Stuart E. G. Findlay

Stream denitrification across biomes and its response to anthropogenic nitrate loading

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20–25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.

Transformation of Freshwater Ecosystems by Bivalves

B ivalves (clams and mussels) are among the most familiar of aquatic organisms. Many have been used by humans for centuries as important sources of food and ornament, and some species are economically important pests, fouling water intakes and other structures. It is only recently, however, that ecologists have begun to understand that bivalves also play many important roles in ecosystems (e.g., Dame 1996). The functional importance of bivalves, especially in fresh water, is still not fully appreciated. For example, recent fresh water ecology I textbooks (Wetzel 1983, Horne and Goldman 1994, Allan 1995, Petts and Calow 1996) scarcely mention the ecological roles of bivalves (the words “bivalve, ” “clam,” and “mussel” do not even appear in the index of any of these books). By contrast,

Aquatic ecosystems: interactivity of dissolved organic matter

Preface. Section I: Sources and Composition Supply of DOM to Aquatic Ecosystems: Autochthonous Sources. Sources, Production and Regulation of Allochthonous Dissolved Organic Matter Inputs to Surface Waters. Trace Organic Moieties of Dissolved Organic Material in Natural Waters. The Role of Monomers in Stream Ecosystem Metabolism. Molecular Indicators of the Bioavailability of Dissolved Organic Matter. Large-Scale Patterns in DOC Concentration, Flux, and Sources. The Speciation of Hydrophobic Organic Compounds by Dissolved Organic Matter. Elemental Complexation by Dissolved Organic Matter in Lakes: Implications for Fe Speciation and the Bioavailability of Fe and P. Section II: Transformation and Regulation The Contribution of Monomers and Other Low Molecular Weight Compounds to the Flux of DOM in Aquatic Ecosystems. Photochemically-Mediated Linkages Between Dissolved Organic Matter and Bacterioplankton. The Importance of Organic Nitrogen Production in Aquatic Systems: A Landscape Perspective. The Role of Biofilms in the Uptake and Transformation of Dissolved Organic Matter. Microbial Extracellular Enzymes and Their Role in DOM Cycling. Linkages between DOM Composition and Bacterial Community Structure. Bacterial Response to Variation in Dissolved Organic Matter. Section III: Approaches to Synthesis Physiological Models in the Context of Microbial Food Webs. Patterns in DOM Iability and Consumption across Aquatic Systems. Integrating DOM Metabolism and Microbial Diversity: An Overview of Conceptual Models. Dissolved Organic Carbon: Detrital Energetics, Metabolic Regulators, and Drivers of Ecosystem Stability of Aquatic Ecosystems. Synthesis.

ZEBRA MUSSEL INVASION IN A LARGE, TURBID RIVER: PHYTOPLANKTON RESPONSE TO INCREASED GRAZING

Changes in the biomass of benthic bivalves can cause dramatic changes in total grazing pressure in aquatic systems, but few studies document ecosystem-level impacts of these changes. This study documents a massive decline in phytoplankton biomass con- current with the invasion of an exotic benthic bivalve, the zebra mussel ( Dreissena poly- morpha), and demonstrates that the zebra mussel actually caused this decline. In the fall of 1992 the zebra mussel became established at high biomass in the Hudson River Estuary, and biomass of mussels remained high during 1993 and 1994. During these 2 yr, grazing pressure on phytoplankton was over 10-fold greater than it had been prior to the zebra mussel invasion. This increased grazing was associated with an 85% decline in phyto- plankton biomass. Between 1987 and 1991 (pre-invasion), summertime chlorophyll aver- aged 30 mg/m 3 ; during 1993 and 1994 summertime concentrations were ,5 mg/m 3 . Over this same period, light availability increased, phosphate concentrations doubled, some planktonic grazers declined, and average flow was not different from the pre-invasion period. Thus, changes in these other factors were not responsible for phytoplankton declines. We developed a mechanistic model that reproduces the spatial and temporal dynamics of phytoplankton prior to the invasion of the zebra mussel (1987-1991). The model ac- curately predicts extreme declines in phytoplankton biomass after the invasion (1993-1994). The model demonstrates that zebra mussel grazing was sufficient to cause the observed phytoplankton decline. The model also allows us to test which features make the Hudson River sensitive to the impact of benthic grazers. The model suggests that the fate of light- scattering inorganic particles (turbidity) is a key feature determining the impact of benthic grazers in aquatic systems.

