Artificial streams containing algal communities derived from a spring and an agriculturally impacted stream were used to assess the effect of 0.1, 1, and 10 mg kg−1 atrazine [2‐chloro‐4‐(ethylamino)‐6‐(isopropylamino)‐s‐triazine], trifluralin (α,α,α‐trifluoro‐2,6‐dinitro‐N,N‐dipropyl‐p‐toluidine), MSMA (monosodium methanearsonate), and paraquat (1,1′‐dimethyl‐4,4′‐bipyridylium cation) on the productivity (photosynthesis and respiration) of stream algae. The importance of induced resistance to the herbicides was evaluated by comparing the responses of the spring and stream algae and by determining if allowing the streams to be colonized for 3 weeks in the presence of 0.01 mg kg−1 of the herbicide under test would modify response by selecting for a more resistant flora. Productivity was measured by open‐water oxygen methods for 3 weeks before and 3 weeks after herbicide injection. The communities derived from both the spring and the stream were dominated by diatoms, and responded similarly to trifluralin, atrazine, and MSMA, but had divergent responses to paraquat. Trifluralin had no effect on productivity. Photosynthesis was significantly depressed by 1 and 10 mg kg−1 atrazine and by 10 mg kg−1 MSMA, and there were indications of a slight inhibition by 0.01 mg kg−1 atrazine. Paraquat at 10 mg kg−1 had little effect on the spring‐derived communities but caused severe inhibition of the stream‐derived algae. There was little evidence that exposure to 0.01 mg kg−1 herbicide during colonization modified the response of the algae to any of the herbicides.
Several methods for the estimation of the reaeration coefficient were compared by determining the ability of the methods to recover the correct K value from a computer-simulated stream oxygen record affected by a variety of non-ideal conditions. Noisy data and long observation intervals were not a serious problem for most methods. Saturating photosynthesis, fluctuating light intensity, afternoon depression and temperature variation caused failures by some methods but were well handled by others. Serious impairment of all methods occurred with low productivity or high K. In general, the best-performing methods were the modified hysteresis, nighttime regression, daytime regression, Odum and Hornberger-Kelly daytime methods.
A series of spreadsheet simulations using SEIS, SEIR, and SEIRS models showed that different durations of effective immunity could have important consequences for the prevalence of an epidemic disease with COVID-19 characteristics. Immunity that lasted four weeks, twelve weeks, six months, one year, and two years was tested with pathogen R0 values of 1.5, 2.3, and 3.0. Shorter durations of immunity resulted in oscillations in disease prevalence. Immunity that lasted from three months to two years produced recurrent disease outbreaks triggered by the expiration of immunity. If immunity faded out gradually instead of persisting at full effectiveness to the end of the immune period, the recurrent outbreaks became more frequent. The duration of effective immunity is an important consideration in the epidemiology of a disease like COVID-19.
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