Size dependent horizontal migration ofMysis relicta

Quantitative seasonal population estimates ofMysis relicta at shallow and deep stations in Emerald Bay, an isolated (forMysis) embayment of Lake Tahoe, California-Nevada, Donner Lake and Fallen Leaf Lake show thatM. relicta exhibits size dependent horizontal migrations. Adults inhabit deep water where they release young in early spring. The newly released juveniles migrate to shallow water where they remain until late summer when they return to deep water. Adults stay in deep water throughout the year.     

The recurrence of Daphnia rosea in Lake Tahoe: analysis of a population pulse

Following the disappearance of all Lake Tahoe cladocera in 1970,Daphnia rosea reappeared in the lake during the summer and fallof 1983. Since the introduction and population increase of theomnivorous opossum shrimp, Mysis relicta, was heavily implicatedin the initial disappearance, we investigated the D. rosea recurrencewith respect to fluctuating M. relicta abundance and steadilyincreasing primary productivity. An analysis of D. rosea populationdynamics, fecundity, feeding characteristics and M. relictafeeding selectivity, suggested that D. rosea was able to coexistwith M. relicta in 1983 only because D. rosea birth rates weresubstantially higher than historical levels due to the increasein algal productivity since 1970. The variation of cladoceransuccess in lakes of M. relicta introductions may be due, inlarge part, to variations in primary productivity.     

Contributions of satellite remote sensing to analysis of spatial variability in zooplankton distributions

Zooplankton populations in Lake Tahoe and other large lakes often exhibit considerable variability due to changing predator assemblages and interactions between littoral and pelagial regions. Wind-induced advection of Emerald Bay populations of Bosmina longirostris into Lake Tahoe was hypothesized to be the mechanism for Bosmina's reappearance in the main body of that lake in late 1978 following an absence of more than three years (Threlkeld 1981).Landsat satellite imagery is examined here to determine if a significant spatial-temporal interaction in water quality in Emerald Bay and Lake Tahoe consistent with this hypothesis occurred during the period when Bosmina reappeared. Densitometer measurements of Landsat MSS band 4 and 5 images were compared by ANOVA; significant station-season interactions were detected, and enlarged Landsat film images revealed plumes, streaks and other surface features which may have been associated with the hypothesized mixing events in Lake Tahoe. A major limitation of this method is the need for pre-flyover water truth data for interpretation of detectable surface features.     

The effect of prey composition and abundance on the predation rate and selectivity ofMysis relicta

The predation rate and selection of specific prey byMysis relicta in Lake Tahoe depends on both the total and the relative densities of prey classes. Functional responses of mysids to changes in the density of two species of calanoid copepod prey were curvilinear in laboratory experiments. In the field and in 2-prey treatments, preference was usually positive forEpischura and negative forDiaptomus. However, the preference forEpischura was greatest at the lowest combined densities of prey in the laboratory and decreased at densities >100Epischura / m3 in the field. Several hypotheses to explain the inverse relationship between selectivity and prey density are discussed. Field assemblages of prey vary vertically, horizontally and seasonally. Potential effects of this hetrogeneity on both predator and prey populations are examined.     

Annual variation in biomass and the community structure of crustacean zooplankton over 5 years in Lake Toya, Japan

The biomass and population dynamics of crustacean zooplankton were determined in oligotrophic Lake Toya in Japan over 5 years from May 1992 to May 1997. In 1992 and 1993, zooplankton biomass was up to 4.3 g dry weight m^?2, whereas it decreased to <1 g dry weight m^?2 after 1994. This extreme change in biomass was associated with the succession of dominant species from larger ones, such as Daphnia longispina and Cyclops strenuus (s. lat.), to smaller ones, such as Eubosmina tanakai and Bosmina longirostris. Consequently, this biomass change seemed to cause an increase in the chlorophyll a concentration in the euphotic zone and a decline in lake transparency. Because the birth rates of the dominant species were somewhat higher after 1994, the decline in the populations of larger crustaceans seemed to depend more on their rate of death rather than rate of birth, and this higher death rate is not considered to be attributed to food shortage. Although these results strongly suggest a top-down cascading effect of fish predation upon crustaceans, annual catches of two commercially important planktivorous fish species have also decreased in the lake, coincidentally with decreases in zooplankton biomass. This may be attributable to fishing regulations that prohibit catching smaller fish, implying that such smaller fish affect zooplankton and phytoplankton, as well as lake transparency.     

