Heterotrophic Dinoflagellates Research Articles

Effects of glacier melting on the planktonic communities of two Antarctic coastal areas (Potter Cove and Hope Bay) in summer

Climate change has contributed to the regional retreat and melting of glaciers in the Antarctic Peninsula. This phenomenon is expected to affect the composition of plankton communities. The aim of this study is to analyze the influence of glacier melting on the composition and structure of phytoplankton and micro- and mesozooplankton assemblages in two coastal areas located in the northwest (Potter Cove) and in the northern end (Hope Bay) of the Antarctic Peninsula during summer 2013. We expect differences in plankton assemblages between PC and HB related to the fact that these areas are subjected to different climatic and environmental conditions and that the glacier meltwater is accumulated in lakes in HB whereas the meltwater directly reaches the adjacent sea at PC. However, plankton communities differed according to their proximity to the glaciers in both areas investigated, with a transition from herbivorous to microbial plankton food webs towards the glacier. The microbial food web was favored under the effects of glacier meltwater due to the stratification of the water column that favors the development of small eukaryotic algae. These are potential prey for ciliates and heterotrophic dinoflagellates which, in turn, favors the development of small omnivorous copepods. Further away from the glaciers, the herbivorous food web consists of large phytoplankton and a greater representation of herbivorous mesozooplankton organisms. It can be inferred from this study that increases in atmospheric temperatures may favor the development of microbial plankton food webs in the Antarctic coastal seas during summer due to increased glacier meltwater.

Microzooplankton Distribution and Dynamics in the Eastern Fram Strait and the Arctic Ocean in May and August 2014

Microzooplankton community structure, distribution, growth and herbivory were examined in the eastern Fram Strait and Arctic Ocean shelf affected by the Atlantic water inflow in May (during the spring bloom, chlorophyll up to 9 µg l-1) and August (post-bloom, 0.13 – 1.7 µg l-1) 2014. Microzooplankton were collected along two longitudinal transects at 79°N and 79.4 °N from the slope to the ice edge and crossed the Western Spitsbergen Current (WSC). In May, integrated microzooplankton biomass in the upper 100 m ranged from 0.16 g C m-2 above the slope to 2.3 g C m-2 within WSC (0.71 g C m-2 on average). Mixotrophic oligotrich ciliates from the genus Strombidium dominated in the spring and formed a surface bloom (79 x 103 cells L-1, 206 µg C L-1). This is the largest microzooplankton biomass recorded in the Arctic so far. The heterotrophic dinoflagellates Gyrodinium and Protoperidinium were abundant at the diatom-dominated stations in the marginal ice zone. In the summer, a more diverse community included a large proportion of heterotrophic and mixotrophic dinoflagellates, tintinnids, and other ciliates. Microzooplankton biomass increased to the average of 1.27 g C m-2. At the ice-covered and open water stations in the Yermak shelf and deep basin, microzooplankton grew at 0.04 to 0.38 d-1; their species-specific growth rates were up to 1.79 d-1. Microzooplankton herbivory on average removed 72% (36 to >100%) of daily primary production with the exception of Phaeocystis pouchetii colonies. These results indicate that the microbial food web plays a central role in the carbon cycle in this Atlantic-influenced polar system.

Tracking Holocene palaeostratification and productivity changes in the Western Irish Sea: A multi-proxy record

The Western Irish Sea preserves an exceptionally thick (ca. 40 m) Holocene succession that is ideally suited to understanding the pattern of palaeostratification and water mass productivity changes in the region, and their relationship with sea level, sedimentation, and biota. Additionally, the presence of shallow-buried methane provides an opportunity to explore its potential impact on the local pattern of Holocene marine environmental change. Multi-proxy investigation of a cored borehole succession through the Holocene interval tracks changes from mixed to seasonally stratified conditions. In the earliest Holocene (11.2–10 ka), high productivity, mixed water conditions prevailed, with abundant and diverse foraminifera and dominant heterotrophic dinoflagellate cysts. Productivity was probably driven by high nutrient fluxes related to high rates of sedimentation (>1600 cm/kyr), in turn influenced by relatively low sea level and restricted sediment accommodation space across shelf areas to the east of the borehole site (eastern Irish Sea Basin). With rising sea level in the later part of the Early Holocene, the region evolved into a relatively lower productivity mixed water mass system, with significant changes in ecology revealed by dinoflagellate cysts and foraminifera. In the latest Early Holocene and earliest Mid Holocene (ca. 8.4–8.2 ka) a return to higher productivity is signalled by dinoflagellate cyst data; a result of seasonal stratification becoming established, evidenced by sharply increased summer sea surface temperature estimates (typically 16–17 °C) that contrast with an opposite (more positive) trend in δ18O values for benthic foraminifera. Reductions in turbulent mixing associated with stratification might have exacerbated the palaeoecological impact of shallow-buried methane associated with the borehole site, potentially evidenced by a significant change in dominant benthic foraminifera and strong, localised excursions in the benthic δ13C/δ18O record.

