C. Lisa Dent

Biogeochemical Hot Spots and Hot Moments at the Interface of Terrestrial and Aquatic Ecosystems

Rates and reactions of biogeochemical processes vary in space and time to produce both hot spots and hot moments of elemental cycling. We define biogeochemical hot spots as patches that show disproportionately high reaction rates relative to the surrounding matrix, whereas hot moments are defined as short periods of time that exhibit disproportionately high reaction rates relative to longer intervening time periods. As has been appreciated by ecologists for decades, hot spot and hot moment activity is often enhanced at terrestrial-aquatic interfaces. Using examples from the carbon (C) and nitrogen (N) cycles, we show that hot spots occur where hydrological flowpaths converge with substrates or other flowpaths containing complementary or missing reactants. Hot moments occur when episodic hydrological flowpaths reactivate and/or mobilize accumulated reactants. By focusing on the delivery of specific missing reactants via hydrologic flowpaths, we can forge a better mechanistic understanding of the factors that create hot spots and hot moments. Such a mechanistic understanding is necessary so that biogeochemical hot spots can be identified at broader spatiotemporal scales and factored into quantitative models. We specifically recommend that resource managers incorporate both natural and artificially created biogeochemical hot spots into their plans for water quality management. Finally, we emphasize the needs for further research to assess the potential importance of hot spot and hot moment phenomena in the cycling of different bioactive elements, improve our ability to predict their occurrence, assess their importance in landscape biogeochemistry, and evaluate their utility as tools for resource management.

SPATIAL HETEROGENEITY OF STREAM WATER NUTRIENT CONCENTRATIONS OVER SUCCESSIONAL TIME

Nutrient availability in ecosystems is patchy both in space and in time. Whereas temporal trends have often been studied, less information exists on spatial patterns of nutrient availability, particularly in aquatic ecosystems. The goals of this study were (1) to describe and quantify patterns of nutrient concentration in surface waters of an arid land stream and (2) to compare spatial patterns of nutrient availability across nutrients and over a successional sequence. We describe changes in the spatial pattern of stream water nutrient concentrations over successional time (between floods) using quantitative measures of heterogeneity. Samples were collected from the middle of the channel every 25 m over a 10-km section of a Sonoran Desert stream during three periods: early succession (2 wk post-flood), middle succession (2 mo post-flood), and late succession (9 mo post-flood). Nutrient concentrations were extremely variable in space (coefficients of variations as high as 145%). Coefficients of variation increased over successional time and were consistently greater for nitrate-nitrogen than for soluble reactive phosphorus. Semi-variogram analysis showed that nutrient con- centrations were spatially dependent on all dates, but to different degrees and over different distances. The distance over which nutrient concentrations were spatially dependent, as measured by the semi-variogram range, tended to decrease with successional time. The strength of spatial dependence, as measured by the slope of the ascending limb of the semi- variogram, increased with successional time. The limiting nutrient, nitrogen, was consis- tently more spatially heterogeneous than phosphorus or conductivity, both in terms of patch size (range) and strength of spatial dependence. In streams, downstream transport combined with nutrient transformation produces patch- es of similar nutrient concentrations that are elongated compared with nutrient patches in terrestrial soils. Variation in nutrient concentration is likely to affect the spatial distribution of organisms and rates of ecosystem processes.

Spatial Extrapolation: The Science of Predicting Ecological Patterns and Processes

Abstract Ecologists are often asked to contribute to solutions for broadscale problems. The extent of most ecological research is relatively limited, however, necessitating extrapolation to broader scales or to new locations. Spatial extrapolation in ecology tends to follow a general framework in which (a) the objectives are defined and a conceptual model is derived; (b) a statistical or simulation model is developed to generate predictions, possibly entailing scaling functions when extrapolating to broad scales; and (c) the results are evaluated against new data. In this article, we examine the application of this framework in a variety of contexts, using examples from the scientific literature. We conclude by discussing the challenges, limitations, and future prospects for extrapolation.

Merging aquatic and terrestrial perspectives of nutrient biogeochemistry.

Although biogeochemistry is an integrative discipline, terrestrial and aquatic subdisciplines have developed somewhat independently of each other. Physical and biological differences between aquatic and terrestrial ecosystems explain this history. In both aquatic and terrestrial biogeochemistry, key questions and concepts arise from a focus on nutrient limitation, ecosystem nutrient retention, and controls of nutrient transformations. Current understanding is captured in conceptual models for different ecosystem types, which share some features and diverge in other ways. Distinctiveness of subdisciplines has been appropriate in some respects and has fostered important advances in theory. On the other hand, lack of integration between aquatic and terrestrial biogeochemistry limits our ability to deal with biogeochemical phenomena across large landscapes in which connections between terrestrial and aquatic elements are important. Separation of the two approaches also has not served attempts to scale up or to estimate fluxes from large areas based on plot measurements. Understanding connectivity between the two system types and scaling up biogeochemical information will rely on coupled hydrologic and ecological models, and may be critical for addressing environmental problems associated with locally, regionally, and globally altered biogeochemical cycles.

