Showing papers in "Journal of Plankton Research in 2013"

Misuse of the phytoplankton-zooplankton dichotomy : the need to assign organisms as mixotrophs within plankton functional types

Abstract: The classic portrayal of plankton is dominated by phytoplanktonic primary producersand zooplanktonic secondary producers. In reality, many if not most planktontraditionally labelled as phytoplankto ...

Trait-based approaches to zooplankton communities

Abstract: Author(s): Litchman, E; Ohman, MD; Kiorboe, T | Abstract: Zooplankton are major primary consumers and predators in most aquatic ecosystems. They exhibit tremendous diversity of traits, ecological strategies and, consequently, impacts on other trophic levels and the cycling of materials and energy. An adequate representation of this diversity in community and ecosystem models is necessary to generate realistic predictions on the functioning of aquatic ecosystems but remains extremely challenging. We propose that the use of trait-based approaches is a promising way to reduce complexity while retaining realism in developing novel descriptions of zooplankton in ecosystem models. Characterizing zooplankton traits and trade-offs will also be helpful in understanding the selection pressures and diversity patterns that emerge in different ecosystems along major environmental gradients. Zooplankton traits can be characterized according to their function and type. Some traits, such as body size and motility, transcend several functions and are major determinants of zooplankton ecological strategies. Future developments of trait-based approaches to zooplankton should assemble a comprehensive matrix of key traits for diverse groups and explore it for general patterns; develop novel predictive models that explicitly incorporate traits and associated trade-offs; and utilize these traits to explain and predict zooplankton community structure and dynamics under different environmental conditions, including global change scenarios. © 2013 The Author.

Microzooplankton grazing in the oceans: impacts, data variability, knowledge gaps and future directions

Abstract: Here we review all published data on phytoplankton growth and microzooplankton grazing using the dilution technique to better understand the role of this group of grazers in different regions of the oceans, and to identify the knowledge gaps that require future efforts. A total of 1525 data points assimilated from 110 studies were included and grouped using the biogeographic subsets defined by Longhurst et al. [(1995) An estimate of global primary production in the ocean from satellite radiometer data. J. Plankton Res., 17, 1245–1271]. Total median phytoplankton growth rates in each of the subsets varied between 0.15 (Polar Southern) and 0.83 day (Trades Atlantic), with the corresponding microzooplankton grazing rates ranging between 0.07 (Polar Southern) and 0.48 day (Trades Indian). The median percentage of primary production (PP) grazed by microzooplankton was relatively constant among the regions and ranged from 49

The effects of changing climate on microzooplankton grazing and community structure: drivers, predictions and knowledge gaps

Abstract: Microzooplankton dominate trophic interactions and biogeochemical processes at the base of pelagic marine food webs and so their responses to a changing ocean environment have potentially large implications for ocean ecosystem functioning. This diverse array of mostly protistan species constitutes an important source of phytoplankton and bacterial mortality, and contributes significantly to the food available to higher trophic levels by packaging minute prey into larger particle sizes that can be consumed by metazooplankton. Microzooplankton are pivotal species in oceanic food webs and nutrient remineralization and so it is essential that we understand the effects that changing climate may have on the biomass, species composition and trophic activities of these assemblages. Yet, our present understanding of this topic is derived from experimental studies of relatively few species subjected to specific environmental variables (e.g. changes in temperature, CO2, pH) in isolated culture. Most experiments and models employed to predict the effects of climate change have focussed on primary productivity and phytoplankton community structure, with less attention paid to microbial heterotrophy. Here we outline some of the major direct and indirect changes in environmental variables that are anticipated to accompany global climate change, and our present state of knowledge regarding their potential impacts on natural microzooplankton assemblages. We highlight a few specific areas for studies to address

Resistance of dormant stages of planktonic invertebrates to adverse environmental conditions

Abstract: Formation of highly resistant dormant forms is a well-known feature that allows inhabitants of unstable water bodies to survive periods of adverse conditions. The resistance of dormant forms of some aquatic invertebrates to different biotic and abiotic factors has been well characterized in recent decades, while the tolerance to desiccation of some rotifers has been recognized since the beginning of the 18th century. Here, I review the literature on the longevity and resistance to environmental extremes of dormant stages of planktonic animals (i.e. cladocerans, copepods, ostracods, monogonont rotifers) and some other aquatic invertebrates. Mechanisms of resistance and the evolutionary forces that shape them are also discussed.