Towards an Understanding of Diel Feeding Rhythms in Marine Protists: Consequences of Light Manipulation

Temporal programs synchronised with the daily cycle are of adaptive importance for organisms exposed to periodic fluctuations. This study deepens into several aspects of the exogenous and endogenous nature of microbial grazers. We investigated the diel rhythms of cell division and feeding activity of four marine protists under different light regimes. In particular, we tested if the feeding cycle of protistan grazers could be mediated by a light-aided enhancement of prey digestion, and also explored the consequences of cell division on diel feeding rhythms. Cell division occurred at night for the heterotrophic dinoflagellates Gyrodinium dominans and Oxyrrhis marina. In contrast, the mixotrophic dinoflagellate Karlodinium armiger and the ciliate Strombidium sp. mostly divided during the day. Additionally, a significant diurnal feeding rhythm was observed in all species. When exposed to continuous darkness, nearly all species maintained the cell division rhythm, but lost the feeding cycle within several hours/days (with the exception of O. marina that kept the rhythm for 9.5days). Additional feeding experiments under continuous light also showed the same pattern. We conclude that the feeding rhythms of protistan grazers are generally regulated not by cell division nor by the enhancement of digestion by light. Our study, moreover, indicates that the cell division cycle is under endogenous control, whereas an external trigger is required to maintain the feeding rhythm, at least for most of the species studied here.

Decadal shifts of coastal microphytoplankton communities in a semi-enclosed bay of NW Mediterranean Sea subjected to multiple stresses

Long-term evolution of microphytoplankton communities remains poorly studied in anthropized coastal zones submitted to multiple stressors. Here, we investigate decadal (2005–2017) microphytoplankton community changes, focusing on abundance and biovolume of major taxa related to both local abiotic conditions (rainfall rate, temperature and salinity) and regional convection events (wintering deep mixing) in the highly urbanized and semi-enclosed Toulon Bay (NW Mediterranean Sea). Results showed that persistent variations in local rainfall regime were followed by major changes in microphytoplankton community composition. Wet period (P2) (increase of wintering precipitations observed between late 2008 and early 2015) was associated to an increase of large heterotrophic dinoflagellates and disappearance of dominant diatom taxa, while dry periods (2005–2008 (P1) and 2015–2017 (P3)) promoted diatoms, microflagellates and small mixotrophic/heterotrophic dinoflagellates including potentially toxic species. Concomitance between intense deep mixing events, reported in open Ligurian Basin (particularly during winters 2005 and 2006) and higher values in the total microphytoplankton abundance and in spring diatom abundance regardless of rainfall conditions, presents this meso-scale process as the main fertilization mechanism in Toulon Bay. Although no change was detected in the chlorophyll a concentration during the 2006–2017 period, its trend was negatively correlated to the total microphytoplankton abundance. This negative relation as well as a change of size in dinoflagellates suggested a shift in the primary producer nature, from large autotrophic cells (diatoms and microflagellates) to smaller ones, driven by a runoff intensification. Finally, different communities composition were observed during both dry periods (i.e. diatoms-dominated and autotrophic microflagellate-dominated communities during P1 and P3, respectively), suggesting another environmental driver of change for phytoplankton communities of this coastal ecosystem.

Gyrodinium jinhaense n. sp., a New Heterotrophic Unarmored Dinoflagellate from the Coastal Waters of Korea.

Four unarmored heterotrophic dinoflagellates were isolated from the coastal waters of southern Korea. The rDNA sequences of four clonal cultures were determined, and the morphology of one of the four strains was examined using light and scanning and transmission electron microscopy. The large subunit (LSU) and small subunit (SSU) rDNA sequences of each of the strains differed by 0-0.9% from those of the other strains, and the SSU rDNA sequence of the strain differed by 1.8-4.4% from those of other Gyrodinium species, whereas the LSU (D1-D2) rDNA sequence differed by 12.4-22.2%. Furthermore, phylogenetic trees showed that Gyrodinium jinhaense n. sp. formed a distinctive clade among the other Gyrodinium species. Meanwhile, microscopy revealed an elliptical bisected apical structure complex and a cingulum that was displaced by approximately one-quarter of the cell length, which confirmed that the dinoflagellate belonged to the genus Gyrodinium. However, the cell surface was ornamented with 16 longitudinal striations, both on the episome and hyposome, unlike other Gyrodinium species. Furthermore, the cells were observed to have pusule systems and trichocysts but lacked mucocysts. Based on morphology and molecular data, we consider this strain to be a new species in the genus Gyrodinium and thus, propose that it be assigned to the name G.jinhaense n. sp.

