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Krill and salps play a pivotal role in marine food webs and ocean biogeochemistry, yet little is known about their stoichiometry, which is crucial for our understanding of the consequences of key grazer population shifts for ecosystem functioning. To assess differences in stoichiometric requirements, we compared the elemental composition of carbon (C), nitrogen (N) and phosphorus (P) in body tissue and gut content of Euphausia superba and Salpa thompsoni collected along the Northern Antarctic Peninsula and conducted a systematic literature review on the stoichiometry of 33 euphausiid and 19 salp species. Our data revealed significantly lower body C:P and N:P ratios but higher gut content N:P in S. thompsoni compared with E. superba, suggesting higher P uptake. In contrast, literature data showed higher C:N, C:P and N:P ratios in salps, potentially indicating lower nutrient demand compared with krill. However, literature data exhibited high intra- and interspecific variability potentially driven by organism size, seasonal or latitudinal variations, emphasizing that nutritional requirements are not only related to the target species, but also to its habitat. Our study provides valuable stoichiometric information for future experimental and modeling studies, fostering our ability to predict the future role of krill and salps for ocean biogeochemistry.

Zooplankton community structure is often characterized by using traits as a function of environmental conditions. However, trait-based knowledge on Southern Ocean mesozooplankton is limited, particularly regarding size and elemental composition. Nine stations around the northern Antarctic Peninsula were sampled during austral autumn to investigate the spatial variability in mesozooplankton taxonomic composition, size structure and stoichiometry in relation to environmental predictors, but also to the abundance of Antarctic krill and salps. The mesozooplankton communities around the South Shetland Islands were dominated by small copepods, mainly Oithonidae and Oncaeidae, while stations along the frontal zones and the Weddell Sea revealed a higher proportion of larger organisms. Spatial differences in taxonomic composition and size structure were significantly altered by salp abundance, with stronger impact on small-sized copepods. Furthermore, taxonomic composition was significantly related to temperature and total carbon but not chlorophyll a, indicating reduced relevance of phytoplankton derived food during autumn. Bulk mesozooplankton stoichiometry, however, showed no significant relation to environmental conditions, mesozooplankton size structure or dominant taxa. Our results indicate that aside from bottom-up related drivers, top-down effects of salps may lead to mesozooplankton communities that are more dominated by larger size classes with potential consequences for trophic interactions and nutrient fluxes.

Harmful algal blooms (HABs) are globally increasing in number and spatial extent. However, their propagation dynamics along environmental gradients and the associated interplay of abiotic factors and biotic interactions are still poorly understood. In this study, a nutrient gradient was established in a linear meta‐ecosystem setup of five interconnected flasks containing an artificially assembled phytoplankton community. The harmful dinoflagellate Alexandrium catenella was introduced into different positions along the nutrient gradient to investigate dispersal and spatial community dynamics. Overall, total algal biovolume increased, while community evenness decreased with increasing nutrient concentrations along the gradient. Alexandrium was able to disperse through all flasks. On the regional scale, diatoms dominated the community, whereas on the local scale the dinoflagellate showed higher contributions at low nutrient concentrations and dominated the community at the lowest nutrient concentration, but only when initiated into this flask. A control treatment without dispersal revealed an even stronger dominance of Alexandrium at the lowest nutrient concentration, indicating that dispersal and the associated nutrient exchange may weaken dinoflagellate dominance under low nutrient conditions. This study presents a first approach to experimentally investigate spatial dynamics and ecological interactions of a harmful dinoflagellate along an environmental gradient in a meta‐ecosystem setup, which has the potential to substantially enhance our understanding of the relevance of dispersal for HAB formation and propagation in combination with local environmental factors., Volkswagen Foundation http://dx.doi.org/10.13039/501100001663

