Response to basal resources by stream macroinvertebrates is shaped by watershed urbanization, riparian canopy cover, and season

Abstract

Riparian reforestation is a common restoration action in urban streams, but relatively little is known about the influence of local riparian vegetation in the face of watershed-scale urban land cover. Allochthonous organic matter and benthic algae are important basal energy resources in streams, but the roles of watershed urbanization vs near-stream vegetation in the availability of these resources are not well understood. Our goal was to understand how the interaction of land cover at 2 spatial scales (watershed vs reach) and seasonal dynamics shape basal resources and their effects on macroinvertebrate communities. We assessed relationships between seasonal patterns in standing stocks of particulate organic matter (POM) and benthic periphyton and macroinvertebrate community composition in openand closed-canopy reaches of 4 urban and 4 reference streams in northern Kentucky, USA. POM standing stocks were not strongly influenced by watershed or riparian condition. Benthic algal biomass was greater in urban than in reference streams in all seasons and in open than in closed riparian canopies in summer when light levels are most affected by a deciduous canopy. Relationships between macroinvertebrate functional feeding group (FFG) biomass and their primary food resources were influenced by watershed land use and season, but riparian canopy effects were minor. The proportion of collectors varied by season, whereas the proportion of shredders was higher in reference than urban streams. Scraper biomass was influenced by benthic algal biomass and varied seasonally, whereas predator biomass was driven by prey-insect biomass. Periphyton density was affected by the interaction of watershedand reach-scale land cover and was the only basal resource strongly related to consumer taxa. Watershed land use influences the stream ecosystem, but local riparian canopy may be important in limiting benthic algal accumulation.

Keywords: aquatic macroinvertebrate; organic matter; periphyton; riparian; trophic interaction; urban streams; watershed.

Figure 1.

Conceptual diagram of expected influences of watershed, riparian canopy, and season on stream benthic basal resource availability and benthic macroinvertebrate functional feeding group (FFG) biomass. CPOM 5 coarse particulate organic matter, FBOM 5 fine benthic organic matter.

Figure 2.

Scatterplots of coarse particulate organic matter (CPOM) standing stock by watershed, canopy, and season. Dots represent benthic CPOM dry mass for individual study reaches during the specific sampling season. Lines connect the mean value for each canopy treatment by watershed type. Fall 5 autumn, Ref 5 reference, Urb 5 urban.

Figure 3.

Scatterplots of fine benthic organic matter (FBOM) standing stock by watershed, canopy, and season. Dots repre-sent FBOM dry mass for individual study reaches during the specific sampling season. Lines connect the mean value for each canopy treatment by watershed type. Fall 5 autumn, Ref 5 reference, Urb 5 urban.

Figure 4.

Scatterplots of benthic chlorophyll a (Chl a) stand-ing stock by watershed, canopy, and season. Dots represent Chl a dry mass for individual study reaches during the specific sam-pling season. Lines connect the mean value for each canopy treatment by watershed type. Fall 5 autumn, Ref 5 reference, Urb 5 urban.

Figure 5.

Scatterplots of macroinvertebrate taxonomic rich-ness by watershed, canopy, and season. Dots represent taxo-nomic richness for individual study reaches during the specific sampling season. Lines connect the mean value for each canopy treatment by watershed type. Fall 5 autumn, Ref 5 reference, Urb 5 urban.