phytoplankton, and benthic diatoms. To an undetermined extent certain 

 microbes may be involved in the production mechanisms of the marine system. 

 The major difference between the production of the marine and estuarine 

 systems is that the productivity of the marine system is dominated by 

 phytoplankton, whereas phytoplankton, macroalgae, and rooted aquatic plants 

 dominate estuarine productivity. The components, habitat preferences, and 

 potential fates of primary production will be described here. 



Phytoplankton . Phytoplankton is a collective term for the several major 

 groups of microscopic algae that are suspended in the water column. In Maine, 

 diatoms occur more frequently and in larger numbers than the other groups. 

 These organisms occur in chains and single cells and in bizarre shapes and 

 sizes, ranging from about 10 to 100 ^m. Their cell walls are composed of 

 silica and they have very low motility. Chaetaceros , Skeletonema costatum , 

 and Thalassiosira are dominant diatoms in temperate waters. 



Dinoflagellates are the next most abundant group and they occur most 

 frequently offshore and in waters south of Maine. They exist mostly as 

 solitary cells; however, some species form chains. Their cell walls are 

 cellulose and generally they have a flagellum used in active swimming. Their 

 sizes range from about 30 to 100 Mm and the blooms of one species are noted 

 for occurring in swarms known as red tides (see "Red Tides," page 12-31 in 

 chapter 12) . 



Another group of phytoplankton, coccolithophores , are more common to the open 

 ocean than to the Gulf of Maine. Their cell wall is composed of calcium 

 carbonate and they are smaller in size (10 to 15 y m) than diatoms or 

 dinoflagellates. Another group of much smaller (about 5 y m) phytoplankton 

 occurs but very little is known about it. These organisms do not appear to 

 take part in the massive seasonal changes common to the other forms. 



Over vast expanses of ocean, phytoplankton are the only producers; hence, the 

 size of higher trophic levels is limited by phytoplankton productivity. 



Growth of phytoplankton is controlled by nutrient availability (see "Nutrient 

 Cycle" below) water temperature, salinity, mixing of the water column, and 

 availability of sunlight. These factors and their interrelationships are 

 discussed in "Interactions affecting productivity and distribution of biota" 

 below. 



Phytoplankton are consumed by animals, such as zooplankton, which also occur 

 in the water column. They also sink to the benthic environment, where 

 phytoplankton cells are consumed by benthic invertebrates or remineralized in 

 the sediments . 



Macroalgae and rooted vegetation . Macroalgae, commonly called seaweeds, 

 are the largest forms of algae and are found in great abundance in marine and 

 estuarine areas along the coast of Maine. They range in size from minute 

 plants to plants of 3 feet (1 m), such as the intertidal rockweeds, or several 

 meters in length, such as the kelps. Seaweeds form the bands of attached 

 nonrooted vegetation that extend from between tide marks to shallow waters 

 (<60 ft; 20 m) along rocky coasts. Seaweeds are divided into three phyla, the 

 brown algae (Phaeophyta) , the red algae (Rhodophyta) , and the green algae 

 (Chlorophyta) , based upon taxonomic considerations. 



4-30 



