No abstract

Food habits and net-spinning activity of Hydropsyche betteni, Hydropsyche slossonae, and Hydropsyche sparna were examined over 1 year at three stations in the Humber River, Ontario. Hydropsyche slossonae and H. sparna nets were similar in mesh size; H. betteni net meshes were much smaller; however, the H. betteni net had the greatest total surface area. Differences in retreat design may relate to prey specialization and (or) flow pattern.Gut content analysis indicated seasonality in food types ingested. Animal material in guts increased in spring and continued to be important in summer; but it decreased in fall and, except in H. betteni, remained low during winter. In winter, no nets were constructed yet larvae continued to feed on epilithic material from the substrate.Food habits were more similar between species in third instars than in later instars, and later instars usually contained more animal material and less detritus than third instars. The most significant difference between species was the degree of carnivory, H. betteni being more carnivorous than the other species at two of the three stations. Avoidance of the filamentous diatom Melosira sp. and inorganic material, which at times were abundant in the environment, suggested some degree of food selectivity.

1. Stream ecosystems are the products of interactions between hydrology, geomorphology and ecology, but examining all three components simultaneously is difficult and rarely attempted. Frequently, either geomorphology or hydrology is treated as invariable or static. 2. To examine the validity of treating either hydrology or geomorphology as static, we studied the individual and combined effects of hydrology and channel geomorphology on coarse particulate organic matter (CPOM) storage. Using data from an experimental leaf release in a hydrologically regulated stream we created a simple numerical model. This allowed us to quantify the relative influence of CPOM trapping and CPOM retention on total long-term CPOM storage under variable regimes of flood frequency and geomorphic structure. 3. CPOM storage is a function of supply, flood frequency and the type and frequency of in-stream structures. In-stream structures perform two distinct functions, trapping and retention, whose relative importance in leaf storage changes with stream hydrology. Trapping is more important for CPOM storage in streams with few floods, while retention is more important in streams with frequent floods. Different structures (e.g. boulders, large wood, small wood) perform these functions at different efficiencies. We found that large wood trapped two to three times more leaves than the bank, but that the bank retained leaves two to three times more efficiently. 4. A modelled channel with five times the amount of large wood as the study channel (a 'wood restoration') initially stored 14% more leaves than the modelled 'natural' channel. After six floods, however, the modelled wood restoration channel stored 50% less CPOM than the natural channel as the large wood had high trapping but poor retention. The modelled natural channel contained structures that could both trap and retain. Thus, as different structures performed different functions, the structural complexity buffered the stream allochthonous energy base against changes in hydrology through its balance of trapping and retention. 5. As the frequency of floods increased, the spatial distribution of CPOM became increasingly patchy as storage was driven entirely by structures with high retention. Thus, the coupling of flood frequency and geomorphic structure influenced CPOM availability, which in turn has ramifications for the entire stream food web.

A 20-m section of stream riffle was covered with black plastic to decrease light intensities and reduce algal standing crops. After four weeks, chlorophyll a concentrations were near zero and Baetis tricaudatus nymphs were far fewer in the experimental riffle than in control riffles above and below the shaded area. Also, Baetis adults, reared from pre-emergent nymphs collected in the experimental riffle, were significantly smaller than reared adults from the control riffles. After three months, we removed one half of the black plastic from the experimental riffle. The algae and B. tricaudatus nymphs recolonized the open section of the experimental riffle within 11 days. These results suggest that algae are an extremely important resource for B. tricaudatus, a collector-gatherer. Reduction of epilithic algae did not greatly influence Simulium, a filter feeder. Other macroinvertebrates that apparently were not influenced by the algal reduction included Asellus, Gammarus, Hyallela, Dugesia and Chironomidae.