Steven A. Thomas

Local adaptation in Trinidadian guppies alters ecosystem processes

Theory suggests evolutionary change can significantly influence and act in tandem with ecological forces via ecological-evolutionary feedbacks. This theory assumes that significant evolutionary change occurs over ecologically relevant timescales and that phenotypes have differential effects on the environment. Here we test the hypothesis that local adaptation causes ecosystem structure and function to diverge. We demonstrate that populations of Trinidadian guppies (Poecilia reticulata), characterized by differences in phenotypic and population-level traits, differ in their impact on ecosystem properties. We report results from a replicated, common garden mesocosm experiment and show that differences between guppy phenotypes result in the divergence of ecosystem structure (algal, invertebrate, and detrital standing stocks) and function (gross primary productivity, leaf decomposition rates, and nutrient flux). These phenotypic effects are further modified by effects of guppy density. We evaluated the generality of these effects by replicating the experiment using guppies derived from two independent origins of the phenotype. Finally, we tested the ability of multiple guppy traits to explain observed differences in the mesocosms. Our findings demonstrate that evolution can significantly affect both ecosystem structure and function. The ecosystem differences reported here are consistent with patterns observed across natural streams and argue that guppies play a significant role in shaping these ecosystems.

Fish distributions and nutrient cycling in streams: can fish create biogeochemical hotspots?

Rates of biogeochemical processes often vary widely in space and time, and characterizing this variation is critical for understanding ecosystem functioning. In streams, spatial hotspots of nutrient transformations are generally attributed to physical and microbial processes. Here we examine the potential for heterogeneous distributions of fish to generate hotspots of nutrient recycling. We measured nitrogen (N) and phosphorus (P) excretion rates of 47 species of fish in an N-limited Neotropical stream, and we combined these data with population densities in each of 49 stream channel units to estimate unit- and reach-scale nutrient recycling. Species varied widely in rates of N and P excretion as well as excreted N:P ratios (6-176 molar). At the reach scale, fish excretion could meet >75% of ecosystem demand for dissolved inorganic N and turn over the ambient NH4 pool in <0.3 km. Areal N excretion estimates varied 47-fold among channel units, suggesting that fish distributions could influence local N availability. P excretion rates varied 14-fold among units but were low relative to ambient concentrations. Spatial variation in aggregate nutrient excretion by fish reflected the effects of habitat characteristics (depth, water velocity) on community structure (body size, density, species composition), and the preference of large-bodied species for deep runs was particularly important. We conclude that the spatial distribution of fish could indeed create hotspots of nutrient recycling during the dry season in this species-rich tropical stream. The prevalence of patchy distributions of stream fish and invertebrates suggests that hotspots of consumer nutrient recycling may often occur in stream ecosystems.

COUPLED CYCLING OF DISSOLVED ORGANIC NITROGEN AND CARBON IN A FOREST STREAM

Dissolved organic nitrogen (DON) is an abundant but poorly understood pool of N in many ecosystems. We assessed DON cycling in a N-limited headwater forest stream via whole-ecosystem additions of dissolved inorganic nitrogen (DIN) and labile dissolved organic matter (DOM), hydrologic transport and biogeochemical modeling, and laboratory experiments with native sediments. We sampled surface and subsurface waters to understand how interaction among hydrologic exchange, DIN, DON, and dissolved organic carbon (DOC) influence stream N losses at summer baseflow. Added DON was taken up rapidly from the water column at rates exceeding DOC and DIN. A significant fraction of this DON was mineralized and nitrified. Combined DON and NO 3-N uptake lengths resulted in spiraling lengths of ;210 m, suggesting the potential for multiple transformations of labile N loads within catchment boundaries. Simultaneous addition of DIN increased DOM uptake, but more so for C, resulting in an upward shift in the C:N ratio of uptake. Sediment incubations also showed a strong biotic influence on DOC and DON dynamics. Despite efficient uptake of added DOM, background DON and high mo- lecular mass DOC concentrations increased downstream, resulting in higher DOM loads than could be accounted for by groundwater discharge and suggesting net release of less bioavailable forms from the channel/hyporheic zone. At the same time, subsurface DOM was characterized by very low C:N ratios and a disproportionately large DON pool despite rapid hydrologic mixing with dilute and high C:N ratio surface waters. Analysis of expected DON loads from conservative hyporheic fluxes indicated that watershed losses of DON would have been seven times greater in the absence of apparent benthic demand, suggesting tight internal cycling of subsurface DON. Our study further demonstrates the potential for significant transformation of N in headwater streams before export to downstream ecosys-

