TY  - JOUR
A1  - Bell, Elanor M.
A1  - Laybourn-Parry, J.
T1  - The plankton community of a young, eutrophic, Antarctic saline lake
N2  - A shallow, saline lake (Rookery Lake) close to the sea and surrounded by a penguin rookery was investigated during the austral spring and summer of 1996/1997. The proximity to the sea means that the lake is likely to have been formed recently during isostatic uplift. Inputs of carbon and nutrients from the penguin rookery have rendered Rookery Lake eutrophic compared with other brackish and saline lakes in the Vestfold Hills. Chlorophyll a concentration, bacterioplankton, heterotrophic nanoflagellate and phototrophic nanoflagellate abundances were all significantly higher than in other non-enriched lakes. The high productivity created seasonal anoxia during winter and spring below ice cover. The ciliate community resembled the marine community, and was dissimilar to that seen in older saline lakes within the Vestfold Hills. Thus Rockery Lake provides valuable evidence of the impact of natural eutrophication on an Antarctic lake, as well as of the evolution of the typical microbial community which dominates the older lakes of the Vestfold Hills.
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Bell, Elanor M.
A1  - Laybourn-Parry, J.
T1  - Annual plankton dynamics in an Antarctic saline lake
N2  - 1. The plankton dynamics of Ace Lake, a saline, meromictic basin in the Vestfold Hills, eastern Antarctica was studied between December 1995 and February 1997. 2. The lake supported two distinct plankton communities; an aerobic microbial community in the upper oxygenated mixolimnion and an anaerobic microbial community in the lower anoxic monimolimnion. 3. Phytoplankton development was limited by nitrogen availability. Soluble reactive phosphorus was never limiting. Chlorophyll a concentrations in the mixolimnion ranged between 0.3 and 4.4 mu g L-1 during the study period and a deep chlorophyll maximum persisted throughout the year below the chemo/oxycline. 4. Bacterioplankton abundance showed considerable seasonal variation related to light and substrate availability. Autotrophic bacterial abundance ranged between 0.02 and 8.94 x 10(8) L-1 and heterotrophic bacterial abundance between 1.26 and 72.8 x 10(8) L-1 throughout the water column. 5. The mixolimnion phytoplankton was dominated by phytoflagellates, in particular Pyramimonas gelidicola. P. gelidicola remained active for most of the year by virtue of its mixotrophic behaviour. Photosynthetic dinoflagellates occurred during the austral summer, but the entire population encysted for the winter. 6. Two communities of heterotrophic flagellates were apparent; a community living in the upper monimolimnion and a community living in the aerobic mixolimnion. Both exhibited different seasonal dynamics. 7. The ciliate community was dominated by the autotroph Mesodinium rubrum. The abundance of M. rubrum peaked in summer. A proportion of the population encysted during winter. Only one other ciliate, Euplotes sp., occurred regularly. 8. Two species of Metazoa occurred in the mixolimnion; a calanoid copepod (Paralabidocera antarctica) and a rotifer (Notholca sp.). However, there was no evidence of grazing pressure on the microbial community. In common with most other Antarctic lakes, Ace Lake appears to be driven by 'bottom-up' forces.
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Laybourn-Parry, J.
A1  - Bell, Elanor M.
A1  - Roberts, E. C.
T1  - Growth of Protozoa in Antarctic lakes
N2  - The growth rates of heterotrophic nanoflagellates (HNAN), mixotrophic cryptophytes, dinoflagellates and ciliates in field assemblages from Ace Lake in the Vestfold Hills (eastern Antarctica) and Lakes Fryxell and Hoare (McMurdo Dry Valleys, western Antarctica), were determined during the austral summers of 1996/1997 and 1997/1998. The response of the nanoflagellates to temperature differed between lakes in eastern and western Antarctica. In Ace Lake the available bacterial food resources had little impact on growth rate, while temperature imposed an impact, whereas in Lake Hoare increased bacterial food resources elicited an increase in growth rate. However, the incorporation of published data from across Antarctica showed that temperature had the greater effect, but that growth is probably controlled by a suite of factors not solely related to bacterial food resources and temperature. Dinoflagellates had relatively high specific growth rates (0.0057-0.384 h(-1)), which were comparable to Antarctic lake ciliates and to dinoflagellates from warmer, lower latitude locations. Temperature did not appear to impose any significant impact on growth rates. Mixotrophic cryptophytes in Lake Hoare had lower specific growth rates than HNAN (0.0029-0.0059 h(-1) and 0.0056-0.0127 h(-1), respectively). They showed a marked seasonal variation in growth rate, which was probably related to photosynthetically active radiation under the ice at different depths in the water column. Ciliates' growth rates showed no relationship between food supply and mean cell volume, but did show a response to temperature. Specific growth rates ranged between 0.0033 and 0.150 h(-1) for heterotrophic ciliates, 0.0143 h(-1) for a mixotrophic Plagiocampa species and 0.0075 h(-1) for the entirely autotrophic ciliate, Mesodinium rubrum. The data indicated that the scope for growth among planktonic Protozoa living in oligotrophic, cold extreme lake ecosystems is limited. These organisms are likely to suffer prolonged physiological stress, which may account for the highly variable growth rates seen within and between Antarctic lakes.
