are vital to the growth of phytoplankton, macroalgae, and emergent wetland 

 plants, which are the basis of the food web of the coastal Maine ecosystem. 



Energy flow through a system has been described by H. T. Odum (1966) using a 

 circuit language (energese) that he developed. Energy flow modeling by cir- 

 cuit language clarifies relationships among the climatic, biological, 

 hydrological, geological, and socioeconomical components of a system. 



The flow of energy through a generalized ecological system is illustrated in 

 figure 1-6. The symbols employed are explained in figure 1-7. Photosynthesis 

 by producer organisms is controlled by certain forcing functions (e.g., 

 sunlight) and the availability of stored compounds (i.e., nutrients). 

 Producers provide energy to consumers either directly, through herbivory, or 

 indirectly, through the feeding of some consumers on partially decomposed 

 plant material (detritus). These consumers, in turn, are preyed upon by 

 higher level consumers. Some energy is recovered through excretion and death. 

 Energy is lost at each transfer to heat (respiration). 



In order to compare the relationships in a generalized ecosystem to those in a 

 natural community, the energy flow in an eelgrass community is examined brie- 

 fly below. The forcing functions that drive photosynthesis in eelgrass beds 

 are sunlight, currents, and salinity. Required nutrients are derived from the 

 water column or sediments. The eelgrass is consumed directly by herbivores 

 (e.g., snails and brant). Plant parts and dead eelgrass decompose through mi- 

 crobial action and are consumed by detritivores (e.g., clams). Secondary 

 consumers, such as fish and black ducks, feed on those primary consumers. 

 Nutrients are returned through excretion and death via the detrital-microbial 

 complex. When a system is impacted by functional change or by the addition or 

 elimination of a component, the effects on its energy flow are many and 

 complex. In the estuarine system, for example, if the characteristics of the 

 tides (height, frequency, and duration) are changed (tidal dams), then the 

 habitats that require the previous tidal regime (e.g., salt marshes, mud 

 flats, sand flats, eelgrass, and rocky shores) will change and the food webs 

 associated with those habitats will change. 



Biogeochemical Cycling 



As energy flow represents the transfer of energy along food chains, bio- 

 geochemical cycles illustrate the mode and direction of the transfer of 

 materials through systems. Biogeochemical cycles are closed and represent 

 continuous movement of materials (e.g., carbon, oxygen, water, and nutrients) 

 among the living and nonliving parts of the ecosystem. Many of these elements 

 are essential to living processes and structures. 



Meteorological, geological, biological, and oceanographical forces or vectors 

 drive biogeochemical cycles. Wind, rain, snow, ice, fog, and gasses bearing 

 dissolved and particulate matter are meteorological vectors. Gravity (e.g., 

 streamflow) is a geological vector. Tides and currents and changes in density 

 due to temperature and salinity distributions are oceanographic vectors. 

 Biota are biological vectors. 



1-8 



