Phytoplankton Grazing Research Articles

Bottom-up behaviourally mediated trophic cascades in plankton food webs.

Our traditional view of the interactions between marine organisms is conceptualized as food webs where species interact with one another mainly via direct consumption. However, recent research suggests that understudied non-consumptive interactions, such as behaviourally mediated indirect interactions (BMIIs), can influence marine ecosystems as much as consumptive effects. Here, we show, to our knowledge, the first experimental evidence and quantification of bottom-up BMIIs in plankton food webs. We used observational, modelling and experimental approaches to investigate how behavioural responses to resource availability influence predation mortality on grazers with different foraging strategies (ambushing versus active foraging). A three-level food chain was used: phytoplankton as resource, copepod nauplii as grazers of phytoplankton and a large copepod as a predator. Ambushers showed little change in foraging activity with resource availability, whereas active foragers decreased their foraging activity with increasing resources, which led to a decrease (24-50%) in predation mortality. Therefore, an increase in resources ('initiator') causes behavioural changes in active grazers ('transmitter'), which ultimately negatively affects predator ('receiver') consumption rates. Consequently, increase in resource abundance may result in decreasing energy transfer to higher trophic levels. These results indicate that behaviourally mediated interactions drive marine food web dynamics differently from that predicted by only density-mediated or consumptive interactions.

Nitrogen and Isotope Flows Through the Costa Rica Dome Upwelling Ecosystem: The Crucial Mesozooplankton Role in Export Flux

AbstractThe Costa Rica Dome (CRD) is an open‐ocean upwelling ecosystem, with high biomasses of picophytoplankton (especially Synechococcus), mesozooplankton, and higher trophic levels. To elucidate the food web pathways supporting the trophic structure and carbon export in this unique ecosystem, we used Markov Chain Monte Carlo techniques to assimilate data from four independent realizations of δ15N and planktonic rate measurements from the CRD into steady state, multicompartment ecosystem box models (linear inverse models). Model results present well‐constrained snapshots of ecosystem nitrogen and stable isotope fluxes. New production is supported by upwelled nitrate, not nitrogen fixation. Protistivory (rather than herbivory) was the most important feeding mode for mesozooplankton, which rely heavily on microzooplankton prey. Mesozooplankton play a central role in vertical nitrogen export, primarily through active transport of nitrogen consumed in the surface layer and excreted at depth, which comprised an average 36–46% of total export. Detritus or aggregate feeding is also an important mode of resource acquisition by mesozooplankton and regeneration of nutrients within the euphotic zone. As a consequence, the ratio of passively sinking particle export to phytoplankton production is very low in the CRD. Comparisons to similar models constrained with data from the nearby equatorial Pacific demonstrate that the dominant role of vertical migrators to the biological pump is a unique feature of the CRD. However, both regions show efficient nitrogen transfer from mesozooplankton to higher trophic levels (as expected for regions with large fish, cetacean, and seabird populations) despite the dominance of protists as major grazers of phytoplankton.

Zooplankton in Bays of the Novaya Zemlya Archipelago: Composition, Distribution, and Role in Phytoplankton Grazing and Biosedimentation

The zooplankton community in two bays of the eastern coast of the Novaya Zemlya archipelago (Blagopoluchiya Bay and Tsivolki Bay) has been studied for the first time. Copepods of the genus Calanus—C. glacialis/finmarchicus and C. hyperboreus—dominate the zooplankton of the bays in terms of biomass (76–93%). These copepods reached the highest abundance in Blagopoluchiya Bay. The total abundance (70 000 ind./m2) and biomass (60 g/m2) of the population were many times higher than the maximum values recorded from waters of the Kara Sea shelf. The ration values for these copepods ranged from 1.5 to 20 µg C ind.–1 day–1 at different copepodite stages. The rate of grazing on phytoplankton biomass and production by zooplankton varied within 1.5–15.6% and from 3 to 127%, respectively. In the considered bay, the flux of organic carbon and suspended particulate matter contained in Calanus fecal pellets reached 157.5 mg Corg m–2 day–1 and 1574 mg m–2 day–1, respectively. In Tsivolki Bay, the total zooplankton biomass (6.6 g/m2) was several times lower. This has led to a decrease in its role in phytoplankton utilization (0.6–2.4% of phytoplankton biomass and 1.9–18% of phytoplankton production), as well as to a less intensive flux of suspended particulate matter (47.8 mg m–2 day–1) and organic carbon of copepod fecal pellets (2.6 mg Corg m–2 day–1) in the bay.