Comparative energetics and life cycle of the opossum shrimp ( Mysis relicta ) in native and non-native environments

1. Life history, fecundity and energy ingestion were compared for non-native Mysis relicta in Flathead Lake, Montana, and in Waterton Lake, Montana/Alberta, where the mysid is native. 2. Based on estimates from stomach contents, M. relicta in Flathead Lake consumed three to four times more energy per hour while foraging than M. relicta from Waterton Lake. The primary prey of M. relicta in Flathead Lake was Daphnia thorata. Diaptomus sicilis was the primary prey for M. relicta in Waterton Lake. 3. Mysis relicta in Flathead Lake had a 1-year life history. It took 2 years for M. relicta in Waterton Lake to complete a generation. Mysis relicta from Flathead Lake produced significantly (P < 0.05) more eggs per female (19.9 ± 4.7) compared to M. relicta in Waterton Lake (13.7 ± 3.6). The lipid content in eggs from M. relicta in Flathead Lake was also significantly (P < 0.05) higher than eggs from M. relicta in Waterton Lake. 4. In Flathead Lake, M. relicta is able effectively to exploit Daphnia thorata, consuming greater biomass than M. relicta in Waterton Lake. Mysis relicta efficiently assimilates the greater biomass, which reduces generation time, increases fecundity, and increases lipid reserves of offspring. These results provide evidence of energetic mechanisms that enable M. relicta to colonize and affect foodwebs when introduced into lakes.     

A morphological study of two populations of Bosmina longispina exposed to different predation

Two populations of Bosmina longispina existing in two similarand closely situated Norwegian mountain lakes were investigatedwith respect to morphological variations. The two lakes areinhabited by different predators, Lake Gopollen is dominatedby whitefish and Lake Djupen by a sparse population of browntrout and invertebrate predators. The B. longispina in LakeDjupen were both larger and also longer at maturation than theB. longispina of Lake Gopollen. In Lake Djupen the mucronesof B. longispina were twice as long and their relative antennulalength was also larger than in Lake Gopollen. However, the relativeeye size did not differ between the two populations. The observationsfit the present hypothesis on morphological predator avoidanceadaptations in B. longispina, fairly well. Invertebrate predatorsfavour large B. longispina with long protuberances, body sizeand length of mucro being the most important features.     

Feeding habits ofMysis relicta — an overview

Mysis relicta has been recognized as an important component of many limnetic food webs. The first approach to studying the feeding habits ofM. relicta was the analysis of stomach contents. Assumptions regarding stomach content analysis have been tested and seasonal feeding estimates ofM. relicta have been reported recently. Laboratory grazing and/or predation experiments have recently been completed usingM. relicta primarily from Lakes Michigan and Tahoe.Mysis relicta is concluded to be opportunistic, capable of utilizing a variety of food resources. It may play an important role in structuring limnetic food webs.     

Comparison of sampling methods for semipelagic animals in two deep basins of Lake Saimaa

The efficiency of different methods for sampling semipelagic animals was compared in the deep basins of Lake Paasivesi and Lake Puruvesi of the complex Lake Saimaa, Finland, during 1990–92. Certain semipelagic animals, such as Mysis relicta, amphipods and larvae of Chaoborus fiavicans, are important bioindicators of water and benthic quality. However, owing to their high mobility and vertical migrations they are difficult to sample quantitatively with traditional benthic methods. The equipment used for sampling was comprised of:

1. Vertical nets (0.56 and 0.13 m²) placed on the bottom for 5 minutes or 12 hours and hauled up vertically.
2. Plexiglass traps placed on the bottom for 12–14 hours.
3. Mysis-trawl hauled horizontally c. 250–300 metres on the bottom.
4. Ekman-grab.