Y1  - 2000
ER  - 
TY  - JOUR
A1  - Laybourn-Parry, J.
A1  - Roberts, E. C.
A1  - Bell, Elanor M.
T1  - Mixotrophy as a survival strategy in Antarctic lakes
N2  - Mixotrophy is a widespread phenomenon among planktonic protists. It involves the combination of autotrophy and heterotrophy in varying degrees. Many phytoflagellate species ingest bacteria as a means of obtaining nutrients for photosynthesis or for supplementing their carbon budget under light limitation. Ciliates either sequester the plastids of their algal prey or harbour endosymbiotic algae. In the saline lakes of the Vestfold Hills and in Lakes Hoare and Fryxell in the McMurdo Dry Valleys the dominant phytoflagellates ingest bacteria, and there is evidence to suggest that during the winter months they lack chlorophyll and may become entirely heterotrophic. In Lake Fryxell phagotrophic pyhtoflagellates (cryptophytes) made a significant impact on bacterial production, removing up to 13% of the bacterial biomass day-1. These cryptophytes suffered predation from Plagiocampa (a ciliate), which appears to harbour them for a significant period before digesting them. We suspect that this may be equivalent to an intermediate stage in the evolution of mixotrophy. A significant number of the planktonic ciliates in Antarctic lakes were mixotrophic. The final evolutionary end point is the situation seen in Mesodinium rubrum, which now relies entirely on its cryptophycean endosymbiont and no longer ingests food. Mesodinium is the dominant ciliate in many of the saline lakes of the Vestfold Hills, which are of marine origin. It can reach abundances in excess of 60,000l-1 in Ace Lake, This ciliate is a ubiquitous member of the marine plankton worldwide and has successfully adapted to the lacustrine environment in Antarctica. The evidence suggests that among the survival strategies seen in Antarctic lake plankton, mixotrophy plays and important role among a number of the dominant protozoan species.
Y1  - 2000
ER  - 
TY  - JOUR
A1  - Bell, Elanor M.
A1  - Weithoff, Guntram
T1  - Benthic recruitment of zooplankton in an acidic lake
N2  - In recent years most studies of the benthic microbial food web have either been descriptive or were restricted to the measurement of within sediment process rates. Little is known about benthic-pelagic coupling processes such as recruitment. We, therefore, developed an ex situ core incubation procedure to quantify the potential for microbial recruitment from the benthos to the pelagic in an acidic mining lake, Mining Lake 111 (ML 111; pH 2.6), in eastern Germany. Our data suggest that considerable zooplankton recruitment from the benthos takes place. Heliozoan and rhizopod recruitment in both summer and winter sediment cores was highest when they were incubated at 20°C. Maximum heliozoan recruitment was 23 (± 9 s.e.) individuals cm-2 d-1 (40% initial standing stock daily) in the winter 20°C incubation. Maximum rhizopod recruitment was 6 (± 2 s.e.) individuals cm-2 d-1 in the summer 20°C incubation. Little or no recruitment was apparent for either taxa when winter cores were incubated at 5°C, implying a temperature cue. Conversely, the rotifer, Cephalodella hoodi, exhibited a maximum recruitment of 6 (± 2 s.e.) individuals cm-2 d-1 during the winter 5°C incubation, representing 30% of initial standing stock daily, but little recruitment when incubated at 20°C. Cephalodella may have responded to an increased winter benthic food supply; in situ winter Chl a concentrations in the benthos were 3.4 times higher than those in the summer. The importance of this was reinforced by the poor pelagic food supply available in ML 111. In situ, Heliozoa, rhizopods and Cephalodella were first observed in the epilimnion of ML 111 in spring or early summer, suggesting active or passive recruitment following lateral transport from littoral sediments. Benthic-pelagic coupling via recruitment is potentially important in understanding the pelagic food web in ML 111 and warrants further investigation in this and other aquatic environments.
