Natural Phytoplankton Assemblages Research Articles

Heatwave‐mediated decreases in phytoplankton quality negatively affect zooplankton productivity

Abstract Climate change is expected to increase the frequency of extreme temperature events. The effect of heatwaves on phytoplankton is of particular concern because they are a key source of C, N, P and essential fatty acids to aquatic ecosystems. Laboratory studies have demonstrated that phytoplankton grown at warmer temperatures are a lower quality food source, but how heatwaves affect phytoplankton quality at the community scale is currently unclear. Here we address this knowledge gap by growing natural assemblages of freshwater phytoplankton at “ambient”, “constant warming” or “heatwave” conditions. We next fed these phytoplankton communities to natural assemblages of zooplankton to test the prediction that zooplankton that consume heatwave‐exposed phytoplankton will exhibit reductions in biomass. Our experiment demonstrated that zooplankton that consumed “heatwave” phytoplankton attained lower community biomass than those fed “constant warming” or “ambient” phytoplankton. Additionally, despite receiving similar total heat input, phytoplankton exposed to “heatwave” conditions contained lower C, N, P and fatty acid concentrations compared to phytoplankton grown in “constant warming” conditions. Correlations between zooplankton biomass and all measured phytoplankton traits revealed that decreases in zooplankton biomass were best explained by low quantities of C, N and monounsaturated fatty acids in “heatwave” phytoplankton. Our study demonstrates that the effects of heatwaves on phytoplankton quality are clearly distinct from those caused by constant warming temperatures and that heatwave‐mediated decreases in resource quality have immediate effects on consumer productivity. Read the free Plain Language Summary for this article on the Journal blog.

Effects of bifenthrin on microcrustaceans grazing behavior on a phytoplankton assemblage dominated by Cyanobacteria.

Insecticides are widely used for pest control and frequently reach aquatic systems, imposing a risk to the biota. In this work, the effect of environmental concentrations of bifenthrin on the grazing capacity of Simocephalus vetulus (Cladocera) and Argyrodiaptomus falcifer (Copepoda) on phytoplankton was evaluated. Fifteen microcrustacean individuals and a natural phytoplankton assemblage dominated by Cyanobacteria were exposed during 46 h to three concentrations of bifenthrin (C0 0 μg L-1, C1 0.02 μg L-1, and C2 0.05 μg L-1). A significant decrease in both microcrustaceans grazing rates on total phytoplankton was observed in C2 compared to C0 and C1. The filtration rate (ml ind-1 h-1) of S. vetulus decreased significantly for the cyanobacteria Anabaenopsis arnoldii, Dolichospermum circinale, and Glaucospira sp. in C2 compared to C0 and C1. The ingestion rate (org ind-1 h-1) of A. falcifer decreased significantly in C1 and C2 compared to C0 only for A. arnoldii. Regarding phytoplankton morphological groups, the filtration rate of S. vetulus decreased in C1 and C2 compared to C0 for Colonies and Coenobiums in C2 concerning C0 and C1 for Filaments and in C2 compared to C0 for Silicified. For A. falcifer, the ingestion rate was reduced in C2 compared to C0 for Silicified, Flagellated, and Sessile. The results showed that bifenthrin affected both microcrustaceans grazing capacity on phytoplankton, especially at the highest insecticide concentration.

