Urbanization of watersheds leads to myriad changes to streams, including modified sediment and streamflow regimes that can result in altered fluvial geomorphic processes and channel structure. Hydrogeomorphic features have been linked to community composition of aquatic biota, as well as to stream ecosystem functioning. Biotic communities in urban stream ecosystems can be markedly different than their counterparts in more natural streams, often exhibiting reduced abundance, diversity, and shifts in assemblage composition, though the specific mechanisms through which urban land use and subsequent hydrogeomorphic modification effects these changes remain unresolved. Hydrogeomorphic modifications may impact both instream habitat as well as connectivity to the surrounding landscape, influencing both biotic assemblage composition as well as ecological connectivity between streams and their adjacent riparian zones. In 23 small urban stream reaches in the Columbus Metropolitan Area (CMA), Ohio, USA, I investigated potential linkages between urban-induced hydrogeomorphic characteristics and: (1) fish assemblage compositional changes over time (3-5 years); (2) fish assemblage trophic dynamics; (3) aquatic-to-terrestrial nutritional subsidies to a common riparian consumer (spiders of the family Tetragnathidae); and (4) downstream drift of larval macroinvertebrates in the water column.
Hydrogeomorphic features related to instream habitat, the hydraulic environment (e.g., slope, shear stress, D50 [median bed sediment particle size]) and stream-floodplain connectivity (e.g., entrenchment ratio, sinuosity, incision ratio) emerged as common influences on fish assemblage composition and trophic dynamics, aquatic-terrestrial connectivity, and invertebrate drift. At a subset of 12 study reaches, several hydrogeomorphic variables showed significant changes over 3-5 years, with many decreasing (e.g., discharge [by 39%], slope [by 0.1%], and shear stress [by 29%, which decreased in concert with slope]). Fish assemblage evenness decreased over the study period in study reaches with higher incision ratio (t = 2.16, p = 0.039), accompanied by species-specific relationships with hydrogeomorphic characteristics. These findings explicitly link hydrogeomorphic and fish-community changes over time, showing that fluvial geomorphic forms in urban streams are not static, and can exert effects on fish assemblages over relatively short time periods, likely via shifts in instream habitat.
In a wider suite of 23 study reaches, hydrogeomorphic characteristics – including slope and bankfull discharge – influenced fish-assemblage diversity (H’), density, and proportion of generalist foragers. Mean reliance on aquatically-derived energy (i.e., reliance on energy pathways derived from benthic algae) of Creek Chub (Semolitus atromaculatus) was 0.60, which increased with slope (Z = 2.27, p = 0.023), suggesting that channel gradient, a measure highly associated with shear stress, may moderate the relative abundance of autochthonous vs. allochthonous basal resources consumed by stream fish. Sunfish species (Lepomis cyanellus and L. macrochirus) relied on aquatically-derived energy for 0.62 of their nutrition, which was positively related to discharge (Z = 1.98, p = 0.048), suggesting an underlying mechanism of discharge-mediated bed disturbance as a control of basal resource availability. Hydrogeomorphic variables did not influence mean trophic position or food-chain length (FCL; ranging from 1.83-4.69 across all study reaches), which was more closely related to water nutrient concentrations (i.e., total N).
Tetragnathid spider trophic position (x ̅ = 2.41) was negatively influenced by D50, while tetragnathid reliance on aquatically-derived energy (x ̅ = 0.43) trended positively with sinuosity (t = 2.10, p = 0.054), suggesting that tetragnathid spider trophic position is influenced by the impact of instream habitat on emergent aquatic insects, while aquatically-derived energy was more closely related to stream-floodplain connectivity. Tetragnathid density decreased with slope (t = -2.51, p = 0.023), as did the flux of emergent aquatic insects into the riparian zone (t = -2.27, p = 0.037). The relative abundance of Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa in emergent aquatic insect assemblages was positively associated with sinuosity (t = 6.84, p < 0.001), potentially as a result of increased habitat heterogeneity and stream-floodplain connectivity.
Larval macroinvertebrate drift was characterized by very low densities (x ̅ = 0.061 individuals m-3) relative to previous studies (but note that few urban studies of drift are available), with fewer than one-third of samples containing larval macroinvertebrates. The probability of a sample containing macroinvertebrates was positively associated with incision ratio (Z = 2.29, p = 0.022), while drift density trended with many hydrogeomorphic variables (e.g., entrenchment ratio [negative trend; Z = -1.9, p = 0.059], sinuosity [positive trend; Z = 1.91, p = 0.066]). Drift density was also highly influenced by chemical water quality and nutrients (e.g., t = 1.96, p = 0.059 with a water quality principal component axis largely driven by total N, total P, and total dissolved solids). Drift assemblage attributes (e.g., diversity [H’], family richness) were not related to any physico-chemical parameters, but rather to benthic macroinvertebrate assemblage characteristics (e.g., positive relationships between family richness and H’ [R2 = 0.11, F = 4.62, p = 0.038]; family richness and %EPT [R2 = 0.20, F = 9.52, p = 0.004]. Thus, a complex suite of factors appears to drive macroinvertebrate drift in urban streams, including degrees of entrenchment and sinuosity, implicating the important role stream-riparian connectivity and the potentially strong influences of terrestrial organic matter on benthic macroinvertebrate assemblages, which serve as the “source pool” available to enter the drift.
Collectively, these results highlight how urban-land development cascades, through hydrologic and sediment-related alterations (e.g., discharge magnitude, shear stress), to impact physical channel (geomorphic) form (e.g., sinuosity, floodplain connectivity) and, in turn, instream habitat (e.g., D50, slope) and biotic community structure and function (e.g., assemblage composition, food-web architecture, and energy flows). An ecogeomorphic approach, such as the one adopted in my research is, therefore, well supported as an avenue toward further understanding and potentially restoring critical physical-biological processes that are key to forming and maintaining resilient urban stream ecosystems. Incorporating hydrogeomorphic surveys into management and restoration activities may thus lead to a more mechanistic understanding of the underlying sources of stream impairment in cities.