Potential of submerged macrophytes to support food webs in lowland agricultural streams

Submerged plants are often abundant in lowland streams in agricultural landscapes, but little is known of their role in stream ecosystems compared to riparian vegetation. We investigated the importance of submerged macrophytes as a basal resource of food webs in stream reaches with good and poor riparian vegetation condition, using mixing model analysis with stable carbon and nitrogen isotopes. Epilithic periphyton and terrestrial detritus were important basal resources in good condition reaches, although where macrophytes were present they contributed to food webs. Higher assimilation of either the macrophyte Cycnogeton huegelii or conspicuous epiphytes on C. huegelii leaves was associated with poor riparian condition. Where Potamogeton ochreatus and Ottelia ovalifolia occurred in poor condition reaches, these macrophytes contributed moderately to the food web, but were probably of greater importance as substrates for epiphytic algae. Mixing models indicated invertebrates commonly had generalist feeding strategies, feeding on the most available resource at each reach. Thus, where riparian vegetation is limited, submerged macrophytes may support opportunistic consumers both directly and as a substrate for epiphytes, thereby partially compensating for the loss of allochthonous resources in lowland agricultural streams.