Viruses in the plankton of freshwater and saline Antarctic lakes

1. Virus‐like particle (VLP) abundances in nine freshwater to saline lakes in the Vestfold Hills, Eastern Antarctica (68° S) were determined in December 1999. In the ultra‐oligotrophic to oligotrophic freshwater lakes, VLP abundances ranged from 1.01 to 3.28 × 10⁶ mL^–1 in the top 6 m of the water column. In the saline lakes the range was between 6.76 and 36.5 × 10⁶ mL^–1. The lowest value was found in meromictic Ace Lake and the highest value in hypersaline Lake Williams. Virus to bacteria ratios (VBR) were lowest in the freshwater lakes and highest in the saline lakes, with a maximum of 23.4 in the former and 50.3 in the latter. 2. A range of morphologies among VLP was observed, including phages with short (Podoviridae) and long tails, icosahedric viruses of up to 300 nm and star‐like particles of about 80 nm diameter. 3. In these microbially dominated ecosystems there was no correlation between VLP and either bacterial numbers or chlorophyll a. There was a significant correlation between VLP abundances and dissolved organic carbon concentration (r=0.845, P < 0.01). 4. The data suggested that viruses probably attack a spectrum of bacteria and protozoan species. Virus‐like particle numbers in the freshwater lakes were lower than values reported for lower latitude systems. Those in the saline lakes were comparable with abundances reported from other Antarctic lakes, and were higher than most values published for lower latitude lakes and many marine systems. Across the salinity spectrum from freshwater through brackish to hypersaline, VLP concentrations increased roughly in relation to increasing trophy. 5. Given that Antarctic lakes have a plankton almost entirely made up of bacteria and protists, and that VLP abundances are high, it is likely that viruses play a pivotal role in carbon cycling in these extreme ecosystems.     

Response of zooplankton to nutrient enrichment and fish in shallow lakes: a pan-European mesocosm experiment

1. Responses of zooplankton to nutrient enrichment and fish predation were studied in 1998 and 1999 by carrying out parallel mesocosm experiments in six lakes across Europe. 2. Zooplankton community structure, biomass and responses to nutrient and fish manipulation showed geographical and year‐to‐year differences. Fish had a greater influence than nutrients in regulating zooplankton biomass and especially the relative abundances of different functional groups of zooplankton. When fish reduced the biomass of large crustaceans, there was a complementary increase in the biomasses of smaller crustacean species and rotifers. 3. High abundance of submerged macrophytes provided refuge for zooplankton against fish predation but this refuge effect differed notably in magnitude among sites. 4. Large crustacean grazers (Daphnia, Diaphanosoma, Sida and Simocephalus) were crucial in controlling algal biomass, while smaller crustacean grazers and rotifers were of minor importance. Large grazers were able to control phytoplankton biomass even under hypereutrophic conditions (up to 1600 μg TP L^?1) when grazer biomass was high (>80–90 μg dry mass L^?1) or accounted for >30% of the grazer community. 5. The littoral zooplankton community was less resistant to change following nutrient enrichment in southern Spain, at high temperatures (close to 30 °C), than at lower temperatures (17–23 °C) characterising the other sites. This lower resistance was because of a greater importance of nutrients than zooplankton in controlling algal biomass. 6. Apart from the reduced role of large crustacean grazers at the lowest latitude, no consistent geographical patterns were observed in the responses of zooplankton communities to nutrient and fish manipulation.     

Comparative ecology of plankton communities in seven Antarctic oasis lakes

Similarities and differences in the biological limnology ofseveral perennially ice-covered, amictic lakes within a 100x 100 km grid of an Antarctic desert oasis have been identified.Most of the lakes possessed the cryptophyte Chroomonas lacustrisas the dominant phytoplankter which was accompanied by otheralgae, bacteria, yeasts, and ciliates. Maximum phytoplanktondensities and extractable particulate chlorophyll a usuallyoccurred well below the poorly transparent ice covers. The sevenlakes varied among themselves but all displayed more oligotrophicthan eutrophic values for five trophic state indicators. Thehigh dissolved O₂/CO₂ ratio in these lakes caused by supersaturatedoxygen and the previously reported high proportion of extracellularphotosynthate production by the phytoplankton raise the possibilityof a significant amount of photorespiration compared to inorganiccarbon incorporation for growth. In addition to the supersaturatedoxygen, the plankton communities could be limited by low lightintensities, hypersalinity (in some lakes), available nutrients,and grazing by protozooplankton.     

