Algal biovolume is a standard measurement for limnological and oceanographic studies, which is used to proxy biomass and productivity. Manual and automatic methods can be applied to estimate biovolume from phytoplankton images. We examined the effects of algal shape and taxon (largely genus) on the accuracy of automatic image-based biovolume estimates for samples from a large oligotrophic embayment (Georgian Bay, Lake Huron). Specifically, we compared biovolume estimates based on equivalent spherical diameter, Feret-based diameter and two shape-based estimates (cylinder and prolate spheroid) against the manual biovolume measurements using correlation, linear regression and analysis of variance. The automatic biovolume estimates were all moderately to strongly correlated with the manual measurements, but the automatic cylinder method was the most accurate, overall. The automatic cylinder method overpredicted the manual total biovolume measurement of the community by 59%, but it had the strongest linear relationship with manual total biovolume, and the strongest correlation based on the biovolumes of individual genera. When algal taxa were assigned to 11 general shape categories, we observed significant differences in the accuracy with which they were measured. Results suggest that the cylinder biovolume method provides a reasonable estimate of total biovolume in algal communities that have a diversity of elongated shapes.

ABSTRACT Laboratory cultures are foundational to marine zooplankton and mixoplankton ecology because they enable controlled, replicated experiments, life-cycle closure and mechanistic inference for theory, modeling, aquaculture and ecotoxicology. Their transferability to nature, however, is filtered by a small number of processes. First, a cultivability bottleneck restricts long-term cultures to a biased subset of handling-tolerant taxa, leaving many oceanic, fragile, or diapause-programmed species underrepresented. This is further complicated by strain-specific divergence and cryptic species. Second, sustained laboratory regimes impose interacting effects of selection and domestication-like adaptation that can shift trait means and reaction norms, particularly for variance- and risk-sensitive traits. Third, chronic laboratory crowding constitutes a distinct selective and plasticity-shaping environment. Fourth, holobiont reassembly in culture alters host–microbiome interactions linked to feeding, detoxification and stress responses. Importantly, phenotypic responses measured in wild populations are also conditional on genetic structure, recent acclimation and carry-over effects; thus, neither cultured nor wild measurements represent neutral baselines. Because grazing and tolerance parameters derived from cultures and short-term field assays dominate uncertainty in ecosystem and biogeochemical models, uncalibrated bias in either direction can propagate into large projection errors. I outline some bridging strategies to map laboratory-derived traits explicitly onto wild trait space.

ABSTRACT Respiration, as one of the most important processes sustaining life of all organisms, has been the subject to extensive study. Measurements of respiration rates of Arctic copepods at their habitat temperatures are scarce. The present study filled this gap with measurements of respiration at −1.5 to 6°C in onboard experiments (428 measurements) in the Siberian Arctic seas. For the first time thermal dependence of respiration of Calanus glacialis and Calanus hyperboreus was obtained during the state of diapause, which makes up an important part of their life cycle. Activation energy in Calanus spp. was higher (0.85–0.87 eV) than usually assumed (0.6–0.7 eV). Higher thermal sensitivity is suggested to be related to a stenothermic life history mode. Carbon-specific respiration rate in diapausing Calanus spp. was 3.6–3.7 times lower than in active copepods, while thermal sensitivity (activation energy and Q10) was similar. Potential duration of diapause of the older stages of the Arctic Calanus spp. is more than a year at −1.5°C and decreasing with increasing temperature.

