settle quite rapidly. However, planktonic organisms, fine particles, and 

 dissolved compounds may be carried long distances, frequently into estuaries. 

 The emergence of immature aquatic insects that have terrestrial adult forms is 

 a significant form of energy and nutrient export that is common to both 

 standing and flowing water systems. Thus, energy and nutrient cycling at any 

 point along the stream's course is affected by export downstream and to the 

 terrestrial system. Much of this loss is compensated for by the input of 

 organic matter from terrestrial vegetation in the headwaters and the input of 

 organic and inorganic nutrients through runoff, throughflow, and leaching 

 along the entire water course. 



BIOTA 



The types of organisms that inhabit riverine systems vary according to biotic 

 and abiotic factors. In the upper perennial or intermittent subsystems the 

 biota is characterized by the dominant decomposers of organic matter and their 

 predators. Aquatic producers and primary consumers play a minor role in these 

 subsystems. Lower stream subsystems (tidal, lower perennial) are 

 characterized by a producer community of aquatic plants and their consumers, 

 including zooplankton, fishes that feed on zooplankton, and predators on 

 fishes . 



Producers 



Aquatic plants of riverine systems in Maine include phytoplankton, algae, 

 mosses, and nonpersistent emergent plants. Phytoplankton are not considered 

 integral to the riverine system even though they make an energy contribution 

 to the system. Their presence is usually dependent on a lacustrine/palustrine 

 source upstream and is a function of time, passage of nutrients, light, and 

 temperature. Attached plants in the riverine system include periphyton and 

 rooted, vascular plants. Attached plants may have local importance if 

 conditions for growth are suitable. The habitation zone of the periphyton is 

 restricted to the immediate water-substrate interface at high current 

 velocities. Periphyton algae may form a green cover that is several cells 

 thick on rocks and other objects. Rooted vegetation may occur in areas of 

 reduced flow and shallow depth. Linear-leaved forms (e.g., Sparganium spp. , 

 Carex spp., Zizania sp., and Scirpus spp.) and mosses are most predominant. 

 Vegetation in the Merrymeeting Bay riverine tidal wetland subsystem is 

 described in detail by Spencer (1959, 1960, and 1965). Physical factors, 

 especially shear stress, limit development of persistent vegetation. 

 Palustrine systems in which aquatic plants grow occur within rivers and 

 streams as islands of persistent vegetation (see chapter 8, "The Palustrine 

 System"). 



Consumers 



Invertebrates typical of riverine systems and their role in the food web are 

 discussed under "Energy Flow" above (table 6-4). Invertebrate consumers 

 include collectors, scrapers, shredders, and predators. It is difficult to 

 confine any organism to only one classification. Feeding habits may change 

 with development of organisms (Cummins 1973) . Fishes serve as primary and 

 secondary consumers in the food web, and as parasites on other fishes and 

 decomposers (table 6-5). The distribution of fishes in Maine rivers is poorly 

 understood and data are incomplete. Those fishes that have been recorded in 



6-25 



10-80 



