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Preview: Journal of Plankton Research - current issue

Journal of Plankton Research Current Issue

Published: Wed, 13 Sep 2017 00:00:00 GMT

Last Build Date: Tue, 03 Oct 2017 12:48:41 GMT


Overwintering effects on the spring bloom dynamics of phytoplankton


The influence of winter on the selection of dominant taxa for the phytoplankton spring bloom was studied in batch culture experiments. Different natural phytoplankton assemblages from different phases of the temperate zone winter were exposed to varying periods of darkness (0, 6/7, 13 and 19 weeks) followed by a re-exposure to saturating light intensity for 14 days to experimentally simulate the onset of spring. The results showed that dark incubation has a strong effect on shaping the phytoplankton community composition. Many taxa disappeared in the absolute darkness. Dark survival ability might be an important contributing factor for the success of diatoms in spring. Different phytoplankton starting assemblages were dominated by the same bloom-forming diatoms, Skeletonema marinoi and Thalassosira spp., after dark incubation for only 6 weeks, irrespective of the high dissimilarities between phytoplankton communities. The growth capacity of surviving phytoplankton is almost unimpaired by darkness. Similar growth rates as that before darkness could be resumed for the surviving taxa with a potential lag time of 1–7 days dependent on taxon and the duration of darkness.

Coccolithophore haploid and diploid distribution patterns in the Mediterranean Sea: can a haplo-diploid life cycle be advantageous under climate change?


Coccolithophores are unicellular pelagic algae, capable of calcification. In the Mediterranean Sea, several species have a well-known haplo-diploid life cycle, alternating the production of different types of calcite plates, the holo- and hetero-coccoliths. We analyzed the distribution of both phases along a W-E Mediterranean transect during April 2011 and May 2013 (spring season), following strong environmental gradients in salinity, oxygen and nutrient concentration, temperature, carbonate chemistry and fluorescence. The proportion of holococcolithophores:heterococcolithophores of selected species varies not only vertically through the water column, but also longitudinally, following the main environmental gradients. Based on the environmental affinities of the coccolithophore life phases, we conclude that a dimorphic life cycle might provide the ability to adapt to the south-eastern (SE) Mediterranean environment, in conditions characterized by surface water with relatively high calcite saturation state, high temperature, stratification and nutrient limitation, and support the survival of species whose diploid phases are in contrast adapted to Atlantic or south-western (SW) Mediterranean conditions. Thus, a haplo-diploid life cycle could provide a way to adapt to environmental changes.

Contrasting diel vertical migration patterns in Salpa fusiformis populations


Vertically stratified zooplankton sampling at three locations in the Catalan Sea demonstrated that the coexistence of two diel vertical migration patterns in Salpa fusiformis populations. Salps migrated to the surface during the day (reverse migration) at one station while they swam to surface layers at night (nocturnal migration) at the other two stations. While nocturnal migration was significant at the deepest station, reverse and nocturnal migrations at shallow stations were weak. Our data support the idea of reproductive, surface aggregation in this species, with a possibility that the larger individuals hit the bottom at the shallower stations, resulting in an apparently random migration pattern.

Assessing an efficient “Instant Acclimation” approximation of dynamic phytoplankton stoichiometry


The variable elemental ratios of carbon to essential nutrients in marine organic matter affect the productivity of marine food-webs and the sequestration of carbon in the deep ocean. It is important that models of these systems are able to correctly reproduce observed trends. “Dynamic Quota” models have achieved some success in this regard, but the computational expense of transporting each state variable in ocean models has prevented many large-scale models from moving beyond a simpler “Fixed Stoichiometry” formulation. This article compares the Dynamic Quota and Fixed Stoichiometry models to a recent “Instant Acclimation” model, which combines the stoichiometric flexibility of the Dynamic Quota model with the computational efficiency of the Fixed Stoichiometry model. The Instant Acclimation model is mathematically equivalent to the Dynamic Quota model at equilibrium, and provides an accurate approximation under a wide range of dynamic conditions. The accuracy and computational efficiency of the Instant Acclimation model recommend it as a candidate for incorporating flexible stoichiometry into marine ecosystem models, especially in situations where the number of model state-variables is restricted.

Modelling rates of random search over the transition from diffusive to ballistic movement of plankton


The rate of search for food (i.e. maximum clearance rate), F, of a plankter is essential to the prediction of encounter rates, and is dependent on movement. Classic encounter rate models assume diffusive or ballistic movements, which represent opposing extremes of directional persistence. From the perspective of the predator, the directional persistence of prey is determined by the ratio of the persistence length (i.e. “run length” of a random walker), λ, and the radius of prey detection, r. We developed an individual-based model to (i) describe variation in F due to λ/r and time, and (ii) evaluate the utility of published corrections (that take into account the effect of λ/r on F) to the classic models. Our results illustrate that classic models overestimate F when their assumptions of movement are invalid, and indicate that the effect of time variation in F on food consumption is most substantial near the middle of the diffusive to ballistic transition (i.e. λ/r ≈ 1). At λ/r ≪ 1, predators may exploit high clearance rates by “jumping”, provided that the far-field concentration of prey is sufficiently high. We recommend a published Michaelis–Menten type correction to the classic models, and discuss the assumptions and applications of our model system.

