Paul A. Bukaveckas

Spatial Variation among Lakes within Landscapes: Ecological Organization along Lake Chains

ABSTRACT Although limnologists have long been interested in regional patterns in lake attributes, only recently have they considered lakes connected and organized across the landscape, rather than as spatially independent entities. Here we explore the spatial organization of lake districts through the concept of landscape position, a concept that considers lakes longitudinally along gradients of geomorphology and hydrology. We analyzed long-term chemical and biological data from nine lake chains (lakes in a series connected through surface or groundwater flow) from seven lake districts of diverse hydrologic and geomorphic settings across North America. Spatial patterns in lake variables driven by landscape position were surprisingly common across lake districts and across a wide range of variables. On the other hand, temporal patterns of lake variables, quantified using synchrony, the degree to which pairs of lakes exhibit similar dynamics through time, related to landscape position only for lake chains with lake water residence times that spanned a wide range and were generally long (close to or greater than 1 year). Highest synchrony of lakes within a lake chain occurred when lakes had short water residence times. Our results from both the spatial and temporal analyses suggest that certain features of the landscape position concept are robust enough to span a wide range of seemingly disparate lake types. The strong spatial patterns observed in this analysis, and some unexplained patterns, suggest the need to further study these scales and to continue to view lake ecosystems spatially, longitudinally, and broadly across the landscape.

Movement of Road Salt to a Small New Hampshire Lake

Runoff of road salt from an interstate highway in New Hampshire has led to contamination of a lake and a stream that flows into the lake, in spite of the construction of a diversion berm to divert road salt runoff out of the lake drainage basin. Chloride concentration in the stream has increased by over an order of magnitude during the 23 yr since the highway was opened, and chloride concentration in the lake has tripled. Road salt moves to the lake primarily via the contaminated stream, which provides 53% of all the chloride to the lake and only 3% of the total streamflow to the lake. The stream receives discharge of salty water from leakage through the diversion berm. Uncontaminated ground water dilutes the stream downstream of the berm. However, reversals of gradient during summer months, likely caused by transpiration from deciduous trees, result in flow of contaminated stream water into the adjacent ground water along the lowest 40-m reach of the stream. This contaminated ground water then discharges into the lake along a 70-m-wide segment of lake shore. Road salt is pervasive in the bedrock between the highway and the lake, but was not detected at all of the wells in the glacial overburden. Of the 500 m of shoreline that could receive discharge of saly ground water directly from the highway, only a 50-m-long segment appears to be contaminated.

N STORAGE AND CYCLING IN VEGETATION OF A FORESTED WETLAND: IMPLICATIONS FOR WATERSHED N PROCESSING

Pools and fluxes of N in wetland vegetation and soils were compared with an adjacent upland site to assess the relative importance of wetland versus upland landscapes in watershedN-retention in the Adirondack Mountains of New York (U.S.A.).The majority of N storage occurred in forest soils and wetlandpeat deposits (96 and 99% of total N in upland forests andwetlands, respectively). Annual N-uptake (49 kg N ha-1yr-1) was greater for wetland vegetation than that ofupland vegetation (30 kg N ha-1 yr-1). In the wetlandthe supply of N from mineralization (36 kg N ha-1yr-1) was less than N-uptake; in contrast, upland Nmineralization (76 kg N ha-1 yr-1) exceeded Nvegetation uptake. Annual N-storage in peat was small due to low peat accretion rates. Wetlands acted as a sink for N andstored a disproportionally high fraction (15%) of catchment Nin relation to their relatively small surface area (∼4%)within the catchment.

Effects of Point Source Loadings, Sub-basin Inputs and Longitudinal Variation in Material Retention on C, N and P Delivery from the Ohio River Basin

Spatial variability in material fluxes within large river basins may arise from point source inputs, variable contributions from sub-basins and longitudinal variation in material transformation and retention. By measuring instantaneous fluxes throughout the Ohio River basin, we were able to draw inferences about the importance of these factors in determining the overall export of C, N and P from the basin. Our study spanned the lower 645 km of the Ohio River and included all tributaries that contributed at least 1% of the volume of the Ohio River at its confluence with the Mississippi. The intensively cultivated northern sub-basin (Wabash River) contributed a large fraction of N and P entering the Ohio River. In the southern sub-basins (Tennessee and Cumberland Rivers), impoundments and less intense cultivation appear to diminish and delay material delivery particularly with respect to N. The southern rivers account for a proportionately larger fraction of the water entering the Mississippi River during low discharge conditions and this fraction has increased during the past 50 years. The upper portion of the study reach was found to be a net source of CHLa and DOC and a net sink for inorganic N suggesting that this portion of the river provided a generally favorable environment for autotrophic production. Point source loadings of NH4 were significant inputs to the upper sub-reach but a relatively small component of the overall budget for dissolved inorganic N.

Seasonal and interannual variation in nutrient fluxes from tributary inputs, consumer recycling and algal growth in a eutrophic river impoundment

We measured tributary inputs, algal nutrient demand and excretion rates of consumers (gizzard shad and zooplankton) at a eutrophic river impoundment. During two summers with contrasting flow regimes, tributary inputs accounted for 38% (1998) and 3% (1999) of algal N demand and 95% (1998) and 17% (1999) of algal P demand. Gizzard shad contributions averaged 14% and 20% of algal demand for N whereas P contributions were 31% and 58% (1998, 1999; respectively). Zooplankton recycling accounted for a comparable fraction of algal P demand (47%) but a larger fraction of N demand (43%) because their excretia were N rich (N:P = 13:1) compared to fish (7:1). Nutrient release by one of the consumers (gizzard shad) was compared with tributary loading over a nine-year period to assess inter-annual variation in their relative importance. Historical records of inflow chemistry, discharge and gizzard shad biomass showed that variation in tributary inputs was the primary determinant of seasonal and inter-annual variation in nutrient loading. Consumer-derived nutrients were important in late-summer and during years when tributary inputs were low. We propose a conceptual model in which primary production is regulated by external nutrient loading and consumer recycling acts to stabilize and sustain production during periods of diminished external inputs.