Seasonal modulation of mesoscale processes alters nutrient availability and plankton communities in the Red Sea

Hydrographic and atmospheric forcing set fundamental constraints on the biogeochemistry of aquatic ecosystems and manifest in the patterns of nutrient availability and recycling, species composition of communities, trophic dynamics, and ecosystem metabolism. In the Red Sea, latitudinal gradients in environmental conditions and primary production have been ascribed to fluctuations in Gulf of Aden Water inflow, upwelling/mixing, and regenerated nutrient utilization i.e. rapidly recycled nitrogen in upper layers. However, our understanding of upper layer dynamics and related changes in plankton communities, metabolism and carbon and nitrogen export is limited. We surmised that stratification and mesoscale eddies modulate the nutrient availability and taxonomic identity of plankton communities in the Red Sea. Based on remote-sensing data of sea level anomalies and high resolution in situ measurements (ScanFish) we selected stations for hydrographic CTD profiles, water sampling (nutrients, seawater oxygen stable isotopes [δ18OSW]), phytoplankton and zooplankton collections. In fall 2014, strong stratification subjected the plankton community to an overall nitrogen and phosphorus shortage. The nutrient deficiency increased numbers of heterotrophic dinoflagellates, microzooplankton, and diazotrophs (Trichodesmium, diatom-diazotroph associations [DDAs]), albeit largely decreased phytoplankton and mesozooplankton abundances. In spring 2015, mesoscale eddies increased the nutrient availability, and the thermohaline characteristics and low δ18OSW point to the interaction of eddies with Gulf of Aden Surface Water (GASW). Cyclonic eddies and, most likely, the availability of nutrients associated with the GASW, increased the abundances of autotrophs (diatoms, Prasinophytes) and supported larger numbers of zooplankton and their larvae. We demonstrate that the interplay of stratification, advection of Gulf of Aden water and mesoscale eddies are key elements to better understand changes in plankton community composition, ecosystem metabolism, and macronutrient export in the Red Sea in space and time.

Shelf sea subsurface chlorophyll maximum thin layers have a distinct phytoplankton community structure

The Western English Channel is a seasonally stratified temperate coastal sea where a subsurface chlorophyll maximum (SCM) is typically detectable within the seasonal thermocline. The SCM often develops as a thin layer (< 5 m) that may contain elevated concentrations of phytoplankton (subsurface chlorophyll maximum thin layer; SCMTL). During summer 2013 a study was conducted offshore of Falmouth, UK to assess spatial and short-term temporal variability in SCM thickness in relation to water column structure and physical conditions and to evaluate any associated changes in phytoplankton community structure. SCMTLs were observed in 18 of 52 vertical profiles, typically characterised by higher chlorophyll concentrations than broader SCMs. SCMTLs were generally associated with a ‘stepped’ thermocline, likely representing the presence of one or more shallow mixed layers forming above/within the seasonal thermocline, and related to increased stratification compared to broader SCMs. Pseudo-nitzschia was almost exclusively the dominant diatom taxon in SCMs, yet statistically distinct differences in community structure existed between SCMTLs and broader SCMs. Within the phytoplankton, the distinction was largely due to a greater biomass of Proboscia alata and other rhizosolenid diatoms, and the dinoflagellate Ceratium lineatum in SCMTLs, and a smaller population of the diatom Chaetoceros spp. There was also a distinction amongst heterotrophic dinoflagellates, with enhanced biomass of Gyrodinium spp. in SCMTLs and a reduction in Diplopsalis lenticula. We propose that this observed difference resulted from promotion of phytoplankton better adapted to environmental conditions more specific to SCMTLs compared to broader SCMs. With more intense and prolonged stratification projected for the NW European shelf, there may be increased prevalence of SCMTLs and the associated larger-sized specialised taxa, with implications for increased carbon export. This study adds to a growing body of evidence of the importance of SCMTLs in coastal and shelf seas, and highlights the requirement for improved understanding of physical forcing and the ecology and physiology of key taxa, particularly as predicted changes in stratification could alter the role of SCM phytoplankton in a future influenced by climate change.

