David F. Bird

Bacterial grazing by planktonic lake algae

Six common species of lake algae were found to ingest bacteria. The ingestion rates measured were of the same magnitude as those recorded for marine microflagellates totally dependent on external sources of carbon. A large biomass of Dinobryon species removed more bacteria from the water column of a lake than crustaceans, rotifers, and ciliates combined.

Trichodesmium Blooms and New Nitrogen in the North Pacific Gyre

A method of protecting plant life from injury due to frost or sub-freezing temperatures which comprises applying to the plant an effective, but non-phytotoxic amount, having regard to the plant being treated of a compound of the formula: wherein R1 is lower-alkyl; R2 is selected from the group consisting of:

Critical depth and marine primary production

The concept of critical depth, as formulated by Sverdrup (J. Cim. perm. int. Explor. Mer. 18, 287 (1953)) to explain the onset of the spring bloom of phytoplankton, is discussed. Simplifying assumptions (use of a linear photosynthesis-light curve, use of daily averaged irradiance), adopted by Sverdrup in his computation of the growth term, are removed. An exact expression is found for the critical depth in terms of a generalized, biomass-specific, loss term. The loss term is calculated as the sum of contributions from algal respiration and excretion, grazing by micro- and macro-zooplankton, and sedimentation. Critical depth is then estimated using figures typical of the North Atlantic, and a sensitivity analysis is done. The respiratory costs of algal growth and metabolism dominate the generalized loss term. It is pointed out that the Sverdrup critical depth criterion is a necessary, but not sufficient, condition for the initiation of phytoplankton blooms. As a diagnostic tool, its value is therefore limited. It tells only whether net growth is possible: it tells nothing about how rapidly the phytoplankton will increase. But from the information used to calculate critical depth, it is possible to deduce the rate of increase in algal biomass (or, equivalently, the net production) for the mixed layer. As long as the growth rate is positive, the Sverdrup criterion is respected. This leads to a characteristic timescale for the development of a bloom. When the timescale is short relative to the time between mixing by storms, a bloom can be expected to occur.

Coupling Between Rates of Bacterial Production and the Abundance of Metabolically Active Bacteria in Lakes, Enumerated Using CTC Reduction and Flow Cytometry

A bstractIn natural bacterioplankton assemblages, only a fraction of the total cell count is active, and, therefore, rates of bacterial production should be more strongly correlated to the number of active cells than to the total number of bacteria. However, this hypothesis has seldom been tested. Herein we explore the relationship between rates of bacterial production (measured as leucine uptake) and the number of active bacteria in 14 lakes in southern Québec. Active bacteria are defined as those cells capable of reducing the tetrazolium salt CTC to its fluorescent formazan; these cells were enumerated using flow cytometry. Bacterial production varied two orders of magnitude in the lakes studied, as did the number of active bacteria, whereas the total number of bacteria varied by only sixfold. The number and proportion of active bacteria were similar among lake strata, but rates of bacterial production were highest in the epilimnion and lowest in the hypolimnion. As expected, bacterial production was better correlated to the number of active cells, and bacterial growth rates calculated for active cells ranged from 0.7 to 1.8 day−1, on average threefold higher than those calculated on the basis of total bacterial abundance. Growth rates scaled to active cells were, on average, similar among lake strata and did not show any pattern along a gradient of increasing chlorophyll concentration, so there was no systematic change of bacterial growth rates with lake productivity. In contrast, growth rates scaled to the entire bacterial assemblage were positively correlated to chlorophyll, were tenfold more variable among lakes than growth rates of active cells, and showed larger differences among lake strata. Scaling bacterial production to either the total number or the number of active cells thus results in very different patterns in bacterial growth rates among aquatic systems.

Role of Predatory Bacteria in the Termination of a Cyanobacterial Bloom.

Changes in cyanobacterial abundance and in the occurrence of bacteria of bacteria capable of lysing cyanobacteria were monitored over a period of 6 months (May to October 1998) in eutrophic Brome Lake (Quebec, Canada), in which dense cyanobacterial blooms recur regularly. By screening lake water, we isolated two strains of lytic bacteria, from the family Cytophagaceae. When tested on 12 cyanobacteria and 6 heterotrophic bacteria, strain 1 lysed only Anabaena flos-aquae and strain 2 lysed only Synechococcus cedorum, Synechococcus leopoliensis, Synechococcus elongatus, and Anacystic nidulans: both liquid and agar-grown cultures of these cyanobacteria were lysed. The number of plaque forming units of bacteria increased dramatically during the decline of the bloom. The results are consistent with an important role for these host-specific lytic bacteria in control and elimination of cyanobacterial blooms in this lake.

Some misconceptions about the spurious correlation problem in the ecological literature

SummaryIt is a common misconception that correlations between variables that share a common term are statistically invalid. Although the idea that such relationships are wholly or partially spurious was rejected decades ago by statisticians, ecologists continue mistakenly to exclude legitimate hypotheses on this basis. Besides directing attention to the statistical literature on the subject, we briefly reconsider the problem from 3 viewpoints: first, the confusion between spurious correlation and spurious inference, second, the problem of concept familiarity and definition, with particular reference to the self-thinning rule for plants, and third, a legitimate concern with measurement error of shared variable components.