ABSTRACT The Southern Ocean (SO) plays a central role in regulating global heat and carbon exchange, yet the Indian sector remains one of its least observed regions despite spanning major oceanic fronts. Phytoplankton dynamics here are strongly influenced by light availability, nutrient supply and vertical mixing, all of which are being reshaped by rapid climate-driven changes. This study examines interannual variability in Subsurface Chlorophyll Maximum (SCM) characteristics and phytoplankton community structure during the austral summers of 2018 and 2020 across the Subtropical Front (STF), Subantarctic Front (SAF), Polar Front (PF) and the region south of the PF (SPF). Phytoplankton pigments were quantified using high-performance liquid chromatography (HPLC), and community composition was assessed via flow cytometry and microscopy. Modest interannual changes in upper-ocean structure resulted in pronounced differences in SCM depth, intensity and community composition. In 2018, shallow mixed layers (25–35 m) and deep euphotic zones (60–80 m) supported shallow, intense SCMs at the STF and SAF (Chlmax ≈ 1.4 mg m−3; Dmax 30–40 m), dominated by small flagellates and dinoflagellates. In 2020, deeper mixing (MLD 55–75 m) and reduced light penetration (Zeu 40–55 m) produced weaker, deeper SCMs at the STF (Chlmax 0.25–0.38 mg m−3), while the SAF developed a stronger, shallower SCM (Chlmax ≈ 1.6 mg m−3). Enhanced stratification at the PF and SPF in 2020 yielded stronger, shallower SCMs (Chlmax up to 0.74 mg m−3), accompanied by surface cooling and freshening indicative of melt water influence. Phytoplankton communities shifted accordingly: diatoms such as Fragilariopsis and Chaetoceros increased by 40–60% south of the PF in 2020, while picophytoplankton (Synechococcus, picoeukaryotes) peaked at the STF under warm, high-light conditions. Principal Component Analysis revealed a shift in dominant environmental controls—from density and salinity in 2018 to light and oxygen in 2020—indicating a transition from physically to radiatively regulated SCMs. These findings highlight the sensitivity of the Indian Sector of the Southern Ocean to small perturbations in stratification and light climate, with implications for regional carbon export, ecosystem functioning and future productivity under continued climate forcing.

ABSTRACT A large and small strain of a new taxon of Brachionus have very different morphological responses to a kairomone produced by the predatory rotifer Asplanchna. The large, less susceptible strain has no posterolateral spines without Asplanchna and only short (46 μm) ones, together with a larger lorica, with it. The small, more readily ingested strain has short (40 μm) posterolateral spines without Asplanchna and extremely long (103 μm) ones, together with a greater increase in lorica length, with it. In this rotifer, large body size provides an effective defense against Asplanchna predation that reduces selection for the possibly costly development of greater spine and lorica lengths in the presence of the predator.

ABSTRACTCordylophora caspia, is considered an invasive species in America. We isolated this hydrozoan from the brackish water section of the River Tuxpan (State of Veracruz, Mexico) and cultured it in the laboratory on mixed prey of copepods and rotifers at a salinity of 20 g L−1. C. caspia is tolerant of a wide range of salinities. Most studies on C. caspia report its feeding and population growth on Artemia nauplii. Here we tested the effect of salinity on functional response and prey preference on Brachionus plicatilis (Rotifera), Apocyclops panamensis (Copepoda, Cyclopoida) and Nitokra lacustris (Copepoda, Harpacticoida). Experiments were conducted at 25°C. The hydrozoan killed several prey during functional response studies, which we assumed that it would eventually consume. On all prey test species, C. caspia showed a Type II functional response. Prey consumption, in terms of biomass, and preferences was higher on copepods than on rotifers. Prey consumption was higher at salinities of 10 and 20 g L−1 than of 30 g L−1.

ABSTRACT Lake Maï-Ndombe is a large natural shallow lake of the Congo River basin. We investigated the composition and abundance of plankton and the diet of Nannothrissa stewarti that is at the basis of an important fishery. The lake is not stratified, phytoplankton being well developed during low water (LW) conditions with higher biomass (average value ~ 2.5 ± 1.1 μg chlorophyll a L−1) than in high water (HW) conditions (0.14 ± 0.1 μg chlorophyll a L−1). It is oligotrophic but with an autochthonous algal primary production, in contrast to earlier reports. The metazooplankton comprises 15 species (excluding bdelloid rotifers), among which the dipteran Chaoborus sp. Its biomass is low (~0.20 ± 0.1 g C m−2 in LW vs 0.130 ± 0.02 g C m−2 in HW). Based on gut contents analysis, Nannothrissa is a highly selective feeder in Lake Maï-Ndombe, feeding mostly on Cladocera despite their relatively low abundance. This suggests that only part of the metazooplankton production is being exploited by the sprat. However, stable carbon and nitrogen isotope data are inconsistent with the dominance of zooplankton in stomach contents, and further work should elucidate which 13C-depleted sources further support the diet of this key consumer in Lake Maï-Ndombe.

