7 



Local Habitats. 



Communities, and 



Succession: 



Ponds, 



Marshes, Sivamps, 



and Bogs 



Pond is a jMspular term for lakes of the third 

 order that are small, shallow, and, when mature, have 

 rooted vegetation over most of the hottom. Tiiere is 

 no clear distinction between ponds and lakes of the 

 first and second orders. The littoral zone of eutrophic 

 lakes, for instance, is pond-like in habitat and or- 

 ganisms. Floating and emergent vegetation com- 

 monly occurs around the margin of ponds to form 

 extensive tracts of marsh. The pond habitat may 

 originate as a shallow basin, as a large pool in a 

 stream, as the result of the filling in of a lake, or 

 from a stream dammed by beavers, man, or landslide. 

 Slow-flowing rivers are essentially elongated ponds, 

 and have a similar fauna (Kofoid 1908, Richardson 

 1928). Because of the slight water movement in 

 ponds, the surface film becomes an important micro- 

 habitat for some species. Pondwater temperature is 

 often uniform at all depths, but during warm sunny 

 weather, i)onds well protected from the wind may 

 show considerable stratification, not only in tempera- 

 ture, but also in oxygen content and other character- 

 istics (Wallen 1955). Daily and seasonal variations 

 in temperature may be great because of the .small 

 volume of water. Ice forms earlier and lasts longer 

 in ponds than in lakes, freezing shallow ponds to the 

 bottom in severe winters. Light penetrates to all 

 depths, encouraging growth of vegetation except in 

 high turbidity. Young ponds may have rocky, sandy, 

 clay, or mud bottoms ; in mature ponds, there is or- 

 dinarily an accumulation of organic matter and silt. 



The dissolved o.xygen content of ponds varies 

 widely from temporary supersaturation when there is 

 excessive photosynthesis of plants to near depletion 

 when decomposition predominates. Oxygen content 

 is often highest in the spring ; very low in late sum- 

 mer ; and sometimes low again under the winter ice 

 cover. Oxygen content is usually higher during day- 

 light hours than during the night because of the 

 daytime photosynthetic cycle of plants. Oxygen may 

 become so low at night as to become critical, espe- 

 cially for fish. Decomposition of organic matter 

 evolves carbon dioxide and, at times, considerable 

 methane, hydrogen sulphide, and other gases. In 

 ponds, as in lakes, there is as wide variation in hy- 

 drogen-ion concentration. As ponds mature and ac- 

 cumulate humus, pH value decreases. 



PLANT SERE 



The plant hydrosere, or pond sere, typically 

 contains the following stages and characteristic spe- 



Submerged vegetation: water weed, pondweed, 

 milfoil, hornwort, naiads, buttercup, bladderwort, eel- 

 grass, and the herb-like alga Chara. 



79 