Beyond the jellyfish joyride and global oscillations: advancing jellyfish research

Abstract: There has been debate in the literature recently about increases in jellyfish populations in response to anthropogenic change, and this has attracted widespread media interest. Despite an international collaborative initiative [National Center for Ecological Analysis and Synthesis (NCEAS) working group on jellyfish blooms] to investigate trends in global jellyfish numbers, interpretations from the data remain ambiguous. Although this is perhaps to be expected given the diversity of potential drivers, the debate has not been helped by a general lack of rigorous data and loose definitions. There is a need for the community to refocus its attention on understanding the implications of jellyfish blooms and managing them, because regardless of global trends, jellyfish are a problem in some coastal marine ecosystems. Here, we provide recommendations for advancing jellyfish research. These include directing research toward better managing jellyfish impacts, expanding research into socio-economic consequences to grow the money available for research, building more operational and ecosystem models for tactical and strategic management, filling in the gaps in our biological knowledge for supporting models, improving surveillance using observing systems and making jellyfish research more rigorous. Some vehicles to address these recommendations include international standardization of methods, a discipline-specific journal for jellyfish research and an international science program on the global ecology and oceanography of jellyfish.

Salinity and nutrients influence species richness and evenness of phytoplankton communities in microcosm experiments from Great Salt Lake, Utah, USA

Abstract: Harsh environments are typically characterized by some dominant variable limiting diversity, making them interesting systems for studying how species diversity patterns change with abiotic conditions. Several environmental factors with the potential to influence phytoplankton diversity in hypersaline lakes were examined with microcosm experiments using inoculum from the Great Salt Lake, Utah. Experimental combinations of salinity and nutrient supply were run at three different temperatures. Results confirm salinity as a strong determinant of phytoplankton diversity, while also demonstrating the importance of nutrient supply, where species richness decreased with increasing salinity and increased with nutrient enrichment. Community evenness decreased with nutrient enrichment, indicating few species were favored by nutrient enrichment, becoming very abundant. Community biomass was positively correlated with richness and negatively correlated with evenness. Additionally, the abundance of particular species, most notably Dunaliella sp., was strongly affected by salinity, temperature and nutrient enrichment following patterns observed in the Great Salt Lake, where its abundance increased with salinity, cooler temperatures and increased nutrient enrichment. These results add to growing evidence that while salinity is a dominant factor influencing diversity in

CO2-driven ocean acidification reduces larval feeding efficiency and changes food selectivity in the mollusk Concholepas concholepas

