This article reports the development of an underwater imaging system and its trial on a moored surface buoy for in situ plankton monitoring of coastal waters. The imager features shadowless white light illumination by an orthogonal lamellar lighting design, resulting in high-quality underwater darkfield color imaging of planktonic particles in the size range of ∼200 μ m to 40 mm and effective reduction of zooplankton phototaxis. Through raft and buoy trials, 46 804 plankton and suspending particle images have been annotated through a human–machine mutually assisted effort into a data set with 90 categories. In the meanwhile, a deep learning model based on a triclassification VGGNet-11 and multiclassification ResNet-18 convolutional neuron networks in a two-staged hierarchy has also been trained and developed. The model has been applied with human supervision to semiautomatically analyze a total of 1 545 187 images obtained from a buoy trial for six months from late spring to early winter of 2020. The high temporal resolution results well documented the variation of the mesoplankton community structure in two time series of 38 days in summer and 54 days in autumn of the target sea region. In addition, the dominant species in the trial period and a zooplankton outbreak that had threatened the safety of the nearby nuclear power plants were quantitatively analyzed. The system is expected to become a new entry into the toolkit for marine surface buoy platforms to upscale their capabilities for more comprehensive in situ plankton monitoring.

Summer hypoxia and harmful algal bloom occurred sometimes in the nearshore of the northern Yellow Sea in recent years. Based on seven multidisciplinary investigations conducted from March to November 2016, except for April and October, the phytoplankton community and its association with ambient seawater physicochemical parameters in coastal waters of the northern Yellow Sea were comprehensively examined. In total, 39 taxa belonging to 4 phyla and 24 genera were identified. Diatoms and dinoflagellates were the dominant groups, which accounted for 64.1% and 30.8% of total species, respectively. An algal blooming event dominated by the diatom (Thalassiosira pacifica) occurred in March, which affects the shifting of diatom–dinoflagellate dominance. A notable dinoflagellate dominance occurred especially in surface water throughout the whole summer but changed to diatom dominance again from September. Hypoxic zones (<2 mg l^-1)were observed in the bottom water in August, with minimum dissolved oxygen (DO) of 1.30 mg l^-1. This low DO zone in August was clearly associated with the diatom blooming event (Thalassiosira pacifica) in March, as diatoms in surface waters sank into bottom waters and decomposed by the microbial community resulting in oxygen consumption. After the early-spring diatom bloom, thermohaline stratification occurred and prevented exchanges of dissolved oxygen, which eventually led to hypoxia in bottom waters. The effects of algal blooms on phytoplankton composition and hypoxia could have a cascaded effect on the fishery sustainability and aquaculture in nearshore waters of the northern Yellow Sea.

The feeding relationship between copepods and phytoplankton has immense ecological significance. This study investigated the feeding behavior of copepods by studying the feeding selectivity of Paracalanus parvus, a key small copepod species, using a high-speed camera. The feeding behavior of P. parvus separately fed on three algae, Prorocentrum minimum, Alexandrium minutum, and Thalassiosira weissflogii, was studied at five different concentrations. The factors characterizing feeding behavior, including the beating frequency (BF), beating time (BT), and rejection behavior, were analyzed. The average BT and BF of P. parvus fed on toxic algae were significantly lower than those of copepods fed on nontoxic algae, indicating that the toxic algae negatively affected their feeding behavior. There were no significant differences in feed rejection among the three algae during the short period of experimentation, indicating that the rejection behavior was insignificant in the early period (within 20 min) of feeding on toxic algae. The feeding behavior was inhibited when the concentration reached 250 cells/mL. The BT was initially affected at increasing concentrations followed by the BF, and P. minimum and A. minutum reduced the BF at concentrations of 250 and 1000 cells/mL, respectively. Analysis of the average BFs revealed that P. parvus was more significantly affected by P. minimum containing diarrheal shellfish poison than by A. minutum containing paralytic shellfish poison. The BF of copepods fed on P. minimum was significantly lower than that of copepods fed on A. minutum at 250–500 cells/mL but was not significantly different from that at 1000 cells/mL. This indicated that the inhibitory effect of P. minimum on the feeding behavior was more significant at concentrations observed at the onset of red tide blooms (0.25–0.5 × 10² cells/mL), but insignificant at concentrations reaching those in advanced red tides (>10³ cells/mL). This study demonstrates that toxic dinoflagellates alter the feeding behavior of copepods and describes the variations in their feeding response to different algal species and concentrations. The findings provide crucial insights for further studies on the feeding relationship between copepods and phytoplankton and on functional assessment of plankton ecosystems.

The mullet Liza haematocheila is widely distributed in low-salinity waters around the world and has high economic value. However, details regarding the foraging ecology of mullet larvae remain unclear. Larvae of L. haematocheila were sampled in Laizhou Bay of the Bohai Sea, China, in May 2016, and diet composition was detected using gut content analysis to compare differences in feeding parameters and diet shift in dominant prey during ontogeny. The results showed no linear relationship between gape size (GS) and standard length (SL) at larva length 7 mm (post-flexion larvae, PFL). Maximum prey size (MPS) overlapped with GS during the FL stage but was never higher than GS during the PFL stage. Trends of increasing MPS and prey size range (PSR) during the PFL stage were lower than those during the FL stage, but prey number (PN) increased significantly during the PFL stage. Diet composition analyses in mullet larvae showed a total of 10 mesozooplankton species (or categories), of which 8 species were copepods (including copepods nauplii and copepodites), and showed the dominance of 4 small copepods (<1 mm). Analyses of the numerical proportion of dominant copepods showed that the largest prey (Paracalanus parvus) gradually increased as GS increased; conversely, the smallest prey (nauplii of Calanus sinicus) decreased. Collectively, these results suggest that PFL tends to exhibit increased PN but not prey size or size range, and diet shifts from smaller to larger prey during ontogeny in mullet larvae. All these indicate that PFL has higher prey selection ability compared with FL, specifically switching the diet to include larger small copepods during the PFL stage and increasing the prey number instead of increasing prey size. These determine the importance of small copepods in mesozooplankton as dominant prey and facilitate predictions of the impact of climate change on mesozooplankton and fish larvae.