Wadden Sea macrozoobenthos has access to food sources of different quality. In addition to microphytobenthos (MPB) and macroalgae (e.g. Fucus vesiculosus Linnaeus, 1753), the invasive C4 grass Spartina anglica C.E. Hubbard has become an abundant primary producer in the Wadden Sea. To elucidate the role of different primary producers for the nutrition of typical Wadden Sea consumers, we conducted a mesocosm experiment with four food treatments ( S. anglica, F. vesiculosus , MPB and all three resources together) under a simulated tidal cycle. We used consumers of different feeding types—the omnivorous polychaete Hediste diversicolor (O.F. Muller, 1776), the interface-feeding bivalve Limecola balthica (Linnaeus, 1758) and the surface deposit-feeding snail Peringia ulvae (Pennant, 1777)—and analyzed their biomass, δ13C and δ15 N stable isotope values and fatty acid (FA) profiles. After 4 weeks, the biomass of H. diversicolor increased in treatments that contained F. vesiculosus, but decreased in the treatment with MPB. δ13C values of H. diversicolor reflected those of the food sources provided and FA profiles varied strongly between the treatments, indicating feeding on copepods and bacteria as well as macrophytes. The biomass of P. ulvae significantly increased in treatments with S. anglica and all three resources. However, δ13C values of P. ulvae differed significantly between the treatments and were unrelated to the food sources except for MPB. FA profiles confirmed the assimilation of MPB in this species. δ13C values and FA composition of L. balthica were similar in each of the treatments and did not resemble the resources provided. Instead, the high proportion of respective marker FAs indicated feeding on plankton . δ 15N values of the consumer species declined significantly in the order H. diversicolor > L. balthica > P. ulvae in each of the treatments. These results indicate different strategies in food source use of the three consumer species with the performance of the generalist feeders H. diversicolor and P. ulvae exceeding that of the selective feeder L. balthica.

Trait variation among heterospecific and conspecific organisms may substantially affect community and food web dynamics. While the relevance of competition and feeding traits have been widely studied for different consumer species, studies on intraspecific differences are more scarce, partly owing to difficulties in distinguishing different clones of the same species. Here, we investigate how intraspecific trait variation affects the competition between the freshwater ciliates Euplotes octocarinatus and Coleps hirtus in a nitrogen‐limited chemostat system. The ciliates competed for the microalgae Cryptomonas sp. (Cry) and Navicula pelliculosa (Nav), and the bacteria present in the cultures over a period of 33 days. We used monoclonal Euplotes and three different Coleps clones (Col 1, Col 2, and Col 3) in the experiment that could be distinguished by a newly developed rDNA‐based molecular assay based on the internal transcribed spacer (ITS) regions. While Euplotes feeds on Cry and on bacteria, the Coleps clones cannot survive on bacteria alone but feed on both Cry and Nav with clone‐specific rates. Experimental treatments comprised two‐species mixtures of Euplotes and one or all of the three different Coleps clones, respectively. We found intraspecific variation in the traits “selectivity” and “maximum ingestion rate” for the different algae to significantly affect the competitive outcome between the two ciliate species. As Nav quickly escaped top‐down control and likely reached a state of low food quality, ciliate competition was strongly determined by the preference of different Coleps clones for Cry as opposed to feeding on Nav. In addition, the ability of Euplotes to use bacteria as an alternative food source strengthened its persistence once Cry was depleted. Hence, trait variation at both trophic levels codetermined the population dynamics and the outcome of species competition., Intraspecific variation in the traits “selectivity” and “maximum ingestion rate” for their prey significantly affected the competition between two freshwater ciliate species. Exclusion of either one of the ciliates or long‐term persistence depended on which of the competitors benefited more from feeding on a less abundant algal resource of high food quality, on severe nutrient limitation that affected the food quality of the dominant algal resource, and on the ability of one competitor to use bacteria as an alternative food source. Hence, trait variation at both trophic levels codetermined the population dynamics and the outcome of species competition.

Pronounced atmospheric and oceanic warming along the West Antarctic Peninsula (WAP) has resulted in abundance shifts in populations of Antarctic krill and Salpa thompsoni determined by changes in the timing of sea-ice advance, the duration of sea-ice cover and food availability. Krill and salps represent the most important macrozooplankton grazers at the WAP, but differ profoundly in their feeding biology, population dynamics and stoichiometry of excretion products with potential consequences for the relative availability of dissolved nitrogen and phosphorus. Alternation of the dissolved nutrient pool due to shifts in krill and salp densities have been hypothesized but never explicitly tested by using observational data. We therefore used the Palmer LTER dataset in order to investigate whether the dominance of either grazer is related with the observed dissolved nitrogen:phosphorus (N:P) ratios at the WAP. Across the whole sampling grid, the dominance of salps over krill was significantly correlated to higher concentrations of both N and P as well as a higher N:P ratios. Using actual long-term data, our study shows for the first time that changes in key grazer dominance may have consequences for the dynamics of dissolved nitrogen and phosphorus at the WAP.

