Journal of Plankton Research

Zooplankton play an important role in the trophic dynamics of coral reef ecosystems. Detailed vertical and temporal distribution and biomass of zooplankton were evaluated at four heights off the bottom and at six times throughout the diel cycle over a coral reef in the Florida Keys (USA). Zooplankton abundance averaged 4396 ± 1949 SD individuals m−3, but temporal and spatial distributions varied for individual zooplankton taxa by time of day and by height off the bottom. Copepods comprised 93–96% of the abundance in the samples. Taxon-based zooplankton CHN values paired with abundance data were used to estimate biomass. Average daily biomass ranged from 3.1 to 21.4 mg C m−3 and differed by both height off the bottom and by time of day. While copepods were the numerically dominant organisms, their contribution to biomass was only 35% of the total zooplankton biomass. Our findings provide important support for the new emerging paradigm of how zooplankton are distributed over reefs.

Mycosporine-like amino acids (MAAs) were studied in zooplankton from 13 Argentinian lakes covering a broad range in altitude, maximum depth and physico-chemical properties of the water. Four to nine different MAAs (predominantly porphyra-334 and shinorine) were found in the copepods Boeckella gibbosa, B. gracilipes, B. meteoris and Parabroteas sarsi, and in the ciliate Stentor amethystinus, while MAAs were undetectable in the cladoceran Daphnia middendorffiana. Among the different copepods, maximum MAA concentrations accounted for 0.25-1.31% of the dry weight, and contents were generally about three to seven times (up to 43 times) higher in the animals living in the clearest lakes compared to those occurring in low-UV systems. This variability in the content of MAAs was related to the lake altitude (r(2) = 0.71), and the fraction of the water column to which 1% of the surface UV radiation at 320 nm penetrated (r(2) = 0.57). Our data therefore underscore the role of MAAs as sunscreens to decrease the potential negative effects of solar radiation, but they also indicate that other environmental factors besides UV transparency play a role in determining MAA concentrations. One lake was selected to obtain additional information on the qualitative composition of MAAs in seston of <100 μm between two sampling sites and over a 2 month study period (austral summer). Six different MAAs were detected in the samples, with porphyra-334 and palythine being predominant. In the copepods collected simultaneously, there was low variation in MAA concentrations between the two sites and over time. Thus, our results suggest that under similar UV exposure conditions MAA contents of planktonic organisms show low temporal variation.

The harmful alga, Karenia brevis, produces a suite of polyether neurotoxins, brevetoxins or PbTx, that cause marine animal mortality and neurotoxic shellfish poisoning (NSP). A characteristic of K. brevis blooms is associated airborne toxins that result in severe respiratory problems. This study was undertaken to determine the composition of aerosolized brevetoxins and oxidative derivatives to which beachgoers are exposed during a K. brevis bloom. The suite of brevetoxins and derivatives in seawater is comprised of intra-cellular (IC) and extra-cellular (EC) compounds. We hypothesized that aerosolized compounds are generated primarily from EC, hydrophobic compounds in seawater by bubble-mediated transport. Thus the composition of aerosolized brevetoxins and derivatives, to which beachgoers are exposed, would reflect the EC composition of the source matrix (the local surf zone). Brevetoxins were extracted from water collected along the shore and from marine aerosols along Siesta Beach and Lido Beach in Sarasota, FL, USA, during K. brevis blooms. Water samples were further processed into IC and EC components. The primary brevetoxins observed in water and air included PbTx-1, -2, -3, -PbTx-2-carboxylic acid, and brevenal. Oxidation and/or hydrolysis products of PbTx-1, -2, -3 and -7 were also found in EC water and in aerosol, but not IC.

The freshwater crustacean Daphnia is an emerging model system in the biology of aging. Diversity in aging patterns is thought to be caused by ecological variation in selection on age-specific performance. Previous work in Daphnia has shown a strong correspondence between selective differences and genetic variation in aging in the Daphnia pulex species complex. However, recent evidence suggests obligate asexuality could account for the more rapid aging found in pond genotypes compared with lake genotypes without invoking differences in selection. Evolutionary biologists have to date assumed equivalent operation of neutral processes when comparing aging across populations, but a shift in the breeding system changes the basic dynamics of neutral evolution. To test the hypothesis that the breeding system could explain the short lifespans of pond-dwelling Daphnia, we compared aging of sexual and asexual Daphnia clones from temporary ponds. Our data contradict the breeding system hypothesis. Differences in aging between the breeding systems were slight, and trended in the opposite direction from that predicted: asexual clones had longer lifespans and appeared to age more slowly than sexual clones. We conclude that divergent selection between habitats remains the best explanation for differences in aging between Daphnia species.

The recurrent depth preference of three ciliate species (two prostomatids and one haptorid) in a transparent alpine lake indicates the existence of niche partitioning among them involving potential factors such as avoidance of high ultraviolet radiation levels and zooplankton predation, as well as competition for food resources.

We present an accurate, fast, simple and non-destructive photographic method to estimate wax ester and lipid content in single individuals of the calanoid copepod genus Calanus and test this method against gas-chromatographic lipid measurements.