Using fluorescence and bioluminescence sensors to characterize auto- and heterotrophic plankton communities

High-resolution autonomous sensors routinely measure physical (temperature, salinity), chemical (oxygen, nutrients) and biological (fluorescence) parameters. However, while fluorescence provides a proxy for phytoplankton, heterotrophic populations remain challenging to monitor in real-time and at high resolution. Bathyphotometers, sensors which measure the light emitted by bioluminescent organisms when mechanically stimulated, provide the capability to identify bioluminescent dinoflagellates and zooplankton. In the coastal ocean, highly abundant dinoflagellates emitting low-intensity flashes generate a background bioluminescence signal, while rarer zooplankton emit bright flashes that can be individually resolved by high-frequency sensors. Bathyphotometers were deployed from ships and onboard autonomous underwater vehicles (AUVs) during three field campaigns in Monterey Bay, California. Ship-based in situ water samples were simultaneously collected and the plankton communities characterized. Plankton concentrations were matched with concurrent datasets of fluorescence and bioluminescence to develop proxies for autotrophic and heterotrophic dinoflagellates, other phytoplankton such as diatoms, copepods, larvaceans (appendicularians), and small jellies. The method extracts the bioluminescence background as a proxy for dinoflagellates, and exploits differences in bioluminescence flash intensity between several types of zooplankton to identify larvaceans, copepods and small jellies. Fluorescence is used to discriminate between autotrophic and heterotrophic dinoflagellates, and to identify other autotrophic plankton. Concurrent fluorometers and bathyphotometers onboard AUVs can thus provide a novel view of plankton diversity and phytoplankton/zooplankton interactions in the sea.

Dinoflagellate cyst production in the Cariaco Basin: A 12.5 year-long sediment trap study

Seasonal and interannual variability in dinoflagellate cyst production were assessed using a 12.5 year-long sediment trap time series from the Cariaco Basin (southern Caribbean Sea). This study constitutes the longest such time series published to date, providing robust patterns of variability for individual dinoflagellate cyst taxa as well as for major phytoplanktonic and (micro-)zooplanktonic groups at the site. Cyst production is interpreted in the context of physico-chemical properties measured in situ at the mooring site (primarily reflecting seasonal upwelling), and considering potential interactions with other major components of the pelagic food web (e.g., diatoms, ciliates, copepods).The time series consists in >300 sediment trap samples, each representing ∼14 days of sedimentation, collected at the CARIACO station between Nov. 8, 1996 and May 19, 2009. Mass fluxes of biogenic silica, calcium carbonate and organic carbon reflect dominantly the timing and strength of wind-driven, seasonal upwelling that brings colder, nutrient-rich waters to the surface, fostering primary productivity and the export of biogenous materials to the depths. On seasonal time scales, dinoflagellate cyst production is closely coupled with upwelling strength, with higher cyst fluxes consistently observed under active upwelling conditions (average of 50.5 × 103 cysts m−2 day−1) compared to non-active upwelling intervals (29.0 × 103 cysts m−2 day−1). Year-to-year variability is characterized by a large increase in cyst production observed over the last ∼4 years of the time series (2006–2009) and minimum cyst fluxes recorded in the years 1998 and 1999, following the strong 1997/98 El Niño event.Dinoflagellate cyst assemblages are dominated by Brigantedinium spp. (59.1%), accompanied by Echinidinium delicatum (10.8%), Bitectatodinium spongium (8.4%), Spiny brown type A (2.9%) and Echinidinium spp. (2.4%). Cyst produced by both autotrophic and heterotrophic dinoflagellates generally respond positively to upwelling in the basin. Most cyst taxa are associated with active upwelling conditions (e.g., Bitectatodinium spongium, Brigantedinium spp., Echinidinium delicatum, Quinquecuspis concreta, Selenopemphix quanta, Spiny brown type C), with some showing higher fluxes under active but weak upwelling conditions (e.g., Echinidinium granulatum, Echinidinium spp., cyst of P. fukuyoi, Spiny brown type A). Other cyst taxa tend to show higher abundances at the onset of upwelling conditions (e.g., Echinidinium aculeatum, cyst of Protoperidinium stellatum) or following active upwelling intervals (e.g., Lejeunecysta marieae, Selenopemphix nephroides). The detailed response of each dinoflagellate cyst taxon to environmental variability is presented in the form of an atlas, providing photomicrographs and detailing overall monthly production, contribution to the total trap assemblage as well as cyst production over the 12.5 years of the time series.

Trophic interactions of micro- and mesozooplankton in the Amundsen Sea polynya and adjacent sea ice zone during austral late summer