Contribution of Crenarchaeota and Euryarchaeota to the prokaryotic plankton in the coastal northwestern Black Sea

The NW Black Sea is an area with peculiar hydrological, hydrochemicaland biological features (river plume dynamics, coastal upwelling,intense eutrophication, blooms algae). We determined the abundanceand distribution of the two major groups of Archaea (Crenarchaeotaand Euryarchaeota) in three contrasting sites of the NW BlackSea from May to August 2004 to estimate the contribution ofnon-extremophilic Archaea to the coastal prokaryotic plankton.Using catalyzed reporter deposition-fluorescence in situ hybridization,Euryarchaeota were found to be particularly prominent (11–22%of total prokaryotic plankton) in the low-salinity waters duringlate-spring and summer, whereas Crenarchaeota contributed only5% to the prokaryotic community in these waters. The abundanceof Crenarchaeota was highest in nutrient- and particle-richwaters and lowest in offshore Black Sea surface waters. Theseresults confirm recent studies on the distribution of planktonicArchaea with Euryarchaeota dominating the archaeal communityin surface waters. Statistical analyses indicated significantcorrelations between crenarchaeotal abundance, ammonia and nitrateconcentrations at each sites. Our data show that the archaealplankton constitute an important and dynamic component of theprokaryotic plankton in the coastal oxygenated waters of theNW Black Sea and might play a significant role in the nitrogencycle.     

Contribution of Crenarchaeota and Euryarchaeota to the prokaryotic plankton in the coastal northwestern Black Sea

Journal of Plankton Research, 29,     

The Antarctic coastal current in the southeastern Weddell Sea

Summary Between January and March 1989 during EPOS leg 3, a hydrographic survey was carried out in the southeastern Weddell Sea on transects across the continental shelf and slope off Kapp Norvegia and Halley Bay. This data set represents oceanographic conditions during Antarctic summer. Winter observations were obtained during the Winter Weddell Gyre Study in September and October 1989. During summer the water in the surface layer is relatively warm and of low salinity. In the area of Halley Bay exceptionally warm conditions were encountered with sea surface temperatures of nearly +1°C. Over the upper continental slope a frontal zone separates Eastern Shelf Water from Antarctic Surface Water in the near surface layer and from Warm Deep Water in the deeper layers. The horizontal pressure gradient associated with the front produces the high velocity core of the Antarctic Coastal Current. In winter Antarctic Surface Water is replaced by colder Winter Water of higher salinity. Measurements from current meters moored off Kapp Norvegia and Vestkapp are used to describe the mean features of the current field and its fluctuations. At Kapp Norvegia annual mean current speeds range from 10 to 20 cm/s. The geostrophic current shear indicates that the speed of the current core decreases towards Halley Bay. The currents show significant seasonal variations with strong interannual differences. These compare well with the variations of the wind field observed at the Georg von Neumayer Station. Superimposed are higher frequency fluctuations with an energetic range between 5 and 15 days which is found in the wind measurements as well. A considerable part of the current velocity variance is due to the tides. The oceanographic conditions are strongly influenced by the local bottom topography. A topographic rise at the shelf edge off Kapp Norvegia reduces horizontal advection and allows a patch of cold Winter Water to be preserved into the summer. In contrast, a patch of Warm Deep Water was found on the shelf of Halley Bay. This illustrates rather heterogeneous conditions in the near bottom layers due to differences in the exchange rate with the open ocean as well as with the near surface layers.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Responses of epilimnetic phytoplankton to experimental nutrient enrichment in three small seepage lakes

