Drivers of phytoplankton blooms in the northeastern Black Sea

Phytoplankton competition and resilience under fluctuating temperature

Environmental variability is an inherent feature of natural systems which complicates predictions of species interactions. Primarily, the complexity in predicting the response of organisms to environmental fluctuations is in part because species' responses to abiotic factors are non-linear, even in stable conditions. Temperature exerts a major control over phytoplankton growth and physiology, yet the influence of thermal fluctuations on growth and competition dynamics is largely unknown. To investigate the limits of coexistence in variable environments, stable mixed cultures with constant species abundance ratios of the marine diatoms, Phaeodactylum tricornutum and Thalassiosira pseudonana, were exposed to different temperature fluctuation regimes (n = 17) under high and low nitrogen (N) conditions. Here we demonstrate that phytoplankton exhibit substantial resilience to temperature variability. The time required to observe a shift in the species abundance ratio decreased with increasing fluctuations, but coexistence of the two model species under high N conditions was disrupted only when amplitudes of temperature fluctuation were high (±8.2°C). N limitation caused the thermal amplitude for disruption of species coexistence to become lower (±5.9°C). Furthermore, once stable conditions were reinstated, the two species differed in their ability to recover from temperature fluctuations. Our findings suggest that despite the expectation of unequal effect of fluctuations on different competitors, cycles in environmental conditions may reduce the rate of species replacement when amplitudes remain below a certain threshold. Beyond these thresholds, competitive exclusion could, however, be accelerated, suggesting that aquatic heatwaves and N availability status are likely to lead to abrupt and unpredictable restructuring of phytoplankton community composition.

Co-occurrence of chromophytic phytoplankton and the Vibrio community during Phaeocystis globosa blooms in the Beibu Gulf

Accumulating research evidence has revealed that harmful algal blooms (HABs) can substantially affect the community structures of phytoplankton and heterotrophic bacteria in marine ecosystems. However, little is known about their species-specific interactions between phytoplankton and heterotrophic bacteria during the HABs period and about their interaction shifts in response to blooms. From this perspective, we investigated the co-occurrence of chromophytic phytoplankton and Vibrio during Phaeocystis globosa blooms in the Beibu Gulf. The results showed that Vibrio communities were distinct during the blooms, and P. globosa blooms resulted in a decline in phytoplankton alpha diversity, revealing that the blooms could affect their community compositions. The regression lines between the Shannon indices and Bray-Curtis distances of phytoplankton and Vibrio showed positive correlations with each other (p < 0.001), suggesting that they may have intrageneric symbiotic interactions overall. In addition, network analysis further demonstrated that relationships between phytoplankton and Vibrio were dominated by positive correlations, and more interaction modules were observed during the blooms, revealing that the blooms intensified synergistic association and mutual symbiotic interactions between them. Environmental factors (SiO₃^2-, NH₄⁺, NO₃^- and TN,) and P. globosa density more deeply affected network interactions between phytoplankton and Vibrio during the periods of P. globosa blooms than those before the blooms and after the blooms. This study provided new insight to elucidate community structure and interaction relationships between phytoplankton and Vibrio in response to P. globosa blooms and their ecological effects in marine ecosystems.

Seasonal and Long-Term Variability of Coccolithophores in the Black Sea According to Remote Sensing Data and the Results of Field Investigations

Based on satellite data from the SeaWiFS, MODIS-Aqua, and MODIS-Terra scanners, the long-term dynamics of coccolithophores in the Black Sea and their large-scale heterogeneity have been studied. During the twenty years in May and June, mass development of coccolithophores population of different intensities was recorded annually. Summer blooms of coccolithophores reached peak levels in 2006, 2012, and 2017, after abnormally cold winters. It was noted that in conditions of low summer temperatures, the blooming of coccolithophores could be significantly reduced or acquire a local character (2004). In the anomalous cold summer of 2001, coccolithophore blooms was replaced by the mass growth of diatoms. Over twenty years, numerous signs of coccolithophores mass development in the cold season have been revealed. Winter blooms develop mainly in warm winters with periods of low wind activity. The formation of a thermocline and the surface layer’s stability are essential factors for initiating winter blooms of coccolithophores. It was noted that after the winter blooms of coccolithophores, their summer growth was poorly expressed. It is shown that during periods of rapid growth, the bulk of coccolithophores is concentrated in the upper mixed layer and thermocline. During the blooming period, the share of coccolithophores in phytoplankton biomass constituted 70–85%. The intensity of coccolithophore’s blooms is associated with the previous diatoms’ growth level. The effect of eddies circulation on the distribution and growth of coccolithophores is considered.

