Phytoplankton Reproduction Research Articles

Triazine herbicide reduced the toxicity of the harmful dinoflagellate Karenia mikimotoi by impairing its photosynthetic systems

Triazine herbicides are common contaminants in coastal waters, and they are recognized as inhibitors of photosystem II, causing significant hinderance to the growth and reproduction of phytoplankton. However, the influence of these herbicides on microalgal toxin production remains unclear. This study aimed to examine this relationship by conducting a comprehensive physiological and 4D label-free quantitative proteomic analysis on the harmful dinoflagellate Karenia mikimotoi in the presence of the triazine herbicide dipropetryn. The findings demonstrated a significant decrease in photosynthetic activity and pigment content, as well as reduced levels of unsaturated fatty acids, reactive oxygen species (ROS), and hemolytic toxins in K. mikimotoi when exposed to dipropetryn. The proteomic analysis revealed a down-regulation in proteins associated with photosynthesis, ROS response, and energy metabolism, such as fatty acid biosynthesis, chlorophyll metabolism, and nitrogen metabolism. In contrast, an up-regulation of proteins related to energy-producing processes, such as fatty acid β-oxidation, glycolysis, and the tricarboxylic acid cycle, was observed. This study demonstrated that dipropetryn disrupts the photosynthetic systems of K. mikimotoi, resulting in a notable decrease in algal toxin production. These findings provide valuable insights into the underlying mechanisms of toxin production in toxigenic microalgae and explore the potential effect of herbicide pollution on harmful algal blooms in coastal environments.

Marine ecological effect of typhoon influenced by the frequency-phases of ENSO and anthropogenic activity: the East China Sea example

In coastal regions, typhoons are one of the most frequent natural disasters, and instrumental data indicate that they can greatly boost marine primary productivity. Typhoons' long-term ecological repercussions, however, are still little understood. To investigate the ecological effects of typhoons since the mid-Holocene, we focused on the sediment cores in the East China Sea, with key parameters of organic CN and Bio-Si to reveal the paleo-productivity variation. Over a millennium, the occurrence of El Niño-Southern Oscillation (ENSO) has had a substantial impact on primary productivity in the southern East China Sea. The primary productivity in the southern East China Sea was risen by humid climate conditions in ∼3.0–2.0 ka and frequent typhoons in 1.5–0 ka. With a millennial period, ENSO frequency has been significantly influencing primary productivity in the southern East China Sea since the mid-Holocene. During 1.5–0 ka, frequent ENSO events have led to more frequent typhoons, which caused terrestrial materials input and water vertically mix in the southern East China Sea, ultimately increasing primary productivity. Particularly on a centennial time scale, ENSO phases determined how much primary productivity was increased, and this magnitude was bigger during the La Niña-like periods than it was during the El Niño-like periods since La Niña is when typhoons occur more frequently. During the Little Ice Age, the cold climate hindered the reproduction of phytoplankton, but increased human activities in Taiwan resulted in more soil erosion and the supply of terrestrial-derived nutrients into the sea, then amplified the typhoon-induced increase in primary productivity. As a result, this stage showed the highest primary productivity since the mid-Holocene. This study reveals that frequent typhoons promote marine primary productivity on centennial-millennial timescales and emphasizes that human activities amplify the ecological effect of typhoons in recent centuries.

The Mechanisms Controlling the CO2 Outgassing of a Karst Spring–River–Lake Continuum: Evidence from Baotuquan Spring Drainage Area, Jinan City, Northern China