Before man hunted the large baleen whales to near extinction by the end of the nineteenth century, Arctic ecosystems were strongly influenced by these large predators. Their main prey were zooplankton, among which the calanoid copepod species of the genus Calanus, long considered key elements of polar marine ecosystems, are particularly abundant. These herbivorous zooplankters display a range of adaptations to the highly seasonal environments of the polar oceans, most notably extensive energy reserves and seasonal migrations to deep waters where the non-feeding season is spent in diapause. Classical work in marine ecology has suggested that slow growth, long lifespan and large body size in zooplankton are specific adaptations to life in cold waters with short and unpredictable feeding seasons. Here, we challenge this understanding and, by using an analogy from the evolutionary and contemporary history of the avocado, argue that predation pressure by the now nearly extinct baleen whales was an important driving force in the evolution of life history diversity in the Arctic Calanus complex.

Intraspecific variation in vertical distribution, timing of vertical migration, and colouration of the mesopelagic shrimp Sergestes arcticus were studied in the >400 m deep part of Masfjorden, Norway. Very few individuals were caught in the upper strata during daytime, and larger individuals occurred deeper during the day than smaller ones. Vertical migration was prominent and no overall trend of increasing length with depth was found at night. Small individuals arrived in the upper layers earlier than larger ones. Animal colouration assessed by digital photography revealed significant variance in individual redness. Depth of capture was the most important factor explaining colouration, with increasing degree of redness with depth. Assessing the gut fullness of the transparent shrimps provided a rapid way of estimating feeding activity and showed that feeding took place mainly at night.

Here we present a nutrient-phytoplankton-zooplankton (NPZ) model that has arbitrary size-resolution within the phytoplankton- and zooplankton-state variables. The model assumes allometric scaling of biological parameters. This particular version of the model (herbivorous zooplankton only) has analytical solutions that allow efficient exploration of the effects of allometric dependencies of various biological processes on the model's equilibrium solutions. The model shows that there are constraints on the possible combinations of allometric scalings of the biological rates that will allow ecosystems to be structured as we observe (larger organisms added as the total biomass increases). The diversity (number of size classes occupied) of the ecosystem is the result of simultaneous bottom-up and top-down control: resources determine which classes can exist; predation determines which classes do exist. Thus, the simultaneous actions of bottom-up and top-down controls are essential for maintaining and structuring planktonic ecosystems. One important conclusion from this model is that there are multiple, independent ways of obtaining any given biomass spectrum, and that the spectral slope is not, in and of itself, very informative concerning the underlying dynamics. There is a clear need for improved size-resolved field measurements of biological rates; these will both elucidate biological processes in the field, and allow strong testing of size-structured models of planktonic ecosystems.

A bottom mounted upward looking Simrad EK60 120-kHz echo sounder was used to study scattering layers (SLs) and individuals of the krill Meganyctiphanes norvegica. The mooring was situated at 150-m depth in the Oslofjord, connected with an onshore cable for power and transmission of digitized data. Records spanned 5 months from late autumn to spring. A current meter and CTD was associated with the acoustic mooring and a shore-based webcam monitored ice conditions in the fjord. The continuous measurements were supplemented with intermittent krill sampling campaigns and their physical and biological environment. The krill carried out diel vertical migration (DVM) throughout the winter, regardless of the distribution of potential prey. The fjord froze over in mid-winter and the daytime distribution of a mid-water SL of krill immediately became shallower associated with snow fall after freezing, likely related to reduction of light intensities. Still, a fraction of the population always descended all the way to the bottom, so that the krill population by day seemed to inhabit waters with light levels spanning up to six orders of magnitude. Deep-living krill ascended in synchrony with the rest of the population in the afternoon, but individuals consistently reappeared in near-bottom waters already <1 h after the ascent. Thereafter, the krill appeared to undertake asynchronous migrations, with some krill always being present in near-bottom waters even though the entire population appeared to undertake DVM.

Three-dimensional data on the mesoscale distribution of hydrography and mesozooplankton were collected at the Polar Front, northwestern Barents Sea, in spring 2008 (29 April-15 May) using a combination of multinet and towed instrument platform equipped with Laser Optical Plankton Counter, fluorometer and CTD. Trophic levels (TLs) within the zooplankton community (whole community and size-separated) were analysed for three consecutive periods using biovolume spectrum theory, which proved to be a powerful tool in the physically and biologically variable frontal system. Trophic structure was highly variable in time and across the Polar Front, but was mostly related to the phytoplankton bloom (as determined by fluorescence). High TLs of 5.5 within the zooplankton community were observed outside bloom situations (mostly in Atlantic Water) and were likely due to increased omnivory of Calanus spp., which dominated the large zooplankton size group that had a lower TL (2.2) during the bloom than outside blooms (max. TL 5.6). A strong input of herbivorous barnacle nauplii (Cirripedia) into the upper layer (35 000 ind. m(-3) in net samples) substantially decreased mean TL in the marginal ice zone. Differences in TL estimates based on biovolume spectrum theory and other methods (stable isotopes, lipid markers, dietary analyses) are discussed.

Diel studies of an Emiliania huxleyi bloom within a mesocosm revealed a highly dynamic associated viral community, changing on small times scales of hours.

While mapping algal blooms from space is now well-established, mapping undesirable algal blooms in eutrophicated coastal waters raises further challenge in detecting individual phytoplankton species. In this paper, an algorithm is developed and tested for detecting Phaeocystis globosa blooms in the Southern North Sea. For this purpose, we first measured the light absorption properties of two phytoplankton groups, P. globosa and diatoms, in laboratory-controlled experiments. The main spectral difference between both groups was observed at 467 nm due to the absorption of the pigment chlorophyll c3 only present in P. globosa, suggesting that the absorption at 467 nm can be used to detect this alga in the field. A Phaeocystis-detection algorithm is proposed to retrieve chlorophyll c3 using either total absorption or water-leaving reflectance field data. Application of this algorithm to absorption and reflectance data from Phaeocystis-dominated natural communities shows positive results. Comparison with pigment concentrations and cell counts suggests that the algorithm can flag the presence of P. globosa and provide quantitative information above a chlorophyll c3 threshold of 0.3 mg m−3 equivalent to a P. globosa cell density of 3 × 106 cells L−1. Finally, the possibility of extrapolating this information to remote sensing reflectance data in these turbid waters is evaluated.