In 1992, parallel samples were taken with vertically hauled nets (12 h and plexiglass traps baited with fish or liver or provided with a chemical light. Large vertically hauled nets were the most efficient equipment for catching Gammaracanthus lacustris, M. relicta and larvae of Ch. flavicans whilst baited or lighted nets were less effective. Lighted plexiglass traps were most effective in catching Pallasea quadrispinosa and water mites, but baited and control traps sampled inefficiently. The trawl sampled M. relicta and P. quadrispinosa relatively well, but was less efficient in sampling other groups. The Ekman-grab was effective only for sampling Monoporeia affinis. The semipelagic community differed between the studied basins. The dominant semipelagic animals in the humic, deep L. Paasivesi were M. relicta, amphipods and larvae of Ch. flavicans whilst the number of water mites was low in the catches. In the shallower, clear-water L. Puruvesi the densities of crustaceans and Chaoborus-larvae were low whilst some water mites, such as Piona pusilla, occurred in high numbers. Nocturnal sampling was more effective, especially for M. relicta.     

Spatial distribution, biomass and population dynamics of Mysis relicta in Lake Michigan

The abundance, biomass, and life history traits of Mysis relicta were evaluated in the spring, summer, and autumn of 2000 at 8 nearshore (45 m) and 8 offshore (75–110 m) stations throughout central and southern Lake Michigan. Abundance was also evaluated on a smaller scale during June 2000 at 4 nearshore and 4 offshore stations in southeastern Lake Michigan. For large-scale sampling, the abundance of M. relicta did not differ among locations in the spring. In the summer and autumn the abundance of M. relicta was similar among offshore stations with the exception of one station each season; for nearshore stations, abundance was generally highest off Pentwater, Michigan. The abundance of mysids was not consistently high for central or southern basin sites, although overall biomass was higher in the southern basin each season. Abundance of Mysis was positively correlated with bottom depth, but not with bottom water temperature, surface water temperature, or mean chlorophyll concentration. Within the smaller region in southeast Lake Michigan, the abundance of M. relicta differed among locations for both nearshore and offshore stations. Brood size and size of reproductive females did not differ among lake wide locations, but the proportion of females with broods and the size distribution of M. relicta did.     

Population dynamics of Bosmina longirostris (Crustacea: Cladocera) in Lake Kariba,Zimbabwe

The population dynamics of Bosmina longirostris in a large tropical reservoir, Lake Kariba, Zimbabwe, are described. Population density showed two peaks associated with periods of high nutrient fluxes. Fecundity was always low. Two eggs per female was the maximum ever recorded. Instantaneous rates of birth, death and population growth calculated from field data using the egg ratio method are discussed. The rate of biomass change fluctuated in a regular pattern, probably linked to food availability or predation by a clupeid, Limnothrissa miodon.     

Significance of a low oxygen layer for a Daphnia population in Lake Yunoko,Japan

Population dynamics and vertical migration of Daphnia longispina in Lake Yunoko were studied. The Daphnia population was small in spring and early summer, probably because of high predation pressure by fish. The population grew in midsummer, when thermal stratification developed and the dissolved oxygen became very low in the deeper layer of the hypolimnion. In this season, adults of D. longispina concentrated in the daytime near the lake bottom, where fish were absent because of the anoxic conditions, but ascended at night to the upper layer of the hypolimnion, where food was most abundant. The low oxygen layer near the bottom kept out the predators and protected Daphnia from predation, and consequently contributed to the built-up of its population. However, the low oxygen layer was unfavorable for reproduction of Daphnia, as reflected in the low egg ratio and high percentage of males in the population. The population decreased in the fall, when thermal stratification disappeared and predation pressure seemed to increase.     

