ALGAL VS. MACROPHYTE INPUTS TO FOOD WEBS OF INLAND SALINE WETLANDS

Abstract

Invertebrate food webs in wetlands were traditionally thought to be fueled mainly by decaying macrophytes, but recently it has been recognized that microalgae may be more important. In particular, the paradigm that shredders of vascular plant litter dominate food web processes may not apply to many wetlands where shredders are rare and microalgae more abundant. This issue is complicated by potential consumption of flocs of dissolved organic matter (DOM) released from living plants, and of exopolymer secretions (EPS) from both autotrophic and heterotrophic microbes. In Wyoming, we used gut contents and stable isotopes to investigate organic matter sources for the dominant invertebrates in oligosaline (0.5–5 g/L total dissolved solids) and mesosaline (5–18 g/L) wetlands. We examined the trophic importance of microalgae vs. macrophytes in wetlands with and without emergent vegetation (Scirpus acutus), with different growth forms and species of submersed plants (Chara spp. vs. Potamogeton pectinatus), with dominance by different microalgal types (phytoplankton, epiphyton, epipelon), and with different primary consumers (mainly amphipods vs. chironomid larvae). In all wetlands studied, guts of the major primary consumers contained little or no macrophyte tissue, but rather mostly amorphous detritus (organic particles with no recognizable cellular structure). Values of δ13C indicated that organic matter entering foodwebs was not from submersed macrophytes, but that emergent plants might be a source of DOM or EPS in amorphous detritus. However, in some wetlands, amphipods eating mainly amorphous detritus had the same δ13C values as chironomids eating a much higher fraction of diatoms, indicating that amorphous detritus was derived mainly from diatoms. Patterns of temporal change of δ13C in consumers, seston, and emergent plants supported this interpretation. We conclude that microalgae rather than macrophytes provided most organic matter for these food webs via amorphous detritus. Amorphous detritus is often thought to have poor nutrient quality and low assimilation efficiency, but this idea may not be true if amorphous detritus is largely flocs of labile DOM/EPS. Our results suggest that characterizing the origin and nature of amorphous detritus is key to understanding variations in macroinvertebrate production among saline wetlands and a broad range of wetland types.