Production of ice-nucleating particles (INPs) by fast-growing phytoplankton

Abstract. Sea spray aerosol contains ice-nucleating particles (INPs), which affect the formation and properties of clouds. Here, we show that aerosols emitted from fast-growing marine phytoplankton produce effective immersion INPs, which nucleate at temperatures significantly warmer than the atmospheric homogeneous freezing (−38.0 ∘C) of pure water. Aerosol sampled over phytoplankton cultures grown in a Marine Aerosol Reference Tank (MART) induced nucleation and freezing at temperatures as high as −15.0 ∘C during exponential phytoplankton growth. This was observed in monospecific cultures representative of two major groups of phytoplankton, namely a cyanobacterium (Synechococcus elongatus) and a diatom (Thalassiosira weissflogii). Ice nucleation occurred at colder temperatures (−28.5 ∘C and below), which were not different from the freezing temperatures of procedural blanks, when the cultures were in the stationary or death phases of growth. Ice nucleation at warmer temperatures was associated with relatively high values of the maximum quantum yield of photosystem II (ΦPSII), an indicator of the physiological status of phytoplankton. High values of ΦPSII indicate the presence of cells with efficient photochemistry and greater potential for photosynthesis. For comparison, field measurements in the North Atlantic Ocean showed that high net growth rates of natural phytoplankton assemblages were associated with marine aerosol that acted as effective immersion INPs at relatively warm temperatures. Data were collected over 4 d at a sampling station maintained in the same water mass as the water column stabilized after deep mixing by a storm. Phytoplankton biomass and net phytoplankton growth rate (0.56 d−1) were greatest over the 24 h preceding the warmest mean ice nucleation temperature (−25.5 ∘C). Collectively, our laboratory and field observations indicate that phytoplankton physiological status is a useful predictor of effective INPs and more reliable than biomass or taxonomic affiliation. Ocean regions associated with fast phytoplankton growth, such as the North Atlantic during the annual spring bloom, may be significant sources of atmospheric INPs.

Response of Phytoplankton Communities to Variation in Salinity in a Small Mediterranean Coastal Lagoon: Future Management and Foreseen Climate Change Consequences

Mediterranean coastal lagoons are particularly vulnerable to increasing direct anthropogenic threats and climate change. Understanding their potential responses to global and local changes is essential to develop management strategies adapted to these ecosystems. Salinity is a fundamental structuring factor for phytoplankton communities; however, its role under climate change is understudied. We hypothesized that salinity variations imposed by climate change and/or management actions could disturb Mediterranean lagoons’ phytoplankton communities. To test our hypothesis, we performed two 5-day microcosm experiments in which natural phytoplankton assemblages from the Santa Giulia lagoon (Corsica Island) were subjected to three increasing (53–63–73) and decreasing (33–26–20) levels of salinity, to mimic strong evaporation and flash flooding, respectively. Results indicate that over-salinization inhibited growth and modified the assemblages’ composition. Freshening, on the contrary, showed feeble effects, mainly boosting microphytoplankton abundance and depleting diversity at lowest salinity. In both experiments and under freshening in particular, initially rare species emerged, while photosynthetic activity was degraded by salinity increase only. We demonstrated that phytoplankton communities’ structure and metabolism are strongly altered by the predicted implications of climate change. Such impacts have to be considered for future management of coastal lagoons (control of sea exchanges and watershed fluxes). This work constitutes a priority step towards the proactive adapted management and conservation of such as-yet-neglected ecosystems in the context of climate change.

Co-Limitation of Phytoplankton by N and P in a Shallow Coastal Lagoon (Ria Formosa): Implications for Eutrophication Evaluation

AbstractThe Ria Formosa coastal lagoon is a highly productive shallow ecosystem in southern Portugal, subjected to nutrient inputs from anthropogenic and natural sources. Nutrients are major abiotic drivers of phytoplankton in this system, but their effects on phytoplankton assemblages and the occurrence of nutrient limitation are still poorly understood. The main goal of this study was, thus, to evaluate the occurrence, type, and effects of nutrient limitation on phytoplankton community and specific functional groups in the Ria Formosa coastal lagoon. We conducted nutrient enrichment experiments with factorial additions of nitrogen (N) and phosphorus (P) using natural phytoplankton assemblages from distinct locations in the Ria Formosa, throughout a yearly cycle. Phytoplankton composition and abundance were evaluated using inverted and epifluorescence microscopies, and spectrophotometric methods were used for biomass. Limitation was defined as higher phytoplankton growth following enrichment with a particular nutrient in relation to the non-enriched control. The most common type of phytoplankton limitation was simultaneous co-limitation by N and P; diatoms, as r-strategists, were the most frequently limited group. Single N and P limitation, and serial P limitation were also observed, as well as negative responses to nutrient enrichment. Group-specific responses to nutrient enrichment were not reflected in the relative abundance of phytoplankton groups within the whole assemblage, due to the numerical dominance of pico-sized groups (cyanobacteria and eukaryotic picophytoplankton). Ambient nutrient ratios and concentrations did not predict phytoplankton nutrient limitation, given the different nutrient utilisation traits among phytoplankton functional groups. Therefore, nutrient ratios should not be used as indicators of nutrient limitation in eutrophication assessment.