Consumer diversity effects on ecosystem functioning are highly context dependent and are determined by consumer specialization and other consumer and prey specific traits such as growth and grazing rates. Despite complex reciprocal interactions between consumers and their prey, few experimental studies have focused on prey diversity effects on consumer dynamics and trophic transfer. In microbial microcosms, we investigated effects of algal prey diversity (one, two and four species) on the production, evenness and grazing rates of 4 ciliate consumers, differing in grazing preferences and rates. Prey diversity increased prey biovolume in the absence of consumers and had opposing effects on different consumers, depending on their specialization and their preferred prey. Consumers profited from prey mixtures compared to monocultures of non-preferred prey, but responded negatively if preferred prey species were offered together with other species. Prey diversity increased consumer evenness by preventing dominance of specific consumers, demonstrating that the loss of prey species may have cascading effects resulting in reduced consumer diversity. Our study emphasizes that not only the degree of specialization but also the selectivity for certain prey species within the dietary niche may alter the consequences of changing prey diversity in a food web context.

Climatic warming is a primary driver of change in ecosystems worldwide. Here, we synthesize responses of species richness and evenness from 187 experimental warming studies in a quantitative meta-analysis. We asked 1) whether effects of warming on diversity were detectable and consistent across terrestrial, freshwater and marine ecosystems, 2) if effects on diversity correlated with intensity, duration, and experimental unit size of temperature change manipulations, and 3) whether these experimental effects on diversity interacted with ecosystem types. Using multilevel mixed linear models and model averaging, we also tested the relative importance of variables that described uncontrolled environmental variation and attributes of experimental units. Overall, experimental warming reduced richness across ecosystems (mean log-response ratio = -0.091, 95% bootstrapped CI: -0.13, -0.05) representing an 8.9% decline relative to ambient temperature treatments. Richness did not change in response to warming in freshwater systems, but was more strongly negative in terrestrial (-11.8%) and marine (-10.5%) experiments. In contrast, warming impacts on evenness were neutral overall and in aquatic systems, but weakly negative on land (7.6%). Intensity and duration of experimental warming did not explain variation in diversity responses, but negative effects on richness were stronger in smaller experimental units, particularly in marine systems. Model-averaged parameter estimation confirmed these main effects while accounting for variation in latitude, ambient temperature at the sites of manipulations, venue (field versus lab), community trophic type, and whether experiments were open or closed to colonization. These analyses synthesize extensive experimental evidence showing declines in local richness with increased temperature, particularly in terrestrial and marine communities. However, the more variable effects of warming on evenness were better explained by the random effect of site identity, suggesting that effects on species' relative abundances were contingent on local species composition.

The stimulation and dominance of potentially harmful phytoplankton taxa at a given locale and time are determined by local environmental conditions as well as by transport to or from neighboring regions. The present study investigated the occurrence of common harmful algal bloom (HAB) taxa within the Southern California Bight, using cross-correlation functions to determine potential dependencies between HAB taxa and environmental factors, and potential links to algal transport via local hydrography and currents. A simulation study, in which Lagrangian particles were released, was used to assess travel times due to advection by prevailing ocean currents in the bight. Our results indicate that transport of some taxa may be an important mechanism for the expansion of their distributions into other regions, which was supported by mean travel times derived from our simulation study and other literature on ocean currents in the Southern California Bight. In other cases, however, phytoplankton dynamics were rather linked to local environmental conditions, including coastal upwelling events. Overall, our study shows that complex current patterns in the Southern California Bight may contribute significantly to the formation and expansion of HABs in addition to local environmental factors determining the spatiotemporal dynamics of phytoplankton blooms.