Satellite‐Based Estimation of Particulate Organic Carbon Export in the Northern South China Sea

AbstractKnowledge of particulate organic carbon export flux (EP) is the key to understanding the marine biological carbon pump. In this study, a satellite remote sensing algorithm based on the food‐web model established by Siegel et al. (2014; https://doi.org/10.1002/2013GB004743) was used to estimate EP in the northern South China Sea (NSCS), which consists of direct sinking flux of large phytoplankton and associated aggregates by gravity and flux of zooplankton feces by grazing. This is the first time that fine spatiotemporal variation of satellite‐derived EP in the NSCS is unveiled. Compared with the results on 1° spatial scale products, the model results based on the satellite products with 1/3° and 1/12° resolutions showed better consistency with the observations. Validation with in situ EP showed that the model exhibited a good performance in the NSCS basin, but the predicted EP in the shelf regions were smaller compared with measurements. The satellite‐derived annual‐mean EP in the shelf areas was 8.47 and 5.56 mmol C·m−2·d−1 in the NSCS basin, on the 1/3° spatial scale, with relative differences from observations were about −50% and −15% for the shelf and basin, respectively. The flux of feces from zooplankton grazing, especially that from microzooplankton grazing, might account for the major fraction of the EP in the NSCS. Accurate products of ocean net primary production and empirical coefficients in the food web (e.g., export efficiency of zooplankton grazing and specific mortality rate of nongrazing phytoplankton) suitable for the NSCS, are essential for performance improvement of the satellite‐based EP model.

Environmental controls on pteropod biogeography along the Western Antarctic Peninsula

AbstractPteropods are abundant zooplankton in the Western Antarctic Peninsula (WAP) and important grazers of phytoplankton and prey for higher trophic levels. We analyzed long‐term (1993–2017) trends in summer (January–February) abundance of WAP pteropods in relation to environmental controls (sea ice, sea surface temperature, climate indices, phytoplankton biomass and productivity, and carbonate chemistry) and interspecies dynamics using general linear models. There was no overall directional trend in abundance of thecosomes, Limacina helicina antarctica and Clio pyramidata, throughout the entire WAP, although L. antarctica abundance increased in the slope region and C. pyramidata abundance increased in the South. High L. antarctica abundance was strongly tied to a negative Multivariate El Niño Southern Oscillation Index the previous year. C. pyramidata abundance was best explained by early sea ice retreat 1‐yr prior. Abundance of the gymnosome species, Clione antarctica and Spongiobranchaea australis, increased over the time series, particularly in the slope region. Gymnosome abundance was positively influenced by abundance of their prey, L. antarctica, during the same season, and late sea ice advance 2‐yr prior. These trends indicate a shorter ice season promotes longer periods of open water in spring/summer favoring all pteropod species. Weak relationships were found between pteropod abundance and carbonate chemistry, and no long‐term trend in carbonate parameters was detected. These factors indicate ocean acidification is not presently influencing WAP pteropod abundance. Pteropods are responsive to the considerable environmental variability on both temporal and spatial scales—key for predicting future effects of climate change on regional carbon cycling and plankton trophic interactions.