Factors regulating autotrophy and heterotrophy in the main channel and an embayment of a large river impoundment

We characterized seasonal patterns of phytoplankton and bacterial biomass, production and nutrient limitation along a lateral transect within a large river impoundment. We hypothesized that the balance between autotrophy and heterotrophy was related to depth gradients and differences in water residence time (WRT) between the main channel and an embayment. Heterotrophy predominated in the main channel with bacterial production exceeding phytoplankton production by a factor of 3.3. In the embayment, autotrophy and heterotrophy were more closely balanced (ratios of bacterial to phytoplankton production ca. 0.8). Phytoplankton and bacterial biomass were positively correlated with WRT. However, WRT accounted for less than 50% of variation and its predictive power was comparable to models based on nutrient or DOC concentrations. Bacterial production was correlated with phytoplankton biomass and production suggesting that algal-derived C may be an important substrate for bacterial growth even in systems dominated by allochthonous inputs. Our experimental data suggest that nutrient limitation may be important particularly in embayments where biomass was somewhat higher and substrate concentrations were lower. Nutrient limitation in the main channel was rare whereas N and P amendments consistently stimulated phytoplankton growth rates in the embayment. Bacterial cell densities did not respond to nitrogen or phosphorus additions in either the main channel or embayment.

Inter-annual, seasonal and spatial variability in nutrient limitation of phytoplankton production in a river impoundment

We characterize seasonal and spatial patterns in phytoplankton abundance, production and nutrient limitation in a mesotrophic river impoundment located in the southeastern United States to assess variation arising from inter-annual differences in watershed inputs. Short-term (48 h) in situ nutrient addition experiments were conducted between May and October at three sites located along the longitudinal axis of the lake. Nutrient limitation was detected in 12 of the 18 experiments conducted over 2 years. Phytoplankton responded to additions of phosphorus alone although highest chlorophyll concentrations were observed in enclosures receiving combined (P and N) additions. Growth responses were greatest at downstream sites and in late summer suggesting that those populations experience more severe nutrient limitation. Interannual variation in nutrient limitation and primary production corresponded to differences in the timing of hydrologic inputs. Above average rainfall and discharge in late-summer (July–October) of 1996 coincided with higher in-lake nutrient concentrations, increased production, and minimal nutrient limitation. During the same period in 1995, discharge was lower, nutrient concentrations were lower, and nutrient limitation of phytoplankton production was more pronounced. Our results suggest that nutrient limitation is common in this river impoundment but that modest inter-annual variability in the timing of hydrologic inputs can substantially influence seasonal and spatial patterns.

A Comparison of Methods for Deriving Solute Flux Rates Using Long-Term Data from Streams in the Mirror Lake Watershed

Calculation of chemical flux rates for streams requires integration of continuous measurements of discharge with discrete measurements of solute concentrations. We compared two commonly used methods for interpolating chemistry data (time-averaging and flow-weighting) to determine whether discrepancies between the two methods were large relative to other sources of error in estimating flux rates. Flux rates of dissolved Si and SO42- were calculated from 10 years of data (1981-1990) for the NW inlet and outlet of Mirror Lake and for a 40-day period (March 22 to April 30, 1993) during which we augmented our routine (weekly) chemical monitoring with collection of daily samples. The time-averaging method yielded higher estimates of solute flux during high-flow periods if no chemistry samples were collected correponding to peak discharge. Concentration-discharge relationships should be used to interpolate stream chemistry during changing flow conditions if chemical changes are large. Caution should be used in choosing the appropriate time-scale over which data are pooled to derive the concentration-discharge regressions because the model parameters (slope and intercept) were found to be sensitive to seasonal and inter-annual variation. Both methods approximated solute flux to within 2-10% for a range of solutes that were monitored during the intensive sampling period. Our results suggest that errors arising from interpolation of stream chemistry data are small compared with other sources of error in developing watershed mass balances.

Calibration of a Water-Quality Model for Herrington Lake Using Empirically Derived Measurements of Phytoplankton Growth and Nutrient Assimilation

Importance of light limitation, nutrient availability, and hydrology in controlling the abundance and composition of the phytoplankton community of Herrington Lake (KY) was investigated over a two-year period. Selected environmental parameters were measured every two weeks (April-October) at five sampling stations located along the longitudinal gradient of the reservoir. In addition, short-term (48hr) nutrient enrichment experiments were conducted to assess the spatial and temporal variations in nutrient limitation. Phytoplankton growth responses to the combined addition of nitrogen (N03) and phosphorus (P04) were greater than those resulting from the addition of either nutrient alone. These results indicate that phytoplankton production was closely co-limited by the availability of both N and P. The magnitude of the phytoplankton responses to nutrient additions was greatest at downstream stations and in late summer suggesting that those populations experience more severe nutrient limitation. Significant interannual variations in nutrient limitation and primary production were observed during this study period (1995-1996). In 1995, nutrient limitation was more severe than in 1996. Above average rainfall and discharge in 1996 coincided with increased productivity (mg C/rrr /hr) and minimal nutrient limitation. Phytoplankton community composition showed similar patterns of seasonal succession in both years. Focus Categories: NU, HYDROL, ECL