Carol A. Johnston

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.

Alteration of North American Streams by Beaver

organic matter in the channel, create and maintain wetlands, modify nutrient cycling and decomposition dynamics, modify the structure and dynamics of the riparian zone, influence the character of water and materials transported downstream, and ultimately influence plant and animal community composition and diversity (Naiman and Melillo 1984, Naiman et al. 1986). In addition to their importance at the ecosystem level, these effects have a significant impact on the landscape and must be interpreted over broad spatial and temporal scales as beaver population dynamics shift in response to disturbance, food supply, disease, and predation. Although once more prevalent than they are today, beaver-induced alterations to drainage networks are not localized or unusual. Where beaver

MEETING ECOLOGICAL AND SOCIETAL NEEDS FOR FRESHWATER

Human society has used freshwater from rivers, lakes, groundwater, and wetlands for many different urban, agricultural, and industrial activities, but in doing so has overlooked its value in supporting ecosystems. Freshwater is vital to human life and societal well-being, and thus its utilization for consumption, irrigation, and transport has long taken precedence over other commodities and services provided by freshwater ecosystems. However, there is growing recognition that functionally intact and biologically complex aquatic ecosystems provide many economically valuable services and long-term benefits to society. The short-term benefits include ecosystem goods and services, such as food supply, flood control, purification of human and industrial wastes, and habitat for plant and animal life—and these are costly, if not impossible, to replace. Long-term benefits include the sustained provision of those goods and services, as well as the adaptive capacity of aquatic ecosystems to respond to future environmental alterations, such as climate change. Thus, maintenance of the processes and properties that support freshwater ecosystem integrity should be included in debates over sustainable water resource allocation. The purpose of this report is to explain how the integrity of freshwater ecosystems depends upon adequate quantity, quality, timing, and temporal variability of water flow. Defining these requirements in a comprehensive but general manner provides a better foundation for their inclusion in current and future debates about allocation of water resources. In this way the needs of freshwater ecosystems can be legitimately recognized and addressed. We also recommend ways in which freshwater ecosystems can be protected, maintained, and restored. Freshwater ecosystem structure and function are tightly linked to the watershed or catchment of which they are a part. Because riverine networks, lakes, wetlands, and their connecting groundwaters, are literally the “sinks” into which landscapes drain, they are greatly influenced by terrestrial processes, including many human uses or modifications of land and water. Freshwater ecosystems, whether lakes, wetlands, or rivers, have specific requirements in terms of quantity, quality, and seasonality of their water supplies. Sustainability normally requires these systems to fluctuate within a natural range of variation. Flow regime, sediment and organic matter inputs, thermal and light characteristics, chemical and nutrient characteristics, and biotic assemblages are fundamental defining attributes of freshwater ecosystems. These attributes impart relatively unique characteristics of productivity and biodiversity to each ecosystem. The natural range of variation in each of these attributes is critical to maintaining the integrity and dynamic potential of aquatic ecosystems; therefore, management should allow for dynamic change. Piecemeal approaches cannot solve the problems confronting freshwater ecosystems. Scientific definitions of the requirements to protect and maintain aquatic ecosystems are necessary but insufficient for establishing the appropriate distribution between societal and ecosystem water needs. For scientific knowledge to be implemented science must be connected to a political agenda for sustainable development. We offer these recommendations as a beginning to redress how water is viewed and managed in the United States: (1) Frame national and regional water management policies to explicitly incorporate freshwater ecosystem needs, particularly those related to naturally variable flow regimes and to the linking of water quality with water quantity; (2) Define water resources to include watersheds, so that freshwaters are viewed within a landscape, or systems context; (3) Increase communication and education across disciplines, especially among engineers, hydrologists, economists, and ecologists to facilitate an integrated view of freshwater resources; (4) Increase restoration efforts, using well-grounded ecological principles as guidelines; (5) Maintain and protect the remaining freshwater ecosystems that have high integrity; and (6) Recognize the dependence of human society on naturally functioning ecosystems.

The Potential Importance of Boundaries of Fluvial Ecosystems

Boundaries separating adjacent resource patches are dynamic components of the aquatic landscape. This article addresses some fundamental questions about boundary structure and function in lotic ecosystems. We give examples of longitudinal and lateral boundaries associated with stream systems, demonstrate the application of chaos theory to understanding the inherent variability of boundary properties, and compare characteristics of boundaries in an arctic-tropical transect. We conclude that studies of resource patches, their boundaries, and the nature of exchange with adjacent patches will improve our perspective of drainage basin dynamics over a range of temporal and spatial scales.

The cumulative effect of wetlands on stream water quality and quantity. A landscape approach

A method was developed to evaluate the cumulative effect of wetland mosaics in the landscape on stream water quality and quantity in the nine-county region surrounding Minneapolis—St. Paul, Minnesota. A Geographic Information System (GIS) was used to record and measure 33 watershed variables derived from historical aerial photos. These watershed variables were then reduced to eight principal components which explained 86% of the variance. Relationships between stream water quality variables and the three wetland-related principal components were explored through stepwise multiple regression analysis. The proximity of wetlands to the sampling station was related to principal component two, which was associated with decreased annual concentrations of inorganic suspended solids, fecal coliform, nitrates, specific conductivity, flow-weighted NH4 flow-weighted total P, and a decreased proportion of phosphorus in dissolved form(p < 0.05). Wetland extent was related to decreased specific conductivity, chloride, and lead concentrations. The wetland-related principal components were also associated with the seasonal export of organic matter, organic nitrogen, and orthophosphate. Relationships between water quality and wetlands components were different for time-weighted averages as compared to flow-weighted averages. This suggests that wetlands were more effective in removing suspended solids, total phosphorus, and ammonia during high flow periods but were more effective in removing nitrates during low flow periods.

