Effect of Leaf Litter on Phosphorus Retention and ...

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Nov 30, 1999 - Brian E. Haggard. USDA-ARS-PPPSRL!: ..... 47:2351-2357. Mulholland, P.J., J.D. Newbold, J.W. Elwood, A. Ferren, and J.R. Webster. 1985.
Effect and

of Leaf

Hydrological

Litter

on Phosphorus

Properties

in Northeast

Retention

at a First

Oklahoma,

Order

Stream

USA

Brian E. Haggard USDA-ARS-PPPSRL!: 203 EngineeringHall Fayetteville,AR 72701 USA E-mail..haggard@uark:edu and Daniel E. Storm Biosystemsand AgriculturalEngineeringDepaltment OklahomaState University Stillwater,OK 74078 USA ABSTRACT We examined the relationship between phosphorus (P) uptake length (Sw) measured by stable PO4additions and the level of PO4enrichment from the additions during summer and fall in a first order stream in the southwestern portion of the Ozark Plateaus in northeast Oklahoma, USA. The y-intercept of this relation was used to more closely approximate ambient Swand other metrics ofP retention efficiency during summer and fall and determine the effect of leaf litter input on P retention efficiency and hydrological properties. Under similar discharge, the presence of the leaf litter during fall decreased water velocity and the dispersion coefficient, while the transient storage area increased compared to summer. An increase in Swwas generally observed with an increase in the level ofP enrichment, and the linear relation (y-intercept) estimated Swat 117 and 86 m during summer and fall. The slope of the linear relation was variable becauseaS !lg L -I increasein P enrichment resulted in a 27 and 5 m increase in Swestimation during summer and fall. Overall, the input of leaf litter into the stream had a relatively small effect on Sw, but our results suggest p retention was slightly greater during fall compared to summer. INTRODUCTION Over the past two decades,the use of the spiraling concept, including measurementof phosphorus(P) uptake length (Sw sensuNewbold et al. 1981) has been used increasingly to estimate P retention efficiency in streams. In particular, Swhas been measuredusing radiotracers 32pO4or 33pO4( e.g., seeNewbold et al. 1981, 1983, Mulholland et al. 1985, 1990) and stable PO4 additions (e.g., see Mulholland et al. 1990, Munn and Meyer 1990, O' Angelo and Webster 1991, Webster et al. 1991, Mart! and Sabater 1996, Butturini and Sabater 1998, Davis and Minshall1999, Haggard et al. 2001). One condition when estimating Swusing stable PO4 additions is that uptake by the stream benthos is proportional to the level ofP enrichment from the addition (Stream Solute Workshop 1990). Thus, uptake kinetics and P limitation of the stream benthos are important factors in this approachto Swmeasurement. Phosphorusuptake is influenced by a myriad of processesincluding biotic uptake from the algal and heterotrophic bioflim, bryophytes, and macrophytes (Lock et al. 1990, Meyer 1979, Chambers and Prepas 1994), sediment adsorption (Klotz 1988, Haggard et al. 1999), water velocity and discl}arge (0' Angelo and Webster 1991, Mart! and Sabater 1996, Butturini and Sabater 1998), and transient storage (Haggard et al. 2001). Seasonal variations in Swmay result from leaf litter input and the associatedgrowth of microbes during fall (Mulholland et al. 1985) and temperature-related changes in metabolic activity during winter (0' Angelo et al. 1991). Fluctuations in seasonalbase flow and in the interaction between the water column and benthic substratesalso affect Swmeasurements (Bencala 1983). 557 Journal of Freshwater

Ecology, Volume

18, Number 4- December

2003

In this study, the relation between Sw measured by stable PO4 additions and the level ofPO4 enrichment from the additions during summer and fall was examined in a first order stream in the Ozark Plateaus in northeast Oklahoma, USA. We used the y-intercept of this relation to more closely approximate ambient Sw and other metrics ofP retention efficiency during summer and fall. The effect of leaf litter input on P retention efficiency and hydrological properties was ascertained under similar discharge.