The effects of hypolimnetic anoxia on the diel vertical migration of freshwater crustacean zooplankton

AbstractLakes and reservoirs worldwide are increasingly experiencing depletion of dissolved oxygen (anoxia) in their bottom waters (the hypolimnion) because of climate change and eutrophication, which is altering the dynamics of many freshwater ecological communities. Hypolimnetic anoxia may substantially alter the daily migration and distribution of zooplankton, the dominant grazers of phytoplankton in aquatic food webs. In waterbodies with oxic hypolimnia, zooplankton exhibit diel vertical migration (DVM), in which they migrate to the dark hypolimnion during the day to escape fish predation or ultraviolet (UV) radiation damage in the well‐lit surface waters (the epilimnion). However, due to the physiologically stressful conditions of anoxic hypolimnia, we hypothesized that zooplankton may be forced to remain in the epilimnion during daylight, trading oxic stress for increased predation risk or UV radiation damage. To examine how anoxia impacts zooplankton vertical migration, distribution, biomass, and community composition over day–night periods, we conducted multiple diel sampling campaigns on reservoirs that spanned oxic, hypoxic, and anoxic hypolimnetic conditions. In addition, we sampled the same reservoirs fortnightly during the daytime to examine the vertical position of zooplankton throughout the summer stratified season. Under anoxic conditions, most zooplankton taxa were predominantly found in the epilimnion during the day and night, did not exhibit DVM, and had lower seasonal biomass than in reservoirs with oxic hypolimnia. Only the phantom midge larva, Chaoborus spp., was consistently anoxia‐tolerant. Consequently, our results suggest that hypolimnetic anoxia may alter zooplankton migration, biomass, and behavior, which may in turn exacerbate water quality degradation due to the critical role zooplankton play in freshwater ecosystems.

Oil Spills and Dispersants Can Cause the Initiation of Potentially Harmful Dinoflagellate Blooms ("Red Tides").

After oil spills and dispersant applications the formation of red tides or harmful algal blooms (HABs) has been observed, which can cause additional negative impacts in areas affected by oil spills. However, the link between oil spills and HABs is still unknown. Here, we present experimental evidence that demonstrates a connection between oil spills and HABs. We determined the effects of oil, dispersant-treated oil, and dispersant alone on the structure of natural plankton assemblages in the Northern Gulf of Mexico. In coastal waters, large tintinnids and oligotrich ciliates, major grazers of phytoplankton, were negatively affected by the exposure to oil and dispersant, whereas bloom-forming dinoflagellates ( Prorocentrum texanum, P. triestinum, and Scrippsiella trochoidea) notably increased their concentration. The removal of key grazers due to oil and dispersant disrupts the predator-prey controls ("top-down controls") that normally function in plankton food webs. This disruption of grazing pressure opens a "loophole" that allows certain dinoflagellates with higher tolerance to oil and dispersants than their grazers to grow and form blooms when there are no growth limiting factors (e.g., nutrients). Therefore, oil spills and dispersants can act as disrupters of predator-prey controls in plankton food webs and as indirect inducers of potentially harmful dinoflagellate blooms.

Spatiotemporal patterns of phytoplankton composition and abundance in the Maryland Coastal Bays: The influence of freshwater discharge and anthropogenic activities

The spatial and temporal variations in phytoplankton abundance and community structure in the northern and southern parts of the Maryland Coastal Bays (MCBs) that differ in anthropogenic activities and hydrological characteristics were studied in 2012 and 2013 using photosynthetic pigments as biomarkers. Phytoplankton pigment biomass and diversity were generally higher in the northern bays that receive high nutrient input from St. Martin River, than in the southern bays where nutrient levels were comparatively low. Sites close to the mouths of tributaries in northern and southern bays had higher nutrient levels, which favored the development of dinoflagellates, and nano- and picophytoplankton, than sites closer to the inlets. The microplankton dominated the phytoplankton community in spring (>90%) and decreased in relative abundance into fall (<60%) whereas nanoplankton peaked in summer or fall. Picoplankton relative abundance increased from late spring (<10%, March 2012 & 2013) to summer (40%, July 2012 and August 2013) and was correlated positively with NH4+ and negatively with salinity. The observed spatial and seasonal patterns of phytoplankton relative abundance and diversity are likely due to changes in nutrient concentrations and ratios, driven by variations in freshwater discharge, and selective grazing of phytoplankton. Water quality management in the MCBs should continue to focus on reducing nutrient inputs into the bays.