Density-Dependent Multiplication and Survival Rates in Heterodera glycines

Seasonal multiplication and overwinter survival are density-dependent in Heterodera glycines. At low to moderate population densities, the nematode is capable of large population increases on susceptible soybean cultivars and high rates of oversummer or overwinter survival in the absence of a host. To improve estimates of H. glycines multiplication and survival rates, egg densities were monitored for 12 cropping sequences across 10 years. Log-linear regression analysis was used to describe and compare density-dependent relationships. Growing-season change in H. glycines egg densities was density-dependent for all crops (susceptible soybean, resistant soybean, and nonhost), with slope estimates for the density-dependent relationship greater for susceptible soybean compared with a non-host crop. Overwinter population change also was density-dependent, with similar declines in survival rates observed for all crops as population densities increased. Survival was greater following susceptible soybean compared with resistant soybean, with an intermediate rate of survival associated with non-host crops. Survival estimates greater than 100% frequently were obtained at low population densities, despite attempts to account for sampling error. Rates of growing-season multiplication and survival, when standardized for population density, declined with year of the study. Standardized overwinter survival rates were inversely related to average daily minimum temperature and monthly snow cover.     

Population Dynamics of Crustacean Zooplankton in a Mesotrophic Lake,with Emphasis on Boeckella hamata BREHM (Copepoda: Calanoida)

The crustacean zooplankton in Lake Mahinerangi, a polymictic, mesotrophic reservoir, were studied for 19 months. The cladocerans, Bosmina meridionalis and Ceriodaphnia dubia, had unimodal patterns of abundance with peak densities in summer; cyclopoids had a single peak of maximum abundance in autumn and Daphnia carinata showed a bimodal pattern with peak densities in early summer and early winter. The strong seasonality in abundance of zooplankton in Lake Mahinerangi distinguishes this lake from other temperate lakes in New Zealand and may reflect its cool winter temperatures (< 5 °C for 2 months). A calanoid, Boeckella hamata, bred throughout the year and was the dominant species numerically. Instar analysis indicated that there were at least three, and possibly four, generations per year. Instantaneous birth and death rates were low with annual mean values (0.036 and 0.057, respectively) that were 2-3.5 times less than those recorded in a population of the same species in Lake Pounui, but within the range of those recorded in calanoid populations elsewhere in New Zealand. The Boeckella population in Lake Mahinerangi appears to be controlled largely by temperature, parasitism and food, with predation having no, or a negligible, effect.     

Effects of Zinc Fertilization of Corn on Hatching of Heterodera glycines in Soil

Experiments were conducted to determine the effects of zinc fertilizers on hatching and soil population densities of Heterodera glycines. In vitro egg hatching in solutions of reagent-grade zinc sulfate and zinc chloride and fertilizer-grade zinc sulfate was significantly greater than hatching in deionized water, whereas zinc chelate fertilizer significantly inhibited egg hatching relative to deionized water. In greenhouse experiments, no differences in cumulative percentage egg hatch were detected in soil naturally infested with H. glycines amended with fertilizer-grade zinc sulfate and zinc chelate at rates equivalent to 0, 1.12, 11.2, and 112 kg Zn/ha and subsequently planted with corn (Zea mays L.). In a field experiment, no significant differences in H. glycines egg population densities and corn yields were detected among plots fertilized with 0, 11.2, and 22.4 kg Zn/ha rates of zinc chelate. Yields of H. glycines-susceptible soybean planted in plots 1 year after zinc fertilization of corn plots also were not significantly affected. Zinc compounds significandy affected H. glycines egg hatching in vitro, but had no effect on hatching in natural soils.     

Aging in Brachionus plicatilis: The evolution of swimming as a function of age at two different calcium concentrations

Species composition and density of cladoceran populations changed in Lakes Zürich, St. Mortiz and Baldegg as human populations increased in these watersheds. Lake Zürich sediments became annually laminated in the 1890's as a result of increased organic input as the size of the cities surrounding the lake grew. At the same time, the Bosmina species changed from a oligotrophic form (longispina) to a eutrophic form (longirostris). An increase in Daphnia spp. populations also occurred at this time in the lake's history. Bosmina longispina reappeared in the lake in 1965 as the lake's trophic status changed from eutrophic to mesotrophic due to effective sewage treatment facilities. Annual laminations appear in the Lake St. Moritz sediments about 1910. Shortly thereafter, a shift from B. longispina to B. longirostris occurred. This change in trophic status is associated with increased tourism in the area. Lake Baldegg sediments also show annual laminations beginning in 1885 and a similar shift in the Bosmina species. Other cladoceran remains were too scarce to be useful in interpreting the histories of these lakes.     

