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Aquatic invertebrates play a key ecological role in riverine biogeochemical cycles, transferring nutrients incorporated from diverse food sources (e.g. algae or terrestrial detritus) to resident and migratory predators within aquatic and riparian ecosystems. Agricultural land use affects stream aquatic invertebrate communities, generally reducing their diversity and the prevalence of sensitive taxa as a result of the introduction of agro-chemicals (nutrients and pesticides) and other environmental stressors (fine sediments and flow). This thesis aims to understand how agricultural land use affects aquatic invertebrate communities and the role these communities play in the storage and transfer of nutrients within stream food webs. To accomplish this, streams in northwestern New Brunswick were sampled over a three-year period (2010 –2012) for water nutrient concentrations, site characteristics, and aquatic invertebrate biomass and nutrient content. The first study established associations between agricultural land use and stream water nutrient concentrations (positive), aquatic invertebrate community diversity and sensitivity (negative), and total community biomass (negative).The second study determined that the nitrogen and phosphorus content within aquatic invertebrate communities decreased and increased, respectively, as aquatic invertebrate communities lost diversity and sensitive taxa. Using data from a subset of these sites and additional ones sampled in 2012, the final study validated the results of the previous studies and showed similar effects of agricultural land use on aqueous nutrient concentrations, aquatic invertebrate communities, and community nutrient content. Further, this study showed that the quantity of nitrogen stored within the aquatic invertebrate community decreased with increasing agricultural land use while the quantity of phosphorus was unaffected. Together these results demonstrate that as aquatic invertebrate communities change their composition in response to increasing agricultural land cover in the catchment, the amount of nutrients stored by the community decreases. This reduction in nutrient storage capacity likely results from shifts in life history strategies in the aquatic invertebrate community. More broadly, these findings suggest that agriculture affects how aquatic invertebrates cycle nutrients in riverine food webs, which may in turn impact aquatic and terrestrial predators that rely on these communities.
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
Farmers have been encouraged to adopt more sustainable farming practices (BMPs) that mitigate adverse agricultural effects on the natural environment. However, the ability of BMPs to protect or restore riverine systems continues to be questioned due to limited evidence directly linking BMP use with improved ecological conditions. The exclusion of hydrological pathways in previous field studies may explain why a direct link has not yet been established. The goal of this study was to assess the association between benthic macroinvertebrate community structure and the number and location of agricultural BMPs. Macroinvertebrates and water chemistry were sampled in 30 headwater catchments in the Grand River Watershed. Catchments exhibited gradients of BMP use and location as measured by the degree of hydrologic connectedness. Stepwise ordination regressions and variance partitioning were used to determine which environmental variables (i.e., BMP metrics, water chemistry parameters, habitat characteristics, and land use variables) were associated with benthic macroinvertebrate community structure. Water chemistry parameters were negatively associated with BMP metrics suggesting BMPs were mitigating losses of nutrients and sediments. However, BMP abundance and location explained minimal variation in benthic macroinvertebrate structure within the 30 sampled catchments. The absence of a strong association between BMPs and benthic macroinvertebrates may indicate a need for greater numbers and targeted siting of BMPS to improve water quality beyond a threshold point that would allow recolonization of intolerant invertebrate taxa. Focusing of conservation goals on ecological conditions and the promotion of BMPs that enhance in-stream habitat may also be required.
Headwater streams comprise the majority of the stream network, providing important ecological functions to the downstream network. Although we are beginning to understand how network structure may influence fish, our understanding of how it influences benthic macroinvertebrate dispersal and population connectivity is limited. We also know little about how these patterns and processes may be disrupted as a result of human-driven landscape change such as stream barriers to movement and creation of artificial habitats such as stormwater and farm ponds. In this study, I investigated the effect of stream network position, stream size, and local habitat on benthic macroinvertebrates, and determined to what degree road crossings and impoundments may be degrading benthic macroinvertebrate and fish communities in headwater streams. These mechanisms were explored using Maryland Department of Natural Resources, (MDNR) Maryland Biological Stream Survey (MBSS) benthic macroinvertebrate, fish, and environmental data from first-order streams in the Piedmont region of Maryland. Using an Information Theoretic Approach (ITA), models were developed based on the hypothesized relationships between benthic macroinvertebrate and fish community structure and several network and anthropogenic impact variables. Based on my results, aquatic community structure was dependent on local habitat conditions and stream network structure. Both assemblages responded negatively to roads, which may suggest an isolation effect. These results also suggest that impoundments are acting as sources for benthic macroinvertebrates and fish, including non-native species.
Land cover change strongly affects biodiversity in stream ecosystems, with several studies demonstrating the negative impacts of agricultural and urban expansion on local community richness. However, little is known of the effects of land cover on the variation among sets of local communities in stream networks, as well as the drivers of community variation in these systems. Using the metacommunity framework, this study takes a multi-scale approach to understand how macroinvertebrate communities are assembled across three catchment land cover types; native forest, agricultural and urban. Specifically, the aims of this study are to assess; (1) how stream network land cover influences alpha and beta diversity of macroinvertebrate communities and, (2) the relative role of local environmental conditions and spatial dispersal variables in structuring these communities. Benthic macroinvertebrate samples and local in-stream and riparian environmental variables were collected at 20 sampling sites in each of the six study stream networks in Auckland. Spatial distance proxies of macroinvertebrate dispersal in stream networks were calculated using geospatial techniques. Community alpha and beta diversity, environmental and distance variables were analysed using multivariate statistical techniques. Comparisons showed reference forest and impacted (agricultural and urban) networks supported distinct communities, with lower alpha diversity in the impacted stream networks. Unexpectedly, beta diversity in the impacted networks was greater than, or equal to the reference stream networks, with community dissimilarity almost entirely driven by species turnover. Overall, irrespective of land cover, macroinvertebrate communities were largely structured by local environmental conditions. Benthic substrate and the presence and composition of riparian vegetation were the most significant local environmental variables influencing community composition. Spatial dispersal limitation variables had a small, but significant, effect on inter-site community dissimilarity and overall community structure in each catchment. Network distance between local communities explained the greatest variation in community dissimilarity of the three distance types. This study identified potential drivers of macroinvertebrate community variation in Auckland streams, specifically highlighting the relative role of local environmental and spatial dispersal processes. The results of this study have relevance for biomonitoring and state of environment reporting of Auckland’s freshwater systems, as well as future stream rehabilitation projects.