This paper describes the responses of three epilimnetic phytoplanktoncommunities to experimental nitrogen and phosphorus enrichmentas compared to the phytoplankton community in a fourth, unmanipulated,lake. Increased nutrient inputs increased total phytoplanktonbiomass, primary productivity, chlorophytes, cryptomonads andspecies turnover rates in all three enriched lakes; cyanobacteriaincreased in two of the three enriched lakes. However, nutrientaddition also led to declines in previously dominant dinoflagellatesand chrysophytes, and in species diversity. At the species level,there were large changes in community composition from yearto year in both enriched and reference lakes, suggesting thatphytoplankton community composition is highly dynamic even inthe absence of enrichment. Overall, changes in total biomass,productivity and species diversity were consistent among theenriched lakes, while changes in species composition differeddue to variation in the physical, chemical and biotic environmentof each lake. This suggests that aggregated variates are moreuseful for quantitative prediction of nutrient effects, whilespecies responses can be used to signal qualitative differencesin environmental conditions among lakes. ³Present address: Department of Biological Sciences, DartmouthCollege, 6044 Gilman Laboratory, Hanover, NH 03755-3576, USA     

The influence of viral infection on a plankton ecosystem undergoing nutrient enrichment

It is increasingly recognised that viruses are a significant active component of oceanic plankton ecosystems. They play an important role in biogeochemical cycles as well as being implicated in observed patterns of species abundance and diversity. The influence of viral infection in plankton ecosystems is not fully understood. Here we use a number of well-founded mathematical models to investigate the interplay of the ecological and epidemiological interactions of plankton and viruses in the sea. Of particular interest is the role of nutrient on the population dynamics. Nutrient forcing has been suggested as a means of absorbing excess anthropogenic atmospheric carbon dioxide by stimulating increased phytoplankton primary productivity. Here we show that enriching nutrient levels in the sea may decrease the amount of infected phytoplankton species thereby additionally enhancing the efficiency of the biological pump, a means by which carbon is transferred from the atmosphere to the deep ocean.     

Identification of DNA-synthesizing bacterial cells in coastal North Sea plankton

We describe a method for microscopic identification of DNA-synthesizing cells in bacterioplankton samples. After incubation with the halogenated thymidine analogue bromodeoxyuridine (BrdU), environmental bacteria were identified by fluorescence in situ hybridization (FISH) with horseradish peroxidase (HRP)-linked oligonucleotide probes. Tyramide signal amplification was used to preserve the FISH staining during the subsequent immunocytochemical detection of BrdU incorporation. DNA-synthesizing cells were visualized by means of an HRP-labeled antibody Fab fragment and a second tyramide signal amplification step. We applied our protocol to samples of prefiltered (pore size, 1.2 micro m) North Sea surface water collected during early autumn. After 4 h of incubation, BrdU incorporation was detected in 3% of all bacterial cells. Within 20 h the detectable DNA-synthesizing fraction increased to >14%. During this period, the cell numbers of members of the Roseobacter lineage remained constant, but the fraction of BrdU-incorporating Roseobacter sp. cells doubled, from 24 to 42%. In Alteromonas sp. high BrdU labeling rates after 4 to 8 h were followed by a 10-fold increase in abundance. Rapid BrdU incorporation was also observed in members of the SAR86 lineage. After 4 h of incubation, cells affiliated with this clade constituted 8% of the total bacteria but almost 50% of the visibly DNA-synthesizing bacterial fraction. Thus, this clade might be an important contributor to total bacterioplankton activity in coastal North Sea water during periods of low phytoplankton primary production. The small size and low ribosome content of SAR86 cells are probably not indications of inactivity or dormancy.     