Effects of nutrient enrichment and grazing by an invasive filter feeder on phytoplankton biomass in a South West Atlantic coastal lagoon

Invasive species can exert strong effects on structure and function of marine ecosystem. In estuaries, nutrient enhancement due to anthropogenic activities can increase microalgal biomass but invading filter-feeders can eventually regulate their abundance. This is what we hypothesized that is happening in a SW Atlantic coastal lagoon (Mar Chiquita, 37°40′S, 57°23′W, Argentina) invaded by the polychaete Ficopomatus enigmaticus. Here, we experimentally evaluated this hypothesis in a mesocosms experiment. Four treatments were performed: (T1) with nutrients (NO3, PO4 and NH4), (T2) with reefs and nutrients, (T3) with reefs, and (T4) without reefs or nutrients. Water samples were obtained to determine in vivo chlorophyll a (Chla) and nutrients concentration. Additionally, to evaluate the trophic position and particle selectivity by the polychaete, analysis of stable isotopes of particulate organic matter (POM) of the water and individuals were performed. Stable isotopes analysis showed that the main resource used by F. enigmaticus was the fraction of POM between 62 and 250 µm, showing particle selectivity and suggesting that the reefs have the potential to promote shifts in size, composition and biomass of local food source. Mesocosms experiments showed that nutrient supply increased the Chla concentration, but when reefs were added, the Chla decreased. Thus, nutrients increased the phytoplankton biomass but grazing by the reefs counteracted these increments. This indicates that an invader such as F. enigmaticus might modulate an anthropogenic impact via suspension feeding, and therefore its role in mitigate the consequences of eutrophication may be highly important.

Noctiluca and copepods grazing on the phytoplankton community in a nutrient-enriched coastal environment along the southwest coast of India

The relative grazing impact of Noctiluca scintillans (hereafter referred only Noctiluca) and copepods (Acrocalanus gracilis, Paracalanus parvus, Acartia danae and Oithona similis) on the phytoplankton community in an upwelling-mudbank environment along the southwest coast India is presented here. This study was carried out during the Pre-Southwest Monsoon (April-May) to the Late Southwest Monsoon (August) period in 2014. During the sampling period, large hydrographical transformation was evident in the study area (off Alappuzha, Southwest coast of India); warmer Pre-Southwest Monsoon water column condition got transformed into cooler and nitrate-rich hypoxic waters during the Southwest Monsoon (June-August) due to intense coastal upwelling. Copepods were present in the study area throughout the sampling period with a noticeable increase in their abundance during the Southwest Monsoon. On the other hand, the first appearance of Noctiluca in the sampling location was during the Early Southwest Monsoon (mid-June) and thereafter their abundance increased towards the Peak Southwest Monsoon. The grazing experiments carried out as per the food removal method showed noticeable differences in the feeding preferences of Noctiluca and copepods, especially on the different size fractions of phytoplankton. Noctiluca showed the highest positive electivity for the phytoplankton micro-fraction (av. 0.49±0.04), followed by nano-fraction (av. 0.17±0.04) and a negative electivity for the pico-fraction (av. -0.66±0.06). In total ingestion of Noctiluca, micro-fraction contribution (83.7%) was significantly higher compared to the nano- (15.7%) and pico-fractions (0.58%). On the other hand, copepods showed the highest positive electivity for the phytoplankton nano-fraction (av. 0.38±0.04) followed by micro- (av. -0.17±0.05) and pico-fractions (av. -0.35±0.05). Similarly, in total ingestion of copepods, nano-fraction (69.7%) was the highest followed by micro- (28.9%) and pico-fractions (1.37%). The grazing pressure of Noctiluca on the total phytoplankton was found to be 27.7% of the standing stock and 45.6% of the production, whereas in the case of copepods, it was 9.95% of the standing stock and 16.6% of the production. The study showed that the grazing pressure of Noctiluca on the total phytoplankton as well as larger phytoplankton fraction was 2.8- and 8-folds higher than that of the copepods. This suggests the leading role of Noctiluca as an effective grazer of larger phytoplankton along the southwest west coast of India, especially during the Peak/Late Southwest Monsoon.