The significance of CO2 emissions at the water–air interface from inland water bodies in the global carbon cycle has been recognized and is being studied more and more. Although it is important to accurately assess CO2 emission flux in a catchment, little research has been carried out to investigate the spatio-temporal variations in CO2 emissions in view of a water continuum. Here, we systematically compared the differences and control factors of CO2 degassing across the water–air interface of a spring–river–lake continuum in the discharge area of Baotuquan Spring in July 2017, which is a typical temperate karst spring area in Jinan city, northern China, using hydrogeochemical parameters, stable carbon isotope values, and CO2 degassing flux. Affected by the pCO2 concentration gradient between the water and ambient air, the spring water showed a high CO2 degassing flux (166.19 ± 91.91 mmol/(m2 d)). After the spring outlet, the CO2 degassing flux in the spring-fed river showed a slight increase (181.05 ± 155.61 mmol/(m2 d)) due to river flow rate disturbance. The river flow rate was significantly reduced by the “blockage” of the lake, which promoted the survival and reproduction of phytoplankton and provided favorable conditions for aquatic plant photosynthesis, increasing the plankton biomass in the lake to 3383.79 × 104/L. In addition, the significant decrease in the dissolved inorganic carbon (DIC) concentration and the increase in the δ13CDIC values in the lake also indicated that the photosynthesis of the lake’s aquatic plants resulted in a significant decrease in the pCO2 concentration, thus limiting the amount of CO2 off-gassing (90.56 ± 55.03 mmol/(m2 d)).

Study of the Response of Environmental Factors of the Coastal Area in Zhoushan Fishery to Typhoon In-fa Based on Remote Sensing

The response of typical environmental factors in Zhoushan Fishery, including sea surface temperature (SST), sea surface salinity (SSS), and chlorophyll a (Chl-a), before and after Typhoon In-fa was analyzed using satellite data and reanalysis data in this study. Additionally, this study simultaneously elucidated the mechanism by which the typhoon affected these factors. The results showed that: (1) the strong vertical mixing caused by In-fa provoked a decrease in SST, while the asymmetric typhoon wind stress and vertical difference in temperature structure before the typhoon caused a more robust cooling of SST on the right side of the In-fa track; (2) despite the strong mixing and inflow of hypersaline seawater increasing SSS, the combined effect of intense rainfall and diluted water inflow caused an overall decrease in SSS after In-fa’s landing; (3) In-fa caused the Chl-a concentration to decrease first and then increase. The high cloudiness and low Chl-a seawater inflow inhibited the phytoplankton growth during the typhoon, while the abundant light, rich surface nutrients under the upwelling effect, and transport of rich land-based substances induced rapid phytoplankton reproduction after the typhoon; and (4) the change in Chl-a concentration, current, temperature, and salinity induced by a typhoon are essential factors that affect fish behavior and community composition in fisheries. This study provides a point of reference to reveal the response of environmental factors to typhoons and their effects on fishery resources in fisheries located on nearshore estuarine shallow waters with intensive islands.

Impacts of environmental factors on Chlorophyll-a in lakes in cold and arid regions: A 10-year study of Wuliangsuhai Lake, China

The study of seasonal variations in Chlorophyll-a (Chl.a) concentration and its influencing factors is particularly important for lake management. The study showed that there was a strong non-linear relationship between Chl.a and environmental factors in spring and autumn, whereas there was a strong linear relationship in summer and winter. Chl.a was redistributed during the ice period, and its concentration in water generally exceeded that in the ice sheet. Characteristics of the ice sheet, including thermal insulation, transparency, and nutrient migration into the water, positively impact phytoplankton growth and reproduction. Therefore, there was no significant difference between Chl.a in winter water and Chl.a in summer water. Although there were differences in the environmental factors affecting the temporal heterogeneity of Chl.a over the four seasons, conductivity (Cond.) showed a significant linear or nonlinear effect on Chl.a throughout the year, indicating its important role in regulating the growth and reproduction of phytoplankton. Environmental factors driving the spatial heterogeneity of Chl.a included the location of the water inlet channel, the distribution of Phragmites, and hydrodynamic conditions. These findings are valuable for understanding the seasonal variation of Chlorophyll-a and preventing eutrophication and algal blooms in cold and arid lakes.