For the first time the entire sequence of the mating behaviour of Antarctic krill (Euphausia superba) in the wild is captured on underwater video. This footage also provides evidence that mating can take place near the seafloor at depths of 400-700 m. This observation challenges the generally accepted concept of the pelagic lifestyle of krill. The mating behaviour observed most closely resembles the mating behaviour reported for a decapod shrimp (Penaeus). The implications of the new observation are also discussed.

Increasing availability and extent of biological ocean time series (from both in situ and satellite data) have helped reveal significant phenological variability of marine plankton. The extent to which the range of this variability is modified as a result of climate change is of obvious importance. Here we summarize recent research results on phenology of both phytoplankton and zooplankton. We suggest directions to better quantify and monitor future plankton phenology shifts, including (i) examining the main mode of expected future changes (ecological shifts in timing and spatial distribution to accommodate fixed environmental niches vs. evolutionary adaptation of timing controls to maintain fixed biogeography and seasonality), (ii) broader understanding of phenology at the species and community level (e.g. for zooplankton beyond Calanus and for phytoplankton beyond chlorophyll), (iii) improving and diversifying statistical metrics for indexing timing and trophic synchrony and (iv) improved consideration of spatio-temporal scales and the Lagrangian nature of plankton assemblages to separate time from space changes.

We examined how tidal changes and which physical factors affected holo- and meroplankton assemblages in a subtropical estuary in Taiwan in February 1999. A factor analysis showed that during tidal flooding, the water mass properties changed from low salinity (5-16) and high particulate organic carbon (POC, 2.6-4.5 mg L(-1)) content to increasing salinity and high total suspended matter content (29.0-104.5 mg L(-1)). With a receding tide, the water became more saline again, and its velocity increased (from non-detectable to 0.67 m s(-1)). One-way ANOVA showed that the distributions of four dominant taxa were affected by the ebb tide and exhibited two distinct groups. The first group consisted of non-motile invertebrate eggs and weakly swimming polychaete sabellid embryos and larvae (at densities of 1.25-1.40 ind. L(-1)), while the second consisted of better-swimming copepods and polychaete spionid larvae (at densities of 0.70-1.65 ind. L(-1)). A canonical correlation analysis demonstrated that the former group occurred at sites with greater freshwater input, higher POC content and greater depth, whereas the latter group was significantly associated with sites subject to seawater and faster flows. We propose that a two-layered circulation process and tidally induced oscillations in water movements might account for the distributional differences between these two groups.

Strains of the green alga Chlamydomonas acidophila and two chrysomonads, Ochromonas spp., isolated from each of two similar acid mining lakes (AMLs) with extremely low pH (∼2.6) were investigated to consider a possible synergistic stress effect of low pH and unfavourable temperature. We measured flagellate growth rates over a combination of four pH (2.5, 3.5, 5.0 and 7.0) and three temperatures (10, 17.5 and 25°C) in the laboratory. Our hypothesis was that, under highly acidic conditions (pH <3), an obligate acidophil species (C. acidophila) would be less sensitive to the combined stress of pH and temperature than acidotolerant species (Ochromonas spp.). We expected that the difference of the fundamental vs. realized pH niche would be greater in the latter. Another chrysomonad, Poterioochromonas malhamensis strain DS, served as a reference for a closely related neutrophil species. Surprisingly, C. acidophila did not survive temperatures >27°C. The lowest temperature tested reduced growth rates of all three chrysomonad strains significantly. Since all chrysomonads were tolerant to high temperature, growth rate of one Ochromonas spp. strain was measured exemplarily at 35°C. Only at this high temperature was the realized pH niche significantly narrowed. We also recorded significant intraspecific differences within the C. acidophila strains from the two AML, illustrating that the niche width of a species is broader than that of individual clones.

Jellyfish blooms are of increasing concern in many parts of the world, and in Norwegian fjords an apparent increase in mass occurrences of the deep water jellyfish Periphylla periphylla has attracted attention. Here we investigate the hypothesis that changes in the water column light attenuation might cause local retention and thereby facilitate mass occurrences. We use a previously tested individual-based model of light-mediated vertical migration in P. periphylla to simulate how retention is affected by changes in light attenuation. Our results suggest that light attenuation, in combination with advection, has a two-sided effect on retention and that three fjord categories can be defined. In category 1, increased light attenuation turns fjords into dark “deep-sea” environments which increase the habitat and retention of P. periphylla. In category 2, an optimal light attenuation facilitates the maximum retention and likelihood for mass occurrences. In category 3, further increase in light attenuation, however, shoals the habitat so that individuals are increasingly exposed to advection and this results in loss of individuals and decreased retention. This classification requires accurate determinations of the organism's light preference, the water column light attenuation and topographical characteristics affecting advection.

