There is considerable inconsistency in results pertaining to the biomagnification of PAHs in aquatic systems. Zooplankton specifically play an important role controlling the fate and distribution of organic contaminants up the food chain, particularly in large plateau reservoirs. However, it remains largely unknown how secondary factors affect the magnification of organic compounds in zooplankton. The present study assessed plankton species and nutrients affecting the trophic transfer of PAHs through the micro-food chain in plateau reservoirs, Guizhou Province China. Results show soluble ∑PAHs range from 99.9 - 147.3 ng L-1, and concentrations of ∑PAHs in zooplankton range from 1003.2 - 22441.3, with a mean of 4460.7 ng g-1 dw. Trophic magnification factors (TMFs) > 1 show biomagnifications of PAHs from phytoplankton to zooplankton. The main mechanisms for trophic magnification > 1 are 1) small Copepoda, Cladocera and Rotifera are prey for larger N. schmackeri and P. tunguidus, and 2) the δ15N and TLs of zooplankton are increasing with the increasing nutrients TN, NO3- and CODMn. As a result, log PAHs concentrations in zooplankton are positively correlated with the trophic levels (TLs) of zooplankton, and log BAFs of the PAHs in zooplankton are increasing with increasing TLs and log Kow. Temperature further enhances TMFs and biomagnifications of PAHs as noted by temperature related reductions in δ15N. There are also available soluble PAHs in the water column which are assimilated with increasing phytoplankton biomass within the taxa groups, diatoms, dinoflagellates and chlorophytes. Notable TMFs of PAHs in zooplankton in Guizhou plateau reservoirs are not significantly affected by phytoplankton and zooplankton biomass dilutions. The present study demonstrates the important roles of species selection, nutrients and temperature in the environmental fate of PAHs in freshwaters.

This paper presents the results of the research on the overall distribution of selenium (Se) in various aquatic compartments (water, sediment, plankton and macrophytes) at six selected sites of the Croatian part of the Drava and Danube rivers, the connected floodplain lake and the melioration channel system carried out in two sampling periods (flooding in June and the drought period in September). In addition, the physicochemical water properties, plankton composition and biomass were analysed. Our study revealed low mean Se contents in sediments and water, indicating Se deficiency in the studied freshwater systems. The physicochemical environment, including Se distribution, was primarily influenced by hydrology rather than site-specific biogeochemical and morphological characteristics. The flooding period was characterised by higher Se content in water and higher transparency, nitrate and total nitrogen concentrations than drought conditions. At the river sites, sediment Se content was the highest during the flood period, while at all other sites, higher concentrations were found during the drought, reaching the maximum in the lake. Although Se concentrations were below the threshold for aquatic ecotoxicity, they increased in the following order: water (0.021-0.187 μg Se L-1) < sediments (0.005-0.352 mg Se kg-1) < macrophytes (0.010-0.413 mg Se kg-1) < plankton (0.044-0.518 mg Se kg-1) indicating its possible biomagnification at the bottom of the food chain. Species known for high Se accumulation potential dominated the biomass of the main plankton groups and the composition of the macrophyte community, which may provide a more sensitive and accurate steady-state compartment monitor for Se assessment in freshwater biotopes.

The Barents Sea is a highly productive ecosystem within the Arctic Ocean. The overall biological productivity in this region relies heavily on the secondary mesozooplankton production (MZP). Previous research has primarily focused on mesozooplankton abundance (MZA) and biomass (MZB), lacking a comprehensive analysis of the environmental factors that influence MZP in the Arctic marine environment. The primary objective of this study was to examine the key factors responsible for the spatial variability in the community structure and MZP during the summer season. Data were collected from 52 stations in the central Barents Sea, specifically during post-bloom conditions in 2015. Through cluster analysis, two distinct groups of stations were identified, differing in terms of mesozooplankton taxa abundance. Copepods were found to dominate the mesozooplankton assemblages, comprising 89% of total MZA, 83% of MZB, and 68% of MZP. The biomass stocks in the study area varied from 8 to 102 mg dry mass (DM) m-3, with an average of 44 mg DM m-3. MZP rates ranged from 0.34 to 2.33 mg DM m-3 day-1, with an average of 1.16 mg DM m-3 day-1. The highest MZB and MZP values were observed at frontal zones that separated relatively warm and cold waters. Through redundancy analysis, it was determined that the primary environmental factors affected the distribution of mesozooplankton were longitude, latitude, and sampling depth. Temperature, salinity, and chlorophyll a concentration were found to have a less significant impact. These findings emphasize the importance of oceanographic conditions as the main predictors of mesozooplankton distribution during the summer season in the Arctic marine environment. This study highlights the essential role of environmental forcing in determining the productivity of Arctic marine zooplankton. Given the ongoing climatic changes, the results of this report can serve as a valuable tool for monitoring pelagic ecosystems in the Arctic.

