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

 5-35), the phytoplankton consume all or most of the nutrients available. Once 

 the supply of nutrients is exhausted, growth is arrested and mass mortalities 

 occur. 



Optimal environmental conditions for phytoplankton growth (i.e., the 

 intermediate condition between vertical mixing and stability) occur in spring 

 and fall and give rise to spring and fall blooms of algae. 



When phytoplankton production in the characterization area is compared with 

 other temperate regions, general trends are noted. Mean annual and maximum 

 cell densities of phytoplankton and chlorophyll concentrations are higher in 

 many bays and estuaries south of Cape Cod than in Maine waters. The higher 

 productivities in southern New England nearshore waters may be due to the 

 following: water temperatures are higher for a longer period of time, the 

 period of optimal light is longer, and the nutrient levels are probably higher 

 due to municipal sewage in the runoff from the large population centers in 

 these areas. In summary, the limited light intensities (due to higher 

 latitudes) combined with low water temperatures and generally low levels of 

 land-based nutrient inputs result in phytoplankton productivities in nearshore 

 waters of the characterization area that are lower than those of southern 

 waters but higher than those of the Gulf of Maine. 



Estuaries have higher productivity than coastal marine waters. Generally, the 

 maxima of production are similar but production is maintained over a longer 

 time period in estuaries, because of mixing. For most estuaries, it is a 

 combination of the following factors that make estuarine waters richer in 

 terms of phytoplankton production than coastal waters or waters of the Gulf of 

 Maine: 



1. Nutrient enrichment associated with inflow of fresh water; 



2. Updraft of nutrient-rich deep water because of displacement of 

 surface water by freshwater inflow; 



3. High rate of nutrient regeneration associated with shallow water 

 sediments; 



4. Mixing processes associated with tidal energy. 



Macroalgae 



The physical and chemical factors that control macroalgal productivity are 

 varied and may differ substantially between species. The factors governing 

 the productivity of marine macrophyte communities are the availability of 

 light, availability of suitable substrata, suitable salinities and water 

 temperatures, and an adequate nutrient supply. Tides carry life sustaining 

 oceanic compounds to intertidal macroalgal populations. 



Growth of the dominant intertidal alga, rockweed ( Ascophyllum nodosum ) , is 

 limited by low water temperatures (below 50°F; 10°C). Generally, the warmer 

 the water the greater the productivity and biological activity in Maine 

 estuaries. In winter the stress associated with surface freezing probably 

 depresses growth. An indirect effect of warmer (summer) water temperatures 

 (around 68 F or 20 C) in Maine is that grazing activity would increase. 



5-69 



10-80 



