A day in the life of winter plankton: under-ice community dynamics during 24 h in a eutrophic lake

Although diel vertical migration (DVM) in aquatic systems may account for the largest daily migration of biomass globally, our understanding of this process under ice cover is limited, particularly in fresh water. The date of lake ice onset and duration of ice cover is declining globally, therefore determining the extent of plankton migrations under ice is imperative to inform our baseline understanding of seasonal differences in community structure and function, and how conditions may change over time. We investigated whether plankton exhibit DVM under ice and explored interactions between phytoplankton and zooplankton at highly resolved space–time scales across 24 h in a eutrophic system. Despite the dominance of motile taxa, phytoplankton remained vertically segregated based on morpho-functional groups throughout the sampling period. Daphnia mendotae exhibited size-structured DVM with an upward migration at sunset and sunrise, and midnight sinking, presumably to avoid the rise of predatory Chaoborus. We hypothesize that because overwintering daphnids require rich lipid stores, D. mendotae migrated to access small, lipid-rich phytoplankton that were limited to surface waters. Given our study took place during an unusually warm winter in a eutrophic system, our results may represent future under-ice dynamics under increasing eutrophication pressure and climate warming in shallow lakes.

Predator-induced allometric changes in the tail spine length of Daphnia: a distinct resource allocation strategy

Understanding resource allocation strategies underlying inducible defense is a challenging scientific issue, because of the difficulty in measuring resource allocations of defensive traits. We examined allometric changes to evaluate resource allocation strategies on the tail spine of Daphnia within and between species and further explore the allometric changes at different developmental stages and their relationship with growth and reproduction. We found that four Daphnia species (Daphnia magna, Daphnia sinensis, Daphnia galeata and Daphnia mitsukuri) can perform significant inducible defensive responses when exposed to fish kairomone. Different from the other Daphnia species, D. mitsukuri significantly enhanced the allometric slope of its tail spine when exposed to fish kairomone. We also found that allometric changes among different D. mitsukuri clones are significant in adult individuals. Furthermore, the allometric changes show a significant negative interaction with individual growth, indicating that a trade-off may exist between the resource allocations of tail spine elongation and growth. This study highlights the species-specific allometric changes in tail spine elongation and provides an explanation for this from resource allocations.

Patterns in Rhizosolenia- and Guinardia-associated Richelia abundances in the tropical marginal seas of the western North Pacific

Richelia is a widespread, heterocystous, N2-fixing cyanobacterium that occurs in oligotrophic seas. It can be symbiotic within rhizosolenoid hosts, including Rhizosolenia (R-R) and Guinardia (G-R), and occasionally occurs as free-living (FL-R) filaments. We investigated the Richelia abundances in the South China (SCS) and Philippine Seas (PS) during the northeast (NEM) and southwest monsoon (SWM) seasons between 2001 and 2016. Field results showed that G-R was most abundant in the SCS during the SWM. Both R-R and FL-R were more abundant in the PS than the SCS and during the SWM versus the NEM. In general, R-R were more abundant than G-R, except at the SCS-SWM stations where abundances were comparable. Multivariate statistical models indicate that as R-R and G-R were abundant in warmer seawaters, nitrate plus nitrite was a key factor negatively controlling R-R abundance while silicate was more positively related to G-R. Positive correlation between G-R and FL-R abundances and microscopic observations imply that like Rhizosolenia-associated Richelia, filaments released from Guinardia hosts can remain viable outside their diatom hosts for time periods on the order of days. Overall, results reveal different distribution patterns between the two rhizosolenoid-Richelia symbioses and both G-R and R-R likely contribute to FL-R in the field.

The private life of Cystodinium: in situ observation of its attachments and population dynamics

Phytoplankton images were collected using an Imaging Flow Cytobot moored in the mesotrophic lake Lac Montjoie (Quebec, Canada). Cystodinium—an unusual dinoflagellate genus—was found during manual classification of the images into taxonomic groups while building an automated classifier. Cystodinium’s particularity is that while it can take a typical motile dinoflagellate form, it is thought to exist primarily as an immotile photosynthetically competent parasitic cyst in the shape of a crescent moon. Observations presented here are of this immotile lunate cyst. Manually classified images revealed that the majority of the Cystodinium found (86%) were attached to other microalgae or detrital material while the rest were unattached. The established auto-classifier was only able to correctly identify unattached Cystodinium images and thus was used to generate time series as cells per 100 mL for the unattached cell subset. Our observations, coupled with a literature review, lead us to question the parasitic nature of this taxonomic group.

Active prey selection in developing larvae of Atlantic bluefin tuna (Thunnus thynnus) in spawning grounds of the Gulf of Mexico

Bluefin tuna spawn in restricted areas of subtropical oligotrophic seas. Here, we investigate the zooplankton prey and feeding selectivity of early larval stages of Atlantic bluefin tuna (ABT, Thunnus thynnus) in larval rearing habitat of the Gulf of Mexico. Larvae and zooplankton were collected during two multi-day Lagrangian experiments during peak spawning in May 2017 and 2018. Larvae were categorized by flexion stage and standard length. We identified, enumerated and sized zooplankton from larval gut contents and in the ambient community. Ciliates were quantitatively important (up to 9%) in carbon-based diets of early larvae. As larvae grew, diet composition and prey selection shifted from small copepod nauplii and calanoid copepodites to larger podonid cladocerans, which accounted for up to 70% of ingested carbon. Even when cladoceran abundances were <0.2 m−3, they comprised 23% of postflexion stage diet. Feeding behaviors of larvae at different development stages were more specialized, and prey selection narrowed to appendicularians and primarily cladocerans when these taxa were more abundant. Our findings suggest that ABT larvae have the capacity to switch from passive selection, regulated by physical factors, to active selection of presumably energetically optimal prey.

Mesozooplankton biomass, grazing and trophic structure in the bluefin tuna spawning area of the oceanic Gulf of Mexico

We investigated size-fractioned biomass, isotopes and grazing of mesozooplankton communities in the larval habitat of Atlantic bluefin tuna (ABT) in the oceanic Gulf of Mexico (GoM) during the peak spawning month of May. Euphotic-zone biomass ranged from 101 to 513 mg C m−2 during the day and 216 to 798 mg C m−2 at night. Grazing varied from 0.1 to 1.0 mg Chla m−2 d−1, averaging 1–3% of phytoplankton Chla consumed d−1. Carnivorous taxa dominated the biomass of > 1-mm zooplankton (78% day; 60% night), while only 13% of smaller zooplankton were carnivores. δ15N enrichment between small and large sizes indicates a 0.5–0.6 trophic-step difference. Although characteristics of GoM zooplankton are generally similar to those of remote oligotrophic subtropical regions, zooplankton stocks in the ABT larval habitat are disproportionately high relative to primary production, compared with HOT and BATS averages. Growth-grazing balances for phytoplankton were resolved with a statistically insignificant residual, and trophic fluxes from local productivity were sufficient to satisfy C demand of suspension feeding mesozooplankton. While carnivore C demand was met by local processes in the central GoM, experiments closer to the coastal margin suggest the need for a lateral subsidy of zooplankton biomass to the oceanic region.