Tidal flats are subject to dynamic processes, characterized by sedimentation and erosion, which become more important in times of climate change. To investigate impacts of elevation levels on ecosystem functioning, we focused on macrozoobenthos as an essential ecosystem component that links trophic levels and determines sediment structure. Therefore, three elevation levels of six experimental islands (each 2 × 6 m) and six natural reference sites (nearby tidal flats) were sampled for species richness, abundance and biomass in the back-barrier system of the island of Spiekeroog (German Wadden Sea, southern North Sea) from 2014 to 2016. Reference sites had constant biodiversity and abundance over time that were higher compared to the three elevations of the experimental islands. At the lowest elevation, the community became more diverse over time, including marine and terrestrial taxa. At the middle elevation, community shifted from a marine species inventory over an opportunist-dominated assemblage to a less diverse community dominated by Enchytraeidae (Oligochaeta). The community of the highest elevation mostly remained constant, resembling the second, opportunistic community from the middle elevation that was dominated by the snail Peringia ulvae (Pennant, 1777) and the oligochaetes Enchytraeidae spp. and Tubificoides benedii (d'Udekem, 1855). Correspondingly, food web structure was more complex and comprised more trophic interactions at the lowest elevation compared to the other elevations mainly inhabited by deposit feeders. Results demonstrate negative impacts of increased sediment elevation on macrofaunal biodiversity and functional diversity, originating from reduced inundation and vegetation establishment.

Ptacnik, Robert ... et al.-- 10 pages, 4 figures, 1 table, supplementary information https://dx.doi.org/10.1038/srep29286, Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light, The experiment was financed by the European Union Seventh Framework Program MESOAQUA (FP7/2009-2013) coordinated by JCN under grant agreement no. 228224. R.P. further acknowledges financial support by the German Research Council (DFG Pt 5/3-1), JMG by Spanish project REMEI (CTM2015-70340) and AC by project PROTOS (CTM2009-08783) from the Spanish Ministry of Economy and Competitiveness

Two ecological frameworks have been used to explain multitrophic interactions, but rarely in combination: (i) ecological stoichiometry (ES), explaining consumption rates in response to consumers' demand and prey's nutrient content; and (ii) metabolic theory of ecology (MTE), proposing that temperature and body mass affect metabolic rates, growth and consumption rates. Here we combined both, ES and MTE to investigate interactive effects of phytoplankton prey stoichiometry, temperature and zooplankton consumer body mass on consumer grazing rates and production in a microcosm experiment. A simple model integrating parameters from both frameworks was used to predict interactive effects of temperature and nutrient conditions on consumer performance. Overall, model predictions reflected experimental patterns well: consumer grazing rates and production increased with temperature, as could be expected based on MTE. With decreasing algal food quality, grazing rates increased due to compensatory feeding, while consumer growth rates and final biovolume decreased. Nutrient effects on consumer biovolume increased with increasing temperature, while nutrient effects on grazing rates decreased. Highly interactive effects of temperature and nutrient supply indicate that combining the frameworks of ES and MTE is highly important to enhance our ability to predict ecosystem functioning in the context of global change.

Protistan community structure was examined from 6 depths (1.5, 20, 42, 150, 500, 880 m) at a coastal ocean site in the San Pedro Channel, California. A total of 856 partial length 18S rDNA protistan sequences from the six clone libraries were analyzed to characterize diversity present at each depth. The sequences were grouped into a total of 259 Operational Taxonomic Units (OTUs) that were inferred using an automated OTU calling program that formed OTUs with approximately species-level distinction (95% sequence similarity). Most OTUs (194 out of 259) were observed at only one specific depth, and only two were present in clone libraries from all depths. OTUs were obtained from 21 major protistan taxonomic groups determined by their closest BLAST matches to identified protists in the NCBI database. Approximately 74% of the detected OTUs belonged to the Chromalveolates, with Group II alveolates making up the largest single group. Protistan assemblages at euphotic depths (1.5, 20 and 42 m) were characterized by the presence of clades that contained phototrophic species (stramenopiles, chlorophytes and haptophytes) as well as consumers (especially ciliates). Assemblages in the lower water column (150, 500 and 800 m) were distinct from communities at shallow depths because of strong contributions from taxa belonging to euglenozoans, acantharians, polycystines and Taxopodida ( Sticholonche spp. and close relatives). Species richness (Chao I estimate) and diversity (Shannon index) were highest within the euphotic zone and at 150 m, and lowest for protistan assemblages located in the oxygen minimum zone (500 and 880 m). Multivariate analyses (Bray–Curtis coefficient) confirmed that protistan assemblage composition differed significantly when samples were grouped into shallow (≤150 m) and deep water assemblages (≥150 m).

