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 ␦ 13 C 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 ␦ 13 C values as chironomids eating a much higher fraction of diatoms, indicating that amorphous detritus was derived mainly from diatoms. Patterns of temporal change of ␦ 13 C 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.

We compared stable‐isotope (δ13C and δ15N) and gut‐content analyses of macroinvertebrate foodwebs in saline wetlands of the Laramie Basin, Wyoming, USA. Standard assumptions of stable‐isotope fractionation between trophic levels (<1‰ for δ13C, mean of 3.4‰ for δ15N) suggested that zygopteran (damselfly) larvae consumed mainly amphipods. However, the guts of zygopterans contained no amphipods but rather a mix of chironomid larvae and zooplankton. In all wetlands the gut contents of zygopterans indicated that they were secondary consumers (trophic level 3), but enrichment of δ15N between zygopterans and their prey (Δδ15N) varied from 1 to 3.4‰ between wetlands. In other studies, such variation in Δδ15N has been interpreted to mean that food‐chain length differed between aquatic systems. We review alternative interpretations of variable 15N enrichment, namely, varying C:N ratios in food, differential enrichment between consumer species, and habitat‐specific variation of δ15N at the base of foodwebs. We also suggest that variation in the timing and rates of nitrogen cycling can affect measured Δδ15N both within and between foodwebs. For aquatic macroinvertebrates, we urge that stable isotopes be supplemented with independent observations to avoid incorrect conclusions about trophic pathways, trophic levels, and food‐chain lengths in different ecosystems.

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We used stable isotopes and gut-content analysis to compare trophic relations of macroinvertebrates between two types of saline wetlands in the Laramie Basin, Wyoming, USA. Amphipods (Hyalella azteca), chironomid larvae, and predatory insects (mainly zygopteran larvae) occurred in both wetland types. However, in oligosaline wetlands (0.5-5& total dissolved solids) amphipods were dominant, whereas in mesosaline wetlands (5-18&) amphipods were scarce and chironomid larvae and predatory insects were much more abundant. Salinity alone seemed inadequate to explain these differences, so we examined trophic interactions to address three main questions: (1) why are predatory insects more abundant in mesosaline wetlands, (2) why are chironomid larvae less abundant in oligosaline wetlands, and (3) why are amphipods uncommon in mesosaline wetlands? Zygopteran larvae ate mainly chironomid larvae and zooplankton, and did not eat amphipods; chironomid larvae were an average 93% dry mass of zygopteran intake. Guts of amphipods contained no animal parts and little vascular plant tissue, but rather mainly amorphous detritus. Chironomid larvae ate amorphous detritus as well as diatoms. There is potential for competition between amphipods and chironomids, but the nature of amorphous detritus in these wetlands needs further study. Because amphipods appear unavailable as prey for predatory insects, top-down impacts on chironomid larvae may be greater in oligosaline than in mesosaline wetlands. Our results suggest that (1) predatory zygopteran larvae and hemipterans are more abundant in mesosaline wetlands due to more abundant chironomid prey, (2) chironomids are less common in oligosaline wetlands because of both competition with amphipods and greater per capita consumption by predatory insects, and (3) amphipods are scarce in mesosaline wetlands because, lacking a resting stage and mode of direct dispersal, they do not cope well with extremes created by hydrologic instability in mesosaline wetlands. These mechanisms may apply to wetlands in other regions where similar patterns of invertebrate community structure have been reported.

Coarse woody debris (CWD) is an important component of headwater streams, however, few studies have investigated the geomorphic effects of CWD in the southern Appalachians. In the Great Smoky Mountains, debris slides supply large volumes of CWD and sediment to low-order streams. This study investigates the effect of CWD on bankfull channel dimensions and in-channel sediment storage along second-order streams. Comparisons are made between streams that have experienced recent debris slides and those that have not. CWD channel obstructions are larger but less frequent along debrisslide-affected streams. Dendrochronological evidence indicates that CWD can remain in channels for over 100 yr. Relatively short residence times of CWD along debris-slideaffected streams suggest that logs are frequently flushed through these streams. CWD causes channel widening along all study streams, but the volume of sediment stored in the channel behind CWD obstructions is up to four times greater than the volume of sediment represented by bank erosion associated with CWD. Two large log jams formed by debris slides at tributary junctions stored approximately 4000 m 3 of sediment. Sediment stored by CWD was finer than mean bed particle size, and thus represents a significant sediment source when CWD obstructions are breached. [