Anne-Mette Hansen’s research while affiliated with University of Southern Denmark and other places

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Publications (12)


Changes in the abundance and composition of cyclopoid copepods following fish manipulation in eutrophic Lake V??ng, Denmark
  • Article

May 2006

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72 Reads

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18 Citations

Freshwater Biology

ANNE-METTE HANSEN

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1. The seasonal cycle of cyclopoid copepods during and following an approximately 50% reduction in planktivorous fish biomass was studied in shallow, eutrophic Lake Vaeng, Denmark, from 1986 to 1990. 2. The dominant cyclopoid copepods changed from Cyclops vicinus and Mesacyclops leuckarti during 1986–1989 to M. leuckarti and Megacyclops viridis in 1990. The abundance of cyclopoid copepods gradually increased from 1986 to 1988–89, decreased in autumn 1989 and markedly decreased in 1990. 3. The increase in the abundance of cyclopoid copepods from 1986 to 1988 is attributed mainly to the reduction in fish predation pressure, there being no concomitant increase in edible phytoplankton. The appearance of M. viridis in 1990, and the general decrease in cyclopoid copepod density in autumn 1989 and in 1990, are attributed to the appearance of submerged macrophytes. 4. Temperature, predation and availability of edible phytoplankton appear to determine whether C. vicinus or M. leuckarti dominates the cyclopoid copepod population of eutrophic Lake Væng.


Diapause of Cyclops vicinus (Uljanin) in Lake Søbygård: Indication of a Risk-spreading Strategy

May 2006

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32 Reads

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11 Citations

Hydrobiologia

Cyclops vicinus is reported to enter summer diapause triggered by day length in order to survive food scarcity and fish predation. Development ceases and the fourth or fifth copepodid stages persist in the sediment for several weeks. In Lake Søbygård, however, a small eutrophic lake in Denmark, C. vicinus is found in the sediment as well as in the open water throughout the year. We performed laboratory experiments to elucidate the life cycle strategy of this population. In order to find the factors that induce diapause we tested the influence of food composition and light regime on the proportion of copepods entering diapause. Further we examined the diapause pattern of the offspring from diapausing and non-diapausing parents to see whether the co-occurrence of diapausing and non-diapausing copepods results from genetic differentiation within the population. In all experiments some of the copepods developed directly into adults, while others remained at the fourth or fifth copepodid stage and displayed diapause features. The proportion of copepods that developed directly into adults was influenced by food supply. Fewer individuals entered diapause when they received a mixed diet of algae and ciliates or algae, ciliates and seston compared with a pure algal diet. The response to light was different than described in literature: about 30% of the copepods entered diapause under dark conditions, and the diapause frequency was not higher when copepods had been exposed to continuous light. The offspring from diapausing parents showed no higher tendency to enter diapause than the offspring of non-diapausing parents, indicating no genetic differentiation. We conclude that diapause in C. vicinus represents a risk-spreading strategy, modified by food. The simultaneous production of diapausing and non-diapausing offspring ensures survival under harsh conditions; the influence of the food supply on the proportion of individuals entering diapause may adjust the population to the actual environmental condition. This flexible life cycle strategy might contribute to the exclusive dominance of C. vicinus in Lake Søbygård.


The life cycle of Cyclops vicinus in Lake Søbygård: New aspects derived from sediment analyses

December 2003

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45 Reads

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10 Citations

Hydrobiologia

Cyclops vicinus is the only copepod species in the pelagic zone of Lake Søbygård and can be found there all year round. We studied the population dynamics of this copepod over a one-year period. In contrast to earlier studies we included the copepods in the pelagic zone as well as the copepods resting in the sediment. Cyclops vicinus was found not only in the open water, but also in the sediment throughout the year. From the fluctuations of the abundances in both habitats we suggest that the life cycle is more complex than known so far: different diapause pattern appear within the population and the induction of diapause is different than in other populations. We assume that these features contribute to the predominance of C. vicinus in Lake Søbygård


Seasonal pattern in nutrient limitation and grazing control of the phytoplankton community in a non‐stratified lake

