Factors such as ice scouring, wave action, and grazing, which dislodge or 

 consume the rockweed increase the rate of turnover of algae, thus increasing 

 net growth. Wave action is also a factor in determining distribution and 

 abundance of raacroalgal species. Certain species (e.g., Fucus spp . ) thrive in 

 areas of increased wave action, while others (e.g., rockweed) prefer more 

 protected areas. Greater wave action and steeper shores promote the 

 development of more hardy plants (e.g., Fucus spp.) which are resistant to 

 wave drag and abrasion. Biomass per unit of area also tends to be somewhat 

 lower in the high energy, wave-exposed shores along the headlands and islands 

 of the characterization area than in more protected areas. In the sublittoral 

 zone wave action influences productivity only at the uppermost levels. 



Populations of subtidal laminarians, or kelp (the dominant subtidal algae), 

 are restricted to areas of suitable substrata and relatively high salinity 

 and, therefore, are limited to the seaward end of estuaries. As with many 

 subtidal plants light availability greatly influences laminarian growth. 

 Therefore these algae only grow in shallow subtidal areas (to 20 m) . In areas 

 of high light penetration algae grow at greater depths, whereas in turbid 

 areas growth is confined to shallower depths. In summer, when solar radiation 

 is highest, some light-sensitive algae do not grow in shallow water. In 

 contrast to the intertidal fucoids, which grow best at high temperatures, 

 laminarians grow fastest at low water temperatures and appear to grow least 

 when temperatures reach about 68 ° F (20 Sz) . Their major growth period is 

 winter and spring. In addition to the possible stress of higher summer 

 temperatures, lack of available nitrogen (see "Nutrient Cycle," page 4-48 in 

 chapter 4) may limit summer and fall kelp production. 



Zooplankton 



Tidal flushing exports and imports various marine species of zooplankton to 

 estuaries. Certain zooplankton species that live in the offshore waters are 

 introduced into Maine estuaries and become a temporary component of these 

 ecosystems. In estuaries where the circulation is restricted, zooplankton 

 populations are more endemic, but the composition is dependent largely on the 

 distribution of salinity and temperature for a given estuary (Jeffries and 

 Johnson 1973) . These two factors have far greater seasonal ranges in 

 estuaries than in the open coastal waters but the accompanying biological 

 phenomena in both habitats bear much in common. 



Temperature is largely responsible for regulating the species composition of 

 zooplankton that occur in the estuaries. Each species has particular 

 temperature requirements for growth, reproduction, and survival. As one group 

 appears another disappears and production is maintained throughout the year, 

 despite annual temperature ranges that sometimes exceed the reproductive 

 tolerances of all the groups. Within each group, annual propagation is 

 dependent upon overwinter survivors or the maturation of new individuals from 

 resting eggs produced the previous year. Temperature, along with food supply 

 (quantity and quality), also controls the rate of growth of zooplankton. 



The concentration of salt in an estuary forms a fresh- to salt-water gradient 

 that limits the distribution and abundance of various species of zooplankton. 

 This factor separates estuarine from offshore or coastal populations of 

 zooplankton. The interactions of temperature and salinity have very specific 



5-70 