Scaling copepod grazing in a coastal upwelling system: the importance of community size structure for phytoplankton C flux

Crustacean zooplankton, often dominated by copepods and euphausiids, are the major phytoplankton grazers in coastal upwelling systems. It has been argued that grazing by zooplankton is a size-dependent process, such that models incorporating the size structure of zooplankton are appropriate for describing herbivore C-transfer. Here, b ased on the size-spectrum theory and on gut-fluorescence experiments, conducted with numerically dominant copepods from two upwelling sites off the Chilean coast , we show that C-specific ingestion rates of copepods are size-dependent. We further show that the size structure of the copepod community, synthesized by the slope of the normalized size spectrum, determines the impact of grazing on phytoplankton. C-specific ingestion rates, depending on species size, were in the range of 0.02-8.40 (µg C x µg C -1 d -1 ). A modelled normalized biomass-spectra of a copepod community in the size range of 0.5 to 74.0 µg C showed that C-specific grazing impact can increase by a factor of 4 when small-sized species (0.1-10 µg C indiv -1 ), such as Paracalanus cf. indicus , Acartia tonsa , Oncaea spp. and Corycaeus spp., dominate the community in terms of biomass. By contrast, when larger-sized copepods dominate (10-100 µg C indiv -1 ), such as Calanus chilensis , Calanoides patagoniensis and Rhyncalanus nasutus , total zooplankton biomass may increase, but with a sharp decrease in the efficiency of C transfer via herbivores. Our findings indicate that processes affecting the size structure of zooplankton communities can substantially impact on phytoplankton C flux through the pelagic food web, thus controlling production of higher trophic levels.

Seasonal phenology of appendicularian tunicates in the North Water, northern Baffin Bay

Appendicularian tunicates are marine, gelatinous zooplankton that are important grazers of protists and phytoplankton in Arctic polynyas. However, little is known at the seasonal scale about their life cycle and population phenology in the Arctic. The temporal and spatial pattern of abundance and biomass, body size, and maturity stage were determined over two cruises representing an entire productive season of the North Water (NOW), to determine when and where populations spawn, and whether population growth tracks the expansion of open water and the development of diatom blooms. Oikopleura vanhoeffeni began to spawn in April and May, in association with an early diatom bloom caused by the opening of the Jones Sound and Inglefield Bay polynyas in the central NOW. Fritillaria borealis began population growth in June, during the transition from the early diatom bloom to an extended bloom of Chaetoceros socialis (‘C-bloom’). Subsequent spawning and population growth of both species followed the progression of open water and the developing C-bloom, to the north and then south during June–September. Maximum abundance and biomass of both species occurred during September, supported by the C-bloom prey community. Temperature had a strong, positive relationship with the abundance and biomass of both species, and moderate-to-high concentrations of diatoms seemed necessary for the initiation of spawning in O. vanhoeffeni. Appendicularians matured quickly in the early-opening polynyas in the NOW despite very low water temperatures and achieved high population growth rates in response to diatoms produced over an extended growing season (April–September).

