Channels are subjected to a wide range of 

 environmental conditions. Tidal flushing is 

 greatest at the mouth and decreases with 

 distance from the mouth; this general gradient 

 in turn influences water movement, salinity, 

 temperature, nutrients, and dissolved gases. 

 Finer sediments are removed by higher 

 current velocities, so that substrates near the 

 mouth have coarser sediments than in tidal 

 creeks. Nutrients brought into the estuary by 

 tidal flushing are more readily available to 

 organisms near the mouth. Temperature, 

 salinity, and dissolved oxygen are less 

 variable in areas of deep water than in tidal 

 creeks. These environmental factors influ- 

 ence the species composition, distribution, 

 and population dynamics of the channel 

 organisms. 



3.6.1 Algae 



The obvious plants of intertidal and 

 subtidal areas are the macroalgae. Vascular 



plants such as eelgrass (Zostera marina) are 

 absent, perhaps because the area of shallow 

 water is too small or too dynamic for rooted 

 plants to become established. However, large 

 populations of Enteromorpha sp. and/or sea 

 lettuce (Ulva sp.; Figure 3.19) sometimes 

 develop on the channel bottoms and later float 

 to the water surface (Rudnicki 1986). 

 Seasonal distribution patterns are highly 

 variable, but their abundance is usually 

 greatest in spring (Chapter 4). 



Phytoplankton are also variable in species 

 composition and density. Dinoflagellates (e.g., 

 Gymnodinium spp.), diatoms, filamentous 

 blue-green algae (cyanophytes), and 

 unidentified unicells or "monads" are all 

 present in the water column (Figure 3.19; 

 Fong 1986). While most of these algae are 

 typical planktonic species, the diatoms are all 

 pennate forms with bilateral symmetry and 

 longitudinal grooves that allow locomotion on 

 substrates. Fong (pers. comm.) believes that 

 most of these are resuspended from the 

 sediments. 



Figure 3.19. Channel producers include a phytoplankton community (on left) with blue-green 

 algal filaments, dinoflagellates, such as Gymnodinium, and diatoms, such as the sigmoid 

 Pleurosignma and Gyrosigma, and many species of Navicula. In addition, there is often an attached 

 or floating mat of macroalgae (such as Ulva, on the right). Mclntire collection, © 1986 by Zedler. 



46 



