COMMUNITY ORGANIZATION: STRATIFICATION 



451 



prawns common in Bermudian waters is a 

 striking illustration of their cosmopolitan 

 residence in the bathypelagic stratum. The 

 crustaceans of this fauna are characteristi- 

 cally of some shade of red in life and are 

 typically bioluminescent (Beebe, 1934a; 

 Chace, 1940). 



HORIZONTAL STRATIFICATION IN 

 AQUATIC COMMUNITIES 



We have demonstrated a similarity in 

 organization of aquatic communities with 

 respect to vertical gradients. We turn our 

 attention now to the second of two funda- 

 mental patterns in such communities, 

 namely, the horizontal gradients. The sub- 

 ject can be more economically surveyed, 

 since a large part of the inanimate back- 

 ground already outUned applies directly to 

 both vertical and horizontal distributions 

 of organisms. 



In both inland water and marine com- 

 munities there is a general tendency for the 

 higher plants to be stratified in more or 

 less parallel zones on the margins of lakes 

 and seas. This horizontal zonation may 

 vary from an irregular pattern, where spe- 

 cial expression of climatic or edaphic fac- 

 tors retards or inhibits rooted vegetation, 

 to an almost ideal progression of concen- 

 tric strata. 



Inland Waters 



The lake floor is generally divisible into 

 three major horizontal strata or zones 

 (Eggleton, 1931, 1939). The first of these, 

 the littoral (paralimnion), embraces the 

 area lying between the water's edge or 

 shore line and the lakeward extension of 

 rooted vegetation. The second or subht- 

 toral embraces the lake bottom from the 

 lakeward Hmit of rooted vegetation to the 

 average upper Umit of the hypolimnion. 

 The third or profundal covers the bottom 

 from the upper hypolimnial line to the 

 deepest parts of the lake floor. A fourth 

 zone, the abyssal, for the deepest lakes, 

 embracing lake bottom below 600 meters, 

 is theoretically possible, but is not used 

 often among Umnologists since few lakes 

 of this depth exist, and, of those few, all 

 have not been critically examined for pro- 

 fundal-abyssal differentiation. 



A typical lake httoral transect in the 

 north temperate latitudes (Welch, 1935) is 

 summarized as follows: 1. Emergent hy- 



drophytes include bulrushes (Scirpus), 

 cat-tails {Tijpha), wild rice (Zizania), ar- 

 row-head {Sagittaria) , sedges {Carex). 

 These plants occupy the shoreward stra 

 tum, from water edge to about the 2 metei 

 depth fine. Within this zone there may be 

 secondary cleavages, but all such plants 

 have the chief photosynthetic surface 

 raised above the water. 



2. Floating hydrophytes include water 

 liHes {Nyjnphaea, and the like), some 

 smartweed {Polygonum), some pondweed 

 (Potamogeton nutans). These plants usual- 

 ly occupy the second lakeward belt, often 

 interdigitated with emergent vegetation, 

 from 10 centimeters' to 3 meters' depth. 

 They are rooted in the lake bottom, and 

 their foliage, connected by long petioles, 

 floats on the water surface or extends a 

 little above it. 



3. Submerged hydrophytes include most 

 pondweeds {Potamogeton), water milfoil 

 {Myriophyllum) , water weed {Elodea), 

 Vallisneria, and other genera. These plants 

 usually occupy the third lakeward belt, 

 often interdigitated with the floating vege- 

 tation, from 2 to 6 meters in depth. They 

 are rooted into the bottom, and their pho- 

 tosynthetic surface is submerged, so that 

 their carbohydrate production is dependent 

 upon light penetrating the water above 

 them. This third horizontal stratum is only 

 slightly visible from above, but may, and 

 usually does, form a large crop. 



Such horizontal distribution of rooted 

 plants affects the shoreward distribution of 

 lake bacteria (Henrici, 1939). Periphytic 

 and profundal bacteria are quantitatively 

 proportional to the amount of vegetation, 

 being notably more abundant with increase 

 of more complex plants. 



The primary horizontal phyto-gradient of 

 fresh-water littoral areas is of great im- 

 portance in determining similar gradients 

 of animals. Its plants are usually perennials 

 and present a more stable structure than 

 the open water phytoplankton. Among 

 them there is a general tendency to develop 

 aerenchyma, or spongy tissue, which is of 

 special physiological importance to their 

 aquatic life and also to the invertebrates 

 associated with them (Wilson, 1939). 



The zonation of vegetation affects the 

 shelter and food of the several life-history 

 stages of lake animals. These effects are 

 diverse, both direct and indirect, and often 