To elucidate the trophic link between micro- and mesozooplankton in the Amundsen Sea polynya (ASP) and adjacent sea ice zone, we estimated the impact of microzooplankton grazing on major phytoplankton groups, as well as the diet composition, ingestion rate, and prey selectivity of two copepods and Euphausia crystallorophias larvae on microbial communities during the late summer. Phaeocystis antarctica, which was ingested by micro- and mesozooplankton, comprised most phytoplankton biomass. Herbivorous microzooplankton consumed at least half of the phytoplankton production, but the microzooplankton grazing may not contribute strongly to the decline of the phytoplankton bloom. Three mesozooplankton species (Calanoides acutus, Metridia gerlachei, and E. crystallorophias larvae) preferentially grazed on ciliates and heterotrophic dinoflagellates (HDF) with sizes from 20 to >100 µm over phytoplankton. Although microzooplankton comprised only an average of 41.7 ± 3.2% o the total carbon available in the natural prey pool, they accounted for an average of 75.4 ± 2.9% of total carbon ingested by copepods and krill larvae. Heterotrophic food sources made up a substantial proportion of mesozooplankton diets, with strong positive selection for microzooplankton at most locations regardless of phytoplankton size and type. In particular, HDF comprised the major dietary component of mesozooplankton in the study area. The presence of mesozooplankton reduced the grazing pressure on P. antarctica and diatoms through predation on herbivorous microzooplankton. Approximately half of the primary production capacity may have indirectly reached mesozooplankton through microzooplankton consumption. Thus, strongly selective feeding behavior and higher grazing pressure on microzooplankton indicated the importance of microheterotrophic pathways through strong trophic coupling between mesozooplankton and the microbial food web during the decline of phytoplankton bloom. In the highly productive ASP system, food web structure can be classified as multivorous, whereby herbivorous and omnivorous modes both play significant roles in carbon export, enhancing the efficiency of the pelagic food web.

Spatial variability of microzooplankton in the central Arabian Sea during spring intermonsoon

AbstractSpatial variability of microzooplankton (MZP; organisms in size 20–200 μm) along with total heterotrophic bacteria (THB) and picophytoplankton in the upper 120‐m water column of the central Arabian Sea (AS) was studied during the spring intermonsoon period (March 2015). Six stations divided into three regions (northern, southern, and central) were sampled along 68°E transect, between 9°N and 21°N. A total of 25 species of MZP were recorded corresponding to loricate ciliates (11 species), aloricate ciliates (two species), and heterotrophic dinoflagellates (12 species). Along the transect, MZP total abundance that varied between 5 and 9 × 103 org./m3 was largely contributed by ciliates, especially loricate ciliates (30%–62%) followed by heterotrophic dinoflagellates (23%–26%). MZP counts were lower in the central region (SD ± 0.4, 7–8 × 103 org./m3) than in the northern and the southern regions (SD ± 0.7, 8–9 × 103 org./m3) of the Arabian Sea. Heterotrophic bacterial counts were higher in the northern (28–202 × 103 cells/ml) and southern (16–92 × 103 cells/ml) and lowest in the central (13–89 × 103 cells/ml) region. In contrast, picophytoplankton showed higher counts in central (3–54 × 103 cells/ml) and southern (4–113 × 103 cells/ml) regions and lowest in the north (1–45 × 103 cells/ml). Strong positive coupling of MZP–THB (0.05 &gt; p &gt; 0.01) than that of MZP–picophytoplankton density and MZP–chlorophyll a concentration suggests the spatial scale variability and functioning of microbial loop in the central Arabian Sea during the spring intermonsoon period.

Postdepositional aerobic and anaerobic particulate organic matter degradation succession reflected by dinoflagellate cysts: The Madeira Abyssal Plain revisited

We report on the succession of selective degradation of dinoflagellate cyst species that can be considered representative for discrete particulate organic matter (POM) classes of different degradability. The effects of anaerobic and aerobic degradation as well as bioturbation in a natural setting are documented in high resolution by means of palynological and geochemical analyses on Madeira Abyssal Plain A- and F-turbidites. These turbidites are unique as their initial ungraded sediments are affected by a downward penetrating oxydation front.Geochemical analyses document the presence of an active downward penetrating oxidation front in the A turbidite, and a palaeo-oxidation front in the F-turbidite. In this latter turbidite, several zones can be distinguished from top to bottom: an oxidised bioturbated zone, an oxidised but not-bioturbated zone, a visible paleo-oxidation front, and a narrow nitrogenous zone overlying unoxidised sediments.We are the first to report that anaerobic degradation within the nitrogenous zones in both turbidites affects cysts of some heterotrophic dinoflagellates. The cyst species affected (Echinidinium aculeatum, Echinidinium spp., cysts of Protoperidinium monospinum and Brigantedinium spp.) exponentially decrease in this zone that is further characterised by a strong decrease in sulphur content due to sulphide oxidation. Degradation rates are different for each species. These cysts of heterotrophic dinoflagellates consist of a nitrogen-rich glycan in contrast to the cellulosic cysts walls of phototrophic dinoflagellates. Therefore, our observation supports the hypothesis that the quality of organic matter plays an important role in OM degradation in oxygen deficient environments with N-rich OM being more labile than other components in these environments.All heterotrophic species are strongly affected by aerobic degradation with their cyst concentrations exponentially decreasing with increasing oxygen exposure. Degradation rates vary between species and range von highly degradable to slightly degradable. Most strongly affected are cysts of Protoperidinium monospinum, Brigantedinium spp., and Echinidinium spp. Increasingly less affected are: Selenopemphix nephroides, Selenopemphix quanta, other Peridinioids and Echinidinium aculeatum. An effect of aerobic degradation on photosynthetic species could only be observed for Pentapharsodinium dalei. All Impagidinium species appeared to be resistant to aerobic degradation.Despite having analysed only a limited number of samples in the bioturbated zone of the F-turbidite, we have strong indications that the bioturbation can lead to the degradation of POM that is not affected by aerobic degradation alone. The cyst species Spiniferites ramosus, Impagidinium paradoxum, Lingulodinium machaerophorum, Nematosphaeropsis labyrinthus, Spiniferites spp., Impagidinium sphaericum, Spiniferites elongates and Spiniferites mirabilis do not show concentration changes in the non-bioturbated aerobic sediments but their concentrations are considerably decreased in the bioturbated part of the F-turbidite. This supports the hypothesis that bioturbation can increase the degradation of POM by e.g. increasing oxygen exposure time and/or by an alteration of aerobic and anaerobic conditions.We furthermore show that the degradation rates of POM components represented by individual cyst species differ between zones with different redox and biological conditions. This implies that POM degradation reaction rate coefficients are environment dependent. Our observation that dinoflagellate cyst species have different degradation rates ranging from extremely labile to extremely recalcitrant within the individual redox/bioturbation zones supports the hypothesis that cyst walls have a species specific molecular structure. Our results support continuum models of organic matter degradation that assume a continuous distribution of organic matter reactivity.