The Late Cretaceous was a unique period in the history of the Earth characterized by elevated sea levels, reduced land area, and significantly high concentrations of atmospheric CO2 resulting in increased temperatures across the globe-a \'Greenhouse World\'. During this period, calcareous dinoflagellate cysts (c-dinocysts) flourished and became a ubiquitous constituent of calcifying plankton around the world. An acme in calcareous dinocysts during the Albian to the Turonian coincided with the highest recorded seawater surface temperatures and was possibly linked to conditions that favored calcification and a highly oligotrophic system in European shelf seas. This study examines the potential applicability of c-dinocysts as a proxy for paleoenvironmental conditions based on their assemblage changes plotted against foraminiferal occurrences and microfacies analysis. The material was extracted from the upper Turonian chalk of the Dubivtsi region in western Ukraine. An inverse correlation was observed between species diversity and the number of c-dinocyst specimens. Nutrient availability gradients apparently determined important changes in the calcareous dinocysts distribution. These trophic changes were likely caused by the interplay of eustatic sea-level fluctuations and Subhercynian tectonic activity leading to changeable nutrient inputs from the nearby land.

Predicting species distribution in the ocean has become a crucial task to assess marine ecosystem responses to ongoing climate change. In the Humboldt Current System (HCS), the endemic copepod Calanus chilensis is one of the key species bioindicator of productivity and water masses. Here we modeled the geographic distribution of Calanus chilensis for two bathymetric ranges, 0-200 and 200-400 m. For the 0-200 m layer, we used the Bayesian Additive Regression Trees (BART) method, whereas, for the 200-400 m layer, we used the Ensembles of Small Models (ESMs) method and then projected the models into two future scenarios to assess changes in geographic distribution patterns. The models were evaluated using the multi-metric approach. We identified that chlorophyll-a (0.34), Mixed Layer Depth (0.302) and salinity (0.36) explained the distribution of C. chilensis. The geographic prediction of the BART model revealed a continuous distribution from Ecuador to the southernmost area of South America for the 0-200 m depth range, whereas the ESM model indicated a discontinuous distribution with greater suitability for the coast of Chile for the 200-400 m depth range. A reduction of the distribution range of C. chilensis is projected in the future. Our study suggests that the distribution of C. chilensis is conditioned by productivity and mesoscale processes, with both processes closely related to upwelling intensity. These models serve as a tool for proposing indicators of changes in the ocean. We further propose that the species C. chilensis is a high productivity and low salinity indicator at the HCS. We recommend further examining multiple spatial and temporal scales for stronger inference.

Hydropower construction and climate change have aggravated river hydrological changes, which have reduced the water flow regime in the Ruhe River Basin. The reduced flow of the river seriously affected the water supply of nearby residents and the operation of the river ecosystem. Therefore, in order to alleviate the contradiction between water use for hydropower facilities and environmental water use, the urgent need is to explore the ecological flow-threshold of rivers. This study took the Fuhe River Basin as the research object, and summarized the monitoring data of eight hydrological stations from recent decades. Based on this, we explored the response law of P-IBI and flow, a tool to quickly measure the health of the ecosystem. Through the response relationship between alterations in environmental factors of the river and phytoplankton index of biotic integrity (P-IBI), it was determined that environmental flow was the dominant influencing factor of P-IBI. According to P-IBI, the threshold of environmental discharge in the Fuhe River was limited to 273~826.8 m3/s. This study established a regulatory framework for the river flow of large rivers by constructing P-IBI and determining the critical thresholds of environmental flow by constraining the constitution. These results provide a theoretical basis for better planning and improvement of river ecosystem restoration and river utilization.

In recent years, Deep Learning (DL) has been increasingly used in many fields, in particular in image recognition, due to its ability to solve problems where traditional machine learning algorithms fail. However, building an appropriate DL model from scratch, especially in the context of ecological studies, is a difficult task due to the dynamic nature and morphological variability of living organisms, as well as the high cost in terms of time, human resources and skills required to label a large number of training images. To overcome this problem, Transfer Learning (TL) can be used to improve a classifier by transferring information learnt from many domains thanks to a very large training set composed of various images, to another domain with a smaller amount of training data. To compensate the lack of \'easy-to-use\' software optimized for ecological studies, we propose the EcoTransLearn R-package, which allows greater automation in the classification of images acquired with various devices (FlowCam, ZooScan, photographs, etc.), thanks to different TL methods pre-trained on the generic ImageNet dataset.
EcoTransLearn is an open-source package. It is implemented in R and calls Python scripts for image classification step (using reticulate and tensorflow libraries). The source code, instruction manual and examples can be found at https://github.com/IFREMER-LERBL/EcoTransLearn.
Supplementary data are available at Bioinformatics online.