Abstract Understanding the response of phytoplankton communities to environmental variability is essential to predicting ecosystem trajectories under climate change, particularly in ecologically sensitive coastal systems. This study investigates the composition, abundance, and ecological dynamics of phytoplankton communities along the central Algerian coast based on data collected during the SPHYTAL 2015 survey. A total of 55 species were identified, with dinoflagellates clearly dominating both qualitatively (70% of taxa) and quantitatively (77% of total cell abundance), followed by diatoms (25%). Prorocentrum and Ceratium were the most abundant dinoflagellate genera, while Coscinodiscus and Thalassiosira dominated among diatoms. Spatial variations in cell densities were influenced by gradients in sea surface temperature, salinity, and nutrient availability, particularly phosphates and nitrates. Multivariate analysis and diversity indices (Shannon, Simpson, Evenness) revealed a moderately structured and diversified community, indicative of a functional shift in phytoplankton assemblages. A climate-based simulation projected a tipping point by 2100, with dinoflagellates increasingly outcompeting diatoms under both RCP4.5 and RCP8.5 scenarios. These findings suggest a transition toward a more stratified and potentially harmful dinoflagellate-dominated regime, echoing patterns observed in other Mediterranean and Atlantic regions. The study underscores the need for integrating phytoplankton monitoring into coastal management and climate adaptation strategies.

While the invasion of Eurasian cladoceran Bosmina (Eubosmina) coregoni to the Laurentian Great Lakes is well-documented, its spread to the west coast of North America is less understood. We provide evidence that Bosmina (Eubosmina) coregoni was present on the Pacific Coast of North America as early as 1965, ~45 years prior to its first observation in this region in 2008. While investigating dated sediment cores in the Kootenay Lake system, British Columbia, Canada, we found B. (E.) coregoni remains in multiple intervals pre-2008, where it was present in high concentrations (up to 1500 exoskeletons/g dry sediment) and a dominant part of the cladoceran assemblage (25–31.5% relative abundance), though it displayed irregular pulses in abundance. While the method of introduction remains unclear, this research reframes the timeline of B. (E.) coregoni introduction and improves our understanding of zooplankton invasion dynamics.

ABSTRACTUntil the 1950s, large-bodied calanoids and cladocerans dominated the zooplankton community of Lake Victoria, whereas cyclopoid copepods only comprised 10% of microcrustaceans. From the 1960’s onwards, cyclopoid copepods increased to 70–90% of zooplankton and cladocerans, now dominated by small species, decreased to ca. 5%. Concomitantly phytoplankton biomass increased and shifted from dominance of diatoms to Cyanobacteria, which were hypothesized to be of less nutritional quality, causing the shift in zooplankton. We investigated whether the natural assemblage of Cyanobacteria in Mwanza Gulf negatively affected growth and fecundity of cladocerans. In 2010–2011, we performed life-history experiments with the cladoceran Daphnia lumholtzi, feeding it natural seston from Mwanza Gulf from three different locations. A laboratory-strain of the green alga Scenedesmus obliquus, proven to be high-quality food, was used as a control. Growth of D. lumholtzi in the rainy season and at one station in the dry season was just as high as in the control treatment. If there were negative effects of natural seston these were small. Although the evidence is circumstantial, this suggests that Cyanobacteria and/or their detritus could have been better food than expected and that food quality is not limiting the growth of D. lumholtzi in L. Victoria.