Adult Zooplankton Research Articles

Mercury Elevator in Lakes: A Novel Vector of Methylmercury Transfer to Fish via Migratory Invertebrates

Fisheries in highly productive prairie lakes of Canada and the United States frequently have fish consumption advisories due to elevated mercury concentrations. This occurrence is unexpected because such alkaline lakes often exhibit lower methylmercury (MeHg) concentrations in basal trophic levels than those expected in less productive basins with circumneutral pH. As sources of MeHg to fish are largely dietary, components of the food web are expected to be critical controls of the transfer of MeHg to fish. Here, we investigated how Leptodora kindtii, a large (1.5 cm) translucent predatory invertebrate that exhibits pronounced diel vertical migration (in sediments by day, upper water column by night), plays a key role in regulating the contamination of fish with MeHg in a well-studied, eutrophic, prairie lake in western Canada. Estimates of diel fluxes of invertebrate biomass and MeHg revealed that migratory adult Leptodora, but not other adult zooplankton or juvenile Leptodora, acted as a vector, uniquely transferring MeHg from presumptive deepwater sites of methylation to pelagic fish communities. This cryptic process suggests that piscivorous fish may experience greatly elevated exposure to MeHg even in circumstances when biogeochemical features, MeHg sources, and daytime food-web configuration suggest that trophic transfers should be minimal.

Population growth of the Cladoceran, Daphnia magna: a quantitative analysis of the effects of different algal food.

In this study, we examined the effects of two phytoplankton species, Chlorella vulgaris and Stephanodiscus hantzschii, on growth of the zooplankton Daphnia magna. Our experimental approach utilized stable isotopes to determine the contribution of food algae to offspring characteristics and to the size of adult D. magna individuals. When equal amounts of food algae were provided (in terms of carbon content), the size of individuals, adult zooplankton, and their offspring increased significantly following the provision of S. hantzschii, but not after the provision of C. vulgaris or of a combination of the two species. Offspring size was unaffected when C. vulgaris or a mixture of the two algal species was delivered, whereas providing only S. hantzschii increased the production of larger-sized offspring. Stable isotope analysis revealed significant assimilation of diatom-derived materials that was important for the growth of D. magna populations. Our results confirm the applicability of stable isotope approaches for clarifying the contribution of different food algae and elucidate the importance of food quality for growth of D. magna individuals and populations. Furthermore, we expect that stable isotope analysis will help to further precisely examine the contribution of prey to predators or grazers in controlled experiments.

Portage connectivity does not predict establishment success of canoe-mediated dispersal for crustacean zooplankton

Although community structure may be largely determined by local abiotic and biotic conditions under moderate levels of dispersal, anthropogenic activities can enhance dispersal rates far beyond what would otherwise occur in natural systems. We investigated the potential impact of recreational canoeing on crustacean zooplankton community structure in Killarney Provincial Park, Canada, where canoes that are transported between lakes via portage routes may enhance zooplankton community connectivity by providing a dispersal “short-cut.” We conducted a study to (1) quantify zooplankton attachment to canoe hulls after paddling through a lake and assess the importance of canoes to overall seasonal dispersal within a lake relative to other means of dispersal, (2) test the prediction that zooplankton survivorship is negatively correlated with portage duration using a mesocosm experiment, and (3) test whether variation in lake community composition was better explained by models based on reduced portage-corrected distances or true edge-to-edge distances between lakes along popular canoe routes. Here, we report the findings that canoes have the potential to act as frequent dispersal vectors, but appear to have little impact on community structure in portage-connected lakes. Substantial numbers of adult zooplankton became attached to canoe hulls and were able to establish viable populations even after exposure to portage conditions for 30 min. However, canoe-mediated dispersal only accounted for a very small proportion (<1% in this case) of overall seasonal dispersal. Moreover, environmental variables explained the greatest amount of variation in community composition among park lakes. Nevertheless, this study indicates that canoe dispersal could be more effective for specific species such as Sida crystallina than is evident by analysis of entire communities and could facilitate the spread of invasive species amenable to attaching to boat hulls. Thus, the debate about whether community composition is more strongly influenced by local environmental conditions or regional dispersal may vary depending on the scale of consideration (i.e., individual species vs. whole community).

