ponds from wliich they dorivc ;iiul in thi' skiggisli 

 waters near tlie inoiitli of the stream. Larger aquatic 

 [ilants are not characteristic of swift flowing streams ; 

 tliey occur in sluggish pools. Filamentous algae, how- 

 ever, may be abundant in riffles, and a rich micro- 

 flora of diatoms, with scattered protozoans, may 

 furnish a thin slimy film over the surface of rocks. 

 Animals are adapted to these food resources as filter 

 feeders, microflora eaters, or carnivores (Nielsen 

 1950). 



The caddisfly larva Hydropsyche is a fine example 

 of a filter feeder. This species and related forms con- 

 struct silken nets at the entrances of their shelters 

 and strain out food particles brought down by the 

 current. The anterior legs of some caddisfly larvae 

 and mayfly naiads are furnished with brushes of hair- 

 like setae which catch and transfer the detritus to 

 the mouth as the animal faces the current. Black fly 

 larvae have a pair of fans at the anterior end of the 

 body. These fans are of long, curved setae. The larva 

 folds them periodically, and the mandibles comb or 

 brush off the detritus that collects. Clams siphon 

 water through the mantle cavity, and detritus ma- 

 terial and plankton are carried to the mouth through 

 the activity of the cilia of the mantle, gills, and labial 

 palps. Sponges and bryozoans also take detritus into 

 body cavities for feeding purposes. 



Feeding on the microflora and filamentous algae 

 are planaria, snails, and various insects. Some cad- 

 disfly larvae have mouthparts specially adapted to 

 scrape the thin film of microflora from the surface of 

 rocks. The maxillae of mayfly naiads serve as a comb 

 or brush with which diatoms are swept up into the 

 mouth. 



Carnivorous species may also be partly herbivo- 

 rous (Table 5-4). Too, there is apt to be seasonal 

 variation in food habits and there are differences of 

 habit between closely related species. Fall and winter 

 stonefly naiads are largely herbivorous, but spring 

 and summer forms comprise genera that are either 

 carnivorous, herbivorous, or omnivorous. Hellgram- 

 mites are largely carnivorous, feeding on immature 

 insects. Crayfish are omnivorous ; they appear to 

 prefer dead and decaying material. The smaller fish, 

 including the darters, are largely insectivorous, but 

 also consume some plant material. Suckers, carp, and 

 catfish feed on bottom debris as well as small living 

 animals and plants. Young bass and trout are largely 

 dependent on insects for food, but as they grow larger 

 they turn also to young crayfish and small fish. The 

 population density of fishes is ultimately determined, 

 therefore, by the abundance of invertebrates and, 

 when fishes rely on vision for finding their food, also 

 on the turbidity of the water. 



The average weight of food in the stomach of fan- 

 tail darters of all sizes, sampled from October to May 

 in New York State, was found to be 0.01354 g 



( Daiber 1956). If the average biomass of the living 

 food averages 2.83 g/m- of bottom, then one indi- 

 vidual of this species could get 209 full meals from 

 one s(|uare meter if it captured everything that was 

 there. Similarly, mottled sculpins could obtain 130 

 meals from a square meter. It would be interesting 

 to know what actual percentage of the invertebrate 

 population can be readily captured by fish and how 

 frequently the fish feed, for correlation with the 

 density of the fish population. Fish, however, also 

 depend to a considerable extent, especially in summer, 

 on small terrestrial organisms that fall, or are washed, 

 into the stream. 



BIOMASS AND PRODUCTIVITY 



Of the kinds of animals present in one short 

 coastal stream in California, the caddisfly larvae were 

 found to be not the most populous. But when size 

 was considered, they constituted more bulk than any 

 other invertebrate group (Table 5-5). The inverte- 

 brate biomass per unit area of riffles is invariably 

 much greater than in sand-bottom pools, whether 

 biomass be computed in terms of wet weight, dry 

 weight, or volume. However, the abundance of spe- 

 cies within the riffles depends on whether the stones 

 are loose or are fastened to the bottom, and on 

 whether or not they are covered with algae, moss, or 

 other vegetation (Percival and Whitehead 1929). 

 The biomass of mud-bottom pools may sometimes 

 exceed that of the riffles, especially if it contains the 

 burrowing mayfly naiad Hexagenia (Behney 1937, 

 Forbes 1928, Lyman 1943, Needham 1932, O'Connell 

 and Campbell 1953, Pennak and Van Gerpen 1947, 

 Smith and Moyle 1944). In the mud-bottom Silver 

 Springs stream in Florida, the dry weight biomass 

 of plants averaged 809 g/m-, herbivores 37 g/nr', 

 small carnivores 11 g/m-, and large carnivores 1.5 

 g/m- (Odum 1957a). 



Insect populations in streams vary with the sea- 

 son (Table 5-6). Peak populations commonly occur 

 during late spring and again in autumn (Daiber 1956, 

 Lyman 1943, Needham 1934, 1938, Stehr and Bran- 

 son 1938). Populations become reduced in summer 

 because of low water ; in winter, because of low tem- 

 perature and ice. 



Small streams tend to have greater densities of 

 insect populations per unit area than do large streams. 

 In New York State, streams up to width 2 m have 

 biomasses that average 22.2 g/m- wet weight ; from 

 2 to 4 m, 18.0 g/m-; from 4 to 6 m. 10.1 g/m^; and 

 over 6 m, 7.7 g/m- (Needham 1934). In small 

 streams, the distribution of organisms is nearly uni- 

 form from one side to the other, but in large streams 

 there is a decrease in density from the sides toward 

 midstream (Behney 1937). Larger streams actually 



Streams 55 



