William V. Sobczak

Loss of foundation species: consequences for the structure and dynamics of forested ecosystems

In many forested ecosystems, the architecture and functional ecology of certain tree species define forest structure and their species-specific traits control ecosystem dynamics. Such foundation tree species are declining throughout the world due to introductions and outbreaks of pests and pathogens, selective removal of individual taxa, and over-harvesting. Through a series of case studies, we show that the loss of foundation tree species changes the local environment on which a variety of other species depend; how this disrupts fundamental ecosystem processes, including rates of decomposition, nutrient fluxes, carbon sequestration, and energy flow; and dramatically alters the dynamics of associated aquatic ecosystems. Forests in which dynamics are controlled by one or a few foundation species appear to be dominated by a small number of strong interactions and may be highly susceptible to alternating between stable states following even small perturbations. The ongoing decline of many foundation species provides a set of important, albeit unfortunate, opportunities to develop the research tools, models, and metrics needed to identify foundation species, anticipate the cascade of immediate, short- and long-term changes in ecosystem structure and function that will follow from their loss, and provide options for remedial conservation and management.

NITROGEN SATURATION AND RETENTION IN FORESTED WATERSHEDS OF THE CATSKILL MOUNTAINS, NEW YORK

The Catskill Mountains of southeastern New York receive relatively high rates of atmospheric N deposition, and NO3 2 concentrations in some streams have increased dramatically since the late 1960s. We measured the chemistry of 39 first- and second-order streams with forested watersheds to determine the variability of nitrogen concentrations within the Catskill Mountain area. We found that some streams have low NO3 2 concentra- tions throughout the year, some have seasonal cycles of varying amplitude, and some have relatively high concentrations year round. If the concentration and seasonality of NO 3 2 in stream water are used as indices of nitrogen saturation, then most stages of nitrogen sat- uration are evident in our survey of Catskill watersheds. Organic nitrogen was a small portion of the total nitrogen for streams with high NO3 2 concentration, but organic N was the dominant form of N (up to 73% of the total) in the streams with lowest nitrate. Estimated retention of N in these watersheds (based on total N in stream water) ranged from 49% to 90% of the atmospheric input. The variation in stream water NO3 2 concentration and the amplitude of the seasonal fluctuations did not appear to be attributable to differences among watersheds in atmospheric deposition, watershed topography, or groundwater influx to the stream. We hypothesize that differences among watersheds in forest species composition and forest history, which are interrelated, produce most of the variation in NO 3 2 concen- tration that we observed.

Bioavailability of organic matter in a highly disturbed estuary: The role of detrital and algal resources

The importance of algal and detrital food supplies to the planktonic food web of a highly disturbed, estuarine ecosystem was evaluated in response to declining zooplankton and fish populations. We assessed organic matter bioavailability among a diversity of habitats and hydrologic inputs over 2 years in San Francisco Estuarys Sacramento–San Joaquin River Delta. Results show that bioavailable dissolved organic carbon from external riverine sources supports a large component of ecosystem metabolism. However, bioavailable particulate organic carbon derived primarily from internal phytoplankton production is the dominant food supply to the planktonic food web. The relative importance of phytoplankton as a food source is surprising because phytoplankton production is a small component of the ecosystems organic-matter mass balance. Our results indicate that management plans aimed at modifying the supply of organic matter to riverine, estuarine, and coastal food webs need to incorporate the potentially wide nutritional range represented by different organic matter sources.

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.

Relationships between DOC bioavailability and nitrate removal in an upland stream: An experimental approach

The Catskill Mountains of southeastern New York State have among thehighest rates of atmospheric nitrogen deposition in the United States. Somestreams draining Catskill catchments have shown dramatic increases in nitrateconcentrations while others have maintained low nitrate concentrations. Streamsin which exchange occurs between surface and subsurface (i.e. hyporheic) watersare thought to be conducive to nitrate removal via microbial assimilationand/ordenitrification. Hyporheic exchange was documented in the Neversink River inthesouthern Catskill Mountains, but dissolved organic carbon (DOC) and nitrate(NO3−) losses along hyporheic flowpaths werenegligible. In this study, Neversink River water was amended with natural,bioavailable dissolved organic carbon (BDOC) (leaf leachate) in a series ofexperimental mesocosms that simulated hyporheic flowpaths. DOC and N dynamicswere examined before and throughout a three week BDOC amendment. In addition,bacterial production, dissolved oxygen demand, denitrification, and sixextracellular enzyme activities were measured to arrive at a mechanisticunderstanding of potential DOC and NO3− removalalong hyporheic flowpaths. There were marked declines in DOC and completeremoval of nitrate in the BDOC amended mesocosms. Independent approaches wereused to partition NO3− loss into two fractions:denitrification and assimilation. Microbial assimilation appears to be thepredominant process explaining N loss. These results suggest that variabilityinBDOC may contribute to temporal differences in NO3−export from streams in the Catskill Mountains.

Detritus Fuels Ecosystem Metabolism but not Metazoan Food Webs in San Francisco Estuary's Freshwater Delta

Detritus from terrestrial ecosystems is the major source of organic matter in many streams, rivers, and estuaries, yet the role of detritus in supporting pelagic food webs is debated. We examined the importance of detritus to secondary productivity in the Sacramento and San Joaquin River Delta (California, United States), a large complex of tidal freshwater habitats. The Delta ecosystem has low primary productivity but large detrital inputs, so we hypothesized that de tritus is the primary energy source fueling production in pelagic food webs. We assessed the sources, quantity, composition, and bioavailability of organic matter among a diversity of habitats (e.g., marsh sloughs, floodplains, tidal lakes, and deep river channels) over two years to test this hypothesis. Our results support the emerging principle that detritus dominates riverine and estuarine organic matter supply and supports the majority of ecosystem metabolism. Yet in contrast to prevailing ideas, we found that detritus was weakly coupled to the Deltas pelagic food web. Results from independent approaches showed that phytoplankton production was the dominant source of organic matter for the Deltas pelagic food web, even though primary production accounts for a small fraction of the Deltas organic matter supply. If these results are general, they suggest that the value of organic matter to higher trophic levels, including species targeted by programs of ecosystem restoration, is a function of phytoplankton production.