In situ nutrient enrichment experiment in the Bohai and Yellow Sea

Nutrient concentrations and N : P ratios have changed significantlyin the past 40 years in the Bohai and Yellow Sea. How do thesechanges influence or contribute to the growth of phytoplankton?Nutrient enrichment experiments were conducted in 1998 and 1999to shed light on which was the first nutrient to limit algalgrowth and uptake rates of nitrogen and phosphorus. Significantvariance analysis, together with nutrient concentration andratio, demonstrated that phosphorus was the first nutrient tolimit the growth of phytoplankton in the Laizhou Bay (SouthBohai); nitrogen was the first, whilst phosphate might be thepotential, nutrient to limit the growth of phytoplankton inthe West Yellow Sea; the Central Yellow Sea was oligotrophicand any one of nitrogen, phosphorus and silicon would limitthe growth of phytoplankton; and silicon was confirmed not tolimit the growth of phytoplankton, although the silicate concentrationdecreased acutely, in the Laizhou Bay. Moreover, the ratio ofnitrogen to phosphorus in phytoplankton uptake was smaller thanthat in seawater, which suggested that phosphorus was preferentiallyused before nitrogen by the phytoplankton. The preference ofphosphorus over nitrogen indicates a further limitation of phosphorusin the Laizhou Bay, and increases the high possibility thatphosphorus, rather than nitrogen, is the first nutrient to limitthe growth of phytoplankton in the West Yellow Sea. Half-saturationconstants (K_s) of dissolved inorganic nitrogen (DIN) and phosphatein the Bohai and Yellow Sea were 1.80 µM and 0.13 µM,respectively. Compared with other sea areas, the K_s value ofDIN in the Bohai and Yellow Sea was located at the high endof the spectrum.     

Response of submerged macrophytes in Danish lakes to nutrient loading reductions and biomanipulation

During the last two decades the nutrient loading to Danish lakes has been reduced with the aim to improve water quality. However, because of internal P-loading and biological resistance, the expected improvement has been delayed. Therefore, to reduce the duration of the recovery period and to accelerate recolonisation of submerged macrophytes, several lakes have been biomanipulated with the purpose of improving the top-down control by zooplankton. To elucidate the effects of these measures, we undertook an analysis of data on submerged macrophytes monitored annually in 17 lakes for 8 years. The results obtained show that the macrophyte coverage in non-biomanipulated lakes remained relatively stable following the external nutrient reduction. However, a small increase in macrophyte coverage occurred in a few lakes. In two of the four biomanipulated lakes, in contrast, macrophyte coverage increased from 0 up to 80% within 2–4 years following manipulation. In the other two lakes macrophyte colonisation failed. However, in lakes with a successful recolonisation, large inter-annual variations in macrophyte coverage, varying between 2 and 80% among growth seasons, occurred. We conclude that the potential of macrophyte recolonisation after nutrient loading reduction on the short term is higher in biomanipulated lakes than in lakes subjected to loading reduction only, although biomanipulation does not provide a guarantee for macrophyte recolonisation or a stable macrophyte community when colonisation occurs.

     

Predicting chlorophyll vertical distribution in response to epilimnetic nutrient enrichment in small stratified lakes

Light-limited metalimnetic phytoplankton communities are thoughtto be negatively impacted by epilimnetic nutrient enrichmentbecause of shading by increased epilimnetic phytoplankton biomass.We tested this expectation with a dynamic simulation model thatwas calibrated to three lakes undergoing whole-lake nutrientand food web manipulations. Total areal chlorophyll increaseddue to nutrient enrichment in each lake, but the magnitude ofthe response varied between lakes. Modeling experiments, whichallowed analysis of separate components of each lake's responseto nutrient enrichment, indicated that the response to enrichmentdepended on lake water color and food web structure. In weaklystained lakes ({small tilde}10 mg Pt 1^–1, k₄ = 0.4 m^–1),metalimnetic chlorophyll was stimulated by nutrient enrichmentup to moderate levels (1 µg Pt1^–1 day^–1).In more strongly colored lakes (25 mg Pt 1^–1, k₄ = 1.0),metalimnetic chlorophyll responded negatively to nutrient enrichmentat all P loading rates. Food web structure, as expressed byrates of zooplanktivory, interacted with water color in twoways. One impact was through direct grazing losses on metalimneticchlorophyll. The other process involved was indirect impactfrom grazing on epilimnetic phytoplankton, which reduced shadingon metalimnetic chlorophyll. Vertical redistribution of chlorophyllbetween the epilimnion and the metalimnion led to little accumulationof areal chlorophyll with increased P loading over limited rangesof water color and nutrient input rates. Model predictions maybe most effectively tested with whole-lake experiments contrastingfood web structure, water color and nutrient loading.     