Nitrogen limitation prevents the effects of iron or dust additions on biological carbon fixation in the Gulf of California

This paper reports the effect of dissolved iron (dFe) and dust additions on the Biological Carbon Fixation (BCF) rates in the surface layer of the central Gulf of California (GC). Two on deck experiments were conducted in the summer of 2016 at an oceanographic station located in the Guaymas Basin. Each experiment consisted in the incubation of natural phytoplankton assemblages collected at three distinct depths within the upper 50 m layer and amended either with iron (FeCl3) or dust leachate, followed by the quantification of BCF rates using the 13C assimilation method. A very warm (∼30 °C) water column strongly stratified and with reduced phytoplankton biomass (<1 mg chlorophyll a m−3) was indicative of an oligotrophic environment. In addition, the surface levels of NO3−+NO2− (<0.1 μM), dFe (1.71 ± 1.04 nM), as well as the Fe:N (78 ± 47 nmol μmol−1) and N:P (0.04 ± 0.01) ratios revealed that phytoplankton was under conditions of inorganic nitrogen limitation. BCF rates measured during the experiments ranged between 0.46 ± 0.04 and 6.24 ± 0.34 mg C m−3 h−1 with the highest values at the maximum of fluorescence and their vertical distribution was associated with the carbon biomass of picophytoplankton and diatoms. Despite the oligotrophic condition that predominated during the cruise, most of the Fe or dust additions did not show a significant effect (p > 0.05) on the magnitude of BCF rates, which was attributed to the fact that N, rather than Fe, was the limiting nutrient for phytoplankton production. However, Fe limitation cannot be completely ruled out as two samples (one for each treatment) showed a significant increase (p < 0.05) in BCF rates. We suggest that atmospheric supply of dust during the summer could play a relevant biological role maintaining high concentrations of dFe (and other bioactive metals) in the surface waters of central GC.

Effect of Mytilus coruscus selective filtration on phytoplankton assemblages

The feeding selectivity of bivalves can play an important role in shaping the structure of phytoplankton communities of natural waters. This could be particularly true in waters with intensive bivalves farming, like Sungo Bay, Northern China. Understanding the role of bivalve feeding behavior is important for assessing how the dense cultivation of bivalves may affect phytoplankton community composition and food web structure in farm areas. In this study, we investigated the feeding selectivity of blue mussel Mytilus coruscus on natural phytoplankton assemblages in Sungo Bay using both optical microscopy and HPLC-pigment analysis. Results showed that cryptophytes dominated the phytoplankton community and made up 66.1% of the total phytoplankton abundance. A comparison of phytoplankton composition between natural and filtered seawater showed that M. coruscus preferred cryptophytes and dinoflagellates than Chaetoceros spp. and Skeletonema spp. Cryptophytes were not detected in gut contents by microscopic observation, while their marker pigment alloxanthin was present, suggesting they were also consumed by M. coruscus and can be readily digested. This highlights the shortcomings of microscopic methods and the significance of HPLC-pigment analysis in obtaining a comprehensive understanding of feeding selectivity of bivalves. The proportions of Chaetoceros spp. and Skeletonema spp. in gut contents were significantly lower than their proportions in the seawater, and contrastingly, the proportions of Cocconeis spp. and Pinnularia spp. showed opposite patterns. The marker pigments prasinoxanthin and zeaxanthin were detected in the gut of M. coruscus indicating that picophytoplankton (e.g., prasinophytes and Synechococcus) are also food sources for this bivalve. This information furthers our understanding of bivalve aquaculture and environment interactions.