MATERIALS AND METHODS Our solute injections were conducted in a 108-m reach of Willow Branch, a first order tributary to Spring Creek in the Ozark Plateaus in northeast Oklahoma, USA. Baseflow in this perennial headwater stream is generated by a spring. This stream is typical of Ozark Mountain streams in northeastern Oklahoma having a cherty substrate, karst topography in the uplands and underlyinf geology of dolomitic limestone. The dominant anion and cation are HCO3- (>lOO mgL- ) and Ca2+(~33 mg L-I), respectively. On 24 August 1999, soluble reactive P (SRP), nitrate as nitrogen (NO3-N), ammonium as nitrogen (NH4-N) and chloride (CI-) concentrations were 15 ~g L-1, and 0.31, 95%) with little urban or agriculture land use. However, becauseof the karst features, Willow Branch may receive ground water discharge from other land uses in surrounding catchmentscomposed of a pasture and woodland mix that may receive land application of animal manure. We added solutions containing stable PO4and a hydrological tracer, CI-, using a Mariotte Bottle into Willow Branch on seven dates, four in the summer (1,2,3, and 7 September 1999) and three in the fall (30 November, I and 2 December 1999). The injections produced increasing levels ofPO4 enrichment on successivedates during each season. Injections began at approximately 1200 hrs and lasted until conductivity reached steady state over the entire stream reach (approximately 1.25 hrs). Conductivity was manually recorded periodically until conductivity started to increase, then measurements were recorded at 1-2 minute intervals at the most downstream station (YSI Model 30 SCT Meter, Yellow Springs, OH). We assumedthat the system was in steady-statewhen conductivity reached its plateau at the most downstream station. We collected background (pre-injection) and steady-statewater samples at sites 20, 35,53,74 and 108 m from the point of injection. Samples were filtered on site through a 0.7 JlIn glass fiber filter (Whatrnan GF/F), acidified with 6N H2SO4to pH 2, and stored on ice until return to the laboratory .Samples were analyzed for soluble reactive phosphorus (SRP) using the ascorbic acid method (Murphy and Riley 1962) and Cl- on a QuickChem Latchet 9000 (Method 10-117-07-l-C, Milwaukee, WI, USA ). In the fall experiments, NO3-N and NH4-N were also determined using Cd-Cu reduction (QuickChem Method 10-107-04-I-A) and the alkaline phenol, Na hypochlorite and nitroprusside reaction (QuickChem Method 10-107-06-1-B), respectively, at the most upstream and downstream sites. Steady-state(plateau) SRP concentrations were corrected for background SRP concentrations and dilution via Cl- data ( e.g., see Marti and Sabater 1996). Corrected SRP concentrations were then expressedas the proportion remaining in the water column at each site. The distance-normalized P uptake rate was determined from the regression of the natural logarithm of this proportion with distance, and Swis the inverse of the slope (Sw= l/k; Webster and Ehrman 1996). The first order P uptake rate coefficient (Kc, S-I; Stream Solute Workshop 1990) was determined as

Kc = u/Sw

where u is the average water velocity (m S-I). The P uptake rate per unit distance of ambient P (UL, ~g S-Im-1 was esti~ated by UL = Cbg.Q/Sw 558

where Cbg is the average SRP concentrations (~g L -I) measured during the injections and Q is the measured discharge (L S-I). A one-dimensional transport model with inflow and storage (OTIS-P; Runkel 1998) was used to estimate dispersion (D), stream cross-sectional area (A), transient storage area (As), and transient storage exchange rate (a). The model estimated these parameters using the relation between conductivity and time at the most downstream station accounting for the influence of zones where water was moving at slower velocities than the average velocity in the main channel. We used OTIS-P to statistically optimize the model output parameters and report the residual sum of squares (RSS) of observed and predicted results. Simple linear regression was used to determine if significant P retention occurred during solute injections and if relations existed between parameters (significance level = 0.05). When comparing values between summer and fall, values were appropriately transformed ( e.g., In-transformation for concentration and hydrological properties) and compared using analysis of variance for physicochemical properties and using Student's T -test with unequal variances for hydrologic properties. Nitrogen concentrations in background and plateau water samples were similarly compared during fall solute injections at the most upstream and downstream sites. RESUL TS AND DISCUSSION Discharge. Temperature and Water Chemistry Mean discharge during summer and fall was about 4.3 L S"I and increased downstream through the IO8-m study reach in Willow Branch (Table I). Streamflow increased in the downstream direction by an average of9 and 16% in summer and fall, respectively. Mean water temperatures during summer and fall were approximately 15.7 and 14.5DC, respectively, reflecting the dominant influence of groundwater. The pH was similar between summer and fall; pH values were generally between 7.4 and 7.8. Electrical conductivity was significantly greater in fall (195 ~S cm-l) than summer (156 ~S cm-l) at the most downstream sampling site (ANOV A, P