Response of an algal assemblage to nutrient enrichment and shading in a Hawaiian stream

To investigate the effects of nitrate enrichment, phosphate enrichment, and light availability on benthic algae, nutrient-diffusing clay flowerpots were colonized with algae at two sites in a Hawaiian stream during spring and autumn 2002 using a randomized factorial design. The algal assemblage that developed under the experimental conditions was investigated by determining biomass (ash-free dry mass and chlorophyll a concentrations) and composition of the diatom assemblage. In situ pulse amplitude-modulated fluorometry was also used to model photosynthetic rate of the algal assemblage. Algal biomass and maximum photosynthetic rate were significantly higher at the unshaded site than at the shaded site. These parameters were higher at the unshaded site with either nitrate, or to a lesser degree, nitrate plus phosphate enrichment. Analysis of similarity of diatom assemblages showed significant differences between shaded and unshaded sites, as well as between spring and autumn experiments, but not between nutrient treatments. However, several individual species of diatoms responded significantly to nitrate enrichment. These results demonstrate that light availability (shaded vs. unshaded) is the primary limiting factor to algal growth in this stream, with nitrogen as a secondary limiting factor.     

Tracking a century of changes in microbial eukaryotic diversity in lakes driven by nutrient enrichment and climate warming

High‐throughput sequencing of sedimentary DNA (sed‐DNA) was utilized to reconstruct the temporal dynamics of microbial eukaryotic communities (MECs) at a centennial scale in two re‐oligotrophicated lakes that were exposed to different levels of phosphorus enrichment. The temporal changes within the MECs were expressed in terms of richness, composition and community structure to investigate their relationships with two key forcing factors (i.e., nutrient enrichment and climate warming). Various groups, including Apicomplexa, Cercozoa, Chrysophyceae, Ciliophora, Chlorophyceae and Dinophyceae, responded to phosphorus enrichment levels with either positive or negative impacts on their richness and relative abundance. For both lakes, statistical modelling demonstrated that phosphorus concentration ([P]) was a dominant contributor to MECs modifications before the 1980s; after the mid‐80s, the contribution of air temperature changes increased and potentially surpassed the contribution of [P]. Co‐occurrence network analysis revealed that some clusters of taxa (i.e., modules) composed mainly of Dinophyceae and unclassified Alveolata were strongly correlated to air temperature in both lakes. Overall, our data showed that sed‐DNA constitutes a precious archive of information on past biodiversity changes, allowing the study of the dynamics of numerous eukaryotic groups that were not traditionally considered in paleo‐reconstructions.     

Seasonal variation in plankton, submicrometre particles and size-fractionated dissolved organic carbon in Antarctic coastal waters

Concentrations of plankton, suspended particles 0.74–87 μm equivalent spherical diameter and dissolved organic carbon (DOC) were measured from May to February at an Antarctic coastal site. Bacteria-sized particles 0.74–1 μm diameter, and bacterial cells and heterotrophic protists all exhibited a seasonal minimum during winter and maxima in summer. Bacteria composed <10% of the bacteria-sized particles. Release of autotrophic protists from the ice caused water column biomass of autotrophs to reach maximum concentrations in October and November, but maximum cell concentration in the water column was reached in January. Microheterotroph biomass weakly reflected the release of the ice algal community but reached maximum concentration during the water column bloom in January. Total DOC concentrations varied from 0.36 mg C l⁻¹ in July to 3.10 mg C l⁻¹ in October, with a yearly average of 1.51 mg C l⁻¹. Ultrafiltration of DOC revealed that the molecular weight composition of the DOC differed greatly through the year. DOC <5 kDa molecular weight reached a maximum of 1.25 mg C l⁻¹ in October and accounted for up to 60% of total DOC in July. Concentrations of high molecular weight DOC (>100 kDa) were highest in July and November, with the DOC (100 kDa–0.5 μm) fraction reaching a maximum of 1.22 mg C l⁻¹ in November and composing 82% of the total DOC in January. Wet chemical oxidation and high-temperature catalytic oxidation organic carbon analyses were compared. Good correlation was observed between methods during summer but no significant correlation existed in winter, indicating that winter DOC may be refractory. Accepted: 21 March 2000     