Effects of TiO2 and ZnO nanoparticles on the growth of phytoplankton assemblages in seawater

Compounds in sunscreen such as ultraviolet (UV) filters protect human skin from damage caused by UV radiation exposure. However, sunscreen components reach marine ecosystems after their release from human skin during activities such as swimming and washing, and are potentially toxic to marine organisms. TiO2 and ZnO nanoparticles (NPs) are commonly used as inorganic UV filters. In this study, we explored the effects of TiO2 and ZnO NPs on natural phytoplankton assemblages in coastal seawater. Growth rates of natural phytoplankton assemblages were significantly decreased by 10 mg L−1 TiO2 and 1 and 10 mg L−1 ZnO NP treatments. NP addition also modified the size structure of phytoplankton assemblages, and small phytoplankton (mainly cyanobacteria) are vulnerable to NPs. Because herbivore food preferences depend strongly on algal cell size, NP contamination could also affect higher trophic levels. Notably, small phytoplankton are an important component in microbial loop, and this energy transfer pathway may be more vulnerable to NP contamination.

Improvement of In Vivo Fluorescence Tools for Fast Monitoring of Freshwater Phytoplankton and Potentially Harmful Cyanobacteria.

The use of multi-wavelength spectrofluorometers for the fast detection of algal taxa, based on chlorophyll a (Chl-a) emission spectra, has become a common practice in freshwater water management, although concerns about their accuracy have been raised. Here, inter-laboratory comparisons using monoalgal cultures have been performed to assess the reliability of different spectrofluorometer models, alongside Chl-a extraction methods. Higher Chl-a concentrations were obtained when using the spectrofluorometers than extraction methods, likely due to the poor extraction efficiencies of solvents, highlighting that traditional extraction methods could underestimate algal or cyanobacterial biomass. Spectrofluorometers correctly assigned species to the respective taxonomic group, with low and constant percent attribution errors (Chlorophyta and Euglenophyceae 6-8%, Cyanobacteria 0-3%, and Bacillariophyta 10-16%), suggesting that functioning limitations can be overcome by spectrofluorometer re-calibration with fresh cultures. The monitoring of a natural phytoplankton assemblage dominated by Chlorophyta and Cyanobacteria gave consistent results among spectrofluorometers and with microscopic observations, especially when cell biovolume rather than cell density was considered. In conclusion, multi-wavelength spectrofluorometers were confirmed as valid tools for freshwater monitoring, whereas a major focus on intercalibration procedures is encouraged to improve their reliability and broaden their use as fast monitoring tools to prevent environmental and public health issues related to the presence of harmful cyanobacteria.

Occurrence of three dominant epibenthic dinoflagellates (Ostreopsis spp., Coolia monotis and Prorocentrum lima) in relation to biotic substrates and environmental factors in a highly dynamic ecosystem, the Strait of Gibraltar (Southwestern Mediterranean).

No studies have been carried out on the benthic harmful algal blooms (BHABs) along the Strait of Gibraltar in the Mediterranean, and little is known about the diversity of blooming species. Here, epibenthic dinoflagellates were monitored at least biweekly over 18 months (May 2019–November 2020) in Oued Lihoud, Cap Malabata and Dalia on the thalli of five dominant macrophytes and in the water column. This is the first report on the seasonal distribution of BHAB species hosted by natural biotic substrates in the Strait of Gibraltar, which is known for high hydrodynamics, major entry of Atlantic waters and important maritime traffic. Three BHAB dinoflagellates were observed in the surveyed areas: Ostreopsis spp., Coolia monotis and Prorocentrum lima. The analysis of all data at the three sites showed that Dictyota dichotoma was the most favourable macroalgae host for these benthic dinoflagellates. The highest cell densities were observed in Cap Malabata for Ostreopsis spp. (2.7 × 105 cells/g fresh weight in September 2020), P. lima (4.57 × 104 cells/g FW in September 2020) and C. monotis (4.07 × 104 cells/g FW in June 2019). Phosphate and temperature were positively correlated to the abundances of the studied thermophilic BHAB species. In contrast, negative correlations were recorded with salinity, ammonium, nitrite, nitrate, DIN, nitrogen/phosphate ratio and suspended material, attesting of the complex relationships between environmental factors and BHAB species dynamic in each marine ecosystem. Toxin analyses of the natural phytoplankton assemblage during BHABs showed the presence of only lipophilic toxins, namely okadaic acid and dinophysistoxins produced by P. lima. These BHABs species have to be isolated to establish monoclonal cultures for ribotyping and ecophysiological investigations.

Phytoplankton growth and community shift over a short-term high-CO2 simulation experiment from the southwestern shelf of India, Eastern Arabian Sea(summer monsoon).