Terrestrial inputs of organic matter to coastal ecosystems: An intercomparison of chemical characteristics and bioavailability

Dissolved and particulate organic matter (DOM and POM) collected from rivers or groundwater feeding five estuaries along the east and west coasts of the USA were characterized with a variety of biogeochemical techniques and related to bioavailability to estuarine microbes. Surface water was sampled from the Columbia, Satilla, Susquehanna and Parker Rivers and groundwater was sampled from the Childs River. Several geochemical descriptors (percent organic matter of suspended particulate matter, C/N, lignin phenol content, ratio of vanillic acid to vanillin) suggested an ordering of the systems with respect to POM lability: Satilla < Parker < Columbia < Susquehanna.DOC concentrations in these systems ranged from <100 μM for the Columbia River to >2000 μM for the Satilla River. Elemental analysis of DOM concentrates (>1000 D) was used to predict organic matter composition and to calculate degree of substrate reduction using two different modeling approaches. Models predicted aliphatic carbon ranging between 43 and 60% and aromatic carbon between 26 and 36%, with aliphatic content lowest in the Satilla and highest in the Columbia River. The degree of substrate reduction of the organic matter concentrates followed a pattern similar to that for aliphatic C, being lowest in the Satilla (3.5) and highest in the Columbia (4.0). Extracellular enzyme activity varied broadly across the systems, but again ordered sites in the same way as did aliphatic content and degree of substrate reduction. Bacterial growth rates ranged from 1.3 ug mg-1 d-1 DOC in the Satilla to 1.7 ug mg-1 d-1 DOC in the Parker River. Bioassays confirmed patterns of dissolved organic matter lability predicted by the chemical models. Between 67% to 75% of the variation in bacterial growth could be explained by differences in organic matter composition.

Can't See the Forest for the Stream? In-stream Processing and Terrestrial Nitrogen Exports

Abstract There has been a long-term decline in nitrate (NO3−) concentration and export from several long-term monitoring watersheds in New England that cannot be explained by current terrestrial ecosystem models. A number of potential causes for this nitrogen (N) decline have been suggested, including changes in atmospheric chemistry, insect outbreaks, soil frost, and interannual climate fluctuations. In-stream removal of NO3− has not been included in current attempts to explain this regional decline in watershed NO3− export, yet streams may have high removal rates of NO3−. We make use of 40 years of data on watershed N export and stream N biogeochemistry from the Hubbard Brook Experimental Forest (HBEF) to determine (a) whether there have been changes in HBEF stream N cycling over the last four decades and (b) whether these changes are of sufficient magnitude to help explain a substantial proportion of the unexplained regional decline in NO3− export. Examining how the tempos and modes of change are distinct for upland forest and stream ecosystems is a necessary step for improving predictions of watershed exports.

A Cross-System Comparison of Bacterial and Fungal Biomass in Detritus Pools of Headwater Streams

The absolute amount of microbial biomass and relative contribution of fungi and bacteria are expected to vary among types of organic matter (OM) within a stream and will vary among streams because of differences in organic matter quality and quantity. Common types of benthic detritus [leaves, small wood, and fine benthic organic matter (FBOM)] were sampled in 9 small (1st-3rd order) streams selected to represent a range of important controlling factors such as surrounding vegetation, detritus standing stocks, and water chemistry. Direct counts of bacteria and measurements of ergosterol (a fungal sterol) were used to describe variation in bacterial and fungal biomass. There were significant differences in bacterial abundance among types of organic matter with higher densities per unit mass of organic matter on fine particles relative to either leaves or wood surfaces. In contrast, ergosterol concentrations were significantly greater on leaves and wood, confirming the predominance of fungal biomass in these larger size classes. In general, bacterial abundance per unit organic matter was less variable than fungal biomass, suggesting bacteria will be a more predictable component of stream microbial communities. For 7 of the 9 streams, the standing stock of fine benthic organic matter was large enough that habitat-weighted reach-scale bacterial biomass was equal to or greater than fungal biomass. The quantities of leaves and small wood varied among streams such that the relative contribution of reach-scale fungal biomass ranged from 10% to as much as 90% of microbial biomass. Ergosterol concentrations were positively associated with substrate C:N ratio while bacterial abundance was negatively correlated with C:N. Both these relationships are confounded by particle size, i.e., leaves and wood had higher C:N than fine benthic organic matter. There was a weak positive relationship between bacterial abundance and streamwater soluble reactive phosphorus concentration, but no apparent pattern between either bacteria or fungi and streamwater dissolved inorganic nitrogen. The variation in microbial biomass per unit organic matter and the relative abundance of different types of organic matter contributed equally to driving differences in total microbial biomass at the reach scale.