Planktonic microorganisms play an important role in urban aquatic ecosystems; however, environmental changes significantly affect their role in the degradation and transformation of pollutants. The highly artificial North Canal River was chosen as the research area in this study. Seasonal changes in planktonic microbial community structure were studied using 16S rRNA high-throughput sequencing. The seasonal change mechanism of planktonic microbial diversity in urban rivers supplied with reclaimed water and its response relationship with environmental parameters were examined. The results showed that there were significant seasonal changes in the diversity and structure of the planktonic microbial community. The alpha diversity in summer was significantly higher than that in spring, owing to the enhancement of water diffusion capacity caused by seasonal rainfall and physical disturbance of the reclaimed water supply. The beta diversity of the planktonic microbial community in summer was weakened compared to that in spring, also owing to the enhancement of water diffusion capacity. Seasonal runoff and temperature were the main driving factors of the seasonal variation in hydrology and water quality in the highly artificial urban river. The changes in NO2--N and TP caused by seasonal runoff and temperature change were the main reason for planktonic microbial diversity changes in the river. The reductive environment of the river was caused by static and discontinuous flow in the spring. Anaerobic bacteria such as Bacteroidetes related to the degradation of dissolved organic matter and Gracilibacteria related to the denitrification process were dominant in the river. Seasonal runoff and frequent rainfall in summer, as well as the increase in the opening and closing frequency of river sluice gates, enhanced the reoxygenation capacity of the river. This significantly alleviated nutrient pollution in the North River Cannel. Additionally, aerobic bacteria and facultative anaerobic bacteria were dominant species in the river during spring. Cyanobacteria with high temperature characteristics, Chloroflexi and other autotrophic microorganisms, as well as Acidobacteria and Gemmatimonadetes played an important role in the degradation and transformation of pollutants. The results of this study have practical significance for urban river pollution control and ecological restoration with reclaimed water as the recharge water source.

Oligotrophic sea area mainly distributes in tropical and subtropical ocean, which have a profound impact on the marine material cycle and the structure of biological community. Based on the comprehensive survey in M4 seamount area of the Tropical Western Pacific Ocean in August 2017, the nutrients characteristics were explained, the formation mechanism of oligotrophic characteristics in this region was explored, and the influence of M4 seamount on oligotrophic sea area was analyzed. The results showed that the M4 seamount area is a typical oligotrophic sea area. In the water column of 0-100 m, the nutrients levels are extremely low, and NO3-N, PO4-P and SiO3-Si are lower than 0.55, 0.15 and 1.75 μmol/L, respectively; in the water column of 100-500 m, the nutrients concentrations rise rapidly, forming nutriclines. Seawater stratification is one of the main reasons for the formation of oligotrophic characteristics in this area. The thermocline in the water column of 100-500 m and the high-salt area in the water column of 100-230 m form a dual effect, which hinders the upward transport of waters with high nutrients concentrations at the bottom. In addition, Synechococcus and bacteria grow and multiply in a large amount in the water column of 0-100 m, absorb and utilize nutrients, and aggravate the oligotrophic characteristics of this area. There are significant upwellings near the summit of the M4 seamount, which promotes the waters with high nutrients concentrations at the bottom to break through the barriers of the thermocline and high-salt area and to transport upwards into the euphotic layer, which can be better utilized by phytoplankton. This study showed that seamounts may have an important effect on improving the oligotrophic characteristics of local sea areas, and then promote the growth of phytoplankton.

The picoplankton is an important component of aquatic food webs and plays a significant biogeochemical and ecological role in the environment. Little is known about this fraction of the plankton in temperate estuaries and especially in South America. In this article, we study the absolute and relative importance of the picoplankton along an annual cycle, and their relationship with physical and chemical variables in the Río de la Plata estuary. We also review the existing research in estuaries around the world concerning this community and present our results in a global context. The seasonal variation in the abundance of the different groups analyzed was very noticeable. Phycocyanin-rich picocyanobacteria (Pcy) were the main component (in abundance and in biovolume) of the picophytoplankton (PPP) almost during the whole year, with a maximum abundance of 7.3 × 105 cell mL-1 in summer, three orders of magnitude higher compared to autumn-winter. Picoeukaryotes, larger phytoplankton and heterotrophic bacteria showed the same seasonal trend, although with a lower range of variation than that of the Pcy. Considering all the phototrophic planktonic fractions, in terms of biomass, the PPP reached a maximum of 43% of the total biomass in spring. The dynamics of PPP found in this area are consistent with the observed in other temperate estuaries, where temperature is the main variable that influences its development, and with a high seasonal variation. Additionally, the absolute and relative importance of Pcy showed a consistently increasing trend towards lower latitude estuaries. The review also showed us that there is scarce information related to the picoplankton fraction in the Southern Hemisphere, its sanitary implications due to their potential of toxicity or their ecological role in coastal zones. The results presented here show the importance of this fraction, not only in Río de la Plata, but in many estuaries of the world, with a clear increase of relative abundance as we approach the equator.