The southwestern shelf water of India (eastern Arabian Sea) experiences high seasonality. This area is one of the understudied regions in terms of phytoplankton response to the projected ocean acidification, particularly, during the summer monsoon when phytoplankton abundance is high. Here we present the results of a short-term simulated ocean acidification experiment (ambient CO2 424 µatm; high CO2, 843, 1138 µatm) on the natural phytoplankton assemblages conducted onboard (R. V. Sindhu Sadhana) during the summer monsoon (Aug 2017). Among the dissolved inorganic nutrients, dissolved silicate (DSi) and nitrate + nitrite levels were quite low (< 2µM). Phytoplankton biomass did not show any net enhancement after the incubation in any treatment. Both marker pigment analysis and microscopy revealed the dominance of diatoms in the phytoplankton community, and a significant restructuring was noticed over the experimental period. Divinyl chlorophylla (DVChla) containing picocyanobacteria and 19'-hexanoyloxyfucoxanthin (19'HF) containing prymnesiophytes did not show any noticeable change in response to CO2 enrichment. A CO2-induced positive growth response was noticed in some diatoms (Guinardia flaccida, Cylindrotheca closterium, and Pseudo-nitzschia sp.) and dinoflagellates (Protoperidinium sp. and Peridinium sp.) indicating their efficiency to quickly acclimatize at elevated CO2 levels. This is important to note that the positive growth response of toxigenic pennate diatoms like Pseudo-nitzschia as well as a few dinoflagellates at elevated CO2 levels can be expected in the future-ocean scenario. The proliferation of such non-palatable phytoplankton may impact grazing, the food chain, and carbon cycling in this region.

Quantifying impacts of titanium dioxide nanoparticles on natural assemblages of riverine phytobenthos and phytoplankton in an outdoor setting

Impacts of widespread release of engineered titanium dioxide nanoparticles (nTiO2) on freshwater phytoplankton and phytobenthic assemblages in the field, represents a significant knowledge gap. Using outdoor experiments, we quantified impacts of nTiO2 on phytoplankton and periphyton from UK rivers, applied at levels representative of environmentally realistic concentrations (0.05 mg/L) and hot spots of accumulation (5.0 mg/L). Addition of nTiO2 to river water led to rapid temporal size changes in homoagglomerates and many heteroaggregates of nTiO2 with cells in the phytoplankton, including green algae, pennate and centric diatoms, increasing settlement of some cells. Changes in phytoplankton composition were evident after 72-h resulting from a significant decline in the relative abundance of very small phytoplankton cells (1–3 μm), often accompanied by increases in centric diatoms at both concentrations. Significant changes detected in the composition of the phytobenthos after 12 days, following nTiO2 treatments, were not evident when using benthic diatoms alone after 56 days. A lack of inhibition in the maximum quantum yield (Fv/Fm) in phytobenthos after 72-h exposures contrasted with a significant inhibition in Fv/Fm in 75% of phytoplankton samples, the highest recorded in Rutile nTiO2 exposures at both concentrations of nTiO2. After 12 days, strong positive stimulatory responses were recorded in the maximum relative electron transport rate (rETRmax) and the maximum non-photochemical coefficient (NPQmax), in phytoplankton and phytobenthos samples exposed to the higher Anatase nTiO2 concentration, were not measured in Rutile exposed biota. Collectively, these results indicate that the Rutile phase of nTiO2 has more negative impacts on freshwater algae than the Anatase form, at specific time scales, and phytoplankton may be more impacted by nTiO2 than phytobenthos. We caution that repeated release of nTiO2, could lead to significant changes in riverine algal biomass and species composition, dependent on the phase and concentration of nTiO2.

