Library

Notes: 1. The effects of nitric acidification on phytoplankton were studied in a small, eperimentally manipulated, oligotrophic lake (L302N) in the Eperimental Lakes Area of Canada. The focus was altered after 9 years of acidification to investigate the possibility of using nutrient additions to stimulate recovery, followed by a controlled incremental recovery, in which the pH was increased to a predetermined target level.2. Five years of additions of HNO3 to L302N reduced its pH from 6.5 to 6.1. Nitrate concentration increased because the algal community was severely P deficient. The phytoplankton community structure and productivity were not significantly affected by these additions.3. The phytoplankton community was significantly affected when pH was subsequently decreased over three successive years from 6.1 to 5.1 by the addition of HCl. Dominance shifted from chrysophytes to a co-dominance of chlorophytes and dinoflagellates, which altered the size structure of the community. Species diversity significantly decreased, although phytoplankton productivity remained unchanged.4. At pH 5.1 nitrate and sulphate additions were made, creating conditions like those in lakes in eastern North America, which receive high loadings of nitrogen from the atmosphere. The phytoplankton assemblage shifted to dominance by small coccoidal chlorophytes. However, biomass and productivity were unaffected.5. Finally, phosphate, as phosphoric acid, was added, along with nitrate and sulphate, to the epilimnion, which stimulated internal alkalinity generation and productivity. It is concluded that CO2 concentrations and the form of N (nitrate vs. ammonia) affect algal composition but that P determines algal biomass and productivity. Chlorophytes were found to be good competitors for P when N and CO2 were high; it is epected that cyanobacteria would be more competitive for P in low CO2 systems. Conversely, dinoflagellates are most competitive in systems with low pH and high P, such as that which occurred in L302N. Although the P additions reduced N concentrations and created alkalinity, this is not a recommended remedial procedure in acidified lakes because it enhanced dinoflagellate abundance, which has been associated with fish kills.6. When all additions ceased, the pH of L302N recovered from 5.1 to 5.8, chrysophytes and chlorophytes became more abundant and dinoflagellates decreased in abundance. Phytoplankton biomass decreased and species diversity increased. Phytoplankton productivity remained unchanged

Notes: 1. Stoichiometric theory predicts that the nitrogen : phosphorus (N : P) ratio of recycled nutrients should increase when P-rich zooplankton such as Daphnia become dominant. We used an enclosure study to test the hypothesis that an increased biomass of Daphnia will increase the relative availability of N versus P sufficiently to decrease the abundance of filamentous cyanobacteria. The experiment was conducted in artificially enriched Lake 227 (L227) in the Experimental Lakes Area (ELA), north-western Ontario, Canada. Previous studies in L227 have shown that the dominance of filamentous, N-fixing cyanobacteria is strongly affected by changes in the relative loading rates of N and P.2. We used a 2 × 2 factorial design with the addition or absence of D. pulicaria and high or low relative loading rates of N and P (+NH4, –NH4) in small enclosures as treatment variables. If Daphnia can strongly affect filamentous cyanobacteria by altering N and P availability, these impacts should be greatest with low external N : P loading rates. The phytoplankton community of L227 was predominantly composed of filamentous Aphanizomenon spp. at the start of the experiment.3. Daphnia strongly reduced filamentous cyanobacterial density in all enclosures to which they were added. The addition of NH4 had only a small impact on algal community composition. Hence, we conclude that Daphnia did not cause reductions in cyanobacteria by altering the N : P ratio of available nutrients.4. Despite the lack of evidence that Daphnia affected filamentous cyanobacteria by altering the relative availability of N and P, we found changes in nutrient cycling consistent with other aspects of stoichiometric theory. In the presence of Daphnia, total P in the water column decreased because of an increase in P sedimentation. In contrast to P, a decrease in suspended particulate N was offset by an increase in dissolved N (especially NH4). Hence, dissolved and total N : P ratios in the water column increased with Daphnia as a result of differences in the fate of suspended particulate N versus P. There was minimal accumulation and storage of P in Daphnia biomass in the enclosures.5. Our experiment demonstrated that Daphnia can strongly limit filamentous cyanobacterial abundance and affect the biogeochemical cycling of nutrients. In our study, changes in nutrient cycling were apparently insufficient to cause the changes in phytoplankton community composition that we observed. Daphnia therefore limited filamentous cyanobacteria by other mechanisms.

Notes: 1. Lake 302S in the Experimental Lakes Area of Canada was acidified from pH 6.7 (1981) to 5.1 (1986). The pH was further reduced to 4.5 in 1987 and held at that level until 1991. From 1992 to 1995, the pH was allowed to increase to a target value of 5.8.2. The response of the phytoplankton community to decreasing pH from 6.0 to 5.1 was similar to that observed in another experimentally acidified lake (223) and in other atmospherically acidified lakes. Acidification affected species diversity of both the phytoplankton and epilithon. Phytoplankton diversity was positively correlated with pH. Epilithic algal diversity was more variable and did not correlate with pH.3. Phytoplankton biomass was enhanced by acidification as the assemblage shifted from a dominance of chrysophytes to large dinoflagellates (Gymnodinium sp. and Peridinium inconspicuum). Epilithon biomass was unaffected, but dominance shifted from filamentous cyanophytes (Lyngbya) to acidophilic diatoms (Tabellaria quadriseptata and Anomoeonis brachysira).4. The only taxon to be similarly affected in both the phytoplankton and epilithon was the cyanobacteria, being significantly reduced below pH 5.1. During early recovery (pH 5.5–5.8), cyanobacteria increased and species present prior to acidification recolonized both habitats.5. In the early stages of recovery, planktonic and benthic assemblages remained more similar to acidified than natural assemblages, but more profound change began at pH 〉 5.5.

Notes: 1. The effect of penicillamine on the growth rate of an osteogenic sarcoma of rats was investigated and compared with cyclophosphamide.2. Rats were inoculated with a readily transplantable osteogenic sarcoma subcutaneously into the left thigh and treated with penicillamine and cyclophosphamide alone or in combination.3. Cyclophosphamide inhibited tumour growth.4. Penicillamine did not delay the appearance or the growth rate of the tumour. Tumour sizes tended to be larger in the penicillamine-treated rats, but there was no evidence that penicillamine interfered with the antitumour effect of cyclophosphamide given in large doses (100 mg/kg).