Interactions between bivalves and zooplankton: competition or intraguild predation? Implications for biomanipulation in subtropical shallow lakes

Zooplankton and bivalves are the main consumers of phytoplankton in shallow lakes; however, knowledge regarding trophic interactions between them is scarce. Competition for resources appears to be an obvious direct interaction, but the scheme may be more complex. Bivalves can consume all or only part of the zooplankton, constituting an intraguild predation module. In the present study we investigated the interaction between bivalves and zooplankton and its effects on phytoplankton grazing and community structure using an experimental approach in an aquarium. Two bivalve species were considered, namely Diplodon parallelopipedon (native) and Corbicula fluminea (non-native), in addition to a natural zooplankton community dominated by small-sized zooplankton. The findings indicate that phytoplankton consumption by the zooplankton is substantially less than that by the bivalves. Under the experimental conditions, the bivalves actively consumed small-sized zooplankton (rotifers and nauplii), but no consumption of medium-sized individuals (cladocerans and copepods) was observed. The differential consumption may have implications on the size and structure of zooplankton, favouring an average larger-sized community. Thus, phytoplankton may be negatively affected both directly by consumption and indirectly because of the resulting structure of the zooplankton community. The main results of the study are also discussed from the perspective of biomanipulation techniques in tropical and subtropical regions.

Environmental and biological factors controlling the spring phytoplankton bloom at the Patagonian shelf-break front – Degraded fucoxanthin pigments and the importance of microzooplankton grazing

The aim of this study was to investigate the biotic and abiotic factors controlling the spring phytoplankton blooms at the Patagonian shelf-break front (PSBF). Using a CHEMTAX analysis of HPLC pigment data and other methods, the biomass and spatial variability of plankton communities were studied in four sections (39–48°S) across the PSBF during October 2005. Environmental factors and the biomass and composition of plankton communities exhibited a marked spatial heterogeneity. The latitudinal and cross-shelf progression in the timing of the spring bloom initiation and the nutritive properties of the water masses (Subantarctic Shelf Waters and Malvinas Current Waters) seemed to be the key factors. Three plankton regions were distinguished: (a) Outer shelf (OS), (b) Shelf-break front (SBF) and (c) Malvinas Current (MC). At the highly stratified OS region, the post-bloom community showed low-biomasshigh-phytoplankton diversity formed mainly by small cells (haptophytes 30–62%, diatoms 17–49%, chlorophytes 0–34%, and prasinophytes 0–21% of total Chl a). High amounts of degraded fucoxanthin were found associated with the heterotrophic dinoflagellate, Protoperidinium capurroi. Grazing by this microheterotroph on the diatom population seemed to be the most important factor for the spring bloom decay at the OS. A remarkable quasi monospecific bloom (∼90%) of a nanodiatom (Thalassiosira bioculata var. raripora) associated with high Chl a (up to 20mgm−3) occurred along (∼1000km) the SBF and in the most northern extension of the MC. In the southern region, the bloom was developed under absent or incipient density stratification, increasing solar irradiance, high nitrate and phosphate availability, and low numbers of phytoplankton grazers. The average mixedlayer PAR irradiance (<2.0mol quanta PAR m−2d−1) and Si:N ratios (<0.2) were low, suggesting a diatom population limited by light and under progressive silicate limitation. The more stratified northern region of the SBF showed a later stage of the bloom development, but the large population of diatoms under Si limitation was not in senescence and losses from microzooplankton grazing were minor. The observed high proportion of Chl a below a shallow upper mixed layer (up to 85%) could directly reach the bed, favoring the development of epibenthic communities and the formation of seed diatom banks and organic iron-rich sediments. The upwelling along the SBF provides a large source of macronutrients and probably the dissolved iron needed to sustain the intense diatom bloom, but also diatom resting stages that could act as seeds for the next spring bloom. The macronutrient-rich MC region showed low chlorophyll (Chl a<0.8mgm−3) and a highly diverse phytoplankton community, mainly composed of small cells (diatoms 20–70%, haptophytes 20–40%, chlorophytes 2–25%, prasinophytes 2–18%, and cryptophytes 3–12% of total Chl a).