Abstract: fromthe atmosphere into the oceans through a process knownas ocean acidification (OA) (Feely et al., 2004). Theimpact of this process on marine ecosystems has only re-cently been recognized as a human-induced stressor,with potentially serious impacts for the sustainability andmanagement of many economically important marineresources (Miller et al., 2009). Within the various phylaparticularly vulnerable to OA, calcifying organisms, suchas marine mollusks, are exceptionally vulnerable (Byrne,2011). They have been recognized for their great eco-nomic value and as a food source for humans (Leiva andCastilla, 2001). Therefore, any impacts of OA on sensi-tive life-history traits of these organisms will potentiallylead to adverse ecological and economic impacts.In recent years, numerous studies have demonstratedthe negative effects of OA on marine mollusks, includingreduced growth and calcification rates (Gazeau et al.,2007; Miller et al., 2009), reduced fertilization success(Byrne et al., 2010), compromised induced defenses(Bibby et al., 2007) and impairment of immune function(Bibby et al., 2008).Although feeding could be one of thekey physiological processes affected by OA, there havebeen few reports of the effect on feeding behavior in bothadult and/or larval stages of marine mollusks (Stumppet al., 2011;Barton et al., 2012).Feeding processes duringearly life stages of marine invertebrates are likely to bemore sensitive to OA than in adults (Findlay et al., 2008,2010).Moreover, OA has been shown to induce morpho-logical changes in larval invertebrates with significantimplications for feeding performance and food selectivity(Chan et al., 2011).In consequence, since it is well knownthat food quantity and quality greatly influences larvalsurvival, development and growth (Boidron-Me´tairon,1995), the influence of OA on feeding may explainobserved impacts on these physiological traits and onotherenergydependent processes including calcification.Here, we selected an important species of the rockyintertidal and sub-tidal communities of the Chileancoast, in the Southern Pacific Ocean, the carnivorousgastropod known as “loco” (Chilean abalone), Concholepasconcholepas (Brugie`re, 1789) (Castilla, 1999). Concholepasconcholepas is an economically and ecologically importantcomponent of the rocky intertidal and sub -tidal commu-nities along the Chilean coast (Castilla, 1999). After 1 to2 months of intra-capsular development, the veligerlarvae hatch from clumps of benthic egg capsules atabout 240–260 mm protoconch length (DiSalvo, 1988).The competent larval stage (i.e. larvae ready to settle) isreached after about 3 months at a protoconch sizebetween 1600 and 1900 mm(DiSalvo, 1988), which cor-responds to the time lapse between maximal abundanceof egg capsules and peak abundance of competent larvaein the water (Manri´quez and Castilla, 2001). Then, theyrequire from 3 to 12 months of planktonic life to reachcompetence and settling on the rocky shore (Gallardo,1973). We conducted incubation experiments to investi-gate the effect of CO

A 5-year study of seasonal patterns in mesozooplankton community structure in a sub-Arctic fjord reveals dominance of Microsetella norvegica (Crustacea, Copepoda)

Abstract: The study presents data from a multi-year zooplankton sampling programme with year-round monthly sampling in a sub-Arctic fjord in Greenland (Godthabsfjord). A total of 56 zooplankton groups were identified over 5 years, with the copepod Microsetella norvegica dominating the mesozooplankton community. Microsetella norvegica was found to be very abundant (maximum abundance: 408 125+ 161 387 nauplii m and 91 995+ 6 864 copepodites m) and to make up, on average, 87% of the annual copepod assemblage. There was a seasonal zooplankton succession whereby Cirripedia nauplii dominated the biomass in March and April, and Calanus spp. dominated in May and June, followed by M. norvegica from July to September. The total copepod biomass peaked in August (71+ 10 mg C m), mainly (68% on average) due to biomass of M. norvegica, indicating that small copepods are important in this system. This multi-year study describes inter-annual variation in species abundance and seasonal succession. Fjord–ocean interactions, tidal mixing and the extensive freshwater run-off from the Greenland Ice Sheet

Seasonal dynamics of Mesodinium rubrum in Chesapeake Bay

Abstract: The photosynthetic ciliate Mesodinium rubrum is a common member of coastalphytoplankton communities that is well adapted to low-light, turbid ecosystems.It supports the growth of, or competes with, harmful dinoflagellate species forcryptophyte prey, as well as being a trophic link to copepods and larval fish. Wehave compiled data from various sources (n ¼ 1063), on the abundance and distri-bution of M. rubrum in Chesapeake Bay and its tributaries. Because M. rubrumrelies on obtaining organelles from cryptophyte algae to maintain rapid growth,we also enumerated cryptophyte algae in the portion of these samples that we col-lected (n ¼ 386). Mesodinium rubrum occurred in oligohaline to polyhaline regions ofChesapeake Bay and throughout the year. Blooms (.100 cells mL

A review on utilizing Bosmina size structure archived in lake sediments to infer historic shifts in predation regimes

Abstract: Zooplankton are considered excellent indicators of aquatic food web structure, due to their role as grazers on primary producers and their sensitivity to predation by both planktivorous fish and invertebrates. Several key zooplankton taxa also leave identifiable remains that are often well-preserved in lake sediments, providing an opportunity to track changes in predation pressure over timescales of decades to thousands of years. For example, the small-bodied cladoceran zooplankter Bosmina (Branchiopoda, Crustacea) is often highly abundant in lake sediments, and because Bosmina often undergoes cyclomorphosis in response to fish and invertebrate predation, measurements of subfossil Bosmina features can be indicative of predation regime shifts. This review focuses on Bosmina cyclomorphic responses to varying predation regimes and the application of these principles to Bosmina subfossil remains to better understand long-term ecological changes occurring in lakes. We conclude that subfossil Bosmina size structure is a promising indicator of historic changes in predation pressure in response to fish introductions/extirpations/population

