quantity of dead plant material available to the invertebrates. Biomass of poly-
chaetes and bivalves may be slow to increase following such a drawdown, because
they both take several years to reach maximum size. It might be best to retain
some pools in the lagoon during a drawdown, from which polychaetes and
bivalves can re-colonize the rest of the lagoon following re-flooding. The chi-
ronomid life-cycle is very short, and these animals quickly re-colonize from
winged adults. Burning patches of emergent vegetation prior to re-flooding to
increase interspersion of brackish swamp and open water (Section 14.9.1) has
been found to increase the biomass of chironomids, while increasing intersper-
sion by burning had no effect on it (de Szalay and Resh 1997).
The salinity regime will also influence the abundance of plant food for water-
fowl. Saline lagoons support only a limited range of macrophytes species,
although some of these, notably Beaked Tasselweed Ruppia maritimaand charo-
phytes, are important waterfowl food. Macrophytes are absent from high salinity
lagoons, for example in southern France, those with salinities more than 64‰
(e.g. Britton and Johnson 1987).
14.9.4Increasing accessibility of food for birds in shallow water
Water levels can be manipulated to provide suitable shallow water as feeding
areas for waders, dabbling ducks, herons, and other species. Highest numbers of
bird species are typically found in water 15–20 cm deep, with few wading species
using water deeper than 40 cm (e.g. Elphick and Oring 1998). Plovers and some
other species feed mainly on bare mud exposed by falling water levels. The range
of feeding opportunities available at any one time can be increasing by enhanc-
ing topographic variation within the area flooded.
Achieving suitable water depths by lowering water levels has the advantage of
concentrating aquatic prey of birds, particularly fish, and providing bare mud
containing stranded benthic invertebrates on which waders and other birds can
feed. For example, periodic lowering of water levels in fish ponds to allow har-
vesting of commercial fish species attracts large numbers of herons and egrets to
feed on stranded non-commercial fish species and shrimps (Young 1998).
Creating suitable water depths by raising water levels to flood new habitat may
temporarily raise productivity by increasing the availability of detritus (see
previous two sections), and provide a short-lived (and probably one-off )
abundance of displaced terrestrial invertebrates, particularly on grassland
(Section 14.9.2).
Where a number of such waterbodies are under independent hydrological con-
trol, feeding conditions for waterfowl can be optimized by sequentially lowering
water levels in different waterbodies, to provide a continuity of suitable feeding
356 |Habitat management