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Preview: Journal of Plankton Research - current issue

Journal of Plankton Research Current Issue

Published: Thu, 06 Jul 2017 00:00:00 GMT

Last Build Date: Fri, 28 Jul 2017 11:45:36 GMT


Population genetic structure of Calanoides natalis (Copepoda, Calanoida) in the eastern Atlantic Ocean and Benguela upwelling system


The population genetic structure of Calanoides natalis (ex Calanoides carinatus; Copepoda, Calanoida), an ecologically important component of African upwelling systems, was studied in order to (i) search for potential cryptic species, (ii) describe spatial patterns in the distribution of genetic variance and (iii) identify potential barriers to gene flow. Samples were obtained in the eastern Atlantic Ocean from the Iberian Peninsula to Namibia. Analysis of mitochondrial (cytochrome c oxidase subunit I; COI) and nuclear (citrate synthase; CS) marker genes revealed a genetically cohesive population of C. natalis with a prevalent shift in allele frequencies. The discovery of a deep split solely present in the mitochondrial dataset does not point to cryptic speciation, but rather suggests the occurrence of nuclear mitochondrial pseudogenes or incomplete reproductive isolation upon secondary contact. Genetic differentiation between the northern and southern hemisphere was significant, which may point to a potential, but permeable barrier close to the equator. No vertical genetic structuring was detected in the northern Benguela implying that horizontal differentiation was more pronounced than vertical structuring. Retention mechanisms and the oxygen minimum zone did not have a strong impact on genetic differentiation of C. natalis in the Benguela region.

Unravelling diversity of deep-sea copepods using integrated morphological and molecular techniques


Accurate species identification is crucial for ecological studies. For copepods, this is usually based on a few diagnostic morphological characters, which can be highly conserved, resulting in an underestimation of species diversity in many copepod families. We elucidate species richness in the morphologically challenging and ecologically important deep-sea copepod family Spinocalanidae in the tropical Atlantic by applying an integrated taxonomic approach combining morphology, DNA-sequence analyses and proteomic fingerprinting. In total, 28 morphospecies could be discriminated, while 39 putative species were detected using DNA-sequence analyses and 42 using proteomic fingerprinting. This outcome verifies proteomic fingerprinting to simplify and accelerate future biodiversity studies of copepods with high taxonomic resolution. Our findings demonstrate the power of this integrated morphological and molecular taxonomic approach by revealing high numbers of cryptic or pseudocryptic species and thus uncovering the incompleteness of taxonomic guides for this group in the poorly explored mesopelagic realm. Furthermore, our analyses reveal a close relationship of Mospicalanus and Spinocalanus group A and indicate that the genus Spinocalanus may be polyphyletic. The underestimated species diversity suggests complex ecological interactions in terms of predator–prey relationships, interspecific competition and species-specific specializations in the vast, but under-studied mesopelagic realm.

Transfer of ice algae carbon to ice-associated amphipods in the high-Arctic pack ice environment


Sympagic (ice-associated) amphipods channel carbon into the marine ecosystem. With Arctic sea ice extent in decline, it is becoming increasingly important to quantify this transfer of sympagic energy. Recently, a method for quantifying sympagic particulate organic carbon (iPOC) in filtered water samples was proposed based on the abundances of the Arctic sea ice biomarker IP25. Here, we tested the hypothesis that adoption of this method could also provide quantitative estimates of iPOC transfer within Arctic amphipods. We analysed five amphipod species collected north of Svalbard and compared findings to some previous studies. Estimates showed that Onisimus glacialis and Apherusa glacialis contained the most iPOC, relative to dry mass (23.5 ± 4.5 and 9.8 ± 1.9 mg C g−1, respectively), while Gammarus wilkitzkii had the highest grazing impact on the available ice algae (0.48 mg C m−2, for an estimated 24 h), equating to 73% of algal standing stock. Our findings are also broadly consistent with those obtained by applying the H-Print biomarker approach to the same samples. The ability to obtain realistic quantitative estimates of iPOC transfer into sympagic and pelagic fauna will likely have important implications for modelling energy flow in Arctic food webs during future climate scenarios.

Gender-specific feeding rates in planktonic copepods with different feeding behavior


Planktonic copepods have sexually dimorphic behaviors, which can cause differences in feeding efficiency between genders. Copepod feeding rates have been studied extensively but most studies have focused only on females. In this study, we experimentally quantified feeding rates of males and females in copepods with different feeding behavior: ambush feeding (Oithona nana), feeding-current feeding (Temora longicornis) and cruising feeding (Centropages hamatus). We hypothesize that carbon-specific maximum ingestion rates are similar between genders, but that maximum clearance rates are lower for male copepods, particularly in ambush feeders, where the males must sacrifice feeding for mate searching. We conducted gender-specific functional feeding response experiments using prey of different size and motility. In most cases, gender-specific maximum ingestion and clearance rates were largely explained by the difference in size between sexes, independent of the feeding strategy. However, maximum clearance rates of males were approximately two times higher than for females in the ambush feeding copepod O. nana feeding on an optimal motile prey (Oxyrrhis marina), as hypothesized. We conclude that the conflict between mate searching and feeding can cause significant difference in feeding efficiency between copepod genders in ambush feeders but not in feeding-current and cruising feeders.