Conservation of cholera vibrios in complex microcosm containing green microalgae

BACKGROUND: In the summer-autumn period, the peak of phytoplankton reproduction and flowering occurs and results in the deterioration of the quality of river water is noted. At the same time, cases of gastrointestinal diseases in humans are recorded, and epidemic complications of cholera may occur in endemic areas. V. cholerae form biofilm on the surface of chitinous hydrobionts and plastic components can lead to the spread of V. cholerae, possibly explaining the autochthonous mechanism of their existence in water bodies.
 AIM: This study determines the duration of Vibrio cholerae preservation on biotic (chitin) and abiotic (plastic) substrates in the presence of green unicellular algae with a change in cultivation temperature under experimental conditions.
 MATERIALS AND METHODS: This study used bacteriological and molecular genetic methods to achieve its goal.
 RESULTS: The preservation of toxigenic and non-toxigenic strains of V. cholerae O1 El Tor and V. cholerae O139 (ctxАВ+tcpА+csh1 and ctxАtcpАcsh1+) for six months, including three months at a low temperature simulating the autumn-winter period, as part of biofilms in microcosms, where one component is green microalgae. We noted an excess of the concentration of V. cholerae O1 and O139 serogroups by two orders of magnitude in samples where one of the components is chitin and the reproduction of green microalgae in the presence of a chitin substrate. This is probably one stage in the food chain in the ecology of water bodies and, accordingly, can be a reservoir for biofilm forms of cholera vibrios. Preservation of V. cholerae O1 El Tor non-toxigenic strain in a viable state in biofilm samples on plastic at decreased temperatures 82C for three months is possibly due to the presence of a cold shock gene in its genome.
 CONCLUSION: Without a substrate for adhesion, colonization, and biofilm formation, vibrios are incapable of long-term persistence at low temperatures.

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.

Modelling Phytoplankton Behaviour in the North and Irish Sea with Transformer Networks

Climate change will affect how water sources are managed and monitored. Continuous monitoring of water quality is crucial to detect pollution, to ensure that various natural cycles are not disrupted by anthropogenic activities and to assess the effectiveness of beneficial management measures taken under defined protocols. One such disruption is algal blooms in which population of phytoplankton increase rapidly affecting biodiversity in marine environments. The frequency of algal blooms will increase with climate change as it presents favourable conditions for reproduction of phytoplankton. Machine learning has been used for early detection of algal blooms previously, with the focus mostly on single closed bodies of water in Far East Asia with short time ranges. In this work, we study four locations around the North Sea and the Irish Sea with different characteristics predicting activity with longer time-spans and explaining the importance of the input with regard to the output of the prediction model. This work aids domain experts to monitor potential changes to the ecosystem over longer time ranges and to take action when necessary.

Acclimation of phytoplankton to diverse environmental variables at the initial operation stage of a free water surface constructed wetland

Phytoplankton played vital roles in aquatic biodiversity and monitoring environmental variations due to the rapid succession of their communities. Weekly studies on phytoplankton species composition and abundance were conducted at a newly established free water surface constructed wetland (FwsCW) from July to October in 2019. Consequences of diversity investigation exhibited there were 124 species of phytoplankton collected, including 57 species or taxa of Bacillariophyta, with the average relative abundance of 44.33 %, 34 species or taxa of Cyanophyta (28.25 %), 17 species or taxa of Chlorophyta (22.62 %), and some of the less collected freshwater community representatives (i.e., Euglenophyta (2.2 %), Pyrrophyta (1.68 %), and Cryptophyta (0.92 %)). The survey credibly revealed the effect of environmental variables on phytoplankton community structure was obvious, especially in the first month (July) and the fourth month (October), and the coexisting species or taxa in each adjacent month were greatly different. Since they entered the FwsCW, existing communities had transited to more diverse ones. The results of principal component analysis (PCA) showed that overall communities’ acclimation process was controlled by hydrochemical and hydrological indices. The most direct factors affecting the growth and reproduction of phytoplankton were the availability of nutrients and the water temperature. The investigation was devoted to the study of some environmental variables which affect the structure of the phytoplankton community, and revealed the importance of integrated management of artificial aquatic ecosystem. The findings also proposed that the long-term operation effects of FwsCW could be pre-evaluated via the structural variation of phytoplankton communities.