Spatial distribution and feeding of dominant zooplankton species in the Ob River estuary

The distribution and feeding of dominant mesozooplankton species were studied in the estuary of the Ob River and adjacent inner Kara Sea shelf waters in September 2013. It was shown that the spatial distributions of Cyclops sp., Senecella siberica, Limnocalanus macrurus, Mysis oculata, Drepanopus bungei, Jashnovia tolli and Pseudocalanus sp. are related to the specific characteristics of the hydrographic regime in the estuarine frontal zone. The distributions of Cyclops sp., Senecella siberica, and Pseudocalanus sp. are mainly limited by salinity, while other species inhabit an area with a wide range of salinity values without clear preferences. Peaks of their abundance could be either consolidated or distanced in space. The populations of Jashnovia tolli, Drepanopus bungei, and Pseudocalanus sp. permanently inhabit the layer under the pycnohalocline; the populations of Cyclops sp. and Mysis oculata inhabit the upper mixed layer. Limnocalanus macrurus demonstrates a different vertical distribution pattern: the copepod undertakes diel vertical migrations in the southern part of the estuarine frontal zone; in its northern part, the population is concentrated below the pycnocline during day and night. The differences in the distributions of the studied species determine their feeding behavior and their role in phytoplankton grazing. The most intense utilization of biomass and production of autotrophic phytoplankton by zooplankton occur in the freshened water zone and the adjacent southern periphery of the estuarine frontal zone: the total daily phytoplankton consumption makes up 10–18% of the biomass and 60–380% of primary production. Daily zooplankton consumption of phytoplankton in the estuarine frontal zone decreases to 2–7% of the biomass and to 14% of primary production; in inner shelf waters, the values do not exceed 1% for both phytoplankton biomass and production.

Meso‐ and microzooplankton grazing in the Amazon River plume and western tropical North Atlantic

AbstractLargely due to size differences, mesozooplankton are important exporters of carbon and prey for larger organisms, while microzooplankton are important recyclers of nutrients, dominant grazers of phytoplankton, and a key link in the microbial loop. We investigated the relative importance of meso‐ and microzooplankton grazing in the western tropical North Atlantic Ocean (WTNA) and Amazon River plume. Sampling as part of the ANACONDAS project occurred in spring (May–June) 2010 during the peak outflow of the Amazon River and in fall (September–October) 2011 during the plume seasonal retroflection. Mesozooplankton grazing rates decreased with increasing salinity in both seasons, but during the fall both day and nighttime grazing rates were significantly negatively correlated with salinity. Mesozooplankton grazing was highest in plume‐influenced surface waters (0–25 m), and usually dominated by smaller size classes (0.2–0.5 mm and 0.5–1.0 mm). Microzooplankton grazing accounted for approximately 68% of bulk phytoplankton growth across all stations. Comparison of meso‐ and microzooplankton grazing suggests a transition in food web dynamics from a mesozooplankton dominated “export” structure in the plume transitioning to a microzooplankton dominated “retention” structure at mesohaline and oceanic stations above sea surface salinity of 33. Comparison between the seasons suggests a seasonal planktonic succession of low mesozooplankton grazing during the spring peak discharge followed by higher grazing rates and impact by mesozooplankton during the fall retroflection. These results provide important baseline information required for examining effects of climate change on the planktonic food web of the WTNA and for use in biogeochemical models of the region.

Lotic bacterioplankton and phytoplankton community changes under dissolved organic-carbon amendment: evidence for competition for nutrients

During periods of low river discharge, bacterial growth is typically limited by dissolved organic carbon (DOC) and is tightly regulated by phytoplankton production. However, import of allochthonous DOC into rivers by freshwater inflows may diminish bacterial reliance on phytoplankton-produced carbon, leading to competition for nitrogen (N) and phosphorus (P). To investigate phytoplankton–bacterial competition in response to allochthonous inputs, we conducted a mesocosm experiment, comparing microbial responses to the following two manipulation treatments: (1) addition of N and P, and (2) addition of a DOC and N and P. Measurement of chlorophyll-a estimated phytoplankton biomass and microscopic counts were performed to discriminate community change. Bacterial abundance was tracked using flow cytometry and community assemblages were characterised using automated ribosomal intergenic spacer analyses and 16S rRNA-amplicon sequencing. We found that bacterial abundance increased in the leachate addition, whereas chlorophyll-a was reduced and the bacterial community shifted to one dominated by heterotrophic genera, and autotrophic microbes including Synechococcus and Cyclotella increased significantly in the nutrient treatment. These observations indicated that DOC and nutrient inputs can lead to shifts in the competitive dynamics between bacteria and phytoplankton, reducing phytoplankton biomass, which may potentially shift the major pathway of carbon to higher trophic organisms, from the phytoplankton grazer chain to the microbial food web.

