No abstract

In many aquatic ecosystems, most microbes live in matrix-enclosed biofilms and contribute substantially to energy flow and nutrient cycling. Little is known, however, about the coupling of structure and dynamics of these biofilms to ecosystem function. Here we show that microbial biofilms changed the physical and chemical microhabitat and contributed to ecosystem processes in 30-m-long stream mesocosms. Biofilm growth increased hydrodynamic transient storage-streamwater detained in quiescent zones, which is a major physical template for ecological processes in streams-by 300% and the retention of suspended particles by 120%. In addition, by enhancing the relative uptake of organic molecules of lower bioavailability, the interplay of biofilm microarchitecture and mass transfer changed their downstream linkage. As living zones of transient storage, biofilms bring hydrodynamic retention and biochemical processing into close spatial proximity and influence biogeochemical processes and patterns in streams. Thus, biofilms are highly efficient and successful ecological communities that may also contribute to the influence that headwater streams have on rivers, estuaries and even oceans through longitudinal linkages of local biogeochemical and hydrodynamic processes.

Nutrient cycling in streams involves some downstream transport before the cycle is completed. Thus, the path traveled by a nutrient atom in passing through the cycle can be visualized as a spiral. As an index of the spiralling process, we introduce spiralling length, defined as the average distance associated with one complete cycle of a nutrient atom. This index provides a measure of the utilization of nutrients relative to the available supply from upstream. Using 32P as a tracer, we estimated a spiralling length of 193 m for phosphorus in a small woodland stream.Key words: downstream transport, nutrient cycling, phosphorus, spiralling, stream

A study of 16 streams in eastern North America shows that riparian deforestation causes channel narrowing, which reduces the total amount of stream habitat and ecosystem per unit channel length and compromises in-stream processing of pollutants. Wide forest reaches had more macroinvertebrates, total ecosystem processing of organic matter, and nitrogen uptake per unit channel length than contiguous narrow deforested reaches. Stream narrowing nullified any potential advantages of deforestation regarding abundance of fish, quality of dissolved organic matter, and pesticide degradation. These findings show that forested stream channels have a wider and more natural configuration, which significantly affects the total in-stream amount and activity of the ecosystem, including the processing of pollutants. The results reinforce both current policy of the United States that endorses riparian forest buffers as best management practice and federal and state programs that subsidize riparian reforestation for stream restoration and water quality. Not only do forest buffers prevent nonpoint source pollutants from entering small streams, they also enhance the in-stream processing of both nonpoint and point source pollutants, thereby reducing their impact on downstream rivers and estuaries.

/ Maryland, Virginia, and Pennsylvania, USA, have agreed to reduce nutrient loadings to Chesapeake Bay by 40% by the year 2000. This requires control of nonpoint sources of nutrients, much of which comes from agriculture. Riparian forest buffer systems (RFBS) provide effective control of nonpoint source (NPS) pollution in some types of agricultural watersheds. Control of NPS pollution is dependent on the type of pollutant and the hydrologic connection between pollution sources, the RFBS, and the stream. Water quality improvements are most likely in areas of where most of the excess precipitation moves across, in, or near the root zone of the RFBS. In areas such as the Inner Coastal Plain and Piedmont watersheds with thin soils, RFBS should retain 50%-90% of the total loading of nitrate in shallow groundwater, sediment in surface runoff, and total N in both surface runoff and groundwater. Retention of phosphorus is generally much less. In regions with deeper soils and/or greater regional groundwater recharge (such as parts of the Piedmont and the Valley and Ridge), RFBS water quality improvements are probably much less. The expected levels of pollutant control by RFBS are identified for each of nine physiographic provinces of the Chesapeake Bay Watershed. Issues related to of establishment, sustainability, and management are also discussed.Research is sometimes applied to broad-scale environmental issues with inadequate knowledge or incomplete understanding. Public policies to encourage or require landscape management techniques such as riparian (streamside) management will often need to proceed with best professional judgment decisions based on incomplete understanding.Riparian forest buffer systems (RFBS) are streamside ecosystems managed for the enhancement of water quality through control of nonpoint source pollution (NPS) and protection of the stream environment. The use of riparian management zones is relatively well established as a best management practice (BMP) for water quality improvement in forestry practices (Comer-