Y1  - 2003
ER  - 
TY  - JOUR
A1  - Bell, Elanor M.
A1  - Laybourn-Parry, J.
T1  - Mixotrophy in the Antarctic phytoflagellate, Pyramimonas gelidicola (Chlorophyta: Prasinophyta)
N2  - Grazing by the planktonic, phytoflagellate, Pyramimonas gelidicola McFadden (Chlorophyta: Prasinophyta), and heterotrophic nanoflagellates (HNAN) in meromictic, saline Ace Lake in the Vestfold Hills, Eastern Antarctica, was investigated in the austral summers of 1997 and 1999. Up to 47% of the P. gelidicola population ingested fluorescently labelled prey (FLP). Ingestion rates varied with depth. In January 1997 and November 1999, maximum P. gelidicola ingestion rates of 6.95 and 0.79 FLP;cell-1;h-1, respectively, were measured at the chemocline (6-8 m) where a deep chlorophyll maximum composed of phototrophic nanoflagellates (PNAN DCM), predominantly P. gelidicola, persisted all year. During the summers of 1997 and 1999, the grazing P. gelidicola community removed between 0.4 and approximately 16% of in situ bacterial biomass, equivalent to between 4 and >100% of in situ bacterial production. Due to their higher abundance, the community clearance rates of HNAN in Ace Lake generally exceeded those of P. gelidicola but HNAN removed approximately only 3 to 4% of bacterial biomass, equivalent to between 28 and 32% of bacterial production. P. gelidicola growth rates were highest at the PNAN DCM concomitant with the highest ingestion rates. It is estimated that during the summer P. gelidicola can derive up to 30% of their daily carbon requirements from bacterivory at the PNAN DCM. This study confirms mixotrophy as an important strategy by which planktonic organisms can survive in extreme, polar, lacustrine ecosystems.
Y1  - 2003
ER  - 
TY  - JOUR
A1  - Raffaeli, D.
A1  - Bell, Elanor M.
A1  - Weithoff, Guntram
A1  - Matsumoto, A.
A1  - Cruz-Motta, J. J.
A1  - Kershaw, P.
A1  - Parker, R.
A1  - Parry, D.
A1  - Jones, M.
T1  - The ups and downs of benthic ecology : considerations of scale, heterogeneity and surveillance for benthic- pelagic coupling
Y1  - 2003
ER  - 
TY  - JOUR
A1  - Bell, Elanor M.
A1  - Lockyear, Jacqueline F.
A1  - McPherson, Jana M.
A1  - Marsden, A. Dale
A1  - Vincent, Amanda C. J.
T1  - First field studies of an Endangered South African seahorse, hippocampus capensis
N2  - South Africa's endemic Knysna seahorse, Hippocampus capensis Boulenger 1900, is a rare example of a marine fish listed as Endangered by the IUCN because of its limited range and habitat vulnerability. It is restricted to four estuaries on the southern coast of South Africa. This study reports on its biology in the Knysna and Swartvlei estuaries, both of which are experiencing heavy coastal development. We found that H. capensis was distributed heterogeneously throughout the Knysna Estuary, with a mean density of 0.0089 m-2 and an estimated total population of 89 000 seahorses (95% confidence interval: 30 000 to 148 000). H. capensis was found most frequently in low density vegetation stands ( 20% cover) and grasping Zostera capensis. Seahorse density was not otherwise correlated with habitat type or depth. The size of the area in which any particular seahorse was resighted did not differ between males and females. Adult sex ratios were skewed in most transects, with more males than females, but were even on a 10 m by 10 m focal study grid. Only three juveniles were sighted during the study. Both sexes were reproductively active but no greeting or courtship behaviours were observed. Males on the focal study grid were longer than females, and had shorter heads and longer tails, but were similar in colouration and skin filamentation. The level of threat to H. capensis and our limited knowledge of its biology mean that further scientific study is urgently needed to assist in developing sound management practices.