Variability in Removal of Dissolved Organic Carbon in Hyporheic Sediments

Dissolved organic carbon (DOC) is consumed by microbial metabolism as streamwater perfuses through a lateral gravel bar of the East Branch of the Wappinger Creek. The rate of DOC removal was estimated from the decline in DOC and travel time through the bar. Variability in DOC removal together with potential regulatory factors was determined for 14 dates spanning more than a 2-y period. DOC removal was not correlated with temperature, availability of oxygen, or residence time within hyporheic sediments. Hyporheic DOC could be predicted (r2 = 0.68) from streamwater DOC concentrations, with a surprisingly constant 57% (±9% [1 SD]) removal of DOC. This pattern suggests an initial concentration-dependent adsorption of DOC onto surfaces. This mechanism allows for efficient retention of DOC within hyporheic sediments even under conditions (low temperature, high interstitial velocity) that might be expected to minimize biotic consumption of DOC.

Epilithic Bacterial Responses to Variations in Algal Biomass and Labile Dissolved Organic Carbon during Biofilm Colonization

This study experimentally examines potential shifts in epilithic bacterial biomass and productivity in response to variations in epilithic algal biomass and labile dissolved organic carbon (DOC) during stream biofilm colonization. I predicted that epilithic bacteria would respond positively to allochthonous DOC early in biofilm colonization and respond positively to increased algal biomass late in biofilm colonization. Using once-through, experimental-stream channels, a 2 × 2 factorial design was employed in which light (shaded vs. non-shaded) and labile DOC (glucose-amended vs. ambient) were manipulated. Ceramic tiles were used as substrates for biofilm colonization and were sampled at different colonization stages. Shading significantly reduced chlorophyll a, live-algal bio-volume, and ash-free dry mass throughout colonization. Bacterial biomass increased significantly during biofilm colonization, but was not significantly different among treatments. Incorporation of [3H]thymidine into bacterial DNA, which was measured as a surrogate for bacterial productivity, was significantly greater in the glucose-amended channels throughout colonization, but it increased in the unshaded, ambient treatment in late colonization as well. These results suggest that labile DOC in the water column can potentially function as a control for epilithic bacteria throughout biofilm colonization, whereas epilithic algae can stimulate bacteria late in biofilm colonization in productive stream ecosystems.

Relationships between bacterial productivity and organic carbon at a soil—stream interface

Microbial communities at soil-stream interfaces may be particularly important in regulating amounts and forms of nutrients that leave upland soils and enter stream ecosystems. While microbial communities are thought to be responsible for key nutrient transformations within near-stream sediments, there is relatively little mechanistic information on factors that control microbial activities in these areas. In this study, we examine the roles of dissolved organic carbon (DOC) vs. particulate organic carbon (POC) as potential controls on rates of bacterial productivity (measured as incorporation of [3H]thymidine into bacterial DNA) and amounts of bacterial biomass (measured as fatty acid yield) in sediments along a transect perpendicular to a soil–stream interface. We hypothesized that spatial patterns in bacterial productivity would vary in response to strong and persistent patterns in pore-water concentrations of DOC that were observed along a soil-stream transect throughout a 2-year period. Our results did not support the existence of such a link between pore-water DOC and bacterial productivity. In contrast, we found bacterial productivity and biomass were related to small-scale spatial variations in sediment POC on 3 of 4 sample dates. While our results indicate that total bacterial productivity in near-stream sediments is not consistently linked to spatial variations in pore-water DOC, it is likely that DOC and POC are not mutually exclusive and the relative contribution of DOC and POC to sedimentary microbes varies temporally and spatially in different riparian habitats.

Subsurface flowpaths in a forested headwater stream harbor a diverse macroinvertebrate community

Headwater streams and wetlands with a combination of surface and subsurface flows are common features of many upland-forested watersheds. Unlike headwater stream reaches with continuous surface flow, the hydrology and ecology of subsurface stream reaches are poorly studied and not factored into existing wetland legislation. We assessed subsurface habitats and associated biota in a 435-m reach of a first-order, intermittent stream draining a riparian zone dominated by eastern hemlock (Tsuga canadensis) in north central Massachusetts. Stream flow was found only in subsurface flowpaths beneath large boulders and surface root mats over approximately 70% of the total stream length at summer base flow. Temperature, specific conductivity, dissolved oxygen, and dissolved organic carbon concentrations of subsurface water were similar to surface water. Macroinvertebrates were found in subsurface habitats but at a lower abundance and richness per unit area compared to surface habitats. Collectors such as Chironomidae, Polycentropodidae, and Ephemerellidae were generally the most abundant families in both surface and subsurface habitats. Our findings indicate that in some glaciated watersheds, intermittent streams with no visual evidence of surface flow may contain subsurface flowpaths with water chemistry and biota comparable to coupled perennial surface flow reaches. The prevalence and importance of subsurface habitats in some headwater streams may warrant review or revision of existing state and local regulatory definitions of intermittent and headwater streams.