Overwintering populations of Anabaena, Aphanizomenon and Microcystis as potential inocula for summer blooms

Abstract: Overwintering cyanobacterial populations of Nostocales and Microcystis were investigated in six freshwater reservoirs in Northwestern Spain during two consecutive winters. Surface sediments hosted 10–10 akinetes mL and 10–10 Microcystis colonies mL. Sediments from deeper areas close to dam accumulated 2-fold (Microcystis) and 11-fold (akinetes) greater concentrations than those at the shallower upstream areas. Anabaena spp. and Microcystis aeruginosa dominated the sediment pool, with minor amounts of akinetes of Aphanizomenon (Aph. flos-aquae, Aph. gracile) and benthic Nostocales (Cylindrospermum, Nostoc and Trichormus). Our study confirms the dual benthic-pelagic overwintering of Anabaena, Aphanizomenon and Microcystis, found in the pelagial at 7.5–9.88C. This study also provides an insight into the little known annual cycle of potential cyanotoxin-producers Aph. gracile and Anabaena circinalis. Our estimates show that: (i) only a small fraction (,1%) of the sediment pool of akinetes and Microcystis was resupended in the bottom water during winter which, however, may be sufficient inocula to build up the summer maxima under realistic in situ growth rates; and (ii) the time required for the development of summer populations is mainly driven by growth rates, and therefore by the environmental conditions faced by the inoculum, with a lower influence (although greater for Microcystis than for Nostocales) of the inoculum size.

Effects of trophic cascades in dilution grazing experiments: from artificial saturated feeding responses to positive slopes

Abstract: Dilution grazing experiments are the most commonly used method for quantifying microzooplankton grazing rates on phytoplankton. However, in some cases, the outcomes of these experiments yield non-significant grazing impacts, even when microbial grazers are numerous, or result in unexpected non-linear functions or positive slopes. Here, we argue that some of these situations and others that may go unnoticed can be explained by trophic cascades occurring in the community confined within the incubation experiments. When trophic cascades are present, simple linear models relating dilution levels to phytoplankton net growth rates can be distorted into artificial saturation curves, V-shaped responses or a non-significant fit and may even invert the sign of the slope. These artefacts have minor consequences for true estimates of phytoplankton mortality rates when the dilution level is sufficiently high (ca. 5–10%), but they may result in significant underestimation of the microzooplankton grazing impact when working at lower, typical maximum dilution

Coastal upwelling linked to toxic Pseudo-nitzschia australis blooms in Los Angeles coastal waters, 2005–2007

Abstract: Harmful algal blooms dominated by the diatom Pseudo-nitzschia spp. have become a perennial but variable event within surface waters near the greater Los Angeles area. Toxic blooms during spring seasons from 2005 to 2007 varied strongly in their overall toxicity and duration. Differences in bloom dynamics were linked to differences in storm-induced river discharge following episodic rain events and coastal upwelling, both major coastal processes that led to the injection of nutrients into coastal surface waters. Heavy river runoff during early 2005, a record-rainfall year, favored a phytoplankton community mainly comprised of algal taxa other than Pseudo-nitzschia. The spring bloom during 2005 was associated with low domoic acid surface concentrations and minor contributions of (mainly) P. delicatissima to the diatom assemblage. In contrast, highly toxic P. australis-dominated blooms during spring seasons of 2006 and 2007 were linked to strong upwelling events. River discharge quotas in 2006 and 2007, in contrast to 2005, fell well below annual averages for the region. Surface toxin levels were linked to colder, more saline (i.e. upwelled) water over the 3-year study, but no such consistent relationship between domoic acid levels and other physiochemical parameters, such as macronutrient concentrations or nutrient ratios, was observed.

