Grantee Research Project Results
Final Report: Influences of Watershed Land Use on Stream Ecosystem Structure and Function
EPA Grant Number: R824777Title: Influences of Watershed Land Use on Stream Ecosystem Structure and Function
Investigators: Meyer, Judith L. , Couch, Carol
Institution: University of Georgia
EPA Project Officer: Packard, Benjamin H
Project Period: November 1, 1995 through November 1, 1998
Project Amount: $500,000
RFA: Water and Watersheds (1995) RFA Text | Recipients Lists
Research Category: Watersheds , Water
Objective:
This research had two objectives: (1) to determine how differing patterns of land use in the watershed alter stream ecosystem structure and function, and (2) to compare measures of stream ecosystem function (rates of community metabolism and transport and cycling of carbon and nutrients) with traditional chemistry-based water quality assessment and organism-based bioassessment.
Summary/Accomplishments (Outputs/Outcomes):
Objective 1: How do differing patterns of land use in the watershed alter stream ecosystem function? We measured ecosystem function during autumn and spring for 2 years in streams draining eight watersheds in the Chattahoochee River basin that represent the range of land uses observed in the Georgia Piedmont. Two streams drain watersheds in each of four land use categories: forest, agricultural (poultry and livestock production), urban, and suburban. Streams were placed in these categories based on GIS analyses of land use in the watershed. The following were used as measures of ecosystem function: nutrient and sediment exports; stream ecosystem metabolism (gross primary productivity and community respiration); uptake of nutrients; removal of fine and coarse particles from the water; leaf decay rate; organic matter budgets (litterfall inputs, benthic organic matter storage, respiration losses) and indices derived from those budgets.
Quantifying Land Use in the Study Watersheds. We compared land use in whole watersheds and in 100 m riparian zones in the eight watersheds. Riparian areas had 15 percent more forest cover than the rest of the watershed in urban and suburban watersheds. Landscape characterization included calculation of impervious surface from urban land use categories for each watershed. Percent impervious surface varied greatly as a function of dominant land use in the watershed: less than 1 percent in forested and agricultural watersheds, 17?22 percent in suburban watersheds, and 32?37 percent in the urban watersheds.
Export of Nutrients and Sediments. Water samples were collected during both base and storm flow in one gaged stream in each land use class and export calculated using concentration versus discharge regressions. Watershed land use exerts a primary control on the observed annual exports of nutrients and sediments, which can be seen most clearly when yields (kg/km2) of nitrogen, phosphorus and sediments are compared. For all constituents, yields increase in the order forest stream < suburban stream < urban stream < agricultural stream.
Stream Metabolism. Whole-system metabolism varied with both season and land use in the watershed. Gross primary production was generally higher in agricultural streams and lower in urban and suburban streams. Community respiration was generally higher in forest and urban streams than in agricultural or suburban streams. All ecosystems were heterotrophic (net ecosystem metabolism < 0, [i.e., community respiration > gross primary production]), although urban and forest streams were more heterotrophic than agricultural or suburban streams. Stored benthic organic matter appears to be driving respiration in forest streams, whereas anthropogenic inputs of labile carbon are driving urban stream respiration.
Nutrient Uptake. We assessed the ability of the stream to assimilate added ammonium (NH4) and phosphate (PO4) by determining nutrient uptake length, the average distance traveled downstream by a NH4 or PO4 molecule before it is removed from the water column. Uptake length for both NH4 and PO4 was significantly greater in urban streams (1630 ? 250 m and 2030 ? 540 m for NH4 and PO4, respectively) than in forest streams (550 ? 100 m and 590 ? 140 m), meaning that nutrients were removed more slowly from the water column in urban streams. In agricultural and suburban streams, uptake length was intermediate. Uptake length could be related to patterns of water movement in the stream, particularly the amount of time water spent in zones of slower flow such as pools or in sediment interstices. Hence, human activities on the landscape that alter channel morphology and patterns of water movement impact the rate at which a stream can assimilate nutrients.
Particle Removal. We developed a new method to measure the rate at which fine particles were removed from the water column using solutions of fluorescently labeled brewer's yeast. Removal of large particles was measured using gingko leaves. Hydraulic characteristics explain much of the variation in fine particle retention in these streams. The extent to which land use change alters the hydraulics of streams will affect organic matter retention. Urban and suburban streams had greatly reduced fine particle retention, largely because of decreases in hydraulic complexity and in-stream retention structures, which reduced the size of transient storage zones. Retention of leaf material was also lowest in urban streams.
Leaf Decay Rates. Rates of decay of Acer barbatum leaves were highest in urban and agricultural streams. Rates were high in urban streams because of physical fragmentation resulting from altered hydrology, whereas high rates were observed in agricultural streams as a result of increased concentrations of nitrogen and phosphorus in the water and higher densities of leaf-shredding macroinvertebrates.
Organic Matter Budgets. Organic inputs to urban and suburban streams were not reduced unless extensive modifications of the riparian zone had occurred; however, there was little storage of organic matter in these streams. The stream metabolism index enables one to compare the fate of organic matter inputs by calculating the fraction of inputs that are lost to respiration. A greater proportion of organic matter inputs to forest streams was lost via respiration than in either suburban or urban streams; values of this index were least in agricultural streams.
Synthesis of Functional Measures. Measures of both particle removal and nutrient uptake showed that in comparison to forest streams, urban streams had a reduced capacity to assimilate particulate and dissolved substances. Other measures of ecosystem function such as leaf decay rate and gross primary productivity indicated elevated rates in agricultural streams, although the proportion of total inputs lost via respiration was lowest in agricultural streams.