Natural spawning, early development and first successful hatchery production of the bluestreak cleaner wrasse, Labroides dimidiatus (Valenciennes, 1839), with application of an inorganic fertilization method in larviculture

Bluestreak cleaner wrasse, Labroides dimidiatus, is one of the top ten most exported fish to the global trade in marine aquarium fish and a key species for reef fish diversity. However, knowledge on reproduction and early ontogeny of this species in captivity are scarce and incomplete. Daily spawning, early development and appropriate rearing condition (temperature and salinity) for the larviculture of L. dimidiatus were studied for the first time. We raised wrasse larvae by using an inorganic fertilization method to produce natural assemblages of phytoplankton and zooplankton, and compared it to the higher N: P ratio group and traditional rotifer-fed group. A single male dominated a harem of three females, spawning between late afternoon and dusk. L. dimidiatus spawned 305 times from 1 May 2011 to 30 April 2012, 86.9 ± 10.1% of which resulted in fertilized eggs. Fecundity ranged from 53 to 25,108 eggs per spawn with average hatching rate of 72.2 ± 22.6%. Fertilized eggs were spherical, buoyant and had a diameter of 0.68 ± 0.02 mm. Embryonic development lasted 19 h at 27.3 °C. Newly hatched larvae were 1.81 ± 0.06 mm in total length (LT) and had an oil globule in the ventroanterior area of the yolk sac. First feeding took places at 3 days post hatch (dph) which is marked by the widening of the gut. At 41 dph, transformation of larvae to the juvenile stage was completed at 12.60 ± 0.98 mm LT. On the scanning electron microscope observations, the fertilized egg membrane was smooth and lack surface sculpturing. The distribution density of pores was 61 pores 100 μm−2. The micropyle was circular or oval-shaped, and the diameter was 6.90–7.13 μm. Several neuromasts were found on the head and body surface of newly hatched larvae. The results showed that unicellular algae, zooplankton within the size ranges of 48–150 μm and larval survival of L. dimidiatus were all significantly (P < 0.05) higher in the fertilized treatment (maintaining the inorganic nitrogen and phosphorus concentrations of 700 μg N L−1 and 100 μg P L−1, respectively) compared with the others. These results indicated that the optimal conditions for embryo incubation and larval rearing of L. dimidiatus were combinations of temperature 27 °C and salinity 33 psu. Our results of this study may provide a keystone for further studies to determine the successful mass-scale production of juvenile L. dimidiatus.

Temporal and Spatial Signaling Mediating the Balance of the Plankton Microbiome.

The annual patterns of plankton succession in the ocean determine ecological and biogeochemical cycles. The temporally fluctuating interplay between photosynthetic eukaryotes and the associated microbiota balances the composition of aquatic planktonic ecosystems. In addition to nutrients and abiotic factors, chemical signaling determines the outcome of interactions between phytoplankton and their associated microbiomes. Chemical mediators control essential processes, such as the development of key morphological, physiological, behavioral, and life-history traits during algal growth. These molecules thus impact species succession and community composition across time and space in processes that are highlighted in this review. We focus on spatial, seasonal, and physiological dynamics that occur during the early association of algae with bacteria, the exponential growth of a bloom, and its decline and recycling. We also discuss how patterns from field data and global surveys might be linked to the actions of metabolic markers in natural phytoplankton assemblages.

Horseradish Essential Oil as a Promising Anti-Algal Product for Prevention of Phytoplankton Proliferation and Biofouling.

Increased proliferation of algae is a current problem in natural and artificial water bodies. Controlling nutrients is the most sustainable treatment of increased algal proliferation, however in certain cases, it is not sufficiently available, or it does not provide results fast enough. Chemicals derived from natural sources, which could be effective in low concentrations and are biodegradable, may have an advantage over conventional chemical treatments. The main aim of the present study was to investigate the anti-cyanobacterial and anti-algal properties of allyl-isothiocyanate-containing essential oil produced from horseradish roots with a complex approach of the topic: on laboratory strains of cyanobacteria and eukaryotic algae, on microcosms containing natural phytoplankton assemblages, and on semi-natural biofilms. The results show that acute treatment can significantly reduce the viability of all the tested cyanobacteria and eukaryotic algae. Results of microcosm experiments with natural phytoplankton assemblages show that horseradish essential oil from 7.1 × 10−6% (v/v) is applicable to push back phytoplankton proliferation even in natural assemblages. The individual number in the biofilm was dropped down to one-fifth of the original individual number, so 7.1 × 10−6% (v/v) and higher concentration of the essential oil can be considered as a successful treatment against biofouling.

