Agricultural land use alters temporal dynamics and the composition of organic matter in temperate headwater streams

Intensification of agricultural land use leads to riparian clear cutting, which disrupts stream aquatic-terrestrial linkages through the loss of terrestrial particulate organic matter (POM). POM is important for structuring habitats and serves as a basal resource for food webs. We studied the effects of agricultural land use on the fate and temporal dynamics of POM inputs and standing crops by comparing 2 agricultural and 2 forested reference streams for 15 mo. We used the C spiraling metrics downstream velocity of organic C (V-OC) and index of retention (IR) to integrate information on the dynamics of benthic organic matter (BOM) with physical characteristics of the streams. Daily POM inputs into reference streams were 15 to 39x higher than inputs into agricultural streams, and mean standing crops of total BOM were significantly lower in agricultural streams than in reference streams. Agricultural streams had significantly higher standing crops of fine benthic organic matter (FBOM), but 1.8 to 3x lower coarse benthic organic matter (CBOM) than reference streams. The temporal dynamics of BOM standing crops differed between landuse types. BOM varied seasonally in reference streams but varied stochastically over time in agricultural streams. V-OC was significantly faster and IR was significantly lower in agricultural than in reference streams. Further, V-OC was mainly determined by benthic organic carbon (BOC) and transported organic carbon (TOC). Analyses of BOM and carbon spiraling metrics suggested that reference streams were more retentive because of terrestrial POM inputs and higher habitat complexity, whereas high discharge and hydrological variability limited the retentive capacity of the agricultural headwaters. However, total litter decomposition rates were high in agricultural streams. Our use of spiraling metrics to integrate physical stream characteristics with temporal dynamics of BOM provides a mechanistic understanding of how agricultural land use affects POM dynamics in temperate headwaters and highlights the importance of natural riparian vegetation to restoration efforts. This understanding is important in light of the growing concerns about the effects of intensive agriculture on stream ecosystems.