Temporal trends of phytoplankton and zooplankton stable isotope composition in tropical Lake Malawi

Stable isotope ratios of three seston size classes (20–100 μm, 2–20 μm, and 0.2–2 μm) and zooplankton species were analyzed to determine the plankton food web structure of Lake Malawi. Over an annual cycle, seston δ 13C varied between −20.41‰ and −27.43‰ with a mean value of −24.27‰ ± 1.2 while δ 13C values for zooplankton fluctuated between −22‰ and −25‰ with a mean of −23.84‰ ± 0.77. Seston δ 13C fluctuations appeared to be related to changes in physical and meteorological conditions in the lake that ultimately control nutrient availability. The highest seston δ 13C values observed during the rainy and mixed seasons likely result from high phytolankton growth rates. δ 15N of plankton was temporally variable, suggesting short term changes in N cycling dynamics that control the supply of N to phytoplankton. Very low seston δ 15N values recorded during the mixing season suggest excess NO 3 − availability resulting from upwelling and vertical mixing. In contrast to expectations the calanoid Tropodiaptomus cunningtoni appeared to feed at a trophic level higher than that of all other zooplankton species, including the cyclopoid, Mesocyclops aequatorialis aequatorialis. δ 15N values indicate that zooplankton were nearly 2 trophic levels above seston in the early stratified season. This implies that adult zooplankton could be utilizing forms of food other than phytoplankton during this period, such as nauplii or protozoans. This extra step in the food web, and the trophic positions of large zooplankton species, may alter estimates of food web efficiency and potential fish production for Lake Malawi.

Review of two decades of progress in the development of management options for reducing or eradicating phytoplankton, zooplankton and bacteria in ship's ballast water

* Corresponding author Abstract The worldwide transfer and introduction of non-indigenous invasive aquatic organisms via ships' ballast water has been amply demonstrated to cause significant ecological, economic and human health impacts. Possible solutions to the problem include: 1) treating ballast water to remove or destroy unwanted organisms; 2) re-designing new vessels to eliminate the need to discharge ballast water; and 3) retaining ballast water onboard. Ballast water exchange is currently the only widely acceptable and suggested (sometimes even required) procedure to minimise the risk of ballast water mediated invasions but the variable efficacy and operational limitations of this approach have led to significant financial investment in the last two decades in the research and development of more effective shipboard and shore based ballast water treatment technologies. Specific technologies under consideration include mechanical separation, heat treatment, UV irradiation, cavitation, de-oxygenation and active substances. To date, no single treatment option has proved to be universally effective and increasing attention has focused on multi- component treatment systems. The high flow rates and volumes of ballast water that must be treated pose significant technological challenges, and the presence of sediment in ballast tanks reduces the efficacy of many treatment options as this provides a habitat for resistant organisms such as resting stages of phytoplankton and zooplankton. Mechanical separation devices would best be used as a primary stage of a treatment system comprising multiple technologies because free-living organisms and sediment below a certain size are likely to be largely unaffected. UV treatment systems are unlikely to eliminate all ballast water organisms, as they are not able to deliver a stable lethal dose across a wide range of water quality conditions and many organisms are resistant to UV exposure or can recuperate after treatment. At the current stage of development, cavitation would not be considered appropriate for the shipboard treatment of ballast water due to high capital and operating costs and high power requirements. The heating of ballast water using waste heat from ships' engines has been claimed to be a practical and cost effective treatment options for eliminating ballast water zooplankton and phytoplankton (including resting stages) but concerns have been expressed that attainable temperatures may not eliminate all bacterial pathogens, that this approach does not apply to ships traversing colder seas and may impact on the integrity of vessel structures. Promising research has been conducted on several systems that are able to achieve temperatures capable of eliminating bacteria but these technologies are still under development. De-oxygenation by the addition of glucose or reducing agents are not effective treatment options, however de- oxygenation technologies based on the injection of an inert gas are more promising (notably against larval and adult zooplankton) as they could be cost effective and do not impact on the aquatic environment as ballast water is re-oxygenated prior to discharge. Biocide dosing systems have low capital costs and power requirements but the costs of active substances are significant. Chemical treatment costs and space requirements can be significantly reduced by using onboard chemical generators but the capital cost of these systems is significant and all have biological efficacy, safety, operational and environmental (poor biodegradation) concerns. Treatment systems that produce free hydroxyl radicals would be favourable over other chemical treatments as they are claimed to produce less or no toxic by-products at ballast discharge but these technologies have high power requirements. Each treatment option requires further research on their biological and operational efficacy and safety under full-scale shipboard conditions. As of July 2009, 16 promising systems using active substances had received basic approval and 8 systems final approval from IMO, with 4 systems receiving type approval certification and 2 systems receiving national approval certification. Effectively eliminating the risk of ballast water mediated invasions still remains a monumental technological and economical challenge.