The pan-global marine appendicularian, Oikopleura dioica, shows considerable promise as a candidate model organism for cross-disciplinary research ranging from chordate genetics and evolution to molecular ecology research. This urochordate, has a simplified anatomical organization, remains transparent throughout an exceptionally short life cycle of less than 1 week and exhibits high fecundity. At 70 Mb, the compact, sequenced genome ranks among the smallest known metazoan genomes, with both gene regulatory and intronic regions highly reduced in size. The organism occupies an important trophic role in marine ecosystems and is a significant contributor to global vertical carbon flux. Among the short list of bona fide biological model organisms, all share the property that they are amenable to long-term maintenance in laboratory cultures. Here, we tested diet regimes, spawn densities and dilutions and seawater treatment, leading to optimization of a detailed culture protocol that permits sustainable long-term maintenance of O. dioica, allowing continuous, uninterrupted production of source material for experimentation. The culture protocol can be quickly adapted in both coastal and inland laboratories and should promote rapid development of the many original research perspectives the animal offers.

Zooplankton in the ocean respond to visual and hydro-mechanical cues such as small-scale shear in turbulent flow. In addition, they form strong aggregations where currents intersect sloping bottoms. Strong and predictable tidal currents over a sill in Knight Inlet, Canada, make it an ideal location to investigate biological behaviour in turbulent cross-isobath flow. We examine acoustic data (38, 120 and 200 kHz) collected there during the daylight hours, when the dominant zooplankters, Euphausia pacifica have descended into low light levels at ∼90 m. As expected, these data reveal strong aggregations at the sill. However, they occur consistently 10–20 m below the preferred light depth of the animals. We have constructed a simple model of the flow to investigate this phenomenon. Tracks of individual animals are traced in the flow and a variety of zooplankton behaviours tested. Our results indicate that the euphausiids must actively swim downward when they encounter the bottom boundary layer (bbl) to reproduce the observed downward shift in aggregation patterns. We suggest that this behaviour is cued by the small-scale shear in the bbl. Furthermore, this behaviour is likely to enhance aggregations found in strong flows at sills and on continental shelves.

We investigated the effect of different light conditions on primary production and growth rates of three closely related freshwater picocyanobacterial strains from three different ribotypes in laboratory cultures. The primary goal was to test whether not only different pigment types (PC-rich versus PE-rich) but also other physiological characteristics suggested by different phylogenetic positions could affect growth and photosynthetic rates of picocyanobacteria. Secondly, we tested whether photacclimation is strain specific. Experiments were conducted over light intensities ranging from 6 to 1500 μmol photons m−2 s−1 with cultures that were acclimated to low (10 μmol photons m−2 s−1) and moderate (100 μmol photons m−2 s−1) irradiance. The PE-rich strain was sensitive to high light conditions and reached highest photosynthesis and growth rates at low light intensities. The relative effect of photoacclimation was different between the two PC-rich strains, with one strain showing only moderate changes in growth rates in response to the light level used during the acclimation period. Overall, growth rates differed widely in response to light intensity and photoacclimation. Photoacclimation significantly affected both primary production and growth rates of all three strains investigated. We conclude that strain-specific photoacclimation adds to the niche partitioning among closely related freshwater picocyanobacteria.

Although picophytoplankton (PP) (0.2–2 µm) are ubiquitous in lakes and oceans, their importance in rivers has rarely been studied. We examined PP assemblages during the ice-free period in five rivers of a temperate region varying in trophic state (9–107 µg/L total phosphorus) and water discharge (1–87 m3/s). In these rivers, PP abundance reached concentrations as high as those observed in lakes and oceans (∼104–105 cells/mL). The highest density of PP (4.9 × 105 cells/mL) was observed in the most eutrophic river when the water temperature (28°C) and total phosphorus (293 µg/L) were highest. For the most part, PP abundance was dominated by non-phycoerythrin-containing cyanobacteria; phycocyanin-rich cells accounted for ∼75% of PP abundance in all the rivers. In multiple regression analyses, water temperature and nitrate concentrations explained about half of the variation in PP abundance across the rivers. Discharge had no effect on PP abundance or biomass, whereas it had a significant negative effect on total algal biomass among the rivers. The PP contribution to total chlorophyll-a averaged 27% (ranging 16–46%) and did not decline with increasing nutrients as found in lakes and oceans. The PP biomass from microscopic enumerations reached a maximum of 9% of total phytoplankton biomass, comparable with that observed in lakes. The results of this study demonstrate the importance of including picophytoplankton when analysing phytoplankton communities in rivers.

The prospect of estimating primary production (II, mg C m−2d−1) from chlorophyll pigment concentrations (Ck, mg m−3 near the ocean surface is appealing, now that chlorophyll concentrations can be extracted from satellite images. Earlier work has shown a proportionality between II and Ck (II αF·Ck) but with large variance in the proportionality factor F. A cursory overview of global data suggests part of this variability is regional. For example, F for subtropical open ocean regions exceeds that of temperate coastal regions. In the richest waters, F approaches a minimum limit value, of ∼100(mg C m−2d−1)(mg Ckm−3)−1. Some of the variability in the relation over time and space in the Southern California Bight is related to environmental variables. In the simplest systems, F is proportional to insolation. The variability in F may be of ecological interest beyond its utility in relating water column production to near-surface chlorophyll as an additional descriptive characteristic of pelagic ecosystems.

