Interactions Affecting the Productivity and Distribution of Biota 



Phytoplankton . Phytoplankton require adequate sunlight, nutrients, and 

 particular ranges of water temperature and salinity for optimal growth. 

 Specific requirements vary with species; however, conditions occur in coastal 

 Maine waters that affect whole phytoplankton populations. Currents determine 

 the distribution of phytoplankton populations. Seasonal changes in the water 

 column (see "Hydrography" and "Nutrients" above) control the growth of 

 phytoplankton populations. 



In winter the deep mixing of the water column (figure 4-19) may limit 

 phytoplankton growth by carrying the phytoplankton below the level where 

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

 is shallow phytoplankton cannot fall below the illuminated region of the 

 ocean; the mixing action continuously carries them back to light intensities 

 at which photosynthesis can take place. 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 value). 

 The critical depth of Maine coastal waters is about 130 to 200 feet (40 to 60 

 ra; personal communication from C. S. Yentsch, Bigelow Laboratory, West 

 Boothbay Harbor, ME; June, 1977). 



Winter water temperatures also limit phytoplankton growth. With every change 

 of approximately 18°F (10°C) in water temperature within the range of 32°F to 

 68°F (0 to 20°C), the photosynthetic rate is altered by about 50% (Yentsch et 

 al. 1974). These authors conclude that the major effects of temperature would 

 be felt and would be detectable in early winter; after this period temperature 

 effects would be difficult to distinguish from other influences, such as the 

 presence or absence of nutrients. 



In summer, when the thermocline develops and waters become stratified (figure 

 4-19), the phytoplankton consume all or most of the nutrients available. Once 

 all the nutrients are exhausted, growth of the phytoplankton is arrested and 

 mass mortalities occur. 



Optimal environmental conditions for phytoplankton growth, that is, the 

 intermediate condition between vertical mixing and stability, occur in the 

 spring and fall and give rise to spring and fall blooms (i.e., sharp increases 

 in abundance) of algae. 



When phytoplankton production in the characterization area is compared with 

 that in other temperate regions, general trends can be noted. Mean annual 

 cell densities and maximum cell densities of phytoplankton and chlorophyll 

 concentrations are lower in Maine waters than in many bays and estuaries south 

 of Cape Cod. The higher productivities in southern nearshore waters may be 

 due to the following: water temperatures are higher for a longer period of 

 time; the period of optimal light intensity is longer; and the nutrient levels 

 are probably higher, because of runoff from the large population centers in 

 these areas. Phytoplankton growth may not reach its full potential in these 

 southern areas, however, because of increased turbidity from land-based 

 discharges, which may limit light penetration. Although phytoplankton 

 productivities in nearshore waters of coastal Maine are lower than those in 

 southern waters, they are higher than those of the offshore Gulf of Maine for 

 the above reasons. 



4-53 



10-80 