Relationships between phytoplankton thin layers and the fine-scale vertical distributions of two trophic levels of zooplankton

Abstract: Thin layers of phytoplankton are well documented, common features in coastal areas globally, but little is known about the relationships of these layers to higher trophic levels We deployed the In Situ Ichthyoplankton Imaging System (ISIIS) to simultaneously quantify the three trophic levels of plankton, including phytoplankton, primary consumers (copepods and appendicularians) and secondary consumers (gelatinous zooplankton) Over a 2-week sampling period, phytoplankton thin layers, primarily composed of Pseudo-nitzschia spp, were common on two of the five sampling days Imagery showed copepods aggregating in zones of lower chlorophyll-a fluorescence, while appendicularians were more common at greater depths and higher chlorophylla levels All gelatinous zooplankton generally increased in abundance with depth Bolinopsis spp ctenophores underwent a ‘bloom,’ and they were the only species observed to aggregate within phytoplankton thin layers The vertical separation between copepods, phytoplankton and gelatinous zooplankton suggests that copepods may use the surface waters as a predation refuge, only performing short migrations into favorable feeding zones where gelatinous predators are much more abundant Thin layers containing dense diatom aggregates obstruct light reaching deeper waters (10 m), which may allow gelatinous zooplankton to avoid visual predation as well as improve the effectiveness of contact predation with copepod prey

The cost of predator-induced morphological defense in rotifers: experimental studies and synthesis

Abstract: In many small planktonic rotifers, predator kairomones induce the development of longer spines, which can provide effective post-contact defense against the predators. This review briefly discusses potential trade-offs associated with pronounced spine development, focuses on an evaluation and synthesis of experimental work comparing the reproductive potential of basic (non-induced) and Asplanchnainduced morphs and suggests areas for future research. To date, the only convincing evidence for an association between induced spine development and reduced reproduction is for Keratella testudo. This rotifer has a dense lorica (skeleton), so that elongated spines increase its sinking rate and thus presumably an offsetting swimming effort. In contrast, several studies with Brachionus calyciflorus and Keratella tropica, which have a thin lorica, showed that morphs with pronounced spine development reproduced at the same rate as basic morphs. In other studies with several different species, and also with B. calyciflorus, the presence of Asplanchna did inhibit reproduction. However, this inhibition generally was not clearly related to Asplanchna density or the degree of spine development, and could not be attributed to a defense cost. While predator-induced morphs of some species do not have a

Spatial patterns of plankton biomass and stable isotopes reflect the influence of the nitrogen-fixer Trichodesmium along the subtropical North Atlantic

Abstract: C) and with the abundance of the nitrogen-fixer Trichodesmium. Diazotrophy was estimated to account for >50% of organic nitrogen in the central zone, and even >30% in the eastern and the western zones. The impact of diazotrophy increased with the size of the organisms, supporting the wide participation of all trophic levels in the processing of recently fixed nitrogen. These results indicate that atmospheric sources of carbon and nitrogen prevail over deep water sources in the subtropical North Atlantic and that the zone influenced by diazotrophy is much larger than reported in previous studies.

Competitive interaction between diatom Skeletonema costatum and dinoflagellate Prorocentrum donghaiense in laboratory culture

Abstract: Skeletonema costatum and Prorocentrum donghaiense co-exist and appear alternately as dominant species in the natural phytoplankton community in the coastal area of the East China Sea. The mechanism that determines which of the two species dominates the succession remains unknown. We conducted a series of experiments to investigate the inter-specific relationship between S. costatum and P. donghaiense. The results of the P. donghaiense and S. costatum bialgal culture experiments indicated that S. costatum dominates P. donghaiense under high-nutrient conditions, and the interaction was independent of the initial chlorophyll a ratios. Adding S. costatum culture filtrates did not significantly influence the growth of P. donghaiense and S. costatum under high-nutrient conditions. Under low-nutrient conditions, the S. costatum filtrates showed a significant autoinhibitory effect, but they did not inhibit the growth of P. donghaiense because of the tolerance of P. donghaiense to allelochemicals. The results demonstrated that the autoallelopathy of S. costatum plays an important role in the competition and succession between S. costatum and P. donghaiense under low-nutrient concentrations, especially during phosphate deficiency.