A size-based ecosystem model was modified to include periodic upwelling events and used to evaluate the effect of episodic nutrient supply on the standing stock, carbon uptake, and carbon flow into mesozooplankton grazing and sinking flux in a coastal upwelling regime. Two ecosystem configurations were compared: a single food chain made up of net phytoplankton and mesozooplankton (one autotroph and one heterotroph, A1H1), and three interconnected food chains plus bacteria (three autotrophs and four heterotrophs, A3H4). The carbon pathways in the A1H1 simulations were under stronger physical control than those of the A3H4 runs, where the small size classes are not affected by frequent upwelling events. In the more complex food web simulations, the microbial pathway determines the total carbon uptake and grazing rates, and regenerated nitrogen accounts for more than half of the total primary production for periods of 20 days or longer between events. By contrast, new production, export of carbon through sinking and mesozooplankton grazing are more important in the A1H1 simulations. In the A3H4 simulations, the turnover time scale of the autotroph biomass increases as the period between upwelling events increases, because of the larger contribution of slow-growing net phytoplankton. The upwelling period was characterized for three upwelling sites from the alongshore wind speed measured by the NASA Scatterometer (NSCAT) and the corresponding model output compared with literature data. This validation exercise for three upwelling sites and a downstream embayment suggests that standing stock, carbon uptake and size fractionation were best supported by the A3H4 simulations, while the simulated sinking fluxes are not distinguishable in the two configurations.

The dynamics of two plankton population models are investigated to examine sensitivities to model complexity and to parameter values. The models simulate concentrations of nutrients, phytoplankton, zooplankton and detritus in the oceanic mixed layer. In Model 1, zooplankton can graze only upon phytoplankton, whereas in Model 2 they can graze upon phytoplankton and detritus. Both feeding strategies are employed by zooplankton in the ocean, and both are features of models in the literature. Each model here consists of four coupled ordinary differential equations, and can exhibit unforced oscillations (limit cycles) of the four concentrations. By constructing diagrams that show how steady states and oscillations persist as each parameter is varied, a general picture of the dynamics of each model is built up. The addition of the detritus pool to an earlier nutrient–phytoplankton–zooplankton model appears to have little influence on the dynamics when the zooplankton cannot graze upon the detritus (Model 1), but if the zooplankton can graze upon the detritus (Model 2), then the dynamics are affected in a significant way. These results, obtained using the theory of dynamical systems, enhance our knowledge of the factors governing the dynamics of plankton population models.

This paper presents a practical numerical method for separating and estimating growth and mortality coefficients in stage- or size-structured populations using only observations of the relative or absolute abundance of each stage. The method involves writing a system of linear ordinary differential equations (ODEs) modelling the rate of change of abundance. The solution of the differential system can be numerically approximated using standard (e.g. sixth-order Runge-Kutta-Felhberg) methods. An optimization problem whose solutions yield 'optimal' coefficients for a given model is formulated. The ODE numerical integration technique can then be employed to furnish required function and gradient information to the optimization algorithm. The data-fitting software package ODRPACK is then successfully employed to estimate optimal coefficients for the ODE population model. Simulation experiments with four- and eight-stage model populations illustrate that the method results in the successful estimation of coefficients of mortality and growth from abundance data.

The marginal ice zone (MIZ) in the northern Barents Sea is ecologically important because it represents a highly productive area in Arctic water masses north of the Polar Front. During a multi-disciplinary cruise in 1995, ecological and oceanographic processes were investigated at four stations located in a north-south transect in the MIZ. This study was carried out in Arctic water masses north of the Polar Front where ice conditions varied from dense first-year pack ice to open water. Also, the phytoplankton development varied along the transect from a pre-bloom situation at the northern-most station to a post-bloom situation in the open water. This paper includes a study of the zooplankton community and population structure of some of the dominant copepod species. Numerically, the most important mesozooplankton components were the copepods Calanus glacialis, Pseudocalanus minutus and Oithona similis. Copepods of Atlantic origin, such as Calanus finmarchicus and Oithona atlantica, gave evidence of an advection of Atlantic water masses into the area. It is concluded that the occurrence of new cohorts of Arctic copepods coincides with the onset of the phytoplankton bloom in the MIZ, and, that therefore, the spawning relies on stored energy.

The strength of coupling between phyto- and zooplankton was measured from 1961 to 1995 by comparing the grazing effect of zooplankton (visible as clear-water phase only in 1968-1994) and also by excluding zooplankton in limnocorral experiments (1980-1984). Although long-term (1961-1995) measurements show little evidence of temporal changes in total biomass of phytoplankton or zooplankton, there is strong evidence of changes in the strength of coupling due to top-down effects. The ratio of change in biomass caused by cladocerans in the intensive grazing period of each year (May/June) and the recovery of netplankton after this period seems to be strongly influenced by the trophic state of the lake. When Lake Lucerne was mesotrophic (1971-1982), the annual mean of monthly changes in phytoplankton biomass was in the range of ±1-2, indicating that the biomass more than doubled (or halved) from month to month (no change = 0). Under oligotrophic conditions, the annual average of monthly changes in biomass was below 0.5. Grazing measurements in limnocorrals at 2 m depth with labelled food (Rhodomonas lacustris) showed distinct diel rhythms, with maximum community grazing rate at dusk and dawn. These diel changes were caused by vertical migration of the zooplankton. Grazing rate and zooplankton biomass were strongly coupled, with a maximum rate of 100-200 ml day-1 mg-1 (zooplankton biomass) when daphnids were dominant. The decrease in biomass caused by excessive grazing shows parallel trends in nanoplankton and netplankton. However, the increase in biomass after the clear-water phase was largely caused by netplankton.