The structure and genetic diversity of marine protistan assemblages were investigated in the upper 500 m of the water column at a Pacific Ocean time-series station off the coast of Southern California. Deoxyribonucleic acid sequence-based microbial eukaryote diversity was examined in January, April, July, and October of 2001 at four depths (5 m, chlorophyll maximum [CM], 150 m, and 500 m). A total of 2956 partial 18S ribosomal ribonucleic acid gene sequences yielded representatives from most of the major eukaryotic lineages. Notable among the taxonomic groups were recently described lineages of stramenopiles, alveolates, and euglenozoa. A large number of polycystine and acantharean sequences were observed at depth. Pairwise sequence analysis was performed to establish operational taxonomic units (OTUs) that were then used to estimate the unsampled protistan diversity by parametric and nonparametric techniques. A total of 2246 protistan sequences grouped into 377 distinct OTUs, with remaining sequences attributed to metazoa. Protistan richness estimates ranged from , 600 to 1500 OTUs when all depths and seasons were combined into a single data set. Seasonal and depth-related trends in the observed protistan diversity were apparent from comparisons of univariate and multivariate analyses. Cluster analysis combined with nonmetric multidimensional scaling and analysis of similarity testing identified distinct protistan assemblages at the shallowest depths (5 m and CM) for each season, which were significantly different (p , 0.03) from assemblages at the two deepest depths (150 and 500 m) where seasonal changes in the protistan assemblage were not apparent.

Incomplete knowledge of acantharian life cycles has hampered their study and limited our understanding of their role in the vertical flux of carbon and strontium. Molecular tools can help identify enigmatic life stages and offer insights into aspects of acantharian biology and evolution. We inferred the phylogenetic position of acantharian sequences from shallow water, as well as acantharian-like clone sequences from 500 and 880 m in the San Pedro Channel, California. The analyses included validated acantharian and polycystine sequences from public databases with environmental clone sequences related to acantharia and used Bayesian inference methods. Our analysis demonstrated strong support for two branches of unidentified organisms that are closely related to, but possibly distinct from the Acantharea. We also found evidence of acantharian sequences from mesopelagic environments branching within the chaunacanthid clade, although the morphology of these organisms is presently unknown. HRP-conjugated probes were developed to target Acantharea and phylotypes from Unidentified Clade 1 using Catalyzed Reporter Deposition Fluorescence In Situ Hybridization (CARD-FISH) on samples collected at 500 m. Our CARD-FISH experiments targeting phylotypes from an unidentified clade offer preliminary glimpses into the morphology of these protists, while a morphology for the aphotic acantharian lineages remains unknown at this time.

DNA sequence information has increasingly been used in ecological research on microbial eukaryotes. Sequence-based approaches have included studies of the total diversity of selected ecosystems, studies of the autecology of ecologically relevant species, and identification and enumeration of species of interest for human health. It is still uncommon, however, to delineate protistan species based on their genetic signatures. The reluctance to assign species-level designations based on DNA sequences is in part a consequence of the limited amount of sequence information presently available for many free-living microbial eukaryotes and in part a consequence of the problematic nature of and debate surrounding the microbial species concept. Despite the difficulties inherent in assigning species names to DNA sequences, there is a growing need to attach meaning to the burgeoning amount of sequence information entering the literature, and there is a growing desire to apply this information in ecological studies. We describe a computer-based tool that assigns DNA sequences from environmental databases to operational taxonomic units at approximately species-level distinctions. This approach provides a practical method for ecological studies of microbial eukaryotes (primarily protists) by enabling semiautomated analysis of large numbers of samples spanning great taxonomic breadth. Derivation of the algorithm was based on an analysis of complete small-subunit (18S) rRNA gene sequences and partial gene sequences obtained from the GenBank database for morphologically described protistan species. The program was tested using environmental 18S rRNA data sets for two oceanic ecosystems. A total of 388 operational taxonomic units were observed for 2,207 sequences obtained from samples collected in the western North Atlantic and eastern North Pacific oceans.