October 2003

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21 Reads

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19 Citations

Freshwater Biology

1. The relative importance of zooplankton grazing and nutrient limitation in regulating the phytoplankton community in the non-stratified Lake Kvie, Denmark, were measured nine times during the growing season. 2. Natural phytoplankton assemblage bioassays showed increasing importance of nutrient limitation during summer. Growth rates at ambient nutrient concentrations were continually below 0.12 per day, while co-enrichment with nitrogen (N) and phosphorus (P) to above concentration-saturated conditions enhanced growth rates from May to the end of July. 3. Stoichiometric ratios of important elements in seston (C : N, C : P, N : P), in lake water (TN : TP), in external loading (TN : TP) and in internal loading (DIN : DIP) were measured to determine whether N or P could be the limiting nutrient. TN : TP molar ratio of both lake water, benthic fluxes and external loading suggested P limitation throughout the growing season. However, seston molar ratios suggested moderate P-deficiency only during mid-summer. 4. Abundance and community structure of the zooplankton varied considerably through the season and proved to be important in determining the responses of algal assemblages to grazing. High abundance of cladocerans and rotifers resulted in significant grazing impact, while cyclopoid copepods had no significant effect on the phytoplankton biomass. 5. Regeneration of ammonium and phosphate by zooplankton were periodically important for phytoplankton growth. A comparison of nutrient regeneration by zooplankton with nutrient inputs from sediment and external sources indicated that zooplankton may contribute significantly in supplying N and P for the growth of phytoplankton.


The effect of diapause emergence on the seasonal dynamics of zooplankton assemblage

October 2000

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128 Reads

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148 Citations

Freshwater Biology

1. The role of seasonal phenology in the emergence of zooplankton from diapause in patterns of seasonal abundance in the water column was investigated in Oneida Lake, New York. Replicate emergence traps, placed in contact with the lake sediments at two locations (one at a shallow site and one at a deep site), were monitored between May and August. 2. Although six rotifer taxa showed a clear seasonal succession in the water column throughout the study period, all but one taxon emerged exclusively in spring. Three cladoceran and three calanoid copepod species, also present in the water column throughout the study period, again showed predominantly spring emergence. In contrast, three cyclopoid copepod species had distinct seasonal periods of emergence that corresponded, at least in part, to the timing of abundance peaks in the plankton. 3. These results for a single lake are largely consistent with patterns observed or inferred by other investigators for other lakes: variable dependence of abundance in the plankton on diapause emergence for species with long‐lived diapausing eggs (i.e. rotifers, cladocerans and calanoid copepods), and much closer dependence for species with short‐lived diapausing immature stages (i.e. cyclopoid copepods).


Response of ciliates and Cryptomonas to the spring cohort of a cyclopoid copepod in a hypertrophic lake

January 2000

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13 Reads

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42 Citations

Journal of Plankton Research

The impact of a cyclopoid copepod population on the protozoa community (two ciliate categories and Cryptomonas ) was assessed weekly during the spring cohort of Cyclops vicinus (one month duration) in hypereutrophic Lake Søbygård by in situ gradient experiments with manipulation of ambient zooplankton abundance. As C.vicinus always made up >92% of the zooplankton biomass, the response of protozoa is assumed to be a result of predation by the copepod. Significant effects of copepod biomass on protozoa net population growth rates were obtained in the four experiments. Copepod clearance rates were significantly higher on oligotrichs than on prostomatids and Cryptomonas but declined for all three protozoa categories during the first three weeks of the copepod cohort, probably because of the change in developmental instar composition of the copepod population. Grazing impact on protozoa at ambient copepod abundance was considerable (range, 0.05–0.87 day–1) and could, together with the estimated reproductive potential of protozoans (range, –0.20–0.87 day–1), account for the decline in abundance and biomass of protozoa during the cohort development. Carbon flow from the protozoa to C.vicinus (range, 2.8–23.5 μg C l–1 day–1) documents the presence of a trophic link between protozoa and the spring cohort of C.vicinus in Lake Søbygård.