Dinoflagellates are characterized by low maximum photosynthetic rates and high respiratory costs. Recent evidence also suggests that dinoflagellates are disproportionately abundant in the diets of many copepods. This suggests that at least some species are preferred prey types. This begs the question: 'How do dinoflagellates co-exist with other, seemingly competitively superior, algal taxa'? Their motility may enable them to maintain position better in the light-rich surface waters, and more successfully make the return journey into deeper waters in order to replenish their internal nutrient stores when surface nutrients become depleted. This theory is examined using a depth-resolved model of algal dynamics. Nutrients and organic detrital matter are represented on a Eulerian grid, whilst a Lagrangian approach is used to represent dinoflagellates and diatoms. The model indicates that strictly autotrophic dinoflagellates have difficulty balancing their metabolic budgets. Even in the absence of competition from diatoms, motility is almost essential to dinoflagellate persistence. Only extremely motile dinoflagellates are able to compete successfully with diatoms. In reality, an increasing number of dinoflagellate species are being found to be mixotrophic. This model suggests that mixotrophy may often be an obligatory, rather than optional, behaviour.

We investigated experimentally how resources were allocated to reproduction in Daphnia pulex and Daphnia longispina when varying levels of toxic Microcystis were added to higher quality food. We used multiple regression models to estimate mean offspring size and clutch size in relation to maternal size and clutch number, and analysed effects of treatments on residuals from the models. We also measured variation in per offspring investment. At a high cyanobacterial level, D.pulex was virtually unable to reproduce. At a lower level, D.pulex produced small clutches. However, the regression model residuals indicated that the presence of cyanobacteria increased the portion of available resources allocated to reproduction. The observed allocation may be a means to maximize reproduction under diminished longevity. Effects on mean offspring size were marginal in D.pulex but variation in per offspring investment sometimes decreased in cyanobacterial exposures. Daphnia longispina was affected by a higher cyanobacterial level only, where offspring sized was reduced. Deviations from the regression model indicated that effects on maternal size alone do not explain this effect. Clutch size residuals and per offspring investment were unaffected by treatments in D.longispina. The observed responses differ from theoretical models on reproductive allocation under food imitation.

Pulse-amplitude-modulation (PAM) fluorometry was used to investigate the effects of varying the silicate concentration on different fluorescence characteristics of batch and chemostat cultures of the diatom Thalassiosira weissflogii. The fluorescence signals, measured both on- and off-line, were used to calculate the actual (φ P ) and potential (φ P0 ) photochemical efficiencies of the reaction centres of photosystem II (PSII). Also the relative electron transport rate of the reaction centres of PSII (J e ) was calculated. Fluorescence decreased in silicate-limited cells and increased rapidly after silicate addition. The decrease was caused by strong non-photochemical quenching (q N ) in silicate-limited cells. Continuous recording (on-line) of the minimum and maximum fluorescence provided data with high temporal resolution, revealing that the first recovery of silicate-limited cells occurred 20 min after the addition of silicate. Based on these observations, we assume a strong influence of silicate metabolism on the photosynthetic efficiency of the reaction centres of PSII assessed by PAM fluorescence. This implies that silicate, as well as other nutrients, has to be considered as a possible cause for variable photosynthetic efficiency of diatoms.

The vertical migration of a zooplankton community dominated by the euphausiid Meganyctiphanes norvegica was monitored between 16 and 23 September 1997 with a 153 kHz Acoustic Doppler Current Profiler (ADCP) and a MOCNESS net. The sampling period covered a phase in the lunar cycle when the rise of the moon (full moon) coincided initially with sunset and then became progressively later. On 16 September 1997, a lunar eclipse occurred 45 min after sunset, lasting for 2 h. At dusk, the ADCP observed the upward vertical migration of two principal backscattering bands 10 min apart with vertical velocities of up to 7 cm s-1. After a period at the surface, a more diffuse band subsequently sank at a slower rate (1-2 cm s-1) to a depth of 75-100 m. Net samples showed that the earlier band consisted mainly of the pteropod Cavolinia inflexa, whilst the later band was mostly euphausiids, predominantly M.norvegica. This species was also the major constituent of the band that sank. The timing of upward migration was relatively constant over the sampling period, but there was an increasing delay of the secondary sinking until 21 September. This showed as a strong correlation between the onset of sinking and the time of moonrise. The lunar eclipse on 16 September perturbed this pattern, such that animals did not sink soon after their arrival at the surface, as occurred on 17 September but remained at the surface until the end of the umbra. This suggests that M.norvegica can perceive moonlight and that this influences vertical migration. Evidence that the behaviour is not solely mediated by this erogenous factor, however, is seen in the pattern that emerged after 21 September, when midnight sinking occurred at a relatively constant time after sunset and before moonrise. These observations support the hypothesis that moonlight is a Zeitgeber for an endogenous rhythm that synchronizes secondary sinking behaviour with the lunar cycle.

The impact of enhanced and reduced UVB radiation (UVBR) on pelagic ecosystems was studied during two mesocosm experiments in May and June/July 1994. The ambient UVBR exposure was either reduced with mylar foil or artificially enhanced with UVB fluorescent tubes. Developments in the phytoplankton communities were followed during 11 and 8 day periods using several structural and functional parameters. In the May experiment, enhanced UVBR significantly stimulated carbon dioxide fixation, activity of alkaline phosphatase and content of fatty acids. In the June-July experiment, the effects induced by changed UVBR were smaller with some indications of decreased algal biomass with enhanced UVBR. Several of the measured parameters indicated that the two experiments represented different stages in the plankton community development. In the May experiment, the community was in a development stage, moving from nutrient-replete to nutrient-depleted conditions, while the community in June/July was depleted of nutrients from the start. The stimulating effects of UVBR in May are suggested to be the secondary effects of a photochemically induced breakdown of dissolved organic matter, resulting in an increase in available nutrients.