Comparative study on the influene factors of chlorophyll-a and nutrient structure between winter and summer in the central Bohai Sea

Based on the data of chlorophyll a (Chl a), nutrients, and hydrological environment parameters of the central Bohai Sea in January(winter) and June(summer) of 2016, we studied spatial and temporal variations of Chl a, nutrient structure, analyzed the influence factors for the phytoplankton growth and the changing trend of the phytoplankton community structure over long-time scale. The results showed that nutrient structure and Chl a had significant seasonal variation characteristics (<italic>P</italic><0.01). In winter, the concentrations of dissolved inorganic nitrogen (DIN), active phosphate (PO^3-₄-P) and silicate (SiO^2-₃-Si) were (10.51±4.52)μmol·L^-1, (0.28±0.14)μmol·L^-1, and (10.63±2.09)μmol·L^-1, respectively, and NO^-₃-N (92%) was the main form of DIN, and all nutrient contents were above the threshold, meanwhile the nutrient ratios of N/P, Si/P, and Si/N were 69.30±86.80, 64.50±68.80, and 1.20±0.50, respectively, the N and Si limitations were not obvious except that some stations showed P potential limitations because of input of the Northern Yellow Sea cold flow, the sediment resuspension, and the release by the mineralization of organic matters. Affected by seawater temperature (<italic>T</italic><8℃), the phytoplankton reproduction and growth were restricted and the content of Chl a (0.99±0.25) mg·m^-3 was low. In summer, influenced by the Yellow River dilute water, the Northern Yellow Sea cold flow, and phytoplankton uptake. The concentrations of DIN, PO^3-₄-P and SiO^2-₃-Si were (4.72±4.20) μmol·L^-1, (0.04±0.04) μmol·L^-1, and (3.81±1.72) μmol·L^-1, and decreased by 55.0%, 85.7% and 64.1%, respectively, NH⁴⁺-N (48%) and NO^-₃-N (45%) became the main forms of DIN, compared to those in the winter. The nutrient ratios of N/P, Si/P and Si/N were 274.19±328.06, 202.18±182.07 and 1.39±1.00, respectively, increased significantly compared with that of winter. The nutrient limitations showed obvious P limitation, and the contents were below the threshold of DIN and Si in some stations, which would limit the growth of phytoplankton, especially the diatoms. The content of Chl a was (4.58±5.51) mg·m^-3 with the seawater temperature of (22.1±1.88)℃, 4.6 times higher than that in winter, there were some stations in the Yellow River Estuary, the content was beyond 10 mg·m^-3 and showed an outbreak risk of alage bloom.

Bottom-up and top-down effects on phytoplankton functional groups in Hulun Lake, China

The debates about the extent to which phytoplankton in freshwater ecosystems are regulated by top-down or bottom-up forces have been ongoing for decades. This study examines the effects of bottom-up and top-down factors on the phytoplankton functional groups in a eutrophic lake. Phytoplankton and zooplankton were sampled and physical-chemical variables measured from May 2019 to October 2019 in Lake Hulun, China. Approximately 43 phytoplankton species were observed and grouped into 23 functional groups. For the zooplankton, about 27 species were observed and classified into 8 functional groups. The study revealed that the bottom-up effects of physical-chemical variables on some phytoplankton functional groups was stronger than the top-down effects of zooplankton. Water temperature (WT), total phosphorus (TP), total nitrogen (TN), conductivity (Cond), water transparency (SD), and dissolved oxygen (DO) significant influence the biomass of the phytoplankton functional groups. The biomass of phytoplankton functional groups was influenced positively by nutrient availability likely because nutrients influence the growth and reproduction of phytoplankton in freshwater. WT and DO had a positive influence on biomass of phytoplankton functional groups. Conversely, phytoplankton biomass revealed a decreasing trend when SD and Cond significantly increased. This study showed that zooplankton functional groups were positively correlated with phytoplankton biomass implying that the top-down control of phytoplankton by the zooplankton in the lake is not strong enough to produce a negative effect. It is evident that the zooplankton functional groups in Lake Hulun are controlled more by bottom-up force than top-down.