Cascading Trophic Interactions from Fish to Bacteria and Nutrients after Reduced Sewage Loading: An 18Year Study of a Shallow Hypertrophic Lake

May 1998

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244 Reads

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182 Citations

Ecosystems

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Jens Peder Jensen

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The effects of major reductions in organic matter, total phosphorus (TP), and total nitrogen (TN) loading on the chemical environment, trophic structure, and dynamics of the hypertrophic, shallow Lake Søbygård were followed for 18 years. After the reduction in organic matter loading in 1976, the lake initially shifted from a summer clear-water state, most likely reflecting high grazing pressure by large Daphniaspecies, to a turbid state with extremely high summer mean chlorophyll a (up to 1400 μg L− 1), high pH (up to 10.2), and low zooplankton grazing. Subsequently, a more variable state with periodically high grazing rates on phytoplankton and bacteria was established. Changes in zooplankton abundance and grazing could be attributed to variations in cyprinid abundance due to a fish kill (probably as a consequence of oxygen depletion) and pH-induced variations in fish recruitment and fry survival. The results suggest strong cascading effects of fish on the abundance and size of zooplankton and phytoplankton and on phytoplankton production. A comparatively weak cascading effect on ciliates and bacterioplankton is suggested. Due to high internal loading, only minor changes were observed in lake-water TP after a reduction in external TP loading of approximately 80% in 1982; net retention of TP was still negative 13 years after the loading reduction, despite a short hydraulic retention time of a few weeks. TN, however, decreased proportionally to the TN-loading reduction in 1987, suggesting a fast N equilibration. Only minor improvement in the environmental state of the lake has been observed. We suggest that another decade will be required before the lake is in equilibrium with present external P loading.


Food limitation in a wild cyclopoid copepod population: Direct and indirect life history responses

January 1998

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11 Reads

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32 Citations

Oecologia

We studied the effects of food limitation on the population dynamics of the freshwater cyclopoid copepod Diacyclops thomasi in Oneida Lake, New York. In the field population, maximum juvenile abundance coincided seasonally with high phytoflagellate concentration. During the clear-water phase (a seasonal period of low algal density), D. thomasi disappeared from the water column, but fourth-instar copepodids (CIV) were found encysted in developmental arrest in the sediment. Laboratory assays of the effect of the density of two types of food on copepod life history parameters showed that temporal variation in the concentration of relatively small phytoflagellates significantly affected stage-specific development times. This food limitation was most pronounced during the clear-water phase, and supplementation of the diet with a laboratory-cultured phytoflagellate, Chlamydomonas, prevented food limitation. Although developmental arrest appears to be controlled primarily by photoperiod, availability of the larger, more mobile food, Euglena, also influenced the percentage of individuals entering developmental arrest in the laboratory. An investigation of the spatial and temporal emergence pattern in the field revealed that CIV copepodids started to emerge in late autumn and that emergence rates were significantly greater at deep-water locations (9–12 m water depth) compared with shallow-water locations (5–7 m). The clear-water phase in Oneida Lake is an annual event, probably produced by intense grazing by Daphnia pulicaria and Daphnia galeata. Food limitation is thus very likely a recurrent phenomenon for D. thomasi. This apparent seasonal competitive impact of Daphnia on Diacyclops affects both nauplii and immature copepodids. Diacyclops shows two types of responses to the food limitation: (1) the physiological response of slowed active development, and (2) the adaptive response of developmental arrest.


Variable life history of a cyclopoid copepod – The role of food availability

March 1996

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10 Reads

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13 Citations

Hydrobiologia

Populations of the copepod Cyclops vicinus in two Danish lakes differed in their life cycles. In Lake Vng C. vicinus was absent during summer, whereas in Lake Sbygrd it continued producing distinct cohorts throughout summer. A comparison between the lakes showed that in the lake where C. vicinus was absent during summer, food limitation generally existed. In contrast, abundance of C. vicinus during summer in Lake Sbygrd always coincided with available resources sufficient for the whole life cycle. Thus, summer diapause of C. vicinus is suggested to be a strategy to avoid food limitation. Avoidance of fish predation does not seem substantially to influence life cycles.


The influence of food resources on the development, survival and reproduction of the two cyclopoid copepods: Cyclops vicinus and Mesocyclops leuckarti