Increased carbon transport in the Hudson River: unexpected consequence of nitrogen deposition?

Dissolved organic carbon (DOC) is the major form of organic matter transported by large rivers and represents an important movement of carbon from terrestrial to coastal systems. Concentrations of DOC in New Yorks Hudson River have doubled over the past 16 years, implying a substantial increase in net movement of organic carbon from the watershed to New York Harbor and Bight. During the same time period there has been a decline in net consumption of DOC during travel through the tidal freshwater portion of the Hudson. This reduced removal of DOC amplifies the apparent increased load of DOC from the Hudsons watershed, resulting in an overall doubling of DOC delivery to the lower reaches. Either factor changing in isolation could cause a substantial increase in carbon delivery, but the two acting in concert suggest a fundamental change in processes supplying carbon to surface waters. Temperature, water yield, and land cover have not changed in ways that would make these viable causes for the altered DOC. ...

Ecology of freshwater shore zones

Freshwater shore zones are among the most ecologically valuable parts of the planet, but have been heavily damaged by human activities. Because the management and rehabilitation of freshwater shore zones could be improved by better use of ecological knowledge, we summarize here what is known about their ecological functioning. Shore zones are complexes of habitats that support high biodiversity, which is enhanced by high physical complexity and connectivity. Shore zones dissipate large amounts of physical energy, can receive and process extraordinarily high inputs of autochthonous and allochthonous organic matter, and are sites of intensive nutrient cycling. Interactions between organic matter inputs (including wood), physical energy, and the biota are especially important. In general, the ecological character of shore zone ecosystems is set by inputs of physical energy, geologic (or anthropogenic) structure, the hydrologic regime, nutrient inputs, the biota, and climate. Humans have affected freshwater shore zones by laterally compressing and stabilizing the shore zone, changing hydrologic regimes, shortening and simplifying shorelines, hardening shorelines, tidying shore zones, increasing inputs of physical energy that impinge on shore zones, pollution, recreational activities, resource extraction, introducing alien species, changing climate, and intensive development in the shore zone. Systems to guide management and restoration by quantifying ecological services provided by shore zones and balancing multiple (and sometimes conflicting) values are relatively recent and imperfect. We close by identifying leading challenges for shore zone ecology and management.

VARIATION IN BIOAVAILABILITY OF DISSOLVED ORGANIC CARBON AMONG STREAM HYPORHEIC FLOWPATHS

Dissolved organic carbon (DOC) dominates the flux of organic matter in most stream ecosystems, but the proportion susceptible to microbial degradation is often presumed to be low. The fraction of bulk DOC contributing to microbial metabolism was assessed in five streams representing the regional range in surface-water DOC concentration in eastern New York State, USA (range 0.5–7.7 mg/L; n = 82). Transects of shallow wells along two hyporheic flowpaths (i.e., saturated sediments found below and lateral to the open-stream channel in active exchange with surface waters) in each of five streams were sampled monthly at baseflow to determine changes in subsurface DOC and dissolved oxygen concentrations. Hyporheic DOC concentrations ranged from 50% to 100% of surface-water concentrations and decreased along hyporheic flowpaths in four of five streams. Dissolved oxygen losses along hyporheic flowpaths paralleled DOC loss, and bacterial activity on tiles incubated at points along the flowpaths generally declined as hyporheic DOC was depleted. DOC losses along natural flowpaths exceeded the quantity of DOC lost during laboratory bottle incubations, even when samples were amended with inorganic nutrients. Hyporheic mesocosms were used to examine the fate of stream-derived DOC along replicated flowpaths under controlled hydrologic conditions. The overall patterns of DOC losses along mesocosm flowpaths supplied with water from previously studied streams were similar to DOC losses along natural flowpaths. DOC declines were paralleled by declines in bacterial activity and dissolved oxygen. Mesocosm results indicated that variation in percentage of DOC loss along natural flowpaths was not a function of dilution, residence time, or initial DOC concentration and that subsurface DOC dynamics were linked to variation in microbial metabolism. The fraction of total DOC available for metabolism varied markedly among regional streams and was independent of initial DOC concentration. DOC near the end of hyporheic flowpaths was not subject to further degradation, regardless of the bioavailability of surface-water DOC entering these flowpaths. Hence, in streams with significant hyporheic exchange, the amount and bioavailability of DOC transported to downstream ecosystems may be affected by subsurface metabolism. DOC depletion during hyporheic transport may provide a general in situ measure of bioavailable DOC in surface water and be a powerful predictor of rates of heterotrophic activity in sediments at the reach level.