Climate change, diapause termination and zooplankton population dynamics: an experimental and modelling approach

Summary1. Earlier spring warming as predicted for climate change will alter combinations of water temperature and photoperiod that act as emergence cues for zooplankton resting stages. As a result, water temperature cue thresholds will be experienced at shorter photoperiods, a variable independent of weather variations. Also, light intensity, another potentially important cue for zooplankton emergence, could decrease in many lakes if symptoms of climate change resemble those of eutrophication.2. We designed a laboratory experiment to test the effects of three factors, temperature (6, 9 and 12 °C), photoperiod (13L : 11D and 16L : 8D) and light intensity (20 and 35 μE m−2 s−1) on hatchling abundance and timing of hatching of daphniids (Daphnia ambigua) and rotifers (Keratella spp. and Synchaeta pectinata) from resting eggs. Further, we investigated the implications of potential changes in hatching dynamics, following variations in hatching cues, on zooplankton spring population development using predator–prey simulation models.3. For hatchling abundance and timing of hatching, photoperiod had a significant effect for D. ambigua but not rotifers. Daphnia ambigua hatchling abundance decreased by 50% when incubated at conditions mimicking early spring (12 °C + 13‐h photoperiod) compared to a later spring (12 °C + 16‐h photoperiod). Light intensity has a significant effect only for S. pectinata, producing greater hatchling abundance at lower light intensity.4. Simulation models suggest that in contrast to a later spring, an early warming produces a shift in spring zooplankton community composition, from daphniid to rotifer dominance. These patterns are primarily driven by differential zooplankton emergence response with variations in temperature–photoperiod cues.5. Overall, our laboratory experiments and simulation models suggest that lakes with strong dependence on the ‘resting egg‐bank’, characteristic of many shallow north‐temperate lakes or in years with low winter survivorship of adult zooplankton, may be most susceptible to climate change. Further, fewer large grazers such as daphniids with an earlier spring may result in less control of cyanobacterial blooms in eutrophic lakes.

Microcystin production by Microcystis aeruginosa exposed to different stages of herbivorous zooplankton

Microcystin (MC) production by four monoclonal Microcystis aeruginosa strains was evaluated in response to infochemicals (indirect exposure) released from different stages of herbivorous zooplankton (neonate/juvenile and adult Daphnia magna and Moina macrocopa). The intracellular MC and extracellular MC concentrations were significantly different among the control and treatments with zooplankton culture media filtrates ( p<0.05), and in most cases MC production was significantly higher ( p<0.05) in strains exposed to infochemicals released from adult zooplankton rather than those of neonate/juvenile zooplankton in four strains of M. aeruginosa. Compared to intracellular MC (385.0–5598.6 μg g −1 DW), very low concentrations of extracellular MC (9.9–737.6 μg ml −1) were released, but both showed similar temporal patterns over the course of the experiment. This result might be attributed to the fact that adult zooplankton produced more infochemical signals than equal numbers of smaller juveniles and neonates. It is the first study to provide evidence that MC production might be impacted by infochemicals released from different stages of zooplankton, mediated with physiological characteristics, body size, and feeding habits.

Measuring and modeling dispersal of adult zooplankton

Habitat fragmentation poses an inherent problem for metacommunity dynamics, as dispersal among communities is hindered by increasing isolation and the loss of patches. Wetlands are one such system that have undergone excessive destruction and fragmentation in recent years. Zooplankton within these communities have historically been considered frequent and widespread dispersers, but direct, quantitative measures of zooplankton dispersal are rare. In this study, I performed two experiments to quantify zooplankton dispersal and to identify the primary dispersal vectors. I first set up an array of traps at 10, 30, 60, 120 and 180 m around an isolated pond to collect dispersing individuals. Nearly 1,500 adult zooplankton were captured in traps up to 180 m from the pond, with approximately 60% of dispersers being captured in traps at 10 m from the pond. A second experiment using open and animal-excluded traps suggested that large animals were the primary dispersal vector for these zooplankton. Using a subset of these data, I fit four models to describe the shape and magnitude of adult cladoceran dispersal at this site. All models showed the majority of cladocerans were deposited very close to the source pond, with three models suggesting that the trapping area encompassed 67% or more of the dispersal distances. These results suggest that adult zooplankton movement among ponds may be significant in areas where aquatic habitats are plentiful. Yet, in recent years climate change and anthropogenic disturbances have reduced the number and size of aquatic habitats in many regions of the world, likely curtailing effective transport of individuals in many cases. As a result, fragmented zooplankton metacommunities may experience increased dispersal limitation, stronger priority effects, higher levels of inbreeding and selection against traits engendering high dispersability.

