No abstract

Phosphorus-rich deposits of Eocene volcanic rock have caused some lakes in the interior of British Columbia to become eutrophic without direct human influence. In Black Lake the soluble reactive phosphorus (SRP) usually exceeds 250 pg Pmliter-'. When calcite precipitation occurred during blooms of Aphanizomenon, SRP was completely removed from the photic zone. In blooms with no calcite formation, SRP changed little. These observations were replicated by splitting the lake with a curtain. Calcite precipitation was also observed several times in four large enclosures (limnocorrals).The subsequent disappearance of phosphatc was correlated with an increase in pII and a reduction of Ca and alkalinity and not necessarily with primary production or algal biomass.

Subinhibitory levels of antibiotics can promote the development of antibiotic resistance in bacteria. However, it is unclear whether antibiotic concentrations released into aquatic systems exert adequate pressure to select populations with resistance traits. To examine this issue, 15 mesocosms containing pristine surface water were treated with oxytetracycline (OTC) for 56 days at five levels (0, 5, 20, 50, and 250 microg L(-1)), and six tetracycline-resistance genes (tet(B), tet(L), tet(M), ted(O), tet(Q), and tet(W)), the sum of those genes (tet(R)), "total" 16S-rRNA genes, and transposons (Tn916 and Tn 1545) were monitored using real-time PCR. Absolute water-column resistance-gene abundances did not change at any OTC exposure. However, an increase was observed in the ratio of tet(R) to 16S-rRNA genes in the 250 microg L(-1) OTC units, and an increase in the selection rate of Tc(r) genes (relative to 16S-rRNA genes) was seen when OTC levels were at 20 microg L(-1). Furthermore, tet(M) and Tn916/1545 gene abundances correlated among all treatments (r2 = 0.701, p = 0.05), and there were similar selection patterns of tetR and Tn916/1545 genes relative to the OTC level, suggesting a possible mechanism for retention of specific resistance genes within the systems.