Molecular discrimination of taxa within the dinoflagellate genus Azadinium, the source of azaspiracid toxins

Abstract: Molecular probes were developed for the dinoflagellate genus Azadinium to discriminate among three taxa difficult to differentiate by light microscopy. This genus contains azaspiracid toxin-producing Azadinium spinosum, but also non-toxigenic species. Quantitative polymerase chain reaction (qPCR) and fluorescence in situ hybridization (FISH) assays were applied to cultured isolates and Azadiniumspiked field plankton. Molecular methods were highly specific and sensitive in the unambiguous detection of Azadinium, and thus are valuable for routine plankton, biogeographic and phylogenetic investigations.

Ice-related seasonality in zooplankton community composition in a high Arctic fjord

Abstract: Seasonal mesozooplankton community composition was studied monthly from March to October 2007 in the high Arctic, Rijpfjorden (Svalbard), and related to abiotic (hydrography, sea ice) and biotic (ice algae and phytoplankton biomass) environmental conditions. The community was numerically dominated by the cosmopolitan Oithona similis, whereas Arctic Calanus glacialis was the dominant taxon in terms of biomass. Seasonal mesozooplankton community development was largely influenced by the sea ice and hydrographic conditions, which impacted the primary production regime in the fjord. Three distinct periods could be identified based on species and life stages composition: (i) winter–spring transition (March–June), with high ice algal biomass in April–June, characterized by peak abundances of Pseudocalanus minutus, Calanus glacialis females and Clione limacina; (ii) sea ice break-up and phytoplankton bloom (July), with high numbers of Calanus nauplii and young copepodids, as well as larvae of benthic crustaceans such as Cirripedia and Decapoda; and (iii) ice-free post-bloom period (August–October), when the pulse of warm waters into the fjord resulted in development of a community with Atlantic characteristics and peak abundances of Oithona similis, Oithona atlantica, Limacina helicina and Echinodermata larvae within the upper 50 m. At the same time, older copepodids of Calanus glacialis and Calanus finmarchicus had already descended to overwintering depths (.100 m). Despite the 2–3 months delay in the phytoplankton bloom compared with ice-free Svalbard fjords, the Rijpfjorden mesozooplankton managed to fulfill their life cycles in a similar time, likely due to the utilization of the ice algal bloom, and warmer water enhancing species growth and development in late summer.

Sensitivity of copepod populations to bottom-up and top-down forcing: a modeling study in the Gulf of Maine region

Abstract: The spatio-temporal variability of marine copepods, like other aquatic and terrestrial organisms, is controlled by both bottom-up (through changes in food resource) and top-down (through changes in predation) forcing. Canonically, climate-related changes in hydrography, nutrient chemistry and circulation can modulate phytoplankton production, thus imposing a bottom-up control on marine copepods, whereas human activities such as fishing may affect the predation mortality of copepods through food-web re-organization such as trophic cascading. Evaluating the sensitivity of copepod populations to bottom-up and topdown forcing is an essential step toward the prediction of future marine planktonic ecosystem changes. In this study, we used a coupled hydrodynamics/food-web/ population-dynamics model to identify the key processes controlling the observed seasonality and distributional patterns of two copepod groups in the Gulf of Maine (GoM) region, including Pseudocalanus spp. and Centropages typicus. Numerical experiments were conducted to assess the sensitivity of the modeled species to changes in phytoplankton biomass and bloom timing, as well as the changes in mortality regime. The results show that both copepod groups are more sensitive to changes in mortality rates than to food availability and peak timing. Bottom-up processes alone cannot explain the observed variability in Pseudocalanus and Centropages population sizes, while top-down controls play a critical role in copepod population dynamics in the GoM region.