Does Noctiluca scintillans end the diatom bloom in coastal water?

Weekly field observations were conducted in Harima-Nada in the eastern part of the Seto Inland Sea, Japan from 21 August to 26 October 2015 to investigate the role of heterotrophic dinoflagellate Noctiluca scintillans on a coastal ecosystem. We observed a rapid N. scintillans growth just after increasing Chl. a, of which the dominant species were Chaetoceros spp., Thalassionema spp., Thalassiothrix spp., Coscinodiscus spp., Thalassiosira spp., and Stephanopyxis spp. We also observed under a microscope that N. scintillans cells grazed on these species of diatoms. The biomass of N. scintillans was estimated to increase from 4.50 to 470 mgC m−2 in 7 days. On the other hand, phytoplankton biomass drastically decreased from 2110 to 725 mgC m−2. The increase in N. scintillans biomass could account for 33.6% of the decrease in phytoplankton biomass. If we assume all the decrease in phytoplankton biomass was due to the N. scintillans grazing of diatoms, the 33.6% indicates the apparent gross growth efficiency of N. scintillans under a sufficient source of feed. Our results indicated the active feeding of N. scintillans on diatoms ends a diatom bloom in coastal water.

A review of the ecological affinities of marine organic microfossils from a Holocene record offshore of Adélie Land (East Antarctica)

Abstract. Integrated Ocean Drilling Program (IODP) Expedition 318 recovered a ∼ 170 m long Holocene organic-rich sedimentary sequence at Site U1357. Located within the narrow but deep Adélie Basin close to the Antarctic margin, the site accumulated sediments at exceptionally high sedimentation rates, which resulted in extraordinary preservation of the organic sedimentary component. Here, we present an overview of 74 different mainly marine microfossil taxa and/or types found within the organic component of the sediment, which include the remains of unicellular and higher organisms from three eukaryotic kingdoms (Chromista, Plantae, and Animalia). These remains include phytoplanktonic (phototrophic dinoflagellates and prasinophytes) and very diverse zooplanktonic (heterotrophic dinoflagellates, tintinnids, copepods) organisms. We illustrate each marine microfossil taxon or type identified by providing morphological details and photographic images, which will help with their identification in future studies. We also review their ecological preferences to aid future (palaeo)ecological and (palaeo)environmental studies. The planktonic assemblage shows a high degree of endemism related to the strong influence of the sea-ice system over Site U1357. In addition, we found the remains of various species of detritus feeders and bottom-dwelling scavengers (benthic foraminifers and annelid worms) indicative of high export productivity at Site U1357. This study shows the potential of organic microfossil remains for reconstructing past environmental conditions, such as sea-ice cover and (export) productivity.