Population dynamics of Daphnia spp. and implications for trophic interactions in small, monomictic lake

In Lake Oglethorpe, Georgia, USA, herbivorous crustacean zooplankton are abundant and dominate zooplankton biomass in winter, but are scarce throughout most of the summer. We used a 3.5 year study of Daphnia population dynamics to infer when food, predators or temperature constrained growth of this population. Transitions between winter and summer consumer assemblages are concurrent with seasonal changes in water temperature, thermal structure (stratification/destratification), resources (autotrophic/heterotrophic-domained production), and predator abundance and activity (e.g. Lepomis macrochirus and Chaoborus punctipennis. We sampled at weekly or less intervals from April 1992 to September 1995, and determined population abundances for all cladoceran species. For the Daphnia population (Daphnia ambigua + Daphnia parvula, we measured clutch size and length for all individuals. We used average water column temperature (where dissolved oxygen is >1 mg l^-1) to estimate egg development time from an empirical model. Estimates of Daphnia population birth and death rate were thus generated from abundance, egg ratio and temperature/dissolved oxygen data. We compared observed birth rate (bobs) with expected birth rate (bexp>95% CI; predicted for food-saturated conditions at ambient temperature). For variable (1-13 week) periods between later November and March, 1992-1995, water temperature was the primary factor constraining Daphnia population growth (bobs = bexp). From about April to early November, bobs < bexp suggested food-limited population growth. In spring, summer and early fall (March-October), population densities were several-fold lower than in late fall and winter (November-February). However, all else being equal, egg ratio and population birth rate data would have predicted that Daphnia abundance fluctuates over equivalent ranges in spring and fall. We interpret this discrepancy as evidence for increased rates of extrinsic mortality during the growing season and a seasonal shift in the relative importance of resource and predator regulation. The duration of predator suppression of crustacean population abundance in Lake Oglethorpe and other warm-latitude lakes (36N-27°S) is longer (3-7 months) than that observed in north temperate lakes (1-2.5 months; 41-52°N).      

The zooplankton-phytoplankton interface in lakes of contrasting trophic status: an experimental comparison

We report here the results of an experimental study designed to compare algal responses to short-term manipulations of zooplankton in three California lakes which encompass a broad range of productivity (ultra-oligotrophic Lake Tahoe, mesotrophic Castle Lake, and strongly eutrophic Clear Lake). To assess the potential strength of grazing in each lake, we evaluated algal responses to a 16-fold range of zooplankton biomass. To better compare algal responses among lakes, we determined algal responses to grazing by a common grazer (Daphnia sp.) over a range ofDaphnia densities from 1 to 16 animals per liter. Effects of both ambient grazers andDaphnia were strong in Castle Lake. However, neither ambient zooplankton norDaphnia had much impact on phytoplankton in Clear Lake. In Lake Tahoe, no grazing impacts could be demonstrated for the ambient zooplankton butDaphnia grazing had dramatic effects. These results indicate weak coupling between phytoplankton and zooplankton in Clear Lake and Lake Tahoe, two lakes which lie near opposite extremes of lake trophic status for most lakes. These observations, along with work reported by other researchers, suggest that linkages between zooplankton and phytoplankton may be weak in lakes with either extremely low or high productivity. Biomanipulation approaches to recover hypereutrophic lakes which aim only to alter zooplankton size structure may be less effective if algal communities are dominated by large, inedible phytoplankton taxa.     

Seasonality,Population Dynamics and Production of Daphnia longispina in the Subtropical Lake Bhimtal (U.P.), India

Seasonal variation in population density, birth rate, mortality and production of a daphnid was studied in a high, altitude sub-tropical lake. A bimodal growth curve with maxima during summer and autumn was observed. A positive correlation was reported between fecundity and death rate, probably due to a difference in age specific death rate combined with a high relative frequency of juveniles in a growing population. The production of Daphnia longispina correlates better with temperature than with food. The P/B ratio was higher during summer and autumn months.