Ingestion and assimilation rates of simullid (Diptera) larvae feeding on natural seston were estimated in an artificial stream and used to predict the impact of simuliids on seston transport in a 40-m section of the outlet stream of Petit Lac Fraser in southern Quebec. This prediction was then compared with changes in the seston flux above and below the population. Finally, we selectively killed simuliids with a larvicide (Bacillus thuringiensis serovar. israelensis) to observe changes in seston transport after removal of simuliids. The stream population of simuliid larvae ingested between 32 and 55% of the seston flux and assimilated between 17 and 25% of the dry mass of ingested seston. The change in assimilation of seston in the stream section before and after the larvicide treatment was similar to that predicted from assimilation rate of larvae and standing stock. Larvae ingested 0.8–1.4% of the seston per linear meter of stream, about 10–1000 times more than what has been reported for other populations of stream filter-feeders. The high values observed can be explained by high specific ingestion rates, high standing stock, and low discharge. Our results suggest that simuliid populations can be food limited and may help to explain the downstream decrease in abundance and growth rates of filter-feeders in lake outlets.

The aim of this study was to test three main hypotheses about the interannual variability of streamflow downstream from dams: (1) an almost similar long-term trend in interannual variability, (2) low variability of flow, and (3) its independence (no link) from climate variability. To test these hypotheses, the interannual variability of winter and spring streamflow downstream from three reservoirs (Gouin, Manouane, and Matawin) which induce an inversion of the natural cycle of streamflow (maximum flows in winter and minimum flows in spring) was compared to the interannual variability of streamflow in natural rivers (measured at the Matawin and Vermillon stations) over the period from 1932 to 2008 in the St-Maurice River watershed. As far as the interannual variability of flow is concerned, its long-term trend is not homogeneous downstream from the three reservoirs in both seasons. However, downstream from two reservoirs, changes in streamflow were observed to be different from those in natural rivers (no significant trend downstream from the Taureau reservoir, on the Matawin River, and significant decrease in spring flow downstream from the Manouane reservoir). Finally, coefficient of variation values for minimum flows are higher downstream from reservoirs than in natural rivers, despite the fact that watershed surface area is larger for regulated rivers than for natural ones. As for the link with climate variability, analysis of the correlation between climate variables (temperature and precipitation) and mean winter and spring daily streamflow reveals that winter streamflow downstream from the Taureau reservoir is not correlated with any climate variable, whereas spring streamflow is positively correlated with rainfall and negatively Water Resour Manage (2011) 25:3661-3675 correlated with maximum temperature. Thus, downstream from reservoirs, the interannual variability of streamflow depends on climate during the spring, but not during winter.

Winter and summer 7-day low flows of eighteen natural rivers in the St. Lawrence River watershed of Quebec were analyzed over the period from 1934 to 2000. The rivers were first subdivided into three modes using principal component analysis. Two of these modes are located on the south shore of the St. Lawrence River, respectively south (Southeast mode) and north (East mode) of the 47°N, and the third (Southwest mode) is located on the north shore. The Southeast mode shows a significant increase in 7-day low flows in winter and summer due to increasing summer and fall precipitation, whereas the Southwest mode shows a significant decrease in summer 7-day low flows due to an increase in evapotranspiration. No significant change in winter and summer 7-day low flows is observed in the East mode. Results show that the variability of 7-day low flows and their link with climate indices are mainly seasonally dependent. The North Atlantic Oscillation (NAO) is the only index which is correlated to 7-day low flows in all three modes. This correlation is positive for north shore rivers and negative for south shore rivers. Furthermore, only north shore rivers are correlated to NAO in both winter and summer.

Numerous field trials have indicated that the effectiveness of Bacillus thuringiensis var. israelensis (B.t.i.) is limited to a short distance in streams, but the reasons for loss of toxicity have not been identified. Two streams were treated with B.t.i. and experiments were conducted to monitor the transfer of B.t.i. toxicity from the channel water to different compartments within the stream. These compartments included benthic substrates such as sediments, periphyton covering bedrocks, vegetation (moss and grass), and water samples taken from hyporheic probes (15, 35, and 65 cm depth). B.t.i. toxic activity was measured using bioassays with mosquito neonate larvae. Results indicated that loss to the hyporheic zone accounted for a significant fraction of B.t.i. removal from the open-channel water. Additional losses were attributable chiefly to moss, but sediments may play an important role in filtering or retaining B.t.i. toxic particles.

Combining a model of susceptibility of black fly larvae to commercial formulations of Bacillus thuringiensis var. israelensis and a one-dimensional transport equation in a river, the quantity of larvicide that should be applied at a given site in order to achieve a given mortality downstream can be calculated. The obtained relation, a function of biological and hydrological parameters, allowed us to calculate the costs related to the treatment of a section of river. It is then possible to determine an optimal strategy of treatment which minimizes the quantity of an insecticide applied and the logistic movement between injection sites. Optimal solution is found with the aid of a well known algorithm developed in operations research theory to solve this combinatorial problem. An experimental treatment conducted in the Onchocerciasis Control Program in West Africa is analyzed as an example of the model's application to illustrate the savings of insecticide, time, and money using this approach.