Iron acquisition by photosynthetic marine phytoplankton from ingested bacteria

Iron is unique among biologically essential trace metals in having a higher particulate than dissolved concentration in ocean surface waters. Uptake of dissolved iron is generally considered to be the norm for phytoplankton, as even the smallest iron-bearing particles are unavailable for transport into cells,. But the oceanic dissolved fraction is so small, and the particulate fraction so inert, that phytoplankton production is limited by a dearth of available iron in some regions. Here we use incubation experiments to show that Ochromonas sp., a common photosynthetic flagellate from the Pacific Ocean, can obtain iron directly in particulate form, by ingesting bacteria. Iron acquisition is highly efficient; Ochromonas assimilates 30% of the ingested ration, acquiring a high intracellular iron concentration and maintaining a significantly faster growth rate than when iron is provided in the dissolved phase. Phytoplankton capable of such phagotrophy (so-called mixotrophic species) may thus be able to assimilate iron in both particulate and dissolved forms in the ocean. Moreover, when iron availability is limited, the iron ‘cost’ of growth is diminished because Ochromonas derives a greater fraction of its energy from the bacteria. Analysis of standing stocks and clearance rates of plankton in the equatorial Pacific shows that the iron flux through mixotrophic flagellates can amount to 35–58% of the total Fe uptake by the entire autotrophic community. Our results suggest that the phagotrophic ingestion of bacteria may be an effective adaptive strategy for photosynthetic organisms to obtain iron for growth in iron-limited regions of the sea.

Microbial biomass and productivity in the western bransfield Strait, Antarctica during the 1986-87 austral summer

Abstract Studies from December 1986 to March 1987 show that waters of Gerlache Strait and the southwestern Bransfield Strait support a rich seasonal bloom of micro-organisms, consisting primarily of phytoplankton. Chlorophyll a concentrations achieved maximum values (>20 mg Chl a m −3 ) in December, began to decline in January and exhibited a rapid decrease in February and March to concentrations ≤0.3 mg Chl a m −3 . The mean cell size of the planktonic assemblage did change, however, from a predominately microplankton (≥20 μm) crop in December to one dominated by nanoplankton (≤20 μm) later in the season. Total microbial biomass as estimated by measurement of ATP, however, increased between December and January, followed by a rapid decline in February and March. However, the putative microheterotrophs, bacteria, showed no significant increases either during or immediately following the December to January bloom of phytoplankton, in apparent contrast to studies reported from temperature waters. Bacterial cell numbers did not vary systematically during the 4-month period observation and thus appeared to be “uncoupled” from phytoplankton dynamics. Rates of assimilation tritiated organic compounds (glutamate, thymidine, adenine) paralleled the total microbial biomass estimates. All available data (e.g. patterns and rates of organic matter assimilation, Chl/ATP ratios, Chl a concentrations and bacterial cell numbers) suggest that much of the measured heterotrophic activity may be due to the activity of phytoplankton cells. The nature of the microbial food web during the seasonal bloom in Antarctic coastal waters is believed to be a complex function of temperature, photobiological effects related to seasonal deepening of the upper mixed layer, grazing and nutrient regeneration/replenishment. Together these processes determine the composition and total biomass of the microbial assemblage that affects other ecological properties such as the magnitude of new production and the rate of particle flux to deeper waters.

DOES ALLELOPATHY CONTRIBUTE TO CYLINDROSPERMOPSIS RACIBORSKII (CYANOBACTERIA) BLOOM OCCURRENCE AND GEOGRAPHIC EXPANSION?1

Cylindrospermopsis raciborskii (Wołosz.) Seenayya et Subba Raju is a planktonic filamentous cyanobacterium whose sudden worldwide proliferation and ability to produce toxins are a reason for concern. In this paper, we suggest that its ecological dominance might be explained by antagonistic interaction with other phytoplankton species due to production of allelopathic metabolites. To test this hypothesis, experiments were run with exudates of natural phytoplankton and C. raciborskii strains isolated from Lagoa Santa, a small natural lake in southeastern Brazil, where this species has become dominant in recent years. The exudates were added to different algal species obtained from the same environment and maintained in culture. After 24 h incubation, PAM fluorometry was used to compare control and treatment photosynthetic responses (relative electron transport rate) to the dissolved extracellular products. Results indicate that most of the target species were sensitive to C. raciborskii exudates, which showed strong inhibitory effects on their photosynthetic activities. These results provide evidence that allelopathy may offer a competitive benefit to C. raciborskii and contribute to its stable dominance in Lagoa Santa. A potential allelopathic advantage could also help to explain the geographic expansion of this species at midlatitudes.

High concentrations of viruses in the sediments of Lac Gilbert, Québec

Viruses were found to be very abundant in the top layer of the sediments of Lac Gilbert, Québec. Viruses were extracted from the sediments using pyrophosphate buffer, and viruses from the diluted extracts were pelleted onto grids and enumerated using transmission electron microscopy. Viral abundance in the sediments ranged from 6.5 × 108 to 1.83 × 1010 ml−1, which is 10- to 1,000-fold greater than the number observed in the water column. This increase corresponds well with the 100- to 1,000-fold increase in bacterial abundance in the sediments. Viral abundance differed significantly among the surface sediment samples taken at different bottom depths and among samples taken at different depths of the water column. Viral abundance also varied significantly between the oxic and anoxic zones of the water column and the sediments. The virus-to-bacteria ratio varied greatly among the different sediment sites but not among depths in the water column. Viral abundance in the water column was related to bacterial abundance and chlorophyll concentration, whereas viruses in the sediments were most abundant in sediments with high organic matter content. Elevated viral abundance and their erratic distribution in the sediments suggest that viruses might play an important role in sediment microbial dynamics.