Biogeochemistry of organic matter in Lake Geneva: I ? particulate hydrocarbons as biogenic and anthropogenic molecular markers

A study of the biogeochemistry of organic matter in Lake Geneva at a central station (SHL2), was carried out at key periods of the biological cycles from November 1985 to September 1986. The seasonal variability of particulate hydrocarbons was studied in the epilimnion and the hypolimnion, focusing on specific periods such as high phytoplankton productivity and rising herbivorous zooplankton activity (May 1986), clearness of waters (June 1986) and intense mixing of waters (February 1986). Analysis of non-aromatic hydrocarbons (NAH) indicated that their major source is the autochtonous phytoplankton. Year 1986 was characterized by a spring bloom of nanoplankton (chlorophyceae and cryptophyceae) which remained predominant during summer and fall. These biological events were correlated with an increase of biogenic NAH, particulary n-alkane and alkene concentrations; the latter increased by one order of magnitude in May. n-Alkane distributions maximized at n-C17 or n-C29, and the n-C17/n-C29 ratio varied in surface water from 2.8 in May to 0.06 in November of the preceding year, reflecting phytoplankton dominant species. Values of NAH/POC, n-alkanes/POC, alkenes/POC and pristane/phytane ratios underwent seasonal and vertical variations which could be related to grazing and/or settling of particles. NAH, n-alkane and alkene enrichment in hypolimnion particles was interpreted to be a result of zooplankton migrations. A series of 37 monoenes and polyenes having from 15 to 31 carbon atoms, originating from planktonic sources, were recognized in the particulate matter by gas chromatography/mass spectrometry (GC/MS). Their distribution was described in 3 sized particles (> 200 µm, 200-60 µm and < 60 µm, identified as adult zooplankton, young zooplankton and nanoplankton respectively) at two different periods of the biological cycle, May and June 1986. Low level inputs originating from terrestrial higher plants were evident in particles; the value of the Carbon Preference Index (CPI) in the carbon range 23–35 was less than 2.8. In all samples analyzed by GC, an unresolved complex mixture of hydrocarbons (UCM) was present and was interpreted, at least in May, as partly originating from the degradation of organic matter by microorganisms. Typical anthropogenic PAH, such as benzofluoranthene and dibenzo (a) anthracene, were found at a low level (9 and 3 ng l−1 respectively). The absence of hopane-type hydrocarbons, except in November 1985 and in September 1986, and typical combustion PAH associated with particles indicated high temperature pyrolytic contaminants rather than petroleum-derived compounds.

Frequency-domain sensitivity analyses of an estuarine ecosystem simulation model

Abstract A mathematical simulation model of the Narragansett Bay (RI) ecosystem which simulates realistic daily values of phytoplankton, several life stages of zooplankton, and three inorganic nutrients, was modified by removing all seasonally-changing environmental forcing functions. This unforced version of the model generated time-series values for the state variables which resulted only from internal mechanistic formulations, not from environmental inputs. Steady state time-series data for biotic responses were analyzed using variance spectral analysis, which partitions the variance of data among frequencies of oscillation. Periodicities at 0.035 and 0.068 cycles d −1 dominated the response spectra for phytoplankton, and juvenile and adult zooplankton. The dominance of these oscillations generally prevailed in experimental runs using constant 1 January and 30 June environmental conditions, and in runs which perturbed the ecosystem with a random temperature component (±1.5 or ±3.0°C) about 1 January (3°C) or 30 June (18°C) temperatures. Biomass decayed to near-zero levels in a temperature perturbation of a version of the model from which nutrient regeneration by the benthos had been eliminated. This result was compared with the responses of microcosms from which a benthic component had been omitted. The dominance of periodicities at 0.035 and 0.068 cycles d −1 cannot be attributed to any single mechanism within the phytoplankton or zooplankton formulations of the model. The lack of sensitivity of model components to temperature perturbations over a wide band of frequencies, and their insensitivity to gross changes in steady-state temperature values, suggest that further analyses should displace rate coefficients of internal mechanisms in order to identify the stability properties of major material cycles of the system.