Search

A model was developed to explore patterns of carbon flow through the pelagic food web of Lake Fryxell, Taylor Valley, Antarctica. The goals of this study were to quantify patterns of carbon flow and test hypotheses of top-down versus bottom-up controls on this system. The model included seven trophic groups: bacteria, photosynthetic nanoflagellates (PNAN), heterotrophic nanoflagellates (HNAN), ciliates (bacterial feeding and flagellate feeding), phytoplankton and rotifers (the top predator); all inputs were driven by predatory demands. This system has no insects, vertebrates, crustacea or allochthonous inputs from terrestrial plants. Autotrophs and bacteria contributed ca. 94% of total community biomass, with all other heterotrophic organisms representing

Pony Lake is a saline and hypereutrophic coastal pond located on Cape Royds in the McMurdo Sound area in Antarctica. Previous studies have shown that Pony Lake typically has high concentrations of dissolved organic carbon (DOC) concentrations (e.g., 9.2 mM) associated with dense blooms of a chlorophyte, Chlamydomonas intermedia Chodat. We studied different classes of dissolved organic material (DOM) isolated from Pony Lake on two dates that bracketed the transition from ice-covered to ice-free conditions. We used tangential flow ultrafiltration followed by XAD column chromatography to isolate fulvic acids and transphilic acids in three molecular size ranges. The small and large size fractions (passing through a 10,000-amu filter and being retained by a 100,000-amu filter, respectively) accounted for most of the total DOM and fulvic acid pools, whereas most of the transphilic acid was in the small size fraction. Measurement of molecular weight using high-pressure size exclusion chromatography (HPSEC) showed that the fulvic acid and transphilic acid in the medium and large size fractions had molecular weights corresponding to a range of 1260–1470 amu, and those in the small size fraction had molecular weights of about 1000 amu. The 13C nuclear magnetic resonance (NMR) spectra of both the fulvic and transphilic acids showed a predominance of aliphatic and carbohydrate carbons and were similar among size fractions and between the two dates. Although a large decrease in the DOC concentration occurred during the transition to ice-free conditions (from 7.7 to 2.7 mM), the changes in the DOM fractionation were relatively small. The percentage of the DOM accounted for by transphilic acid increased from 11% to 18%, and the peak in the HPSEC chromatograms corresponding to the smaller molecular weight range decreased for all fulvic acid and transphilic acid fractions. The nitrogen content of all fulvic acid and transphilic acid fractions decreased significantly during the transition and the δ15N values became heavier for all fractions. The only discernible change in the 13C NMR spectra of all the fractions was a decrease in a peak in the region corresponding to amide carbons. These results suggest that preferential loss of N-containing molecules of fulvic acid and transphilic acids occurred in association with the considerable loss of DOM in midsummer.

Nitrate loading to coastal waters has increased over recent decades while silicon loading has remained relatively constant or decreased. As the N:Si ratio in coastal waters shifts due to these anthropogenic influences, silicate limitation of diatom biomass may become a feature of the biogeochemistry in coastal waters especially in regions of reduced exchange. Two sets of nutrient enrichment mesocosm experiments were conducted in successive years using a natural planktonic assemblage obtained from the Trondheimsfjord, Norway. The inorganic nutrient concentrations at the start of the experiments were manipulated to give a variety of N:Si concentrations at ratios representative of current and possible future values, should N loading continue. In June 1999 experiments were conducted with a gradient of inorganic N:Si ratios (1:2, 1:1, 2:1, 4:1) to investigate the influence of low and high N:Si ratio conditions and to determine the conditions that would generate Si limitation of diatom growth. In June 2000, based on 1999 data, highly replicated experiments were conducted at N:Si ratios of 1:1 and 4:1 which were expected to result in N and Si limitation of diatom growth, respectively; statistical differences in cellular composition were recorded. N limitation of diatom biomass increase was observed under the three lowest N:Si ratios: particulate carbon (C) accumulation continued to occur following N exhaustion resulting in an increase in the organic C:N ratio. Silicate limitation of diatom biomass increase only occurred at the highest N:Si ratio of 4:1. Silicate exhaustion was followed by continued nitrate uptake for several days, at a slower rate than previously. The resulting increase in organic N was accompanied by an increase in organic C such that the C:N ratio of the organic material at the highest N:Si ratio failed to increase to the extent observed under the N limited conditions. Statistically significant differences in chlorophyll-a yield per unit nitrate, C:chlorophyll-a ratios, C:N ratio and diatom cell yield per unit nitrate or Si were observed in Si compared to N limited conditions. All mesocosms became dominated numerically and in terms of biomass by the diatom Skeletonema costatum. The potential implications of changing N and Si regimes in coastal waters are discussed.

Results are presented from a study into the effects of scuba diving on water column structure in perennially ice-covered Lake Fryxell, Antarctica. General theoretical predictions are compared with results, which assess potential impacts from diving activities on water quality parameters related to disruptions to water column stability and dissolved gas concentrations. Potential mechanisms of water column disturbance are considered including: (1) mixing due to diver motion, (2) mixing by diver-exhaled bubble plumes and (3) changes of water column gas concentrations due to dissolution of diver-exhaled gases. Data (temperature, dissolved oxygen, specific conductance, pH, dissolved organic carbon, salinity) were collected from two control holes and one dive hole immediately before and 2.5 d after dive activities. Variability in measured parameters did not differ significantly between sampling sites either before or after diving; and differences between control holes and dive hole observations, on a depth-by-depth basis, were within the range of expected statistical fluctuations. Pre- and post-dive virus-like particle, bacterial, ciliate, photo- and heterotrophic flagellate densities also did not differ significantly. An apparent lack of long-term trends indicative of water column destabilization over the past 18 ys of diving is discussed, along with recent evidence for deep mixing in this lake. Although all potentially affected parameters have not been considered, theoretical model results and an analysis of field data suggest that diving impacts on physicochemical properties and microbial distributions in a closed-basin, ice-covered lake occur at spatiotemporal scales other than those considered in this study.

Colin Reynolds’ distinguished career began 42 years ago, when, as an undergraduate, he started work on nuisance algae in London reservoirs. Since the publication of his first book, The Ecology of Freshwater Phytoplankton, in 1984 [1], our understanding of planktonic organisms has increased substantially. Indeed, Reynolds acknowledges that many of the assumptions made in his first book no longer hold true. For example, researchers now recognise the crucial role that microbes have in aquatic ecosystems.