Abstract. Vertical distribution of zooplankton in the upper 1000 m was studied from the south-east Arabian Sea in order to determine the variations in zooplankton at different depths. The distribution and migration patterns of calanoid copepod species were given special attention. The mean zooplankton standing stock in the upper 1000 m was 2.1 g dry Wt m−2, of which 97.7% was concentrated in the upper 400 m. Herbivores were generally more abundant at all depths, but did not predominate. An increase in zooplankton at night occurred in the upper 200 m, as well as at the 600–1000 m stratum. The maximum diversity of calanoid copepods also coincided with these two depths. Based on vertical ranges, the calanoid copepod species were assigned to three groups: (i) species occurring predominantly in the epipelagic layer and forming the bulk of the calanoids; (ii) relatively sparser deeper living species confined below 200 m; and (iii) species occurring throughout the water column. Some amount of vertical niche separation among congeneric species was indicated. While some species showed active migration, a good number of species were non- migratory.

Ammonium inhibition of nitrate uptake by natural phytoplankton communities was investigated in two size fractions (<2 and >2 μm) in the northeast basin of the Atlantic Ocean. Kinetics of the inhibition process were assessed in surface waters by measuring nitrate uptake rates over a range of ammonium concentrations with 15N tracer techniques. Extremely low levels of ammonium were shown to significantly inhibit nitrate uptake in both small and large fractions at all seven sampling stations. The half-inhibition constant was significantly lower and the maximum inhibition was generally higher in the <2 μm (Ki = 60 ± 29 nmol L-1, Imax = 0.74 ± 0.13) compared to the >2 μm size fractions (Ki = 132 ± 72 nmol L-1, Imax = 0.67 ± 0.12), demonstrating that the ammonium inhibition was more pronounced in the small size class. Based on the surface ammonium concentrations of the study area, inhibition kinetics parameters predict a mean reduction of nitrate uptake of 27.8% (±12.6) for <2 μm fraction and of 16.5% (± 12.8) for >2 μm fraction which leads to a mean reduction of 20.8% (± 11.9) for the total community. The importance of the inhibition size specificity in regulating new production and the f-ratio is discussed. © The Author 2008. Published by Oxford University Press. All rights reserved.

To assess the influence of grazing by cladocerans on dissolved organic matter (DOM), glycolytic and proteolytic activities and bacterial growth were measured by in situ incubation of lake water from the epilimnion of an oligotrophic reservoir in three different treatments: in absence of zooplankton, and in presence of zooplankton (natural abundance and concentrated four-fold). These experiments were conducted at two periods in the succession of plankton populations (May and June 1998), that differed in the quality of the prey ingested (Eudorina sp. compared to Cryptomonas sp. and Rhodomonas sp.) and their grazing intensity (31.8 ± 2.2 μg C l–1 day–1 compared to 10.2 ± 0.5 μg C l–1 day–1). A systematic increase in bacterial biomass was measured in the treatments containing the highest zooplankton concentrations. The DOM concentrations produced in situ showed few significant differences between the three treatments, but the assimilation of DOM was higher in the presence of zooplankton than in their absence. These results show that the influence of cladocerans on the DOM was more of a qualitative than a quantitative nature. The protein compounds derived from the grazing activities of metazoans seem to be a major nutrient source for growth for bacteria (r = 0.81, P <0.05). In this study, the highest hydrolytic activities were recorded in the presence of high concentrations of metazoan zooplankton. However, the processes that regulated these activities differed between the two experimental dates (repression compared to enzyme stimulation). Grazing activities could lead to an increase in phytoplanktonic excretion during the growth phase, and therefore the production of low molecular weight compounds that are easily assimilated by the bacterial plankton.

δ 13C values in particulate organic C were measured during batch growth of cultures of the phototroph Isochrysis galbana and the heterotroph Oxyrrhis marina . There was an increase in δ 13C during growth of the phototroph which was reversed as the number of predators increased and/or the rate of C-fixation decreased. There was a diurnal change in lsochrysis δ 13C with lower values at the end of the dark phase. Evidence of 12C concentration was apparent in mixed cultures, especially with stable predator and prey numbers where δ 13C values decreased while particulate organic carbon remained constant. In cultures where the phototroph was effectively eliminated, δ 13C values remained stable; changes in δ 13C required the presence of the phototroph. Because δ 13C values in mixed microbial populations can vary in response to various factors, the use of such values in trophic web analyses must be undertaken with caution. Also, care must be taken not to use high concentrations of N or P in batch cultures for either stock maintenance or experimental work because of the implications for C-stress with inadequate dissolved inorganic C (DIC).

The nutrient, phytoplankton, and zooplankton dynamics in three enclosed water columns (1300 m3) are described. Two of the enclosures were mixed using a bubbling chamber at depth. Young chum salmon (Oncorhynchus keta) were added to one of the mixed enclosures and the unmixed enclosure. No other manipulations were imposed. Copepods appeared in large numbers (e.g. especially Pseudocalanus minutus s.l. and Paracalanus parvus) and population growth rates were estimated. Ctenophora did not appear in large numbers despite presumably ideal food environments; it is suggested that in one enclosure this is a consequence of fish predation on the ctenophores. The fish experienced high mortalities and low growth rates presumably due to unsuitable prey size. Weekly collections of sediment permitted isolation of two major sediment contributors, the first from phytoplankton sinking and the second from biogenk fallout associated with herbivore production. It was found that the more oligotrophic enclosure (unmixed) experienced proportionally higher utilization of organic carbon. Some of these results are explained by our data while others require more sophisticated experimentation, both in the design of the containers and in the types of observations.