Acutely Rising Temperature Reduces Photosynthetic Capacity of Phytoplankton Assemblages in Tropical Oceans: A Large-Scale Investigation

Climate changes interacting with human activities are raising the temperature in global oceans. To explore physiological responses of in situ phytoplankton assemblages to increasing temperatures, we conducted a shipboard experiment in tropical regions of the eastern Indian Ocean, Java Sea, and southern South China Sea. Throughout the surveyed areas, phytoplankton biomass (Chla) ranged from 0.09 to 0.86 μg L−1 (median, 0.22 μg L−1) in the surface and from 0.30 to 0.99 μg L−1 (median, 0.50 μg L−1) in maximal chlorophyll layer (DCM), respectively. Picophytoplankton that occupied 27–89% (79%) and 83–92% (88%) of total Chla in the surface and DCM layers, ranged from 0.32 × 104 to 23.10 × 104 cells mL−1 (3.69 × 104 cells mL−1) and from 7.44 × 104 to 25.70 × 104 cells mL−1 (12.60 × 104 cells mL−1), respectively. Synechococcus took up 30–97% (78%) of pico-cells compositions in the surface layer, while, in the DCM layer, Prochlorococcus took up 42–98% (91%). Moreover, the maximal photochemical quantum yield (FV/FM) of photosystem II (PS II) and the rapid light curve (RLC)-derived light utilization efficiency (α) were lower in the surface layer than that in the DCM layer, but the saturation irradiance (EK) was higher. In particular, we found that acutely rising temperature decreased the FV/FM and α in both the surface and the DCM layers but increased the absorption cross-section (σPSII) of PSII photochemistry. Our results clearly indicate that the presently rising temperature adversely affects the photophysiology of natural phytoplankton assemblages in tropical oceans.

Ocean warming and freshening effects on lipid metabolism in coastal Antarctic phytoplankton assemblages dominated by sub-Antarctic species

Marine phytoplankton can utilize different strategies to cope with ocean warming and freshening from glacial melting in polar regions, which are disproportionally impacted by global warming. In the present study, we investigated the individual and combined effects of a 4 °C increase in seawater temperature (T+) and a 4 psu decrease in salinity (S−) from ambient values on biomass, nutrient use, fatty acid composition and lipid damage biochemistry of natural phytoplankton assemblages from Potter Cove (25 de Mayo/King George Island, Antarctica). Experiments were conducted by exposing the assemblages to four treatments during a 7-day incubation period using microcosm located along shore from January 23 to 31, 2016. The N:P ratio decreased in all treatments from day 4 onwards, but especially under high temperature (T+). Lipid damage was mainly detected under S0T+ and S−T+ conditions, and it decreased when the production of the antioxidant α-tocopherol increased. This antioxidant protection resulted in a build-up of phytoplankton biomass, especially at T+. Under the combined effect of both stressors (S−T+), the concentration of ω3 fatty acids increased, potentially leading to higher-quality FA composition. These results, which were related to the dominance of sub-Antarctic species in phytoplankton assemblages, contribute to the understanding of the potential consequences of ocean warming and increase seawater freshening on the trophic webs of the Southern Ocean.

Preliminary Observations of the Effect of Temperature and Food Concentration on the Egg Production Rate and Hatching Success of Acartia amboinensis from the Central Red Sea.

The effects of temperature and food concentration on the egg production rate (EPR) of the tropical calanoid copepod Acartia amboinensis were studied from the coastal waters of the central Red Sea in March 2017. In the first experiment, adult females were incubated in glass bottles that were pre-filled with screened seawater containing a natural assemblage of phytoplankton. In the second experiment, the species were incubated in glass bottles that were enriched with different concentrations of Chaetoceros muelleri along with natural assemblages of phytoplankton. Both the experimental setups were then exposed to different temperatures (21, 24, 27, 30 and 33°C). The daily EPR varied significantly across different temperatures and the various food concentrations (p < 0.05). Within the natural food assemblage (Exp. 1), the EPR increased gradually to a peak mean of 13.7 eggs female-1 d-1 at 27°C, then declined as temperatures increased (at 30 and 33°C). In the second experiment when the water was enriched with algal culture, EPRs were significantly greater (maximum EPR: 63.9 eggs female-1 d-1 at 27°C) than those incubated in ambient water with natural food (maximum EPR: 17.4 eggs female-1 d-1 at 21°C). The hatching rate fluctuated between 42.4% and 88.6%. The present study revealed that the EPR of Acartia amboinensis responded rapidly to changes in food availability, suggesting an extreme food limitation in the central Red Sea.