The limiting role of phosphorus on leaf decomposition and primary producers was investigated in a second—order woodland stream in Tennessee by experimentally enriching, for 95 d, adjacent reaches with an average of 60 and 450 mg PO4—P/L, respectively, over upstream control levels of °4 mg/L. Red oak (Quercus rubra) leaf packs in the enriched sections lost mass 24% faster than control packs (P < .05). Nitrogen content of the enriched packs increased 60% more, and P content increased 83% more than the respective increases in the control packs (P < .05). Differences in mass loss and N and P levels between the low and high enrichments were not significant (P > .05). Respiration rates of subsampled leaf discs were significantly higher than control rates only at the high level of enrichment. The increased respiration rates in the low and high enrichments accounted for 10 and 34% of the increased mass loss in the respective enriched sections, suggesting that the enrichment also produced increases in mechanical breakdown through faster microbial conditioning, increases in macroinvertebrate feeding, or both. Effects of the enrichment on aufwuchs initially consisted of increased chlorophyll a levels, followed by increased aufwuchs biomass levels. Dense growth of filamentous algae, including some Oscillatoria, which may be a nitrogen fixer, developed immediately downstream of P inputs. In addition, Nostoc, a known nitrogen—fixing blue—green alga, sampled after the enrichment, was significantly more abundant in the enriched sections than the control (P < .05). Densities of the snail, Goniobasis clavaeformis, a grazer—shredder sampled after the enrichment, also were significantly greater in the enriched reaches, suggesting that the lack of a sustained response of chlorophyll a to the enrichment may have been a result of increased grazing on algal biomass. These findings indicate that nutrient limitation of detrital processing is a significant factor in natural streams. The apparent increases in densities of benthic macroinvertebrates in the enriched sections, along with reported relationships between detrital food richness and macroinvertebrate growth and survivorship, suggest that nutrient limitation in streams also has ramifications on higher tropic levels.

The term spiralling refers to the interdependent processes of cycling and downstream transport of nutrients in a stream ecosystem. To describe spiralling in Walker Branch, a first—order woodland stream in Tennessee, we released 32PO4 to the stream water and measured its uptake from the water and then followed its dynamics in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), aufwuchs, grazers, shredders, collectors, net—spinning filter feeders, and predators over a 6—wk period. Rates of transfer among compartments and rates of downstream transport were estimated by fitting a partial differential equation model of the ecosystem to the data. With the resulting coefficients, the model was run to steady state to estimate standing stocks and fluxes of exchangeable phosphorus. Phosphorus moved downstream at an average velocity of 10.4 m/d, cycling once every 18.4 d. The average downstream distance associated with one cycle, defined as the spiralling length, was therefore 190 m (10.4 m/d ° d). Spiralling length, at steady state, is approximately the ratio of the total downstream flux of phosphorus per unit width of stream (720 mg°d—1°m—1) to the rate of P uptake from the water (3.90 mg°m—2.d—1). CPOM accounted for 60% of the uptake, FPOM for 35%, and aufwuchs for 5%. Turnover times of P in particulates ranged from 5.6 to 6.7, except for FPOM, which showed a second, slower turnover time of 99 d. Of the P uptake from water by particulates, 2.8% was transferred to consumers, while the remainder returned directly to the water. About 30% of the consumer uptake, in turn, was transferred to predators. The spiralling length was partitioned into: (1) an uptake length associated with transport in the water column (165 m), (2) a particulate turnover length associated with transport in FPOM and CPOM (25 m), and (3) a consumer turnover length associated with animal drift (0.05 m). FPOM transport accounted for 99% of the particulate turnover length. The small consumer turnover length reflected low consumer uptake of P from particulates and slow downstream drift velocity (0.013 m/d). In spite of the low rate of phosphorus uptake, the combined consumer—and—predator community accounted for 25% of the standing stock of exchangeable P in the stream. The retentiveness of this community is attributable both to the low drift rate and to a long turnover time (152 d) for P within the community.

This literature review addresses how wide a streamside forest buffer needs to be to protect water quality, habitat, and biota for small streams (≤~100 km 2 or~5th order watershed) with a focus on eight functions: (1) subsurface nitrate removal varied inversely with subsurface water flux and for sites with water flux >50 l/m/day (~40% avg base flow to Chesapeake Bay) median removal efficiency was 55% (26-64%) for buffers <40 m wide and 89% (27-99%) for buffers >40 m wide; (2) sediment trapping was~65 and~85% for a 10-and 30-m buffer, respectively, based on streamside field or experimentally loaded sites; (3) stream channel width was significantly wider when bordered by~25-m buffer (relative to no forest) with no additional widening for buffers ≥25 m; (4) channel meandering and bank erosion were lower in forest but more studies are needed to determine the effect of buffer width; (5) temperature remained within 2°C of levels in a fully forested watershed with a buffer ≥20 m but full protection against thermal change requires buffers ≥30 m; (6) large woody debris (LWD) has been poorly studied but we infer a buffer width equal to the height of mature streamside trees (~30 m) can provide natural input levels; (7, 8) macroinvertebrate and fish communities, and their instream habitat, remain near a natural or semi-natural state when buffered by ≥30 m of forest. Overall, buffers ≥30 m wide are needed to protect the physical, chemical, and biological integrity of small streams.