Interannual variability of Emiliania huxleyi blooms in the Barents Sea: In situ data 2014-2018

Using in situ data of 2014-2018, the summers Emiliania huxleyi blooms in the Barents Sea were studied. The blooms were recorded in the upper mixed layer in July and August every year, during which they spread to cover large areas and were associated with Atlantic waters. The E. huxleyi abundance revealed interannual variability, with the highest values (up to 12 × 10⁶ cells/L) in July 2016. Bloom is characterized by a sharp seasonal thermocline, water surface temperature of about 7.14-11.7 °C, low silicate (0.45 ± 0.08 μM) and nitrogen (0.74 ± 0.16 μM) concentration, high phosphorus concentration (0.09 ± 0.01 μM) and nitrogen to phosphorus ratio significantly below the Redfield ratio. Data confirming the hypothesis of limiting the growth of diatoms by nitrogen concentration are presented. When E. huxleyi bloomed, its biomass exceeded 70% of the total phytoplankton biomass, species diversity was low, and diatoms were practically absent, and dinoflagellates were usually represented by large species.

Nutrient addition bioassay and phytoplankton community structure monitored during autumn in Xiangxi Bay of Three Gorges Reservoir, China

The increasing freshwater ecosystem nutrient budget is a critical anthropogenic factor promoting freshwater eutrophication and episodic bloom of harmful algae which threaten water quality and public health. To understand how the eutrophic freshwater ecosystem responds in term of phytoplankton community structure dynamics to a sudden rise in nutrient concentrations, a microcosm study by nutrient addition bioassay was implemented in Xiangxi Bay (XXB) of Three Gorges Reservoir, China. Our results showed that dissolved trace elements supply adequately altered the phytoplankton community structure creating a regime shift from cyanobacteria-dominated to essentially Chlorophytes-dominated system, relative abundance (>70%). Combined N, P, and Si led to maximum growth stimulation accompanied by the highest chlorophyll yield (82.7 ± 14.01 μgL^-1) and growth rate (1.098 ± 0.12 μgL^-1d^-1). N separate additions resulted in growth responses which did not differ while P -addition differed significantly (p∠0.05) with the control justifying a P limited system. Si enrichment stimulated diatom growth, relative abundance (20.62%) and maximum utility rate (U_Si = 83.37 ± 0.33%). This study also reveals that increasing nutrient loading from anthropogenic sources adequately decrease the ecological diversity (H < 1) and community overlap (CC ≤ 0.5) intensifying competition and succession which then select the fast-growing taxa to dominate and expand. Result points to the need for multiple nutrient control of N, P and Si loading into XXB through a prudent nutrient management protocol for lasting bloom mitigation in the tributary bay.

Incorporation of Cochlodinium bloom-derived organic matter into a temperate subtidal macrobenthic food web as traced by stable isotopes

Harmful algal blooms involving the dinoflagellate Cochlodinium polykrikoides occur every summer off the Korean peninsula's central southern coast. To determine whether Cochlodinium bloom-derived organic carbon is incorporated into the subtidal macrobenthic food web, we compared the δ¹³C and δ¹⁵N values of suspended particulate organic matter (SPOM) and sedimentary organic matter, and macrobenthic consumers between bloom and non-bloom seasons. Chemotaxonomic analysis revealed the presence of Cochlodinium blooms in summer and a predominance of diatoms in autumn. Both the δ¹³C and δ¹⁵N values of SPOM were higher in the bloom than in the non-bloom seasons. Such temporal shifts in the δ¹³C and δ¹⁵N values were also observed for most macrobenthic consumers collected in both seasons. Consistent temporal isotopic shifts in SPOM and macrobenthos revealed that the Cochlodinium bloom-derived carbon was incorporated into the coastal benthic food web, resulting from its increasing availability during blooms.