Y1  - 2003
ER  - 
TY  - JOUR
A1  - Tittel, Jörg
A1  - Bissinger, Vera
A1  - Zippel, Barbara
A1  - Gaedke, Ursula
A1  - Bell, Elanor M.
A1  - Lorke, Andreas
A1  - Kamjunke, Norbert
T1  - Mixotrophs combine resource use to outcompete specialists: Implications for aquatic food webs
N2  - The majority of species can be grouped into those relying solely on photosynthesis (phototrophy) or those relying solely on the assimilation of organic substances (heterotrophy) to meet their requirements for energy and carbon. However, a special life history trait exists in which organisms combine both phototrophy and heterotrophy. Such 'mixotrophy' is a widespread phenomenon in aquatic habitats and is observed in many protozoan and metazoan organisms. The strategy requires investment in both photosynthetic and heterotrophic cellular apparatus, but the benefits must outweigh these costs. In accordance with the mechanistic resource competition theory, laboratory experiments revealed that pigmented mixotrophs combined light and prey as substitutable resources. Thereby, they reduced prey abundance below the critical food concentration of competing specialist grazers [Rothhaupt, K. O. (1996) Ecology 77, 716-724]. Here, we demonstrate for the first time the important consequences of this strategy for an aquatic community. In the illuminated surface strata of a lake, mixotrophs reduced prey abundance so steeply that grazers from higher trophic levels, consuming both the mixotrophs and their prey, could not persist. Thus, the mixotrophs escaped from both competition and grazing, and remained dominant. Furthermore, the mixotrophs structured the prey abundance along the vertical light gradient creating low densities near the surface and a pronounced maximum of their algal prey at depth. Such deep algal accumulations are typical features of nutrient poor aquatic habitats, previously explained by resource availability. We hypothesize instead that the mixotrophic grazing strategy is responsible for deep algal accumulations in many aquatic environments.
Y1  - 2003
UR  - www.pnas.org/cgi/doi/10.1073/pnas.2130696100
ER  - 
TY  - JOUR
A1  - Kamjunke, Norbert
A1  - Gaedke, Ursula
A1  - Tittel, Jörg
A1  - Weithoff, Guntram
A1  - Bell, Elanor M.
T1  - Strong vertical differences in the plankton composition of an extremely acidic lake
N2  - Vertical differences in food web structure were examined in an extremely acidic, iron-rich mining lake in Germany (Lake 111; pH 2.6, total Fe 150mg L-1) during the period of stratification. We tested whether or not the seasonal variation of the plankton composition is less pronounced than the differences observed over depth. The lake was strongly stratified in summer, and concentrations of dissolved organic carbon and inorganic carbon were consistently low in the epilimnion but high in the hypolimnion. Oxygen concentrations declined in the hypolimnion but were always above 2mg L-1. Light attenuation did not change over depth and time and was governed by dissolved ferric iron. The plankton consisted mainly of single-celled and filamentous bacteria, the two mixotrophic flagellates Chlamydomonas sp. and Ochromonas sp., the two rotifer species Elosa worallii and Cephalodella hoodi, and Heliozoa as top predators. We observed very few ciliates and rhizopods, and no heterotrophic flagellates, crustaceans or fish. Ochromonas sp., bacterial filaments, Elosa and Heliozoa dominated in the epilimnion whereas Chlamydomonas sp., single-celled bacteria and Cephalodella dominated in the hypolimnion. Single-celled bacteria were controlled by Ochromonas sp. whereas the lack of large consumers favoured a high proportion of bacterial filaments. The primarily phototrophic Chlamydomas sp. was limited by light and CO2 and may have been reduced due to grazing by Ochromonas sp. in the epilimnion. The distribution of the primarily phagotrophic Ochromonas sp. and of the animals seemed to be controlled by prey availability. Differences in the plankton composition were much higher between the epilimnion and hypolimnion than within a particular stratum over time. The food web in Lake 111 was extremely species-poor enabling no functional redundancy. This was attributed to the direct exclusion of species by the harsh environmental conditions and presumably enforced by competitive exclusion. The latter was promoted by the low diversity at the first trophic level which, in turn, was attributed to relatively stable growth conditions and the independence of resource availability (inorganic carbon and light) from algal density. Ecological theory suggests that low functional redundancy promotes low stability in ecosystem processes which was not supported by our data.
Y1  - 2004
ER  -