Effects of Artificial LED Light on the Growth of Three Submerged Macrophyte Species during the Low-Growth Winter Season: Implications for Macrophyte Restoration in Small Eutrophic Lakes

Eutrophication of lakes is becoming a global environmental problem, leading to, among other things, rapid reproduction of phytoplankton, increased turbidity, loss of submerged macrophytes, and the recovery of these plants following nutrient loading reduction is often delayed. Artificial light supplement could potentially be a useful method to help speeding up recovery. In this study, three common species of submerged macrophytes, Vallisneria natans, Myriophyllum spicatum and Ceratophyllum demersum, were exposed to three LED light treatments (blue, red and white) and shaded (control) for 100 days (from 10 November 2016 to 18 January 2017) in 12 tanks holding 800 L of water. All the three LED light treatments promoted growth of the three macrophyte species in terms of shoot number, length and dry mass. The three light treatments differed in their effects on the growth of the plants; generally, the red light had the strongest promoting effects, followed by blue and white. The differences in light effects may be caused by the different photosynthetic photon flux density (PPFD) of the lights, as indicated by an observed relationship of PPFD with the growth variables. The three species also responded differently to the light treatments, V. natans and C. demersum showing higher growth than M. spicatum. Our findings demonstrate that artificial light supplement in the low-growth winter season can promote growth and recovery of submerged macrophytes and hence potentially enhance their competitiveness against phytoplankton in the following spring. More studies, however, are needed to elucidate if LED light treatment is a potential restoration method in small lakes, when the growth of submerged macrophytes are delayed following a sufficiently large external nutrient loading reduction for a shift to a clear macrophyte state to have a potential to occur. Our results may also be of relevance when elucidating the role of artificial light from cities on the ecosystem functioning of lakes in urban areas.

Effects of adding sucrose onPenaeus monodon(Fabricius, 1798) growth performance and water quality in a biofloc system

A 60-day indoor growth trial was conducted to study the effects of biofloc on the growth performance of a Penaeus monodon (Fabricius, 1798), water quality and biological indicators including biofloc volume, chlorophyll-a, heterotrophic bacteria and Bacillus quantity. Two concentrations of sucrose (0 and 75%) were added daily to the P. monodon culture systems (2.94 ± 0.11 g), which were conducted indoors in fibre-glass tanks (500 L). Results showed that the final body weight and weight gain of the adding 75% sucrose group were significantly higher (P < 0.05) than that of the control, as well as significantly (P < 0.05) improved specific growth rates and survival rates, and reduced feed coefficient. Adding 75% sucrose promoted heterotrophic bacteria, Bacillus and phytoplankton reproduction, and significantly (P < 0.05) reduced the concentration of ammonia-N (NH4-N), nitrite-N (NO2-N) and nitrate-N (NO3-N). The changes of water quality indicators in the two groups showed the similar trend at the end of the experiment, and the ammonia-N, nitrite-N, nitrate-N and phosphate-P concentrations in the 75% sucrose group were significantly (P < 0.05) lower than those of the control group, Chlorophyll-a concentrations peaked at 389.12 μg/L in the biofloc sucrose group at 18:00 h, and heterotrophic bacteria peaked 8 h after sucrose was added. The addition of sucrose also reduced the pH of the water. Our research showed that adding sucrose promoted biofloc formation and shortened the formation time; increased the number of heterotrophic bacteria and algae which might play a role in improving water quality by assimilating ammonia-N and other harmful substances in the water; supplemented food for P monodon growth; and reduced the feed coefficient.

Seasonally chemical hydrology and ecological responses in frontal zone of the central southern Yellow Sea