Summer phytoplankton assemblages and carbon biomass in the northern south China sea

Phytoplankton assemblage composition, abundance, carbon biomass and distribution were studied in summer 2004 in the northern South China Sea (NSCS; 18°-22°N, 110°-117°E). This is the first report of natural phytoplankton assemblages which were analyzed by Utermöhl method and their biovolume conversion carbon biomass in the NSCS. A taxa of phytoplankton was identified, which belong to 4 classes and 76 genera, and mainly composed of Bacillariophyceae. Most of them belong to nanophytoplankton (Equivalent Sphere Diameter is 10–15 μm). Associated phytoplankton, with a share of 3% UPA (Utermöhl phytoplankton assemblage cell abundance), primarily consisted of Dinophyceae. Chrysophyceae and Cyanophyceae occurred in appreciable amounts only at upper layers waters in the NSCS. Combined the UPC (Utermöhl phytoplankton carbon) with PPC (picophytoplankton carbon) into the BPC (bulk phytoplankton carbon biomass), it ranged from 0.117 to 1467.172 μg C L−1, its average was 54.848 ± 197.578 μg C L−1. The ratios of PPC contribution to BPC ranged from 0.001% to 6.567%, its average was 0.667 ± 1.215%. This indicates that picophytoplankton contribution to phytoplankton carbon pool is much low than that of chlorophyll a partitioning. Two carbon biomass provinces, one was higher biomass province along continental surface and bottom layers, and the other one was lower biomass province in open sea waters, were clearly discerned along the isoline of 20 μg C L−1. The calculated budget amount BPC pool in the NSCS was 1.21 × 104 tC. The calculated Carbon: Chlorophyll a ratio (C: Chl a) in study area ranged from 0.21 to 217.648, its average was 37.31 ± 46.53. Several typical phytoplankton assemblages can be discerned based on Shannon-Wiener diversity index, Pielou's evenness and C: Chl a. Examination of phytoplankton abundances, biomass and environmental factors by multivariate techniques PCA (Principal Component Analysis) suggests BPC distribution in the NSCS was mainly controlled by salinity at Section along PRE (Pearl River estuary) to Dongsha Islands, and nitrate concentration at Section along the Yangjiang to Zhongsha Islands. The keystone phytoplankton species in the NSCS were mainly composed of large or chain form Bacillariophyceae, such as Pseudo-nitzschia delicatissima, Chaetoceros spp. and Asterionellopsis glacialis, and some Dinophyceae also take an important role in the NSCS, such as Gyrodinium spirale and Prorocentrum dentatum. In addition, the features of phytoplankton distribution were according to functional morphology.

Lake Phytoplankton Assemblage Altered by Irregularly Shaped PLA Body Wash Microplastics but Not by PS Calibration Beads

Microplastics are an emerging environmental pollutant, whose global ubiquity is becoming increasingly evident. Conventional wastewater treatment does not completely remove them, and there are growing concerns about microplastics in source water and post-treatment drinking water. Microplastics have been reported to alter the development, physiology, and behavior of various aquatic organisms; however, limited knowledge exists on their effect on natural phytoplankton communities. Many studies also use uniformly spherical plastic beads, while most scrub particles in consumer products and secondary microplastics in the environment have various shapes and sizes. We tested the effects of two types of microplastics, 50 µm polystyrene (PS) calibration beads and polylactic acid (PLA) plastic body wash scrub particles, and one type of plant-derived body wash scrub particle on a natural phytoplankton assemblage through a 7-day incubation experiment in a temperate, mesotrophic lake. The calibration beads and the plant-derived particles generally did not alter the taxonomic composition of the phytoplankton in the mesocosms, while the PLA body wash microplastics eliminated cryptophytes (p &lt; 0.001) and increased chrysophytes (p = 0.041). Our findings demonstrate differential effects of irregularly shaped PLA body wash microplastics vs. PS calibration beads on lake phytoplankters and empirically support potential bottom-up alteration of the aquatic food web by secondary microplastics.