Multiscale effects of surface–subsurface exchange on stream water nutrient concentrations

Stream–riparian ecosystems are landscapes composed of dynamic interacting terrestrial and aquatic patches. Patch composition and configuration affects both the form of transported materials and the amount of nutrient retention and export. We describe spatial patterns of nutrients in the surface water of an arid-land stream using surveys conducted at 3 different scales, ranging from 30 m to 10 km in extent and from 1 m to 25 m in grain. We then relate these patterns to connections with subsurface patches at channel subunit, channel unit, and reach scales. Our objectives were to compare spatial variation in nutrients across scales, to determine the causes of downstream changes in nutrient concentration in terms of intervening patches, and to investigate whether subsurface patches at different scales behaved similarly in terms of net nutrient processing. Nutrients varied spatially at all scales sampled. The highest variation was observed in nitrate-N (NO3-N) in the survey with the smallest grain (CV = 161%) and the lowest was observed in soluble reactive P (SRP) in the same survey (CV = 17%). We hypothesized that downstream changes in nutrient concentrations were caused by upwelling of high-nutrient water from the subsurface. To test this hypothesis, we identified locations of hydrologic inputs to surface water from the subsurface using geomorphic features of the stream such as gravel bar edges (channel subunit scale), riffle-run transitions (channel unit scale), and permanent groundwater sources (reach scale). As surface water passed over these locations, nutrient concentrations generally increased, particularly during late succession when subsurface patches acted as sources of NO3-N at all 3 scales and as sources of SRP at the channel unit and reach scales. A hierarchical approach allowed us to decompose effects of subsurface upwellings at different scales and to consider interactions between them. Processes occurring in subsurface patches influenced surface water nutrient patterns at scales from a few meters to several kilometers.

VARIABILITY OF LAKES ON THE LANDSCAPE: ROLES OF PHOSPHORUS, FOOD WEBS, AND DISSOLVED ORGANIC CARBON

In northern temperate lakes, algal abundance or chlorophyll levels are af- fected by phosphorus loading (P), dissolved organic carbon (DOC), and food web effects from trophic cascades induced by anglers. To investigate how changes in land use and climate might affect future chlorophyll conditions in these lakes, we created a nonlinear model for lake chlorophyll that considers the effects of these factors. Parameters were estimated for northern Wisconsin lakes. We show that resilience of the clear-water state in a single lake is maximized when P inputs are low, DOC is high, and angler pressure is low. We simulated a population of lakes to understand the current distribution of chlorophyll and resilience across lakes in the landscape. Under current conditions of land and lake use in the area, the model indicates that most lakes in the region are resilient clear-water lakes. Low chlorophyll levels, however, do not guarantee resiliency. Resilience shows a bimodal distribution suggesting that, with stochastic shocks or changing conditions, more lakes could shift to a high chlorophyll state that is costly to remediate. We also simulated a limnological comparative study to determine what conclusions would be drawn from a common research method if lacustrine ecosystem dynamics are indeed faithfully generated by our model. We show that phosphorus input will most often appear to be the most significant driver of lake chlorophyll levels, despite the fact that all mechanisms (including DOC and grazing) drive the dynamics. This finding suggests that long-standing debates in limnology about the primary drivers of algal abundance are explainable by differences in research approaches. This work brings together community and ecosystem ecology and shows how their processes can interact to drive higher-order feedbacks.

Modelling nutrient-periphyton dynamics in streams with surface-subsurface exchange

Abstract Stream ecosystems include both surface and subsurface components that are connected by the flow of water. Processes occurring in subsurface sediments affect those in the surface, and vice versa. A model was developed to investigate the effects of nutrient transformations occurring in subsurface sediments on the growth of surface-dwelling periphyton. In the model the stream is divided into three zones: free-flowing surface water, a surface storage zone where flow is minimal and where periphyton growth occurs, and a subsurface zone. Parameters were based on information from Sycamore Creek, Arizona, a nitrogen-limited desert stream that has been extensively studied. The behavior of the model was examined both at steady state and as it approached steady state after periphyton biomass was reduced to low values, simulating the effects of a scouring flood. Previous work has shown that subsurface sediments are a source of inorganic nutrients, mainly nitrogen, to surface water and to periphyton communities in Sycamore Creek. Thus it was expected that in this model, periphyton biomass would increase when exchange between surface and subsurface zones was increased, and biomass would also increase when the rate of nutrient transformation in subsurface sediments was increased. Model results confirmed the expectations and highlighted the important role of organic nitrogen in mediating the periphyton-nutrient feedback. Post-flood recovery of periphyton biomass was particularly sensitive to elevated concentrations of inorganic nitrogen in flood water. The model shows that processes in the subsurface zone can have a large effect on surface organisms, and illustrates how surface periphyton communities interact with subsurface microorganisms through the recycling of nutrients.