Controlling harmful cyanobacterial blooms in a climatically more extreme world: management options and research needs


Cyanobacteria have a long evolutionary history that has been instrumental in allowing them to adapt to long-term geochemical and climatic changes, as well as current human and climatic alterations of aquatic ecosystems; e.g. nutrient over-enrichment, hydrologic modifications and warming. Harmful (toxic, hypoxia-generating, food web altering) cyanobacterial bloom (CyanoHAB) genera are particularly adept at taking advantage of these changes and perturbations. In addition, they have developed numerous mutualistic and symbiotic associations with other microbes and higher flora and fauna, and they modulate positive biogeochemical feedbacks, instrumental in their survival and dominance in diverse ecosystems. CyanoHABs are controlled by the combined and often synergistic effects of nutrient (nitrogen and phosphorus) inputs, light, temperature, water residence/flushing times, and biotic interactions. Accordingly, mitigation strategies are oriented towards manipulating these dynamic factors. Physical, chemical (nutrient) and biological manipulations can be effective in reducing CyanoHABs. However, these manipulations should also be accompanied by nutrient (both nitrogen and phosphorus in most cases) input reductions to ensure long-term success and sustainability. A major research and management goal for long-term control of CyanoHABs is to develop strategies that are adaptive to climatic variability and change, because nutrient-CyanoHAB thresholds are likely to be altered in a climatically more extreme world.

Effects of zooplankton grazing on the bloom-forming Cyanothece sp. in a subtropical estuarine lake


Modern anthropogenic modifications to aquatic environments, specifically through changes in trophic state and hydrodynamic conditions, have facilitated increasing frequency and duration of cyanobacterial blooms world-wide. This study investigated the potential for top-down grazing control by mesozooplankton on a persistent bloom-forming (18 months) cyanobacterium, Cyanothece sp., isolated from Lake St Lucia (South Africa). Seasonal surveys and in situ experiments were conducted to assess the zooplankton community structure and its grazing impact on Cyanothece sp. cells, respectively. Grazing experiments showed that zooplankton can have high grazing impacts on Cyanothece sp., but that increased salinity levels are responsible for the observed decrease in total abundances and the shift to a halotolerant community. Most typical estuarine zooplankton species at St Lucia disappear when salinity increases above 100. Additionally, the halotolerant zooplankton species, specifically Fabrea salina, occurred in low densities at the time of the study. This would result in a major drop of grazing control by zooplankton on Cyanothece sp. The results, which indicate that zooplankton can potentially play a key role in top-down grazing control of cyanobacterial bloom, are relevant in the global context where the occurrence of persistent cyanobacterial blooms is on the rise.

Insights into thermal preferences of copepods in nature using the horizontal gradient method


The final thermal preferendum (FTP) and the lower and upper thresholds of temperatures avoided were defined for the first time for Cyclops vicinus Ulyanin, 1875 (Crustacea: Copepoda). The preferred temperature on day one had a modal value of 13.9°C (range: 11–17°C), which increased on Days 2 and 3 to 20.4°C (range: 17–23°C) and 20.2°C (range: 17–22°C), respectively, and then declined during Days 4 (19.1) to 7 (17.1°C). The selection process occurs with overshoot. On days seven and eight after placement in the thermal gradient the preferred temperature stabilized at 17.1–17.5°C. These temperatures are accepted as the FTP of individual C. vicinus. Cyclops vicinus avoided temperatures below 5°C and above 29°С. We can compare these thermal ranges observed under experimental conditions with the typical temperature values reported for field populations. This study provides a proof of concept of an approach that was used previously primarily for determining species specific thermal preferences in fish and then adapted for cladocerans. This laboratory approach using C. vicinus provides an example of how this method can be used to improve interpretation of copepod phenologies in nature.

First observations of living sea-ice diatom agglomeration to tintinnid loricae in East Antarctica


Tintinnid ciliates are an important link in marine food webs as they feed on phytoplankton and bacteria while providing nutrients to higher trophic levels. Tintinnids are known to agglutinate mineral particles or dead biogenic material such as diatom frustules to their shell-like housing (lorica), however, reasons for this agglutination remain questioned. We report on our observation of agglomeration of the living diatoms Fragilariopsis curta, F. cylindrus, F. pseudonana and F. rhombica to loricae of the Antarctic tintinnid ciliates Laackmanniella naviculaefera and Codonellopsis gaussi. These unusual associations between living diatoms and tintinnids were exclusively observed south of 63.59°S. We discuss the significance of our new finding and generate hypotheses to be tested by future research. It remains unclear where these living diatom–tintinnid associations are initially formed (in or near sea ice or also further north when abundances of L. naviculaefera, C. gaussi, F. curta, F. cylindrus, F. pseudonana and F. rhombica happen to be relatively high); who the beneficiary is in this association; what the exact benefits are; and how they might influence the Southern Ocean carbon cycle. Nevertheless, our observation provides a key step forward towards illuminating the largely unknown ecology of two Southern Ocean-endemic tintinnid species.

Phospholipid-bound eicosapentaenoic acid (EPA) supports higher fecundity than free EPA in Daphnia magna


Nutrition bioassays in which polyunsaturated fatty acids (PUFA)-deficient diets were supplemented with free long-chain PUFA (≥C20) consistently revealed positive effects on somatic growth and fecundity of Daphnia. However, free PUFA are hardly available in natural diets. In general, PUFA are bound to other lipids, especially to phospholipids and triglycerides. Here, we evaluate the potential of free and phospholipid-bound dietary eicosapentaenoic acid (EPA) to support somatic growth and fecundity of Daphnia magna. In a growth experiment, supplementation of a C20 PUFA-deficient diet with free or phospholipid-bound EPA improved somatic growth rates of D. magna equally. However, the increase in fecundity was significantly more pronounced when phospholipid-bound EPA was provided. Free and phospholipid-bound EPA were provided in the same concentrations in our experiment, suggesting that the allocation to reproduction-related processes is affected differently by phospholipid-bound PUFA and free PUFA. Our finding stresses the need to consider the distribution of dietary PUFA in different lipid classes to gain a better understanding of how PUFA influence life history traits of Daphnids in the field.