Structural Composition of Protozooplankton Communities in Relation to Environmental Factors in Shallow Lakes and Reservoirs of Rīga, Latvia

Abstract Protozooplankton are dominant grazers of phytoplankton and an important component of the microbial food web, as a link between pico and nanoplankton to higher trophic levels. Their fast growing rate, relative abundance, biomass and diversity are used as indicators of organic and toxic pollution. The impact of urbanisation on ecosystems and their sustainability and biodiversity have recently been much studied. We studied the protozooplankton ciliate communities during the vegetation period from April to October in two small lakes (Bābelītis, Gaiļezers) and two reservoirs (Bolderāja, Saurieši). The largest peak of biomass (15.7 × 102 mg/l) was found in Gaiļezers Lake in August and of abundance (60.2 × 103 org/l) in Bābelītis Lake in July. The lowest biomass (0.006 mg/l) and abundance (0.12 × 103 org/l) were found in the Saurieši Reservoir station. The most abundant ciliates were from the order Oligotrichida.

Mesozooplankton grazing impact on phytoplankton in the northern regions of the Kara Sea in autumn

The role of mass zooplankton species, copepods Pseudocalanus spp. and Oithona spp. and pteropods Limacina helicina, in grazing impact on phytoplankon in the upper 50 m layer was investigated. Material was collected at 14 stations along two sections crossing the outer shelf, continental slope and adjacent deep-water regions in the western and eastern branches of St. Anna trough (Kara Sea) in September 2011. Feeding rate was measured with fluorescence method. It was shown that total grazing impact on phytoplankton varied in a wide range from 0.2 to 8.7% of biomass and from 2 to 230% of daily primary production. The main contribution to phytoplankton grazing was made by small copepods Oithona spp. and juvenile pteropods. Maximum values of grazing were found in the frontal zones over the slope, minimum–over the outer continental shelf. Significant mesoscale spatial heterogeneity in total zooplankton grazing due to the changes in zooplankton composition and abundance was observed.

Microzooplankton community associated with phytoplankton blooms in the naturally iron-fertilized Kerguelen area (Southern Ocean).

The spatial and temporal community composition of microzooplankton (dinoflagellates and ciliates) was assessed in the Kerguelen area (Southern Ocean) during the KEOPS2 cruise in early spring (October-November) 2011. This naturally iron-fertilized region was characterized by a complex mesoscale circulation resulting in a patchy distribution of phytoplankton blooms. Collectively, 97 morphospecies of dinoflagellates and ciliates belonging to 41 genera were identified by microscopy, and 202 Alveolata-related OTUs (operational taxonomical units) were retrieved with tag-pyrosequencing. Microscopy and pyrosequencing data were in accordance, in that diatom-consuming dinoflagellates were the most enhanced taxa in the blooms. Dinoflagellates also showed significant positive relationships with phytoplankton pigments, while no major differences were found in the ciliate abundances inside and outside the blooms. Cluster analysis showed clear differences in the phytoplankton and microzooplankton community structures between the iron-fertilized and HNLC (high nutrient low chlorophyll) waters, and between the blooms, concerning their location and the fertilization mechanisms. These results were combined with the rates of primary production and mesozooplankton consumption determined for the study area. The potential role of dinoflagellates and ciliates as phytoplankton consumers and as prey for mesozooplankton was then evaluated. Overall, heterotrophic dinoflagellates were probably the most important group of phytoplankton grazers, and a potential food source for copepods.