Copepods and hypoxia in Chesapeake Bay: abundance, vertical position and non-predatory mortality

Abstract: Live and dead copepod abundances and environmental conditions were measured during summer in Chesapeake Bay to determine how population size, vertical position and non-predatory mortality varied with hypoxia. Abundances of copepod nauplii and Acartia tonsa copepodites decreased when low-oxygen water was present. Possible explanations include copepods altering their vertical position to avoid hypoxia, resulting in increased predation and advection losses. Alternatively, copepods residing in hypoxic water may experience increased mortality and sub-lethal effects of hypoxia on growth and reproduction. The vertical position of copepod nauplii did not appear to respond to hypoxia, but the vertical position of A. tonsa copepodites shifted upward in response to lethal hypoxia in bottom water. Nonpredatory mortality of nauplii increased with the severity of hypoxia, but no similar increase was apparent for copepodites. Overall, it appears that hypoxia in Chesapeake Bay can result in lower copepod population abundances. Under moderate hypoxia, sub-lethal effects of low oxygen on growth and reproduction likely contribute to lower abundances, since the copepods do not avoid the hypoxic water. Under severe hypoxia, non-predatory mortality due to low oxygen is likely more important for naupliar stages, and the effects of altered vertical position on predation and advection may be important for copepodites.

Silicate consumption: an indicator for long-term trends in spring diatom development in the Baltic Sea

Abstract: Long-term data sets from 1979 to 2011 were analysed for trends in spring diatom development. As irregular phytoplankton sampling cannot record phytoplankton blooms completely, diatom growth during spring was estimated by silicate consumption. The spring diatom biomass decreased sharply at the end of the 1980s in the Baltic Proper but not in the neighbouring western Baltic area. The diatoms, mainly Thalassiosira spp. and Chaetoceros spp., reappeared after a series of cold winters. A strong negative correlation between the minimum water temperature and the magnitude of the diatom growth in the southern Baltic Proper confirms that diatom growth has decreased after mild winters, when motile phytoplankton, such as dinoflagellates and Mesodinium rubrum, may form blooms instead of diatoms. Silicate limitation did not occur in the Baltic Proper. Decreased convective mixing after mild winters may hamper diatom growth (stratification hypothesis), but this effect could not be proved. Our study supports the hypothesis that increased selective grazing pressure after mild winters may control diatom spring blooms (feeding hypothesis).

Diel vertical migration behaviour in Euphausiids of the northern Benguela current: seasonal adaptations to food availability and strong gradients of temperature and oxygen

Abstract: In the highly productive northern Benguela upwelling system, euphausiids can dominate the mesozooplankton community and may contribute substantially to the vertical flux of organic carbon. The diurnal vertical distribution of four euphausiid species was observed over three seasons from different years. The most abundant, Euphausia hanseni, showed pronounced diel vertical migration (DVM), regularly crossing the thermocline and retreating again to the oxygen minimum zone (OMZ). Nematoscelis megalops was a weak migrant, persisting in the OMZ throughout 24 h. Euphausia recurva showed vertical migration into the OMZ but may have avoided oxygen concentrations below 1 mLO2 L21, Euphausia americana remained in the upper water layers above the OMZ. Thus, euphausiids were divided into different ecological groups using or avoiding the OMZ and were vertically separated, thus avoiding interspecific competition. However, DVM behaviour was adjusted to seasonal variations in water temperature, oxygen and food availability. A conceptual model, combining DVM patterns, environmental parameters such as temperature and food availability and physiological constraints such as species-specific respiration rates, was used to assess the carbon demand of the seasonal DVM behaviours. Energetic considerations based on the DVM mode showed that temperature acted as the controlling and limiting factor with food abundance further modifying vertical positioning of euphausiid species.

Evaluation of ITS2-28S as a molecular marker for identification of calanoid copepods in the subtropical western North Pacific

Abstract: We evaluated the internal transcribed spacer 2 and large subunit ribosomal DNA (ITS2-28S) region as a molecular marker for identifying calanoid copepods in the subtropical western North Pacific. The ITS2-28S sequence was successfully amplified in 232/244 individuals, a much higher rate of success than for cytochrome oxidase I amplification (77/244 individuals). A total of 194 sequences of ITS2-28S were obtained using a single primer pair. A high degree of genetic variability in ITS2-28S was observed in Lucicutia flavicornis (n 1⁄4 13), Nannocalanus minor (n 1⁄4 9), Pleuromamma abdominalis (n 1⁄4 9) and Spinocalanus spinosus (n 1⁄4 2), each showing several sequence types based on phylogenetic analysis of the ITS2-28S. A difference in the body size that corresponded with two sequence types of P. abdominalis was observed. The other species showed consistently lower genetic variability within species (0–0.001) than between species ( 0.005) using ITS2-28S of 810–968 bp aligned in each family. Although further morphological and genetic analysis of a larger sample size are necessary, our results suggest that ITS2-28S is a nuclear marker that does not need to be cloned and is easily amplified and sufficiently variable as a possible marker for DNA barcoding or to identify genetic groups of calanoid copepods.

