Linking characteristics of parasites to host diet-induced shifts in host-parasite interactions is a critical step in predicting the abundance of specific parasites within an ecosystem. A parasite’s infection route reflects a suite of parasite characteristics that could mediate the effects of host diet quality on host-parasite interactions, but the effect of infection route on the response of host-parasite interactions to host diet quality has not been quantified. We quantified these effects for the first time by experimentally manipulating a parasite capable of transmitting itself via multiple pathways utilized by a wide variety of parasites. We altered the diet phosphorus (P) content of Daphnia infected by the mixed mode transmitter Hamiltosporidian tvaerminnensis via one of two infection routes: horizontal, via the ingestion of spores from dead hosts, or vertical, via the infection of host offspring. Then, we quantified the effect of infection route on host diet-induced shifts in parasite load and virulence (i.e. host fecundity and survival). Overall, we found that horizontal infections were more sensitive to host diet than vertical infections. Low P diets decreased the parasite load and fecundity of all hosts, but had stronger negative effects on the loads of horizontally infected individuals than vertically infected ones. Likewise, horizontal infection reduced the survival of hosts fed low P (but not high P) diets while vertical infection did not affect survival. This increased sensitivity of horizontal infections to diet quality empirically supports the intuitive hypothesis that host diet quality exerts stronger effects on parasites that transmit via ingestion into a host’s gut than on parasites that transmit via routes less directly related to host nutrient uptake. Through these effects, host diet quality may shape the ecology and evolution of parasite communities.

Water-mediated linkages that connect landscape components are collectively referred to as hydrologic connectivity. In river-floodplain systems, quantifying hydrologic connectivity enables descriptions of hydrologic function that emerge from complex, heterogeneous interactions of underlying geomorphic, climatic and biologic controls. Here, we measure hydrologic connectivity using field indicators and develop a continuous connectivity metric that represents a vector strength between a source along the North St Vrain river to ten surface water target sites within the river-floodplain system. To measure this connectivity strength, we analyzed hydrometric, injected conservative tracers, and natural occurring geochemical and microbial indicators across streamflows in 2018. We developed empirical models of hydrologic connectivity as a function of river stage to predict daily connectivity strength across multiple floodplain sites for five years between May and September of 2016-2020. Three sites were either consistently connected or disconnected to the river, while seven varied across time in their hydrologic connectivity strength. Of the sites with variable connectivity, some disconnected very quickly and others had a prolonged disconnection phase. By scaling site dynamics to the system scale, we found across-system hydrologic connectivity always increased with streamflow while across-system variance in hydrologic connectivity peaked at intermediate streamflow. At sites with intermittent connections to the river, river stage disconnection thresholds were variable (308 to 650 mm) and their connectivity dynamics were sensitive to inter-annual variation in streamflows, suggesting that future connectivity behavior under climate change will depend on how flow durations change across a range of flow states.

Headwater forest ecosystems of the western U.S. generate a large portion of the dissolved organic matter (DOM) transported across North America. Land cover changes that alter forest structure and forest species composition affect the quantity and composition of DOM transferred to aquatic ecosystems. Clear-cut harvesting effects ~1% of the forest area of North America annually, leaving most forests in varying stages of successional regrowth, and the total area of old-growth forest decreasing. The consequences of this widespread management practice on watershed carbon cycling remain unknown. We investigated the role of land cover change from old-growth subalpine forest to lodgepole pine dominated second-growth on the character and reactivity of DOM hillslope exports. We evaluated inputs of DOM from litter leachates and export of DOM collected at the base of trenched hillslopes during a three-year period (2016-2018) at the Fraser Experimental Forest in northcentral Colorado, USA. Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) were higher in lateral subsurface flow draining old- versus secondary-growth forest. Fluorescence spectroscopy showed that the DOM exported from the old-growth forest was more heterogeneous and aromatic and that proteinaceous, microbially processed DOM components were more prevalent in the second-growth forest. Biological oxygen demand (BOD) assays revealed much lower microbial metabolism of both DOM inputs from litter leachate and subsurface exports from old-growth forest. Old-growth and second-growth forests are co-mingled in managed ecosystems, and our findings demonstrate that the influence of species composition on DOM inputs can affect the reactivity of DOM transferred from terrestrial to aquatic ecosystems.

INTRODUCTION Biofilms are diverse and complex microbial consortia, and, the biofilm lifestyle is the rule rather than the exception for microbes in many environments. Large and small-scale biofilm architectural features play an important role in their ecology and influence their role in biogeochemical cycles . Fluid mechanics impact biofilm structure and assembly , but it is less clear how other abiotic factors such as resource availability affect biofilm assembly. Aquatic biofilms initiate with seed propagules from the planktonic community . Thus, resource amendments that influence planktonic communities may also influence the recruitment of microbial populations during biofilm community assembly. In a crude sense, biofilm and planktonic microbial communities divide into two key groups: oxygenic phototrophs including eukaryotes and cyanobacteria (hereafter “photoautotrophs”), and heterotrophic bacteria and archaea. This dichotomy, admittedly an abstraction (e.g. non-phototrophs can also be autotrophs), can be a powerful paradigm for understanding community shifts across ecosystems of varying trophic state . Heterotrophs meet some to all of their organic carbon (C) requirements from photoautotroph produced C while simultaneously competing with photoautotrophs for limiting nutrients such as phosphorous (P) . The presence of external C inputs, such as terrigenous C leaching from the watershed or C exudates derived from macrophytes , can alleviate heterotroph reliance on photoautotroph derived C and shift the heterotroph-photoautotroph relationship from commensal and competitive to strictly competitive . Therefore, increased C supply should increase the resource space available to heterotrophs and increase competition for mineral nutrients decreasing nutrients available for photoautotrophs (assuming that heterotrophs are superior competitors for limiting nutrients as has been observed ). These dynamics should result in the increase in heterotroph biomass relative to the photoautotroph biomass along a gradient of increasing labile C inputs. We refer to this differential allocation of limiting resources among components of the microbial community as niche partitioning. While these gross level dynamics have been discussed conceptually and to some extent demonstrated empirically , the effects of biomass dynamics on photoautotroph and heterotroph membership and structure has not been directly evaluated in plankton or biofilms. In addition, how changes in planktonic communities propagate to biofilms during community assembly is not well understood. We designed this study to test if C subsidies shift the biomass balance between autotrophs and heterotrophs within the biofilm or its seed pool (i.e. the plankton), and, to measure how changes in biomass pool size alter composition of the plankton and biofilm communities. Specifically, we amended marine mesocosms with varying levels of labile C input and evaluated differences in photoautotroph and heterotrophic bacterial biomass in plankton and biofilm samples along the C gradient. In each treatment we characterized plankton and biofilm community composition by PCR amplifying and DNA sequencing 16S rRNA genes and plastid 23S rRNA genes.