Based on annual-cycle survey data collected in 2006–2007 in the southern Yellow Sea (SYS) and analyses on the seasonally chemical hydrologic characteristics of the boundary front of the Yellow Sea Cold Water Mass (YSCWM) and Yellow Sea Warm Current (YSWC), the seasonal variations in upwelling along the frontal zone were determined, and the ecological impacts of the front were investigated. During the generation and dissipation of the YSCWM, the implied upwelling along its western front exhibited seasonal variation. The upwelling first shifted westward from the deep-water region to its westernmost point in summer then returned eastward. The intensity of the upwelling gradually increased from spring to summer and decreased in autumn. In spring, the existence of cold water west of the YSWC was not conducive to the reproduction of phytoplankton. Additionally, the front to the east of this cold water mass also made the western boundary of the phytoplankton bloom region in the central SYS more obvious, forming a prominent chlorophyll a (Chl-a) front. During the entire stratified season (summer and autumn), the upwelling in the frontal zone of the YSCWM played an essential role in maintaining the relatively high concentrations of Chl-a. In winter, the front that formed at the intersection of the YSWC and coastal cold water was also favorable for the formation of the high-Chl-a region. The distribution of anchovy biomass was closely related to the seasonal variations in the position of the frontal zone. In winter and spring, the tongue-shaped warm water and front associated with the intrusion of the YSWC into the SYS had a significant impact on anchovy. During the stratified season in summer and autumn, the development of a front near the boundary of the YSCWM was an important physical driving mechanism for the dense distribution of anchovy. This work enhanced the study of the seasonal relationships between the physical, chemical and biological processes in the frontal zone of the central SYS and deepened our understanding of the ecological significance of this front.

Characterization of Phytoplankton Communities in Response to Macronutrient Treatments Determined by High-Performance Liquid Chromatography Mass Spectrometry

ABSTRACTThe growth and reproduction of phytoplankton are controlled by macronutrients. Phytoplankton communities are important indicators of marine responses to environmental changes. Thus, it is important to study the relationships between phytoplankton composition and macronutrient concentrations. High performance liquid chromatography (HPLC) coupled with electrospray ionization/triple quadrupole mass spectrometry (MS) was used for the determination of 13 pigments of phytoplankton taxa. Based on the pigments and optimized formulas for algae biomass conversion, the contributions of diatoms, dinoflagellates, chrysophytes, haptophytes, cyanobacteria, chlorophytes, prasinophytes, and cryptophytes to the total phytoplankton biomass were evaluated. The highest concentrations of pigments were found in the phytoplankton community treated with nitrogen/phosphorus/silicon for 9 days. By monitoring cholorophyll a in eight taxa, the phytoplankton biomass was found to increase in the following order: control>silicon>nitrogen/phosphorus/silicon. Diatoms/fucoxanthin and chlorophytes/lutein were the major phytoplankton present in the community.

Recognition on the forming-vanishing process and underlying mechanisms of the hypoxia off the Yangtze River estuary

On the basis of compiled multidisciplinary historical data in 2006–2007 and incorporation of relevant simulation results and remote sensing data, we performed an in-depth study of the generation and dissipation process of the hypoxic zone and its distribution morphology and structure off the Yangtze River estuary. Based on the hydrological circulation dynamics, reproduction of phytoplankton (leading to the decomposition of organic matter), and other factors, we comprehensively and systematically investigated the generation and dissipation of the hypoxic zone and underlying mechanisms for the seasonal variation in its position, explored the multi-factorial synergistic reactions during the generation and dissipation process of the hypoxic zone, and revealed the controlled mechanism for the morphology and structure of the hypoxic zone’s distribution. Our studies indicate that in the winter and spring seasons, the hydrological environment off the Yangtze River estuary provides a water body with relatively low contents of dissolved oxygen (DO), which is the background for the formation of a hypoxic zone. After entering into the summer season, the hypoxic zone gradually develops towards the north and becomes mature. Because of the impact of the terrain, local decomposition of organic matter, and upwelling of the Kuroshio subsurface water in July–August, the hypoxic zone off the Yangtze River estuary exhibits the characteristics of discontinuous distribution in space and has a south and north “dual-core” structure in the inner continental shelf. In addition, there is a hypoxic core in the eastern outer continental shelf. The degrees of hypoxia vary for different areas; they are strongest overall in the north, next strongest in the south; they are weakest on the outer continental shelf. In summer, the hypoxic zone in the north is related to the northward differentiation of the southern hypoxic zone and results from local development and intensification. In August, the hypoxic zone in the north reaches its peak, and after September, it rapidly retreats southward and disappears because of weakening stratification. In the fall, there is hypoxic zone along the coast of Zhejiang in the south, and there is also a low-DO area to the southwest of Jeju Island, with both zones disappearing rapidly. In addition, the change of dynamic environment also causes the low-DO area of the outer continental shelf to move outward in the fall. The variation in the intensity of the stratification and its cumulative effects as a barrier of vertical DO transportation over long periods of time have a significant impact on the degree of hypoxia in the hypoxic zone. In addition, the seasonal variations in the size of the stratified region, intensity of each current system/water mass, upwelling, front, and high-value area of phytoplankton biomass jointly restrict the extension of the hypoxic zone in the inner continental shelf and latitudinal (south-north direction) movement of its location off the Yangtze River estuary. The combined effect of dynamic factors, such as that of the Kuroshio subsurface water, causes a low-DO core in the outer continental shelf. The bottom cold water to the north of the East China Sea is the dynamic basis for the formation of the low-DO area to the southwest of Jeju Island during the fall season. The special seabed topography and mud area distribution off the Yangtze River estuary have a certain degree of influence on the development of the hypoxic zone. The generation and dissipation of the hypoxic zone and its distribution morphology off the Yangtze River estuary, and seasonal variation of its structure and position are a result of the synergistic effects of various factors.