The stable carbon isotope 13C has been used in the open ocean to estimate the inorganic carbon uptake by phytoplankton and this technique has been compared with the 13C tracer method. An overall correlation coefficient of 0.806 and a regression slope of 1.29 were calculated from 50 sample pairs gathered during three cruises in widely different oceanic areas ranging in production rates from 0.01 to 6 mgC m−3 h−1. However, significant differences between the two methods were apparent for cruises located in nutrient-depleted areas. Possible explanations lie either in a volume effect, the high silicate content of the 14C solution which could stimulate the 14C uptake or in errors associated with the particulate carbon measurements which are necessary to convert specific uptake rates to absolute uptake rates and to yield compatible units for the comparison, in laboratory cultures the 14C technique overestimated the net particulate carbon increase by — 16%.

The 14C and the 13C methods are routinely used for measuring phytoplankton production. However, few systematic comparisons of estimates using the two approaches have been conducted. The present comparison is based on 257 pairs of samples, representing 1 year of monthly sampling at 10 depths in the euphotic zone. Data, obtained by the same operators following a standard protocol, were collected at three different stations on the Scotian Shelf (Northwestern Atlantic Ocean). Overall agreement between the two methods was good ( r &equals;0.827). However, relative differences between the two sets of estimates were not randomly distributed in time and space. Three factors (photic depth, station and sampling date), identified to explain the observed differences, were included in a three-way analysis of variance (ANOVA) using relative differences as the covariate. Following this ANOVA, the whole set was split into three subsets. For the two subsets where the above identified factors did not have any significant effect, the distribution of relative differences was narrower than these for the whole data set. Significant effects of the three factors persisted for the third subset and relative differences exhibited wide variations. Possible explanations for the observed differences include (i) the volume of incubation bottles, (ii) the incubation temperature and (iii) the absence of measurements of dark uptake.

δ13C and δ15N analyses show that the three dominant zooplankton species of the Mpenjati Estuary (South Africa) derive their energetic requirements from specific and unique food sources within the same trophic level.

ICES' Statutory Meeting, October 1982, Copenhagen, C.M. 1982/L:36 Biol. Oceanography Comm., 27 Publishing place: Copenhagen

This paper presents the results of plankton studies in the Peruvian upwelling region off Chimbote from November - December, 1977, during the Investigación Cooperativa de la Anchoveta y su Ecosistema (ICANE). Primary productivity, respiratory ETS activity, composition of particulate organic matter, and microplankton cell numbers were determined. Phytoplankton growth, and bacterial and ciliate carbon-uptake rates were computed from cell counts. Inshore waters were dominated by diatoms and were more productive than offshore waters which were dominated by dinoflagellates and ciliates. Particulate primary production averaged 5.26 ± 5.24 g C m⁻²d⁻¹, and the POC standing stock was 7.75 ± 2.74 g C m⁻² for the euphotic layer of 7 shelf stations. On the shelf, microplankton respiration rates were higher in plankton populations dominated by dinoflagellates and ciliates (47% of particulate primary production per 24 h) than those in diatom dominated populations (11%, respectively). The diatom populations, which were dominated by Chaetoceros species, varied in their ecophysiological properties (assimilation numbers, proportion of water soluble carbohydrate, and protein/nitrogen ratios). The relationships between these variations and growth conditions were investigated. A 40 h time-series station revealed patchiness which was superimposed on physiological changes of the plankters. Bacterial numbers of 2 × 10⁶ cells/ml were found in the euphotic layer corresponding to approximately 17 μg C/l bacterial biomass (or 6% of the POC standing stock). Ciliate biomass (Lohmanniella oviformis was the dominating species) ranging from 2 to 9% of the POC standing stock were found even in diatom dominated populations. From a rough carbon balance for the euphotic layer it was deduced that in diatom dominated populations, 36–77% of particulate primary production was potentially available to adult anchoveta grazing.

Primary production, release of extracellular products (EOC) and the distribution of 14C into primary products of photosynthesis (low mol. wt. products, lipids, polysaccarides, and proteins) were investigated during 2 months in spring 1983 in eutrophic Lake Hylke, Denmark. After half days in situ incubations, each fraction was assayed for 14C activity and the standing stock of carbon. Calculation of specific activity revealed non-equilibrium conditions in all fractions. Since the intracellular fractions act as precursors of EOC, this means that the release may be seriously underestimated (3–20 times) by assuming isotopic equilibrium between the inorganic carbon pool and the EOC products in the Extracellular medium. However, measurements of the standing stock of carbon are complicated by inclusion of non-phytoplankton material, thereby making calculation of specific activity uncertain.

The impact of u.v.-A (315–400 nm) on phytoplanktonic C-assimilation has been studied in situ and in the laboratory under artificial light. Water samples from Lake Lucerne were placed in DURAN-glass bottles and incubated either covered or uncovered with u.v. absorbing transparent tubes. Exposure to u.v.-A clearly inhibited 14C-assimilation in the uncovered samples both in situ and in the laboratory. Variations in visible light intensity and filtering of u.v.-B selectively demonstrated small inhibition of 14C-assimilation. U.v.-A inhibition of productivity is the major factor in the well known depression in productivity for surface waters.