Seasonal variation and taxonomic composition of mesozooplankton in the southern Black Sea (off Sinop) between 2005 and 2009

The monthly and long-term fluctuations of mesozooplankton abundance, biomass, and taxonomic composition in the Sinop inner harbor (southern Black Sea) between 2005 and 2009 are presented in the present study. In total, 31 mesozooplankton taxa were identified during the study. The recorded average mesozooplankton abundance and biomass were 34,323 ± 7580 ind. m$^{-2}$ and 1208 ± 460 mg m$^{-2}$ in 2005, 95,063 ± 31,434 ind m$^{-2}$ and 1787 ± 604 mg m$^{-2}$ in 2006, 97,626 ± 12,141 ind m$^{-2}$ and 1034 ± 20 mg m$^{-2}$ in 2007, 91,918 ± 10,476 ind m$^{-2}$ and 775 ± 121 mg m$^{-2}$ in 2008, and 146,918 ± 19,671 ind m$^{-2}$ and 1955 ± 437 mg m$^{-2}$ in 2009, respectively. The highest abundance values of mesozooplankton were encountered in March 2006 (299,110 ind m$^{-2}$) with the contribution of Copepoda nauplii (167,000 ind m$^{-2}$) particularly. Meanwhile, the mesozooplankton biomass was highest in October 2005 (5852 mg m$^{-2}$), due to Parasagitta setosa (3695 mg m$^{-2}$). The heterotrophic dinoflagellate Noctiluca scintillans was a major component of plankton samples in the Sinop region, presenting its highest values in April 2005. Mesozooplankton abundance values showed significant differences (except 2007-2009 and 2008-2009) during the 5-year sampling period. Since the middle of 2006, an increase in abundance, biomass, and biodiversity of mesozooplankton has been detected. Many mesozooplanktonic species/groups showed seasonality in the study (especially Cladocera species). This seasonality was determined to be based on temperature changes.

Feeding by common heterotrophic protists on the mixotrophic alga Gymnodinium smaydae (Dinophyceae), one of the fastest growing dinoflagellates.

Gymnodinium smaydae is one of the fastest growing dinoflagellates. However, its population dynamics are affected by both growth and mortality due to predation. Thus, feeding by common heterotrophic dinoflagellates Gyrodinium dominans, Gyrodinium moestrupii, Oblea rotunda, Oxyrrhis marina, and Polykrikos kofoidii, and the naked ciliate Pelagostrobilidium sp. on G.smaydae was investigated in the laboratory. Furthermore, growth and ingestion rates of O.marina, G.dominans, and Pelagostrobilidium sp. on G.smaydae in response to prey concentration were also determined. Oxyrrhis marina, G.dominans, G.moestrupii, and Pelagostrobilidium sp. were able to feed on G.smaydae, but P.kofoidii and O.rotunda did not feed on this dinoflagellate. The maximum growth rate of O.marina on G.smaydae was 0.411 per day. However, G.smaydae did not support the positive growth of Pelagostrobilidium sp. The maximum ingestion rates of O.marina and Pelagostrobilidium sp. on G.smaydae were 0.27 and 6.91ng C·predator-1 ·d-1 , respectively. At the given mean prey concentrations, the highest growth and ingestion rates of G.dominans on G.smaydae were 0.114 per day and 0.04ng C·predator-1 ·d-1 , respectively. The maximum growth and ingestion rates of O.marina on G.smaydae are lower than those on most of the other algal prey species. Therefore, O.marina may be an effective predator of G.smaydae, but G.smaydae may not be the preferred prey for supporting high growth of the predator in comparison to other species as inferred from a literature survey.

Differential feeding by common heterotrophic protists on 12 different Alexandrium species

The genus Alexandrium often forms harmful algal blooms causing human illness and large-scale mortality of fish and shellfish. Thus, Alexandrium bloom dynamics are primary concerns for scientists, government officials, aquaculture farmers, and the public. To understand bloom dynamics, mortality due to predation needs to be assessed; however, interactions between many Alexandrium species and their potential predators have not previously been reported. Thus, feeding by five common heterotrophic dinoflagellates (Oxyrrhis marina, Gyrodinium dominans, Polykrikos kofoidii, Pfiesteria piscicida, and Oblea rotunda) and a naked ciliate (Strombidinopsis sp.) on 12 Alexandrium species was examined. Furthermore, the growth and ingestion rates of P. kofoidii on A. minutum CCMP 1888 (previously A. lusitanicum), A. minutum CCMP 113, and A. tamarense were measured as a function of prey concentration. The growth rates of P. kofoidii on the other Alexandrium species at single high prey concentrations were measured, at which the growth rates on A. minutum CCMP 1888 and A. tamarense were saturated. Feeding occurrence by these predators on 12 Alexandrium species could be categorized into 6 different prey groups. Each Alexandrium species was consumed by at least one predator; however, there was no Alexandrium species that was eaten by all six predators. Cells of A. minutum CCMP 1888, A. minutum CCMP 113, and A. tamarense were fed upon by four predators, but A. affine and A. pacificum by only one predator species, P. kofoidii or Strombidinopsis sp. Furthermore, A. minutum CCMP 1888 and A. tamarense supported high growth rates of P. kofoidii, but the other Alexandrium species did not support, but rather inhibited P. kofoidii growth. With increasing prey concentrations, the growth and ingestion rates of P. kofoidii on A. minutum CCMP 1888 and A. tamarense increased and became saturated, whereas those on A. minutum CCMP 113 continuously decreased. The maximum growth rates of P. kofoidii on A. tamarense and A. minutum CCMP 1888 were 1.010 and 0.765 d−1, respectively, and P. kofoidii maximum ingestion rates were 26.2 and 11.1 ng C predator−1d−1, respectively. In contrast, the growth rates of P. kofoidii on the other Alexandrium species at single high prey concentrations were almost zero (A. pacificum) or negative. Based on the feeding occurrence and growth and ingestion rates of predators on 12 Alexandrium species, it is suggested that common heterotrophic protistan predators respond differently to different Alexandrium species, and thus ecological niches of the Alexandrium species may be different from each other. These results may provide an insight into the roles of protistan predators in bloom dynamics of Alexandrium species.