Nutrient-specific responses of a phytoplankton community: a case study of the North Atlantic Gyre, Azores


Nutrient concentrations are unevenly distributed in the oceans, influencing the abundance and composition of phytoplankton communities. Even so, the dominant driving factors responsible for variability between phytoplankton communities are still unclear. In the North Atlantic Gyre, the Azores present a good opportunity to study phytoplankton communities of oligotrophic areas that experience nutrient pulses. We followed the development of an enclosed natural phytoplankton community occurring off the coast of Terceira (Azores) and tested the effects of single (nitrate, phosphate, silicate and a mix of the trace metals Fe, Co, Cu, Mo, Zn and Mn) and combined nutrient enrichments on phytoplankton abundance, particulate organic matter (POM) build-up, nutrient drawdown and community composition. Towards the end of the microcosm-based incubation, biomass developed dramatically (430-fold) when all the nutrients considered were added simultaneously. Importantly, the community composition at the end of the incubation was dependent on the combination of nutrients supplied, with diatoms dominating most of the treatments; coccolithophores under Phosphate + Trace Metals; and organisms with characteristics of a nitrogen fixer such as low δ15N under full nutrient enrichment. These results indicate group-specific nutrient requirements and limitations occurring near the Azores with a few taxa dominating the groups’ response to nutrient pulses.

Metacaspases and programmed cell death in Skeletonema marinoi in response to silicate limitation


Diatoms comprise up to 40% of annual productivity in the modern ocean, and some stress factors may alter their physiology and survival. Here, we describe the morphological, biochemical and molecular responses that occur during metacaspase-mediated programmed cell death (PCD) in response to silicate limitation in the ubiquitous diatom Skeletonema marinoi. Transmission electron microscopy revealed that vacuolization occurred while the plasma membranes remained intact. Caspase 3-like activity, antioxidant gene expression, phosphatidylserine externalization and activated caspase-like expression were measured using in vivo staining. The results show that each was significantly increased under silicate limitation conditions. Using quantitative reverse transcription-PCR, we identified six distinct putative metacaspase proteins in S. marinoi, and we show that each contains a conserved caspase-like domain and is differentially expressed and that some of their expression profiles were correlated with PCD. Silicate limitation could trigger PCD in S. marinoi as well as an increase of antioxidant genes. PCD could be employed as a strategy to decrease silicate demand of the whole population and probably regulate the fate of S. marinoi.

Dynamics of Teleaulax -like cryptophytes during the decline of a red water bloom in the Columbia River Estuary


The mixotrophic ciliate, Mesodinium rubrum, is a globally distributed ciliate that relies on the acquisition and use of chloroplasts derived from its cryptophyte prey. The ecology and physiology of the cryptophytes is not well known, nor is it clear how their growth influences M. rubrum blooms. A 4-week survey was conducted in the Columbia River estuary in 2013 during the decline of the annual M. rubrum bloom to better understand how environmental factors influence the dynamics of the cryptophyte prey, Teleaulax amphioxeia. Abundances and division rates of free-living Teleaulax-like cryptophytes were continuously monitored using flow cytometry. Cryptophyte division rates, estimated in situ for the first time using a size-structured division rate model, ranged from 0.2 to 1.5 d−1, with the highest rates observed in accordance with high abundances. These division rates were positively correlated with concentrations of dissolved inorganic nitrogen and phosphorus, suggesting nutrient availability limited the growth of Teleaulax-like cryptophytes at that time. Assuming a minimum ingestion rate of ~1 cryptophyte ciliate−1 day−1, the growth of M. rubrum may have been limited by the low abundance of Teleaulax-like cryptophytes during the M. rubrum bloom decline. Our results highlight the importance of prey availability for understanding the dynamics of red water blooms.

Ecotoxicology of salinity tolerance in Daphnia pulex : interactive effects of clonal variation, salinity stress and predation


Despite decades of research on the impacts of salinity in freshwater systems, the effects of salinity stress on planktivore–zooplankton interactions have received limited attention. We used laboratory-based experiments to examine Daphnia pulex responses to salinity stress and the lethal and non-lethal effects of Chaoborus (a dominant planktivore in fishless ponds). We also examined how D. pulex clonal variation mediates these responses using two clones known to differ in salinity tolerance. Presence of kairomone induced neckteeth formation, increased fertility and increased age and size at maturity relative to controls. As predicted, increasing salinity generally weakened life history responses to kairomone and reduced survivorship in the presence of lethal predation. While some of our results are suggestive of a moderating effect of clonal variation in salinity tolerance, clone effects on responses to increasing salinity were inconsistent. Our study demonstrates that non-lethal levels of salinity stress have the potential to impact Daphnia populations negatively by altering life history and behavioral responses to predators.