March 1995

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20 Reads

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100 Citations

Journal of Plankton Research

Laboratory experiments were conducted to determine whether the two cyclopoid copepods, Cyclops vicinus and Mesocyclops leuckarti, exploit the same food resources. The food requirements of juveniles of the two cyclopoid copepods were investigated. Moreover, the importance of algae for the predaceous adults was studied. Nauplii of both M.leuckarti and C.vicinus successfully developed into copepodites when fed the motile algae Chlamydomonas reinhardtii, Chlamydomonas sphaeroides and Cryptomonas sp. Threshold food concentrations for naupliar development varied between offered algae and between the two cyclopoid species. The food threshold for successful naupliar development, when reared on C.reinhardtii, was lower for M.leuckarti (0.3 mg C l-1) than for C.vicinus (0.5 mg C l-1), whereas a similar food threshold was found using Cryptomonas sp. (0.3 mg C l-1) and C.sphaeroides (<0.2 mg C l-1). Naupliar development time was inversely related to food concentration. Food required for copepodite development differed for the two cyclopoid species. Cyclops vicinus was able to develop to the adult stage on a pure diet of any one of the three algal species, whereas M.leuckarti required a prey supply of the rotifer Brachionus rubens. Food composition, i.e. algal species, algal concentration and rotifer abundance, influenced copepodite survivorship of both cyclopoids and was always higher in the presence of B.rubens. Under similar food conditions, mortality was higher for M.leuckarti than for C.vicinus. Mesocyclops leuckarti females were very dependent on animal food. The predation rate of M.leuckarti was not lower in the presence of algae. Egg production of M.leuckarti was low on a pure algal diet and significantly higher when B.rubens was present. The results were used to discuss the life cycle strategy and the possibility of exploitative competition of the two cyclopoid copepods.


Citations (12)


... The effect of external disturbances on the internal structure of an ecosystem depends on the properties of the disturbance itself, the size of the disturbance, how it is applied and where in the cycle of population dynamics the disturbance is located. The cyclical pattern that emerged in the simulations was similar to the seasonal cycles that occur in natural ecosystems (Hansen & Jeppesen, 1992;Winder & Cloern, 2010), although the annual cycle of phytoplankton appears in the model in cycles of 5-10 simulated years. This periodicity of inter-decadal variation was also manifested in Lake Veluwemeer's changes in vegetation cover in water between 1950(Scheffer, 1998. ...

Reference:

Regime shifts in shallow lakes: palaeoecological and agent-based model approaches
Changes in the abundance and composition of cyclopoid copepods following fish manipulation in eutrophic Lake Væng, Denmark
  • Citing Article
  • January 1992

SIL Proceedings 1922-2010

... Zooplankton "egg banks" have been a target of several studies aiming to explore past communities, assess temporal patterns, population dynamics and seasonal succession among many other processes within this community (Hairston et al., 2000;Brendonck & De Meester, 2003;Gyllström & Hansson, 2004;Nevalainen et al., 2011;Vehmaa et al., 2018). In this regard, the study of passive zooplankton has been useful in diversity studies, paleolimnology, fundamental and applied ecology, climate change effects on aquatic systems, and other related field of research (Jeppesen et al., 2001;Vandekerkhove et al., 2005;Gaikwad et al., 2008;Nielsen & Brock, 2009). ...

The effect of diapause emergence on the seasonal dynamics of zooplankton assemblage
  • Citing Article
  • October 2000

Freshwater Biology

... Mesocyclops leuckarti ( Figure S11), are known to be predators of rotifers (Brandl, 2005;Hansen & Santer, 1995;Šorf & Brandl, 2012;Williamson, 1983), and often feed preferentially on rotifers over phytoplankton (Hansen & Santer, 1995;Kunzmann et al., 2019;Williamson & Butler, 1986). We did not observe a negative correlation between the biomass of Copepoda and Rotifera. ...

The influence of food resources on the development, survival and reproduction of the two cyclopoid copepods: Cyclops vicinus and Mesocyclops leuckarti
  • Citing Article
  • March 1995

Journal of Plankton Research

... For example, in Lake Kasumigaura, Japan (average water depth 3.8 m [7]), although external P loading had decreased from 179 mg m −2 year −1 in 1990 to 139 mg m −2 year −1 in 2020 as sewerage development progressed, the total P (TP) concentration in the lake water had increased from 61 µg L −1 to 92 µg L −1 [8,9]. In Lake Søbygård, Denmark (average water depth 1.2 m [10]), external P loading per catchment had decreased drastically from 28-33 mg m −2 year −1 (1978)(1979)(1980)(1981) to 2-7 mg m −2 year −1 (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995), but the TP concentration in the lake water during the summer had only decreased slightly from 900-1600 µg L −1 to 400-1000 µg L −1 [11]. ...