Phytoplankton Community Dynamic: A Driver for Ciliate Trophic Strategies

Phytoplankton plays a key role as primary producers and mediating biogeochemical cycles in the water column. The understanding of the temporal dynamic of primary grazers channelling energy and carbon from primary producers is important for evaluating aquatic ecosystems functioning. This study investigates the coupling between phytoplankton and ciliates from live samples collected with approximately daily frequency during an almost two-year cycle. The study site is a nutrient-rich temperate estuary, Roskilde Fjord (Denmark). Our aim is to evaluate the importance of protist grazers, especially ciliates, as predators on phytoplankton and to evaluate differences among multiple nutritional strategies through different seasons. The phytoplankton community, was mostly dominated by small organisms (< 20 µm) with few observations of diatoms. In most of observations, heterotrophic dinoflagellates biomass was smaller than biomass of ciliates (< 10%), indicating that ciliates are the main component of microzooplankton. Except for the spring 2016, the ciliate community closely followed the phytoplankton community, showing a tight coupling between the primary producers and grazers during all seasons. This somehow contradicts the general assumption that ciliate dominance is restricted to periods of nutrient limitations dominated by the microbial food web and suggests a year-round key role of ciliates as consumers of phytoplankton biomass. Biomasses of ciliates increased during spring and were highest during summer. Relative importance of mixotrophs were high due to occurrence of Mesodinium rubrum blooms as well as other mixotrophic ciliates in late spring/early summer. M. rubrum biomass had the opposite pattern of the cryptophyte prey Teleaulax spp., and the coupling between the two populations was very strong in late spring. Ciliates that grazed on selected phytoplankton, had a smaller potential grazing impact regarding their biomasses, likely due to food limitation; conversely ciliates that feed on diverse prey items were less constrained by food limitation, and their seasonality appear to be driven by other factors. These findings suggest that the ciliate community structure and dynamics is important in structuring the phytoplankton community on short and seasonal scale.

Dynamics of Noctiluca scintillans (Macartney) Kofoid &amp; Swezy and its Contribution to Mesozooplankton in the Southeastern Black Sea

<div><!--block-->During the last half century, significant changes have been observed in the zooplankton communities in the Black Sea. While some species have almost disappeared, someother species have increased in number. Noctiluca scintillans is a red-tide forming heterotrophic dinoflagellate, which is widely distributed in the world’s oceans. Due to its large cell size (&gt;200 μm) and phagotrophic feeding behavior, Noctiluca scintillans is assessed within mesozooplankton communities. After the 1970s, Noctiluca scintillans became dominant in the mesozooplankton community in the Black Sea. Between May 2015 and April 2016, the population characteristics of Noctiluca scintillans and its contribution to mesozooplankton communities were investigated in surface waters in the southeastern Black Sea. The abundance of Noctiluca scintillans in surface waters ranged between 0–23357 cells/m-3 with the lowest number found in the summer and the highest number found from February to May. Abundance tended to increase from the river mouth toward open waters. During the study, the cell diameter and volume of Noctiluca scintillans varied between 352–629 µm and 2.28 x 107–1.3x108 µm3, respectively. The total mesozooplankton abundance ranged between 10–24020 individuals/m-3. There are no statistically significant differences in Noctiluca scintillans and mesozooplankton abundance between sampling stations (p&gt;0.05). The contribution of Noctiluca scintillans to the mesozooplankton population was the lowest during summer, but from February to May, Noctiluca scintillans comprised 79%±4% and 98%±2% of the total mesozooplankton population in the river mouth and coastal waters, respectively. These results confirm that Noctiluca scintillans is still an important contributor of the mesozoplankton community in the southeastern Black Sea.</div>