Coupling Nutrient Uptake and Energy Flow in Headwater Streams

Nutrient cycling and energy flow in ecosystems are tightly linked through the metabolic processes of organisms. Greater uptake of inorganic nutrients is expected to be associated with higher rates of metabolism [gross primary production (GPP) and respiration (R)], due to assimilatory demand of both autotrophs and heterotrophs. However, relationships between uptake and metabolism should vary with the relative contribution of autochthonous and allochthonous sources of organic matter. To investigate the relationship between metabolism and nutrient uptake, we used whole-stream and benthic chamber methods to measure rates of nitrate–nitrogen (NO3–N) uptake and metabolism in four headwater streams chosen to span a range of light availability and therefore differing rates of GPP and contributions of autochthonous carbon. We coupled whole-stream metabolism with measures of NO3–N uptake conducted repeatedly over the same stream reach during both day and night, as well as incubating benthic sediments under both light and dark conditions. NO3–N uptake was generally greater in daylight compared to dark conditions, and although day-night differences in whole-stream uptake were not significant, light–dark differences in benthic chambers were significant at three of the four sites. Estimates of N demand indicated that assimilation by photoautotrophs could account for the majority of NO3–N uptake at the two sites with relatively open canopies. Contrary to expectations, photoautotrophs contributed substantially to NO3–N uptake even at the two closed-canopy sites, which had low values of GPP/R and relied heavily on allochthonous carbon to fuel R.

Physical factors influencing fine organic particle transport and deposition in streams

The influence of physical factors on the transport and deposition of fine particulate organic matter (FPOM, 53–106 μm) in streams was investigated using 14C-labeled natural detritus. Field estimates of mean FPOM transport distance (SP) were calculated as the inverse of the longitudinal loss rate (kP) of particles. Deposition was determined by standardizing kP for depth and velocity and expressed as the mass transfer coefficient, vdep. SP varied by orders of magnitude (7–1000 m) within and among streams. As expected, vdep behaved more conservatively than SP but still varied by a factor of 18 (0.06–1.10 mm/s). Field-estimated deposition velocities were always less than the quiescent water fall velocity (vfall) but no consistent relationship existed between the 2 (r = 0.26, p = 0.53). Variability in SP was strongly associated with the cross-sectional area of the transient storage zone (AS; r = 0.93, p < 0.01) and the uptake length of water (SW; r = 0.84, p = 0.01). The transfer coefficient was highest in the 2 smallest streams (Q < 15 L/s) but was similar, and unrelated to stream size, among 6 experiments conducted in stream segments where Q exceeded 100 L/s. Variability in FPOM vdep was less related to physical characteristics than SP, although a significant, positive correlation was detected between vdep and AS/A. Evidence from this study suggests that the mechanisms assumed to govern particle transport in gravitational/hydrodynamic models may not be solely responsible for FPOM deposition in streams and that alternative processes, such as hyporheic filtration and biotic retention, may be important.

Epac signaling is required for hippocampus-dependent memory retrieval

Previously we uncovered a critical role for norepinephrine and β1-adrenergic signaling in hippocampus-dependent memory retrieval. Because the β1 receptor couples to Gs, we examine here whether cAMP is also required for contextual memory retrieval. Using pharmacologic and genetic approaches to manipulate cAMP and downstream signaling, we demonstrate that cAMP and two of its targets, protein kinase A (PKA) and exchange protein activated by cAMP (Epac), are both required for retrieval. These findings demonstrate that cAMP signaling through Epac (as well as PKA) plays an essential role in cognition.

Effects of light on NO3− uptake in small forested streams: diurnal and day-to-day variations

Abstract We investigated the effects of autotrophy on short-term variations in nutrient dynamics by measuring diurnal and day-to-day variations in light level, primary productivity, and NO3− uptake during early and late spring in 2 forested streams, the East and West Forks of Walker Branch in eastern Tennessee, USA. We predicted that diurnal and day-to-day variations in NO3− uptake rate would be larger in the West Fork than in the East Fork in early spring because of higher rates of primary productivity resulting from a more stable substratum in the West Fork. We also predicted minimal diurnal variations in both streams in late spring after forest leaf emergence when light levels and primary productivity are uniformly low. Reach-scale rates of gross primary production (GPP) were determined using the diurnal dissolved O2 change technique, and reach-scale rates of NO3− uptake were determined by tracer 15N-NO3− additions. In the West Fork, significant diurnal and day-to-day variations in NO3− uptake were related to variations in light level and primary productivity in early spring but not in late spring, consistent with our predictions. In early spring, West Fork NO3− uptake rates were 2 to 3× higher at midday than during predawn hours and 50% higher on 2 clear days than on an overcast day several days earlier. In the East Fork, early spring rates of GPP were 4 to 5× lower than in the West Fork and diurnal and day-to-day variations in NO3− uptake rates were <30%, considerably lower than in the West Fork. However, diurnal variations in NO3− uptake rates were greater in late spring in the East Fork, possibly because of diurnal variation in water temperature. Our results indicate the important role of autotrophs in nutrient uptake in some forested streams, particularly during seasons when forest vegetation is dormant and light levels are relatively high. Our results also have important implications for longer-term assessments of N cycling in streams that rely on daytime measurements or measurements only under limited weather conditions (i.e., clear days).