Deployment of an imaging system to investigate fine-scale spatial distribution of early life stages of the ctenophore Mnemiopsis leidyi in Chesapeake Bay

Abstract: A high-resolution digital imaging system, the zooplankton visualization and imaging system (ZOOVIS) was deployed in the mid-region of Chesapeake Bay to investigate the fine-scale distribution of small individuals of the lobate ctenophore Mnemiopsis leidyi. Counts of individuals, mostly ,5 mm, were enumerated from each frame of ZOOVIS and merged with data from a conductivity-temperaturedepth to provide corresponding values of depth, temperature and salinity. Mnemiopsis leidyi rarely occurred below the pycnocline depth but commonly occurred in the upper water column. Where the water column was strongly stratified, M. leidyi tended to be more common near the pycnocline, but it was more abundant near the bottom at locations where the water column was well mixed. Horizontally, M. leidyi was more abundant on the shoulders of the middle Bay than in the deeper main channel. In this survey, M. leidyi occurred more frequently in relatively warm and low salinity water. Furthermore, small M. leidyi were patchily distributed and the patch size was ,700 m in horizontal extent. A zero-inflated, general additive model successfully simulated the spatial distribution of M. leidyi, with temperature and salinity as predictors. The ZOOVIS system proved to be a valuable tool to study fine-scale distributions of small and delicate gelatinous zooplankton, even in the relatively turbid conditions that prevailed in this estuarine system.

Diatom fluxes in the NW Mediterranean: evidence from a 12-year sediment trap record and surficial sediments

Abstract: We examined the total diatom flux and species composition, total coccolith flux and total mass flux collected with a sediment trap between October 1993 and January 2006 in the northeastern entrance of the Gulf of Lions (North Western Mediterranean). The average daily diatom and coccolith fluxes (3 x 10(7) valves m(2) d(1) and 6.1 x 10(8) coccoliths m(2) day(-1), respectively) are comparable in magnitude with previously reported data sets in other high productivity areas of the Western Mediterranean. The temporal particle flux pattern reflected the variations in surface oceanographic conditions and primary productivity, which showed strong annual cycling. Highest diatom, coccolith and total mass fluxes always occurred during the winter-spring transition, while minima were observed during summer. Changes in the diverse diatom communities reflected the water column conditions throughout the record. The intensity of the diatom winter-spring blooms seemed to be enhanced in those years with intense and cold winds during winter, whereas years with low winter wind stress were liable to be less productive for diatoms. Coccolith fluxes exhibited a more stable interannual pattern than diatom fluxes. Significant discrepancies were found between the sediment trap and surficial sediment diatom assemblages.

Modelling the life cycle of dinoflagellates: a case study with Biecheleria baltica

Abstract: The cold-water dinoflagellate Biecheleria baltica has increasingly dominated the phytoplankton spring bloom in the Baltic Sea during the past years. Life cycle transitions between bloom forming cells and resting cysts are assumed to regulate the bloom dynamics of this species. We investigate the seasonal cycle and succession of Biecheleria baltica’s life cycle stages using a numerical model with four different stages, vegetative cells, gametes, resting cysts and germinating cells. The transitions among the stages are functions of environmental conditions and endogenous factors. Coupled to a water column model, the model is able to represent the seasonal cycle of Biecheleria baltica with two blooms in spring. The first bloom can be explained by germination of resting cysts in winter, the second by growth of vegetative cells. Sensitivity experiments indicate that temperature is an important factor regulating the composition of Biecheleria baltica life cycle stages; increased or decreased temperature leads to fewer growing cells and more resting cysts during spring. Our newly developed life cycle model can be used to study in more detail cyst formation, cyst distribution and consequences for biogeochemical cycling in the past and future.