Lagrangian coherent structures in the ocean favorable for fishery

Regions rich in nutrients with food grounds for zooplankton, the main feeding resource of pelagic fish, are locations of fishing grounds. Favorable condi� tions are usually formed in the subarctic frontal zones at the boundaries between cold and warm currents and in the periphery of mesoscale eddies. These are regions with high mechanical energy and intense horizontal mixing, in which phytoplankton, the food for zoop� lankton, can concentrate. Numerous investigations in different seas and oceans confirm that fronts and eddies are formations in which favorable conditions are formed for reproduction of plankton and growth of fish [1–3]. In the northwestern part of the Pacific Ocean east of Hokkaido Island (Japan) and the South� ern Kuril Islands, the waters of the cold subarctic Oyashio Current mix with the waters of the warm Kuroshio Current. Owing to the high concentration of nutrients in the cold waters of the Oyashio and high tides facilitating reproduction of phytoplankton in the surface waters, this region is known as one of the rich� est fishing locations in the world. The objective of this work is to distinguish the Lagrangian coherent structures in the ocean at the fronts (boundaries between the waters of different ori� gins), along which favorable conditions for fishery are formed. In the language of the theory of dynamic sys� tems, these structures are called unstable manifolds of hyperbolic points of nonstationary flow. The geo� strophic velocity field was calculated from the data of satellite altimetry. By calculating the displacements D of a large number of particles during a specific time period, one can plot a synoptic chart of this value for any region in which the fronts and the curves of the local minimum of D are clearly pronounced that visu� alize the strongest unstable manifolds in the region. It is shown in this paper that the vessels with the maximum catches of saury in the fall of 2002 and 2004 were located mainly along the unstable manifold depicting the Oyashio front at that period of time. We solve the equation for the advection of a passive admixture

The fate of Cu, Zn, and Cd in the initial stage of water system contamination: The effect of phytoplankton activity

The fate of Cu, Zn and Cd after their entry into an unpolluted fresh water body was studied as a function of a condition of a natural phytoplankton community. In a field experiment, additives of metals entered into light and dark mesocosms twice to simulate effects of primary and repeated contamination. Over 31 days, the reproduction of new generation of a phytoplankton as renewable source of the suspended particles capable to metal biosorption was higher in a light mesocosm. As a result the residual concentration of metals in water decreased faster and their content in the settled deposits was higher in a light mesocosm, than in a dark mesocosm. Efficiency of planktonic channel in deducing of metals from water with settling deposits (in the order of Cu>Zn>Cd) depends on intensity of a phytoplankton reproduction in the initial stage of water ecosystem contamination.