A regression approach to estimating reactive solute uptake in advective and transient storage zones of stream ecosystems

A method is developed, the Regression Partitioning Method (RPM), for estimating the proportion of reactive solute uptake occurring within transient storage zones of streams. The RPM is a technique for analyzing solute addition data in which whole stream uptake (mg m � 2 d � 1 ) is determined from the longitudinal pattern in plateau tracer concentrations. At one location, a time series of samples are collected that define the rising limb of the solute breakthrough curve. The y-intercept estimated by regressing a measure of reactive tracer availability (e.g. NO3– 15 N:Cl ratio) and the percentage of tracer that has resided within, and returned from, the transient storage zone (i.e. hyporheic zone) was used to predict channel-specific NO3 uptake rates. Uptake within the transient storage zone of stream-derived material is calculated by difference. Several numerical steps are developed that link uptake rate estimates to first-order reaction rate constants (kC and kS, min � 1 ) more commonly used to describe solute behavior in onedimensional transport models. The RPM was used to analyze the results of 2 stable isotope additions performed in Snake Den Branch, a small headwater stream in western North Carolina, USA. Channel-specific uptake rates (UC) ranged from 10.6 to 23.0 mg NO3–N m � 2 d � 1 and slightly exceeded uptake in the transient storage zone (US), which varied from 10.1 to 18.2 mg NO3–N m � 2 d � 1 . Uptake within the transient storage zone accounted for 44–49% of the total uptake. kC and kS estimates ranged from 0.023 to 0.034 min � 1 and 0.011 to 0.024 min � 1 , respectively. These processing rates correspond to solute residence times of 30–44 min and 41–90 min in the channel and storage zones, respectively. Finally, we assess the sensitivity of our approach to variation in the subsurface uptake coefficient and differing proportions of uptake occurring within the hyporheic zone. � 2003 Elsevier Ltd. All rights reserved.

ENDOGENOUS AND EXOGENOUS CONTROL OF ECOSYSTEM FUNCTION: N CYCLING IN HEADWATER STREAMS

Allochthonous inputs act as resource subsidies to many ecosystems, where they exert strong influences on metabolism and material cycling. At the same time, metabolic theory proposes endogenous thermal control independent of resource supply. To address the relative importance of exogenous and endogenous influences, we quantified spatial and temporal variation in ecosystem metabolism and nitrogen (N) uptake using seasonal releases of 15N as nitrate in six streams differing in riparian-stream interaction and metabolic character. Nitrate removal was quantified using a nutrient spiraling approach based on measurements of downstream decline in 15N flux. Respiration (R) and gross primary production (GPP) were measured with whole-stream diel oxygen budgets. Uptake and metabolism metrics were addressed as z scores relative to site means to assess temporal variation. In open-canopied streams, areal uptake (U; microg N x m(-2) x s(-1)) was closely related to GPP, metabolic rates increased with temperature, and R was accurately predicted by metabolic scaling relationships. In forested streams, N spiraling was not related to GPP; instead, uptake velocity (v(f); mm/s) was closely related to R. In contrast to open-canopied streams, N uptake and metabolic activity were negatively correlated to temperature and poorly described by scaling laws. We contend that streams differ along a gradient of exogenous and endogenous control that relates to the relative influences of resource subsidies and in-stream energetics as determinants of seasonal patterns of metabolism and N cycling. Our research suggests that temporal variation in the propagation of ecological influence between adjacent systems generates phases when ecosystems are alternatively characterized as endogenously and exogenously controlled.

Atmospheric N Deposition Increases Organic N Loss from Temperate Forests

Atmospheric deposition of nitrogen (N) resulting from fossil fuel combustion has increased N inputs to temperate forests worldwide with large consequences for forest productivity and water quality. Recent work has illustrated that dissolved organic N (DON) often dominates N loss from unpolluted forests and that the relative magnitude of dissolved inorganic N (DIN) loss increases with atmospheric loading. In contrast to DIN, DON loss is thought to be controlled by soil dynamics that operate independently of N supply and demand and thus should track dissolved organic carbon (DOC) following strict stoichiometric constraints. Conversely, DON loss may shift with N supply if soil (SOM) or dissolved organic matter (DOM) is stoichiometrically altered. Here, we assess these two explanations of DON loss, which we refer to as the Passive Carbon Vehicle and the Stoichiometric Enrichment hypotheses, by analyzing patterns in soil and stream C and N in forest watersheds spanning a broad gradient in atmospheric N loading (5–45 kg N ha−1 y−1). We show that soil N and DON losses are not static but rather increase asymptotically with N loading whereas soil C and DOC do not, resulting in enrichment of organic N expressed as decreased soil C:N and stream DOC:DON ratios. DON losses from unpolluted sites are consistent with conservative dissolution and transport of refractory SOM. As N supply increases, however, N enrichment of organic losses is greater than expected from simple dissolution of enriched soils, suggesting activation of novel pathways of DON production or direct N enrichment of DOM. We suggest that our two hypotheses represent domains of control over forest DON loss as N supply increases but also that stoichiometric enrichment of bulk soils alone cannot fully account for large DON losses in the most N-polluted forests.