Enhancing food supplies for waders: inconsistent effects of substratum manipulations on aquatic invertebrate biomass

1. The potential of substratum manipulations as a technique for enhancing f ood supplies for waders was investigated in 98 constructed ponds at six sit es in the Upper Waitaki Basin, New Zealand. Substrata were manipulated by u sing different construction methods, and by adding barley straw or stones t o some ponds. Total invertebrate biomass and chironomid biomass were measur ed 3 and 15 months after ponds were constructed. 2. Food supplies in many newly constructed ponds contained large proportion s of chironomid larvae, an attractive food for waders. However, this patter n was not universal; food supplies in many other newly constructed ponds we re dominated by various nektonic invertebrates, or, at one site, by mollusc s. 3. Food supplies developed rapidly; 3 months after construction most ponds contained similar or greater total invertebrate biomass (mean, range: 1.20, 0.10-7.66 g m(-2) dry mass) and greater chironomid biomass (0.66, 0-5.41 g m(-2)) than nearby 'pre-existing' wetlands (1.79, 0.39-6.18 g m(-2) total invertebrate dry mass; 0.01, 0-0.08 g m(-2) chironomid dry mass). However, ponds with stony substrata contained relatively low total invertebrate and chironomid biomass. 4. Substratum manipulations affected chironomid or total invertebrate bioma ss at all sites. Differences in biomass between substrata in constructed po nds were large at some sites (up to 2.7 g m(-2) dry mass for chironomids, a nd up to 3.39 g m(-2) dry mass for total invertebrates), indicating that su bstratum manipulations have good potential as a tool for increasing food su pplies for wetland birds. 5. Despite the sometimes large effects of substratum manipulations, no tech nique achieved consistently positive results. The absolute and relative eff ects of any given substratum manipulation varied greatly among sites. This inconsistency indicates that, although substratum manipulations can be wort hwhile, they need to be evaluated on a site by site basis.