Effect of nutrient level on phytoplankton community structure in different water bodies

Increasing levels of pollution within water bodies can cause eutrophication and an associated rapid growth in and reproduction of phytoplankton. Although most frequently occurring in bodies of water such as lakes and dams, in recent years an increasing number of river systems in China have suffered serious algal blooms. The community structure of phytoplankton may differ, however, dependent on the hydrodynamic conditions and nutrient levels within the water body. The field investigation results obtained from a stagnant river in Suzhou City and Taihu Lake, China, showed that in water with higher concentrations of nitrogen and phosphorus, Chlorophyta became the predominant species and in water with lower concentrations of nitrogen and phosphorus, Cyanobacteria became the predominant species. Growth experiments with competitive species, Microcystis aeruginosa Kütz and Scenedesmus quadricauda (Turp.), were conducted at three different nutrient levels. The biomass of algae in pure and mixed cultures was measured under conditions of different N/P ratios at oligotrophic, eutrophic and hypertrophic nutrient levels. The results indicated that the most suitable state for the growth and reproduction of M. aeruginosa and S. quadricauda were eutrophic conditions in both pure and mixed cultures. Under competition, however, the lower medium nutrient levels favoured M. aeruginosa, while the higher medium nutrient levels better suited S. quadricauda. Under similar hydrodynamic conditions, the community structure of phytoplankton in the water body was determined by the dominant species in competition for nutrients.

Mussel grazing and the importance of hydrodynamic coupling: reply

Conroy is correct to note that our experimental study did not take into account how hydrodynamic coupling affected the ability of zebra mussels to graze algae. Unfortunately, the size of our mesocosms (18 L) did not allow for, nor were they designed to provide, a thorough quantification of benthic–pelagic coupling. Therefore, we agree with Conroy that the magnitude of zebra mussel effects on phytoplankton within individual systems will depend on site-specific factors such as turbulent mixing, mussel filtering efficiency, phytoplankton reproduction and sinking rates, and nutrient remineralization. As such, we made no attempts to directly quantify zebra mussel filtration or grazing rates in our study reservoir. Instead, we presented our research as “a preliminary study” and suggest that additional research is needed to address among other things “ . . . how zebra mussels affect algal responses to nutrient enrichment in experiments that better mimic natural conditions (e.g. large in situ reservoir enclosures).” With that said, however, we do not agree with Conroy when he states that our proposal is inconsistent with published examples of post-invasion increases in phytoplankton, benthic algae and cyanobacterial toxins. For example, he provides two references (Raikow et al., 2004; Conroy et al., 2005) to show that phytoplankton have increased following invasion. However, as cited in our original manuscript, there are also numerous examples of phytoplankton decreasing following invasion; even Raikow et al. (Raikow et al., 2004) reported that lakes with zebra mussels had lower water column concentrations of total phytoplankton biomass than lakes without zebra mussels. Furthermore, zebra mussels can “decouple” relationships between total phosphorus (TP) and chlorophyll a so that observed yields of algal biomass are less than predicted from TP concentrations (see Nichols et al., 2001 and other references cited in Dzialowski and Jessie, 2009). It is also important to note that we address how zebra mussels may impact both benthic algae (e.g. they may increase) and cyanobacteria (e.g. the effects may be nutrient dependent) in eutrophic reservoirs in light of recently published research [see Dzialowski and Jessie, 2009 for more details]. Combined, we believe that these studies are consistent with our predictions that “ . . . invaded reservoirs may exhibit lower yields of algal biomass than those predicted from TP concentrations . . . ” and that “ . . . the severity of algal bloom events may be lower in invaded reservoirs if zebra mussels are able to directly negate or mask the increasing effects of external nutrient pulses on algal biomass as we have shown here.” We would also like to respond to Conroy’s assertion that the effects of zebra mussels on phytoplankton are most apparent in the 1 m boundary layer at the bottom of lakes. While this appears to be true for the western basin of Lake Erie, it is also important to note that zebra mussels have reduced phytoplankton biomass in the epilimnion of stratified lakes and reservoirs as well (Yu and Culver, 2000; Raikow et al., 2004; Knoll et al., 2008). Therefore, it seems inappropriate to suggest that the effects of zebra mussels on phytoplankton will be restricted mainly to the benthos. As such, additional research is needed to determine how hydrodynamic coupling affects the ability of zebra mussels to impact systems other than the Great Lakes including reservoirs of the Central Plains which often have both highly mixed (e.g. riverine zone) and thermally stratified (e.g. lacustrine zone) regions.