Mixotrophic nanoflagellates (MNF) were quantified in plankton and sea ice of the Ross Sea, Antarctica, during austral spring. Tracer experiments using fluorescently labeled bacteria (FLB) were conducted to enumerate MNF and determine their contribution to total chloroplastidic and total bacterivorous nanoflagellates. Absolute abundances of MNF were typically

Large-scale environmental sensing, e.g., understanding microbial processes in an aquatic ecosystem, requires coordination across a multidisciplinary team of experts working closely with a robotic sensing and sampling system. We describe a human-robot team that conducted an aquatic sampling campaign in Lake Fulmor, San Jacinto Mountains Reserve, California during three consecutive site visits (May 9–11, June 19–22, and August 28–31, 2006). The goal of the campaign was to study the behavior of phytoplankton in the lake and their relationship to the underlying physical, chemical, and biological parameters. Phytoplankton form the largest source of oxygen and the foundation of the food web in most aquatic ecosystems. The reported campaign consisted of three system deployments spanning four months. The robotic system consisted of two subsystems—NAMOS (networked aquatic microbial observing systems) comprised of a robotic boat and static buoys, and NIMS-RD (rapidly deployable networked infomechanical systems) comprised of an infrastructure-supported tethered robotic system capable of high-resolution sampling in a two-dimensional cross section (vertical plane) of the lake. The multidisciplinary human team consisted of 25 investigators from robotics, computer science, engineering, biology, and statistics.We describe the lake profiling campaign requirements, the robotic systems assisted by a human team to perform high fidelity sampling, and the sensing devices used during the campaign to observe several environmental parameters. We discuss measures taken to ensure system robustness and quality of the collected data. Finally, we present an analysis of the data collected by iteratively adapting our experiment design to the observations in the sampled environment. We conclude with the plans for future deployments.

Benthic mixotrophic nanoflagellates (MNF) were studied in coastal sediments in Kiel Fjord, western Baltic Sea. Three types of surface sediment of different grain size were investigated, each along a vertical gradient within the first centimeter depth. A tracer experiment using fluores- cently labeled bacteria (FLB) was conducted to identify MNF and quantify their contribution to total bacterivory in different sediment types and depths. MNF abundances and their phagotrophic activ- ity did not vary significantly with sediment depth and grain size. Overall, mixotrophs contributed less to total nanoflagellates in sediments from Kiel Fjord (max. 2%) than those recorded from comparable pelagic systems, contributing a higher percentage to total phytoflagellates than to the total bacteri- vores (5 to 10 vs. 2 to 5%, respectively). Potential reasons for these findings could be extensive niche segregation for bacterial grazers, osmotrophic feeding strategies in sediments due to high bacterial abundances and/or rarely limiting nutrient conditions.

A new method based on quantitative real-time polymerase chain reaction (qPCR) was developed and applied to quantify the red tide dinoflagellate Lingulodinium polyedrum in natural seawater samples and in laboratory cultures. The method uses a Molecular Beacontrade mark approach to target a species-specific region of the small subunit ribosomal RNA gene. The accuracy of the method was verified by microscopical counts using cultures of the dinoflagellate isolated from coastal waters near Los Angeles, CA, and with natural water samples spiked with cultured L. polyedrum. The method was applied to document the pattern and timing of vertical migration by the dinoflagellate in a 2-m water column on an 11:13 h light/dark photoperiod established in the laboratory. Positive phototaxis of L. polyedrum resulted in dense aggregations of the dinoflagellate within the top few centimeters of the water column during the light period. This pattern of distribution was readily established by both methods, although abundances of L. polyedrum determined using qPCR were higher than abundances determined by microscopy in the morning and lower in the afternoon and evening. These differences may have been a consequence of variability in the DNA content per cell because of synchrony of cell division. Counts using both methods to analyze natural samples collected from coastal waters in the Long Beach-Los Angeles area and adjacent San Pedro Channel were in close agreement. However, the qPCR method exhibited greater sensitivity than the microscopical method when L. polyedrum was present at low abundances, and qPCR had a much higher rate of sample throughput than microscopy. The development of this new approach for enumerating L. polyedrum provides a useful tool for studying the ecology of this important red tide species.