Beaver influences on the long-term biogeochemical characteristics of boreal forest drainage networks

Beaver (Castor canadensis) affect biogeochemical cycles and the accumu- lation and distribution of chemical elements over time and space by altering the hydrologic regime. Aerial photograph analyses of beaver activities on the 298-km2 Kabetogama Pen- insula, Minnesota, were coupled with site-specific studies of soil and pore water concen- trations of nutrients (nitrogen, phosphorus) and other ions (potassium, calcium, magnesium, iron, sulfate, chloride), nitrogen cycling processes (nitrogen fixation and denitrification), and biophysical environmental variables (vegetation, temperature, organic matter, soil structure, pH, and oxidation-reduction potential). Our analyses demonstrate that beaver influence the distribution, standing stocks, and availability of chemical elements by hy- drologically induced alteration of biogeochemical pathways and by shifting element storage from forest vegetation to sediments and soils. Over the 63 yr of aerial photo records (1927-1988), beaver converted 13% of the peninsula to meadows and ponds. Elemental concentrations in soils (in micrograms per cubic centimetre) and in pore water (in milligrams per litre) revealed complex patterns within and among the principal hydrologic zones (e.g., forest, moist meadow, wet meadow, pond, stream). Principal components analysis (PCA) suggested that anaerobic conditions caused by saturation of soil by water was the fundamental control over subsequent altera- tions of biogeochemical pathways. Although few clear statistical trends were detected for mass- or volume-specific elemental concentrations among habitats, organic horizon (O and A) depths were greatest in the wet meadows and ponds (> 15 cm), causing the standing stocks of chemical elements to be greatest there. We argue that the net effect of beaver activities has been to translocate chemical elements from the originally inundated upland forest vegetation to downstream communities and to pond sediments. As the upland vegetation dies and decays after dam construction, only a portion of the chemical elements are exported downstream (except for calcium and magnesium) or returned to the atmo- sphere (C and N only). Consequently, the organic horizons of pond sediments accumulate substantial standing stocks of chemical elements that are available for vegetative growth when dams fail, the ponds drain, and meadows are formed. Since 1927 beaver activities have augmented the standing stock of chemical elements in the organic horizons by 20- 295%, depending on the element. These influences are spatially extensive and long lasting, affecting fundamental environmental characteristics of boreal forest drainage networks for decades to centuries.

Boundary dynamics at the aquatic-terrestrial interface: The influence of beaver and geomorphology

Beaver (Castor canadensis) impoundments are used to illustrate the effect of large animals on the boundary dynamics of ‘patch bodies’, volumetric landscape units which have surficial boundaries with upper and lower strata, and lateral boundaries with adjacent patches within the same stratum. Patch bodies created by beaver impoundments include the beaver pond, the aerobic soil beneath the pond, and the underlying anaerobic soil. Beaver herbivory in the riparian zone creates an additional patch body concentric to the pond. Beaver and water are the primary biotic and abiotic vectors mediating fluxes across lateral patch body boundaries; vegetation and microbes are the primary biotic vectors mediating fluxes across surficial patch body boundaries. Basin geomorphology affects the permeability of pond boundaries (i.e., their ability to transmit, energy and materials) by affecting the kinetic energy of water, the surface-to-volume ratio of the impoundment, and the movement of beaver between the pond and the riparian foraging zone. We suggest that: (1) permeability of lateral boundaries to abiotic vectors is a function of kinetic energy; (2) within-patch retention of particulate matter transferred by abiotic vectors across lateral boundaries is maximized by a decrease in kinetic energy; (3) lateral patch boundaries between safe refuge and a resource used by an animal vector are most permeable when they are narrow; and (4) total amount of energy and materials transferred across surficial boundaries is maximized by increasing surface area.

Potential feedbacks of northern wetlands on climate change

Atmospheric concentrations of many greenhouse gases, in particular carbon dioxide and methane, are rapidly increasing from preindustrial levels. This paper explores the potential feedback from northern wetlands, particularly with regard to carbon dioxide and methane flux, to climate change. Although progress has been made in determining current trace gas fluxes from major biomass, the authors believe that a mechanistic approach is necessary to predict the direct and indirect feedbacks between climate and northern wetlands. Relevant areas covered in the article include the following: feedback mechanisms; carbon and nutrient mineralization; the kinetics of carbon and nutrient mineralization; methane oxidation and the role of plants; field dynamics of gases in a meadow and bog in Minnesota; and extrapolating to larger scales. 80 refs., 8 figs., 1 tab.

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.

Aquatic Patch Creation in Relation to Beaver Population Trends

The creation of aquatic patches by beaver (Castor canadensis) in the boreal forest of northern Minnesota, USA, was studied to determine how the population dynamics of a disturbance-causing animal are linked to rates of patch formation and growth over a period of population expansion and stabilization. Using six series of aerial photographs taken between 1940 and 1986, we determined the size and growth rates of individual patches, and the numbers, area, density, and establishment rate of the patch population. The rate of patch formation was much higher during the first two decades of colonization than during the subsequent two decades. The average area of all pond sites, which included both filled and drained ponds, remained at z4 ha throughout the time period, but the average area of new ponds decreased significantly over time. Ponds established by 1961 constituted 75% of the total number and 90% of the total pond-site area as of 1986. When pond sites of similar age but different pond cohort (i.e., decade of establishment) were compared, the average area per pond site was always significantly larger for the earlier cohort. Although the rate of pond creation paralleled the increase in number of beaver colonies between 1961 and 1986, the rate of new pond creation prior to 1961 greatly exceeded the increase in number of beaver colonies. We conclude that the rate of patch formation after the first two decades of beaver colonization was constrained by geomorphology, which limited the availability of sites at which a beaver dam could impound a large area of water.