September results in a higher biomass of aquatic invertebrates (predominantly
ramshorn snails Planorbidae) the following winter than does re-flooding in
November (Anderson and Smith 2000). In wetlands where the benthic fauna is
dominated by non-biting midge larvae, invertebrate biomass is likely to be higher
in winter if re-flooding is done in autumn while adult midges are still active and
ovipositing.
14.9.3 Increasing food abundance for birds in shallow brackish
and saline water
Invertebrate prey biomass for birds in brackish and saline lagoons can be maxi-
mized by maintaining optimum salinities for growth and reproduction of these
different suites of prey species. This requires regular monitoring of salinity or con-
ductivity. Measurements should be taken more frequently in summer when high
evaporation rates can lead to rapid rises in salinity. Salinity in parts per thousand
(ppt or ‰) is approximately equal to 0.64conductivity in milliesiemens (mS)
( Jones and Reynolds 1996). In temperate lagoons at low salinities (ca. 8‰) the
most abundant invertebrate prey are usually non-biting midge larvae (Chirono-
midae) in the mud, and water boatmen (Corixidae), and opossum shrimps
Neomysisspp. in the water column. At higher salinities (above ca. 8‰ and below
40–70‰) the main prey in the mud are polychaete worms, non-biting midge
larvae, molluscs and amphipods, with opossum and other shrimps the main prey
in the water column (Britton and Johnson 1987; Robertson 1993). Brine shrimps
Artemiaspp. are virtually the only prey present at salinities above ca. 70‰. These
animals are restricted to warm climates and can withstand salinities up to
ca. 320‰ (e.g. Britton and Johnson 1987). Studies of lagoonal invertebrates in
England have revealed that maximum biomass of a non-biting midge larvae/water
boatman and ragworm Hediste diversicolor/Corophium volutatorfauna occur at,
respectively, ca. 6‰ and ca. 24‰ (Robertson 1993).
There is little information on the effects of periodically drying out saline
waterbodies on the invertebrate food supply for birds. As in freshwater wetlands,
drying out kills fish, and high densities of fish can reduce densities of benthic
invertebrates (Robertson 1993). Set against this, the small fish found in saline
lagoons, such as sticklebacks Gasterosteidae and gobies Gobiidae, are themselves
important prey for birds such as herons and egrets. Experimental studies have
found that adding dead plant matter to brackish lagoons can increase the bio-
mass of ragworms, but has little or no effect on that of non-biting midge larvae
(Robertson 1993). Therefore, invertebrate biomass in ragworm-dominated
lagoons could probably be increased by first lowering water levels to allow vege-
tation to colonize, and then re-flooding to kill this vegetation and increase the
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