Particulate matter (detritus) is derived from decaying and fragmented plant 

 material. Zooplankton play an important role in transforming particulate 

 matter through feeding and the release of fecal pellets. During conversion of 

 pesticides, dissolved compounds are released through the liberation of 

 protoplasm (during death or breakage of cells) and excretions. Some simple 

 molecules can be reassimilated by phytoplankton. 



The importance of bacteria in the cycling of organic matter is not understood 

 fully. Bacteria assimilate dissolved organic compounds and release carbon 

 dioxide and other nutrients. Bacteria attach to particles and living cells 

 and hasten their decomposition by liberating enzymes. These colonized 

 particles are eaten by filter-feeding and deposit-feeding animals. 



A schematic diagram of the organic matter cycle of the oceans is presented in 

 figure 5-34. 



INTERACTIONS AFFECTING PRODUCTIVITY AND DISTRIBUTION OF BIOTA 



The interactions of the biota with physical environment associated with Maine 

 estuaries are described below. The interactions associated with fish and 

 birds are described in detail in chapter 11, "Fishes," chapter 14, 

 "Waterbirds," chapter 15, "Waterfowl," and chapter 16, "Terrestrial Birds." 

 In addition, interactions associated with commercially important invertebrates 

 are discussed in chapter 12. 



Phytoplankton 



Phytoplankton require adequate sunlight, nutrients, water temperatures, and 

 salinity for optimal growth. The specific requirements vary with the species. 

 Seasonal changes of the above factors in the water column (see "Hydrography" 

 and "Nutrients" in chapter 4) largely control the species compositon and 

 biomass of the phytoplankton populations. 



In winter, the deep mixing of the water column (figure 5-35) may limit 

 phytoplankton growth by carrying the phytoplankton below the level where 

 sufficient sunlight exists to support growth. If the depth of the water is 

 shallow, phytoplankton usually do not fall below the level of light 

 penetration and constantly are mixed back up into the photosynthetic zone. 

 This "critical mixing depth" is considered to be about three times the depth 

 of the photic zone (the depth of the level at which light intensity is reduced 

 to 1% of its surface intensity). The critical depth in Maine coastal waters 

 is about 130 to 200 feet (40 to 60 m) . Some areas of coastal Maine estuaries 

 are this deep (e.g., the Sheepscot) . 



Winter water temperatures also limit phytoplankton growth. For about every 18° 

 F (IQOC) change in water temperature between 32 to 68°F (0 to 20° C), the 

 photosynthetic rate is changed by about 50% (Yentsch et al. 1974). These 

 authors conclude that the major effects of temperature would occur in early 

 winter but after that the effects of changes in temperature would be difficult 

 to distinguish from other factors such as nutrient availability. 



5-67 



10-80 