Objective 2: How do measures of ecosystem function compare with more traditional measures of ecosystem structure?
Relating Land Use to Measures of Biological Structure. Structural measures of biological integrity frequently used for bioassessments were computed for streams in the four land use classes. Fish assemblages were assessed with an Index of Biotic Integrity (IBI), and the condition of macroinvertebrate assemblages was assessed using total taxa richness, number of EPT taxa (Ephemeroptera, Plecoptera, and Trichoptera), and proportion of taxa as chironomids. Invertebrate measures were highest in forest and agricultural streams and lowest in urban and suburban streams. Total taxa richness was consistently low in urban and suburban streams, averaging half that found in forest and agricultural streams. Suburban and urban streams also had much higher proportions of chironomid taxa. Fish IBI scores paint a slightly different picture, and indicate similar conditions in agricultural and suburban streams with severe degradation in urban streams. Hence, macroinvertebrate indices suggest suburban streams are in poorer condition than do fish indices. EPT taxa richness and fish IBI were compared with patterns of land use in both whole watersheds and in 100-m riparian buffers. Above a threshold of 25?30 percent impervious surface cover, both fish and macroinvertebrate indicators drop sharply. The correlation between biological indicators and forest cover in riparian zones was similar to those calculated for whole watersheds.
Relating Land Use to Measures of Water Quality. Median nutrient and suspended sediment concentrations were consistently highest in agricultural and urban streams and lowest in the forest stream. These differences are particularly apparent during storm flow, which is often not sampled and yet essential for calculating amount of nutrients being exported to downstream ecosystems.
Comparing Structural With Functional Measures. For the measures based on chemistry, agricultural, suburban, and urban streams all showed elevated levels of nutrients and sediments when compared with forest streams. Measures of biological integrity were all lower in these streams with greater human activity in the watershed. Some measures of ecological function (community respiration, stream metabolism index, nutrient and particle uptake rates) were lower in these streams than in forest streams, while other measures were higher (leaf decay and gross primary productivity). The direction of change in ecological function in response to human activities on the landscape is not as consistent as the structural measures, yet the direction of change is predictable given the mechanisms likely to be causing the change.
Relevance of Findings. Ecologists have long recognized that human alteration of the landscape results in structural alterations of streams such as changes in water quality and loss of sensitive stream taxa. This research has shown that there is also an alteration in stream ecosystem function as a consequence of human activities on the landscape. For example, not only have human actions increased the amount of nitrogen and phosphorus being added to urban streams, but the capacity of these streams to remove nitrogen and phosphorus from the water column also has been reduced. This leads to reductions in stream water quality and increased nutrient loading to downstream ecosystems such as larger rivers, lakes, reservoirs, and estuaries, many of which are experiencing eutrophication. Many of the alterations in nutrient uptake capacity of streams appear to be related to hydrologic alteration.
The finding that biological indicators showed a similar relationship to both riparian and whole watershed forest cover implies that in urban watersheds, the success of riparian restoration projects may be compromised unless conditions in the entire watershed are taken into account. In particular, extent of impervious surface cover is critical. Biological integrity of the stream is compromised when impervious surfaces cover more than 25?30 percent of the watershed.
Structural and functional measures of ecological integrity provided somewhat different rankings of streams in the four land use classes. The amount of nutrients and sediments being delivered to downstream ecosystems increased in the order forest < suburban < urban < agricultural streams. Yet agricultural streams ranked much closer to forest streams when bioassessment measures were used, and suburban and urban streams were ranked as much more degraded. Some measures of ecosystem function (e.g., gross primary productivity) showed elevated rates in agricultural streams and reduced rates in urban and suburban streams. Other measures (e.g., nutrient and particle retention) showed highest rates in forest streams, lowest rates in urban streams, and intermediate rates in agricultural and suburban streams. These differences in ranking demonstrate the importance of using more than one indicator when trying to assess ecological condition.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 19 publications | 1 publications in selected types | All 1 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Paul MJ, Hall Jr. RO. Particle transport and transient storage along a stream-size gradient in the Hubbard Brook Experimental Forest. Journal of the North American Benthological Society 2002;21(2):195-205. |
R824777 (Final) |
Exit |
Supplemental Keywords:
water quality, urban streams, agricultural streams, stream metabolism, gross primary productivity, community respiration, bioassessment, index of biotic integrity, nitrogen loading, phosphorus loading, riparian buffer, land use, land cover, leaf decay rate, organic matter budget, spiraling length, transient storage zone., RFA, Scientific Discipline, Water, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Nutrients, Water & Watershed, Ecology, Hydrology, Ecosystem/Assessment/Indicators, Ecosystem Protection, exploratory research environmental biology, Chemical Mixtures - Environmental Exposure & Risk, Environmental Chemistry, State, Chemistry, Ecological Effects - Environmental Exposure & Risk, Zoology, Ecological Effects - Human Health, Drinking Water, Ecological Indicators, Watersheds, EPA Region, ecological effects, ecological exposure, aquatic ecosystem, environmental monitoring, fate and transport, nutrient transport, anthropogenic processes, bioassessment, region 4, carbon cycling, stream ecosystems, other - risk assessment, human-dominated systems, Georgia (GA), watershed land use, ecosystem indicators, biological integrity, toxins, aquatic ecosystems, nutrient cycling, water quality, stream ecosystem, land use, carbon nutrientsRelevant Websites:
http://www.ga.usgs.gov/nawqa Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.