Cascading Trophic Interactions from Fish to Bacteria and Nutrients after Reduced Sewage Loading: An 18Year Study of a Shallow Hypertrophic Lake
  • Citing Article
  • May 1998

Ecosystems

... Rotifers, in contrast to larger microcrustaceans, graze on a narrow size window of phytoplankton, leaving them more sensitive to changes in the phytoplankton community (Padial et al. 2012). The lowest level of congruence was generally found for copepods, which is to be expected, as several of the cyclopoid species present in Danish lakes are predators or facultative predators in their advanced stages (Hansen and Jeppesen 1992). These findings are also consistent with previous studies showing weakening congruence in richness of organisms with increasing difference in body sizes in both terrestrial (Sandom et al. 2013) and aquatic (Velghe and Gregory-Eaves 2013) ecosystems. ...

Changes in the abundance and composition of cyclopoid copepods following fish manipulation in eutrophic Lake V??ng, Denmark
  • Citing Article
  • May 2006

Freshwater Biology

... The development of algal blooms is expected to be a spatially heterogeneous phenomenon Butitta, Carpenter, Loken, Pace, & Stanley, 2017;Pace et al., 2017;Serizawa, Amemiya, & Itoh, 2008) due to both local heterogeneity in nutrient limitation, zooplankton grazing, and temperature (Davis, Berry, Boyer, & Gobler, 2009;Hansen, Andersen, & Jensen, 1997) and population scale heterogeneity due to wind (George & Heaney, 1978). Algal blooms can have a negative effect on ecosystem services, and therefore are often a target for ecosystem monitoring and management. ...

Seasonal pattern in nutrient limitation and grazing control of the phytoplankton community in a non‐stratified lake
  • Citing Article
  • October 2003

Freshwater Biology

... Winter diapausing is considered to be a common strategy to overcome periods of limited food resources, as well as low temperature and oxygen conditions in Northern European lakes (Elgmork 1980, Elgmork et al. 1990Naess et al. 1993). On the other hand, summer diapause in cyclopoids is seen as a long-term decline in metabolic processes providing encysted or passive dormant stages with the capability to survive in cold and oxygen-depleted deep waters of thermally stratified lakes and avoid food resource limitation and fish predation (George 1973;Gliwicz and Rowan 1984;Wyngaard 1988;Santer and Lampert 1995;Hansen 1996;Santer and Boldt 1998;Kobari and Ban 1998;Santer and Hansen 2006;Alekseev et al. 2007). However, diapausing cyclopoids may suffer from predation by invertebrates, such as chaoborids, that migrate and survive in cold and anoxic deepwater zones. ...

Variable life history of a cyclopoid copepod – The role of food availability
  • Citing Article
  • March 1996

Hydrobiologia

... lakes have adaptations to withstand periods of environmental stress (due to a decrease in temperature, an increase in salinity, or desiccation) [14][15][16][17][18]. These adaptations include the production of dormant or diapausing states, as is the case with the resistance eggs of anostraca, notostraca, cladocerans, and rotifers [15,[18][19][20][21], or the embryonic stage of the copepods [22][23][24]. The accumulation of these resistance structures in the lake sediments generates an egg bank, where they can remain viable for long periods and repopulate the lake when conditions become favorable again [17,25]. ...

Diapause of Cyclops vicinus (Uljanin) in Lake Søbygård: Indication of a Risk-spreading Strategy
  • Citing Article
  • May 2006

Hydrobiologia

... Gilbert, 1963;Kuo-Cheng Shan, 1974;Pasternak & Arashkevich, 1999) and food limitation (e.g. Scharfenberg, 1910;D'Abramo, 1980;Hansen & Hairston, 1998;Drillet, Hansen & Kiørboe, 2011) as proximate cues. However, given the typically large vertical extent (amplitude) of the migration, it is inexpedient to view most marine SVMs as a behaviour to avoid harmful irradiance or temperature (see also Banse, 1964;Marshall & Orr, 1972). ...

Food limitation in a wild cyclopoid copepod population: Direct and indirect life history responses
  • Citing Article
  • January 1998

Oecologia

... On one hand, they prey on a wide variety of microorganisms and are major consumers of bacteria, algae, and other protozoa (Šimek et al., 1990;Weisse et al., 1990). On the other hand, ciliates are preyed by various meiobenthos (Hansen, 2000;Kosiba et al., 2017). In addition, microbenthos also competes with meiobenthos due to the food overlapping between them. ...

Response of ciliates and Cryptomonas to the spring cohort of a cyclopoid copepod in a hypertrophic lake
  • Citing Article
  • January 2000

Journal of Plankton Research