperature and a 3 °C increase for the Baltic Sea. This study examined the short-term interactive effects of a realistic future temperature increase (3 °C) on pelagic respiration and bacterioplankton growth and phytoplankton photosynthesis in situ. This study was undertaken throughout a full seasonal cycle in the northern Baltic Sea. We found marked positive short-term effects of temperature on plankton respiration but no significant effect on bacterioplankton growth or phytoplankton photosynthesis. Absolute respiration rates remained similar to other comparable environments at the in situ temperature. With the 3 °C temperature increase, respiration rates in situ increased up to 5-fold during the winter and 2-fold during the summer. A maximum seasonal Q10 value of 332 was observed for respiration during the cold winter months (twater ≈ 0 °C), and summer Q10 values were comparatively high (9.1). Q10 values exhibited a significant inverse relationship to water temperature during winter. Our results thereby suggest that plankton respiration in this coastal zone is more temperature sensitive than previously reported. In addition, field data indicated that plankton respiration switched from being temperature limited to being limited by dissolved organic carbon (DOC) after the simulated temperature increase. Assuming that our observations are relevant over longer time scales, climate change may worsen hypoxia, increase CO2 emissions and create a more heterotrophic food web in coastal zones with a high load of riverine DOC.

Lagged response of phytoplanktonic photosynthetic competence to variation of nitrogen concentration in Jiaozhou Bay, China

Fluorescence-based maximal photochemical efficiency, F v/F m, is widely used as an indicator to photosynthetic competence in marine systems. It has been considered a useful parameter diagnosing the nutrient stress on phytoplankton photosynthesis, but many studies argue its usefulness. In the present study, we try to find a temporal relationship between F v/F m and nitrogen concentration, and provide a possible explanation on the controversy. We continuously measured F v/F m and nitrogen concentration once every 10 days from September 2003 to March 2004 at two stations in Jiaozhou Bay, northern China. It was found that F v/F m did not significantly correlate to synchronous nitrogen concentration, but the variation (i.e. the change between two adjacent cruises) of nitrogen concentration of the previous cruise and the variation of F v/F m of the current cruise were strongly correlated. This result indicates that a time lag exists between the variation of nutrient status and the subsequent F v/F m response. Length of the time lag seems just matched the interval of our measurements (10 days). In the field, direct dependence of F v/F m on nitrogen concentration may not be found because of the lagged response of F v/F m to nitrogen concentration variations or physiological acclimation. Our results provide a possible way to explain the previously reported conflicting results on the relationship between F v/F m and nutrient status. To give a more-accurate estimate about the length of the time lag, an investigation that includes more frequent measurements is needed.

Adaptive responses of photosynthesis in phytoplankton: specificity to time-scale of change in light

The relationship between photosynthesis and light intensity, as expressed by the P-I curve, is fundamental to models of primary production in the oceans. Adaptive responses of phytoplankton are man...