In the western Baltic Sea, the highly competitive blue mussel Mytilus edulis tends to monopolize shallow water hard substrata. In many habitats, mussel dominance is mainly controlled by the generalist predator Carcinus maenas. These predator-prey interactions seem to be affected by mussel size (relative to crab size) and mussel epibionts. There is a clear relationship between prey size and predator size as suggested by the optimal foraging theory: Each crab size class preferentially preys on a certain mussel size class. Preferred prey size increases with crab size. Epibionts on Mytilus, however, influence this simple pattern of feeding preferences by crabs. When offered similarly sized mussels, crabs prefer Balanus-fouled mussels over clean mussels. There is, however, a hierarchy of factors: the influence of attractive epibiotic barnacles is weaker than the factor 'mussel size'. Testing small mussels against large mussels, presence or absence of epibiotic barnacles does not significantly alter preferences caused by mussel size. Balanus enhanced crab predation on mussels in two ways: Additional food gain and, probably more important, improvement in handling of the prey. The latter effect is illustrated by the fact that artificial barnacle mimics increased crab predation on mussels to the same extent as do live barnacles. We conclude that crab predation preferences follows the optimal foraging model when prey belong to different size classes, whereas within size classes crab preferences is controlled by epibionts

Ecosystem functioning is affected by horizontal (within trophic groups) and vertical (across trophic levels) biodiversity. Theory predicts that the effects of vertical biodiversity depend on consumer specialization. In a microcosm experiment, we investigated ciliate consumer diversity and specialization effects on algal prey biovolume, evenness and composition, and on ciliate biovolume production. The experimental data was complemented by a process-based model further analyzing the ecological mechanisms behind the observed diversity effects. Overall, increasing consumer diversity had no significant effect on prey biovolume or evenness. However, consumer specialization affected the prey community. Specialist consumers showed a stronger negative impact on prey biovolume and evenness than generalists. The model confirmed that this pattern was mainly driven by a single specialist with a high per capita grazing rate, consuming the two most productive prey species. When these were suppressed, the prey assemblage became dominated by a less productive species, consequently decreasing prey biovolume and evenness. Consumer diversity increased consumer biovolume, which was stronger for generalists than for specialists and highest in mixed combinations, indicating that consumer functional diversity, i.e. more diverse feeding strategies, increased resource use efficiency. Overall, our results indicate that consumer diversity effects on prey and consumers strongly depend on species-specific growth and grazing rates, which may be at least equally important as consumer specialization in driving consumer diversity effects across trophic levels. Synthesis In a microcosm experiment, we investigated multitrophic consumer diversity and specialization effects using ciliate consumers and microalgal prey. Consumer diversity increased consumer biovolume, which was highest in combinations containing both generalists and specialists. Specialist consumers showed a stronger negative effect on prey biovolume and evenness than generalists. These experimental data were supported by a process-based model, indicating that the large effect of the specialists was based on high per capita grazing rate on the two most productive prey species. Species-specific traits such as growth and grazing rates were equally important for multitrophic diversity effects than consumer specialization.

Summary There is compelling evidence for dispersal limitation among microscopic organisms, including phyto- and zooplankton, especially from studies addressing spatial patterns in taxon richness. This evidence is not in conflict with the widely accepted importance of strong local interactions in the plankton. However, the simultaneous importance of dispersal limitation and strong local interactions can only be understood when taking high temporal turnover rates into account. Current observational and experimental evidence suggests that biodiversity–ecosystem functioning (B–EF) relationships do not differ systematically from those known from higher organisms. Plankton communities are not saturated by default. Although the pelagial has little spatial structure, it is rich in environmental dimensionality when considering the dimensionality in time and chemical and physical properties, resulting in complex biotic interactions. We propose a conceptual model explaining B–EF effects in plankton, which contrasts environmental dimensionality with trait dimensionality of the community. This model, which is applicable to ecological communities in general, predicts that positive B–EF relationships depend on sufficient environmental dimensionality. We show how this model can be applied to understand B–EF relationships along gradients of productivity and stoichiometry. Our major conclusions are that local community dynamics of plankton communities may be better understood when putting them into a wider spatial context, that is, considering regional species pools. Moreover, the framework of environmental and trait dimensionality can be used to make concise predictions for the occurrence and strength of B–EF relationships.