Variability in Calanus finmarchicus egg production rate measurements: methodology versus reality


Egg production rates (EPRs) for freshly caught female Calanus finmarchicus increase with increasing in situ chlorophyll concentration to upper limits, but with considerable scatter for individual points around fitted curves. Here, using time course experiments, we investigated whether females exhibit synchronous diel egg-laying behaviour, leading to variations in measured EPRs for different experimental start times. Also, we compared 24 h EPRs from these experiments with results obtained using two other standard 24 h incubation methods. We found (i) female C. finmarchicus from the Labrador Sea did not exhibit diel egg-laying behaviour in spring, (ii) egg-laying was sometimes more frequent during the first 6 h of incubation than thereafter and (iii) standard 24 h incubations underestimated EPRs in 20–36% of experiments due to egg loss. Using results from this and a previous study Head et al. [(2013a) Characteristics of egg production of the planktonic copepod, Calanus finmarchicus, in the Labrador Sea: 1997–2010. J. Plankton Res. 35, 281–298] we developed a new expression relating in situ EPRs to chlorophyll concentration, temperature, female size and season. Season is represented by empirically derived coefficients, which vary regionally reflecting differences in spring bloom dynamics. For 100 experimental stations predicted and measured EPRs were highly correlated (r2 = 0.56, P < 0.001). Additional sources of variability are discussed and recommendations are made regarding EPR measurement methods.

Glacier melting and response of Daphnia oxidative stress


We analysed the antioxidant response of Daphnia commutata in an oligotrophic North-Patagonian lake (Lake Mascardi) that receives inputs of glacial clay in one extreme, which creates a plume with a consequent gradient in underwater light intensity (including ultraviolet radiation) and suspended solid material. This gradient in light intensity also affects the light:nutrient ratio and hence the C:P ratio of the food for planktonic herbivores. In the field, along a 9 km transparency gradient, we measured the activities of glutathione S-transferase (GST) and catalase (CAT) enzymes involved in protection against UVR. Through laboratory experiments, we tested the possible role of suspended sediment particles as an additional stressor for a filter feeding zooplankter. Our results indicate that the inputs of glacial clay into the lake have antagonistic effects on Daphnia. Glacial clay was a stress mitigating factor to UVR (decrease in the antioxidant response of GST activity), but was also a source of stress that generated feeding interference, increased respiration rates and consequently increased CAT activity. This light gradient also affected the C:P ratio of food and the maximum response in GST is also modulated by food quality (C:P ratio) that limits its activity in the transparent end of the gradient.

Does environmental heterogeneity explain temporal β diversity of small eukaryotic phytoplankton? Example from a tropical eutrophic coastal lagoon


The temporal β diversity of small eukaryotic phytoplankton in a tropical urban coastal lagoon (Rodrigo de Freitas Lagoon, Brazil) was characterized throughout two annual cycles (2012–2103) with twice-weekly sampling. Small eukaryotic phytoplankton (<20 μm) were very abundant (above 1 × 103 and up to 6.2 × 105 cells mL−1). Small eukaryotes were also the most diverse phytoplankton group, showing high temporal variability at the species level that was represented by seven taxonomic divisions. Temporal β diversity was positively related to environmental heterogeneity on a short-time scale. Although abundance peaks of the main species were positively related to NH4 inputs, selection of species ultimately depended on the simultaneous interaction among several physical-chemical variables acting hierarchically on different temporal scales (first seasonally and then on a short-time scale). Considering the numerical importance of small size fraction phytoplankton, information relevant to the management and conservation of coastal lagoons could be incomplete if the floristic composition and temporal species turnover of pico- and nanophytoplankton species are not properly determined.

Spatial variation in dinoflagellate recruitment along a reservoir ecosystem continuum


Physical and chemical gradients across ecosystems, such as stream-to-lake continua within human-made reservoirs, provide valuable opportunities to examine how organisms respond to changing environments. We quantified the rate of dinoflagellate recruitment across a small reservoir to test the hypothesis that organisms are controlled by different factors along a reservoir continuum. We predicted that recruitment would be tightly coupled with reservoir physics in the riverine zone and closely related to water chemistry in the lacustrine zone. For the dominant dinoflagellate genus in the reservoir, Peridinium, recruitment from the sediments accounted for a median of 16% of increases in pelagic cell abundance throughout the summer. As predicted, Peridinium recruitment rates at the riverine site were correlated with physical variables, while at the lacustrine site, recruitment rates were highly correlated with water chemistry (e.g. nutrient ratios and dissolved oxygen). Recruitment patterns of the second most common genus, Gymnodinium, were not correlated with environmental variables, though Gymnodinium’s much lower densities suggest that its dynamics were controlled by other factors. Our results reveal that the physical–biological coupling controlling algal recruitment, which can play a large role in pelagic population growth and bloom formation, can vary substantially on a spatial gradient within even a small reservoir.