94 Natural systems and greenspaces
urban regions, wetlands have been mainly drained or filled, so the remaining
ones are often centers of rare species.
Lakes and ponds usually have sufficient horizontal water flow to remain oxy-
genated, and hence support ample populations of fish and other species (Wetzel
and Likens 2000, Wetzel 2001, Kalff 2002). Internal habitat heterogeneity is espe-
cially important.Littoral zones,theshallowwateredgesoflakes,tendtosup-
port rooted vegetation and numerous microhabitats for a diverse fauna, from
microorganisms to fish. Lakeshores with natural vegetation, which are often
scarce in urban regions, provide valuable inputs and protection for aquatic
ecosystems in small lakes and ponds. Lake bottoms are typically covered by fine
inorganic and organic sediments, which support bottom-dwelling animals. Water
layers in the lake, largely characterized by temperature and oxygen differences,
contain somewhat different fish and other species. Some lakes haveanaerobic con-
ditions,i.e., no oxygen, in the bottom layer permanently or seasonally.Ephemeral
ponds(such as vernal pools) contain surface water for only a portion of the year,
which essentially eliminates fish, but provides habitat for many unusual species
of conservation importance (Colburn2004).
Astream/riversystemreceivesprecipitation water from adrainage basin(catch-
ment or watershed area) (Figure4.3)(Decamps and Decamps2001,Wetzel2001,
Kalff 2002, Wiens2002). In streams and rivers, water velocity is a major determi-
nant of both their habitat structure and aquatic ecosystems. In moist climates,
streams and rivers are differentiated by astream-order system.Inthisthe small-
est perennially flowing streams, called first-order streams, receive groundwater
flows, but only receive surfacewater in tiny intermittent (ephemeral) channels.
Twofirst-order streams combine to form a second-order stream, two second-
orders combine to form a third-order stream, and so forth. Second- to about
fourth-order streams typically are straight to somewhat curvy, flow rather fast,
slowly erode downward, maintain heterogeneous mainly rocky or sandy bot-
toms, and have narrow floodplains alongside. About fifth-order-and-up rivers
commonly are meandering or convoluted, flow slowly, gradually accumulate
sediment, have river bottoms predominantly of silt (relatively fine material),
and flow through wide floodplains.
Theriver continuum concepthighlights the sequence from small first-order
stream to large river, with gradual changes in water velocity, inputs of important
dead leaves, light availability, presence of rooted vegetation, floating-algae pro-
duction, stream-bottom micro-heterogeneity, curvilinearity of streamsides, fish
populations, and much more. Just above where rivers enter the sea, freshwater
is flowing downriver while tidal seawater pushes upriver, producing afreshwa-
ter tidal zonesometimes extending for several kilometers. This unusual habitat