Some nutrients limit growth and reproduction. In estuarine and marine 

 systems, nitrogen availability is of critical importance (see chapters 4 and 

 5, "The Marine System" and "The Estuarine System"). In palustrine, 

 lacustrine, and riverine systems, phosphorus is a limiting element. The most 

 important biogeochemicals , nutrients (nitrogen and phosphorus) and organic 

 matter, are discussed in the systems chapters (4 through 10). 



People affect the distribution and content of materials in systems through in- 

 puts of water pollutants (industrial and municiapl wastes, domestic animal 

 manure, agricultural fertilizers, pesticides, herbicides, heavy metals, and 

 oil), atmospheric pollutants (SO2 and NO2, which cause acid rains), habitat 

 alteration (e.g., construction of causeways in estuarine areas, which alters 

 tidal flow, flushing rates, and, therefore, concentrations of nutrients), and 

 by extracting or harvesting nutrients and other components (water, peat, fish, 

 and birds) of the system. These impacts are discussed in chapter 3, "Human 

 Impacts on the Ecosystem." 



Biota 



The biotic (living) component of an ecosystem is composed of a diverse group 

 of organisms that live in a variety of physical habitats. The biotic commu- 

 nity has three basic components: producers, consumers, and decomposers. 



Producers are autotrophic organisms that manufacture food from inorganic sub- 

 stances using energy from the sun. All green plants are producers. In the 

 Gulf of Maine, phytoplankton are the major producers, while in nearshore 

 marine waters and in estuarine systems macroalgae and intertidal emergent 

 plants are also important. Nonpersistent rooted aquatic submergents (e.g., 

 pond weeds), emergents (e.g., pickerel weed), and phytoplankton are important 

 producers in the lacustrine and riverine systems. Producers in the palustrine 

 system in coastal Maine include persistent nonvascular (e.g., mosses and 

 liverworts) and vascular plants (emergent herbs, shrubs, and trees). In the 

 terrestrial system vascular plants dominate. 



Generally, three consumer levels are present in any system. In coastal Maine, 

 primary consumers in aquatic environments are represented by the herbivorous 

 zooplankton, benthos, birds, and fish, and in terrestrial systems by insects, 

 certain mammals (e.g., hares and hoofed mammals) and birds (e.g., geese and 

 finches). Secondary consumers (or carnivores) in Maine include certain 

 waterfowl, other birds, mammals, fish, and predaceous insects. Tertiary con- 

 sumers are carnivores that feed upon primary and secondary consumers. Large 

 predatory birds (bald eagles and osprey) , mammals (bobcats, harbor seals, and 

 people), and fish (Atlantic cod and Atlantic salmon), are examples of tertiary 

 level consumers in coastal Maine. Tertiary consumers usually have relatively 

 less biomass available to them as prey, exhibit greater mobility, have larger 

 home ranges, and require a greater diversity of habitat than primary or secon- 

 dary consumers. 



Variations in these feeding habits also exist. Omnivores consume both plant 

 and animal material. Scavengers eat dead organic matter, for example, carrion 

 and refuse. 



In the Maine coast ecosystem, the decomposers are fungi, heterotrophic 

 bacteria, and certain insects and other invertebrates. Decomposers process 



1-13 



10-80 



