Document: SHA-3-52-21

Effects of straw management and winter flooding on aquatic food web structure in a rice agroecosystem.

LAWLER, S.P.* and D.A.DRITZ

University of California at Davis, CA ¹

Abstract:
Rice fields harbor diverse aquatic communities that are similar to temporary ponds in many respects. Recent changes in rice straw management and flooding schedules provide an opportunity for large-scale tests of how detritus and flooding frequency affect the structure and dynamics of aquatic food webs. Traditionally, California farmers burned old rice straw, but many now incorporate it into the soil to comply with air quality regulations. Some farmers flood fields in winter to attract waterfowl and/or decay leftover straw. We studied twenty 0.72 hectare rice fields that were treated as follows: burned, burned + winter flooded, straw incorporated, incorporated + flooded, straw rolled down + flooded. We predicted that detritivorous invertebrates would be more abundant in fields where straw had not been burned, and that these would support more predators. It was difficult to predict effects of winter flooding a priori, because flooding would reduce the amount and quality of detritus but might also elevate local populations of some invertebrates or provide stronger oviposition cues. Fields were sampled monthly during the growing season for invertebrates and algae. No pesticides were used on this site, but fertilizer was applied once and herbicides twice near the beginning of the growing season. Leftover detritus had surprisingly few effects on the structure of the invertebrate community, however mosquito larvae were nearly twice as abundant fields where straw was not burned. Mosquito larvae feed on algae and fine particulate organic material, so effects of straw may have been indirect. Winter flooding promoted development of comparatively large populations of midges and predators. Conchostracans were less abundant in winter flooded fields. Flooding may have had negative effects on their diapausing eggs. This study shows that flooding frequency can have strong effects on aquatic food webs. Detritus may have been less important in this agroecosystem than in natural wetlands because of the early-season fertilizer application.

Keywords: aquatic invertebrates, food webs, agroecosystem, hydroperiod, detritus, mosquitoes, rice

This abstract is being presented at: 2:45 PM in session:
Oral Session #31: Agroecology.