The construction of a diatom-based chlorophyll a transfer function and its application at three lakes on Signy Island (maritime Antarctic) subject to differing degrees of nutrient enrichment

1. Canonical correspondence analysis of a diatom and water chemistry dataset from fifty-nine maritime Antarctic lakes situated on Signy and Livingston Islands showed that nutrients and functions of nutrients (NH₄⁺, chlorophyll a) accounted for a significant fraction of the variance in the diatom data. 2. Weighted averaging regression was used to construct a diatom-based transfer function for inferring chlorophyll a concentrations from sediment core diatom assemblages. 3. The transfer function was applied to ²¹⁰Pb-dated sediment cores from three lakes (Moss, Sombre and Heywood) receiving different levels of nutrient input from fur seal populations, i.e. low, medium and high, respectively. 4. Moss Lake showed relatively stable reconstructed chlorophyll a values, and no evidence of recent eutrophication, agreeing with measured chlorophyll a concentrations at the site. 5. Changes in diatom assemblages and results of chlorophyll a reconstructions at Sombre Lake suggested that nutrient enrichment had occurred, which could be clearly linked to fluctuations in the measured water chemistry over the last 10–14 years. 6. Despite recorded increases in recent nutrient inputs there was no apparent diatom response at Heywood Lake.     

The relative importance of light and nutrient limitation of phytoplankton growth: a simple index of coastal ecosystem sensitivity to nutrient enrichment

Anthropogenic nutrient enrichment of the coastal zone is now a well-established fact. However, there is still uncertainty about the mechanisms through which nutrient enrichment can disrupt biological communities and ecosystem processes in the coastal zone. For example, while some estuaries exhibit classic symptoms of acute eutrophication, including enhanced production of algal biomass, other nutrient-rich estuaries maintain low algal biomass and primary production. This implies that large differences exist among coastal ecosystems in the rates and patterns of nutrient assimilation and cycling. Part of this variability comes from differences among ecosystems in the other resource that can limit algal growth and production – the light energy required for photosynthesis. Complete understanding of the eutrophication process requires consideration of the interacting effects of light and nutrients, including the role of light availability as a regulator of the expression of eutrophication. A simple index of the relative strength of light and nutrient limitation of algal growth can be derived from models that describe growth rate as a function of these resources. This index can then be used as one diagnostic to classify the sensitivity of coastal ecosystems to the harmful effects of eutrophication. Here I illustrate the application of this diagnostic with light and nutrient measurements made in three California estuaries and two Dutch estuaries.     

Examination of the meroplankton community in the south-western Ross Sea, Antarctica, using a collapsible plankton net

The meroplankton community in the coastal Antarctic has been poorly studied, in part due to the presence of extensive sea-ice cover during the summer months. In this study, a collapsible plankton net was used to examine the meroplankton community at two sites in the south-western Ross Sea: at Cape Evans on Ross Island, and at Cape Roberts on the Victoria Land coast. Three samples were taken on 5 consecutive days in the early summer at each site (26–30 November and 8–12 December 2001) and 11 operational taxonomic units (OTUs) were quantified from the phyla Annelida, Mollusca, Nemertea, Echinodermata and Sipuncula and the early developmental stages of undetermined phyla. Multivariate analysis showed that there were distinct differences in the meroplankton communities at the two locations, but because there was a 2-week difference in the time of sampling between sites, location and time are confounded variables. On average, a total of 16 larvae/eggs/embryos were collected per tow (3.92 m³) at Cape Evans and 27 larvae/eggs/embryos at Cape Roberts, with a significant difference in total larval numbers and in numbers of 4/6 larval types tested between location and time. This research has shown that a collapsible plankton net can be used for meroplankton work in remote locations in the coastal Antarctic and that a diverse meroplankton community is evident in the south-western Ross Sea in the early summer.