The experimental watershed liming study: Comparison of lake and watershed neutralization strategies

The Experimental Watershed Liming Study (EWLS) was initiated to evaluate the application of CaCO₃ to a forested watershed in an effort to mitigate the acidification of surface water. The objective of the EWLS was to assess the response of the Woods Lake watershed to an experimental addition of CaCO₃. During October 1989, 6.89 Mg CaCO₃/ha was applied by helicopter to two subcatchments comprising about 50% (102.5 ha) of the watershed area. The EWLS involved individual investigations of the response of soil and soil water chemistry, forest and wetland vegetation, soil microbial processes, wetland, stream and lake chemistry, and phytoplankton and fish to the CaCO₃ treatment. In addition, the Integrated Lake/Watershed Acidification (ILWAS) model was applied to the site to evaluate model performance and duration of the treatment. The results of these studies are detailed in this volume. The purposes of this introduction and synthesis paper are to: 1) present the overall design of the EWLS, 2) discuss the linkages between the individual studies that comprise the EWLS, and 3) summarize the response of the lakewater chemistry to watershed addition of CaCO₃ and compare these results to previous studies of direct lake addition. An analysis of lake chemistry revealed the watershed treatment resulted in a gradual change in pH, acid neutralizing capacity (ANC) and Ca²⁺ in the water column. This pattern was in contrast to direct lake additions of CaCO₃ which were characterized by abrupt changes following base addition and subsequent rapid reacidification. Over the three-year study period, the supply of ANC to drainage waters was largely derived from dissolution of CaCO₃ in wetlands. Relatively little dissolution of CaCO₃ occurred in freely draining upland soils. The watershed treatment had only minor effects on forest vegetation. The watershed treatment eliminated the episodic acidification of streamwater and the near-shore region of the lake during snowmelt, a phenomenon that occurred during direct lake treatments. Positive ANC water in the near-shore area may improve chemical conditions for fish reproduction, and allow for the development of a viable fish population. The watershed CaCO₃ treatment also decreased the transport of Al from the watershed to the lake, and increased the concentrations of dissolved organic carbon (DOC) and dissolved silica (H₄SiO₄) in stream and lakewater. The watershed treatment appeared to enhance soil nitrification, increasing concentrations of NO₃ ^– in soilwater and surface waters. However, the acidity associated with this NO₃ ^– release was small compared to the increase in ANC due to CaCO₃ addition and did not alter the acid-base status of Woods Lake. Acid neutralizing capacity (ANC) budgets for 12-month periods before and after the watershed treatment showed that the lake shifted from a large source of ANC to a minor source due to retention of SO₄ ^2–, NO₃ ^–, Al and the elevated inputs of Ca²⁺ associated with the watershed CaCO₃ application. In contrast to the direct lake treatments, Ca²⁺ inputs from the watershed application were largely transported from the lake.     

Role of dissolved organic carbon in the attenuation of photosynthetically active and ultraviolet radiation in Adirondack lakes

1. We surveyed eighty-five lakes located in the Adirondack Mountain Region of New York State, U.S.A., to characterize the attenuation of photosynthetically active (PAR) and ultraviolet radiation (UVR) in relation to dissolved organic carbon (DOC) concentrations and pH. Attenuation of PAR was quantified in situ . Attenuation was also inferred by measuring the light absorption of filtered lake water samples at wavelengths (300, 340 and 440 nm) representing UV-B, UV-A and PAR.
2. Substantial variation in transparency was observed among lakes in this region. Attenuation depths ( z _1%) for PAR ranged from 0.5 to greater than 20 m, while inferred values for UV-B and UV-A ranged from a few centimetres to > 5 m. Median values of UV-A penetration (0.75 m) and UV-B penetration (0.45 m) corresponded to 11% (UV-A) and 6% (UV-B) of lake maximum depth.
3. Much of the variation in PAR and UVR attenuation was explained by differences in lake DOC. Univariate power models based solely on DOC accounted for 85% (PAR), 90% (UV-A) and 91% (UV-B) of the variation in absorption.
4. Attenuation and absorption coefficients were generally lower for recently acidified lakes compared to acidic and circumneutral lakes which have not undergone recent acidification. However, differences among these three groups of lakes were not statistically significant. Our results suggest that the effects of acidification on the optical properties of a regional population of lakes, even in an area experiencing widespread acidification, are relatively subtle in comparison with other factors contributing to inter-lake variability.
5. The presence of near-shore wetlands is probably a key factor influencing regional variability in DOC and light climate among Adirondack lakes. Temporal variability in climatic factors influencing wetland DOC production and export may mask more subtle influences on lake DOC associated with anthropogenic acidification.     

Importance of phytoplankton carbon to heterotrophic bacteria in the Ohio,Cumberland, and Tennessee rivers,USA

The flux of carbon and nutrients through aquatic systems is largely dependent upon interactions between autotrophic and heterotrophic processes. As a means of assessing the relative importance of autotrophy and heterotrophy in large rivers, we compared phytoplankton production, heterotrophic bacterial production and community respiration in three regulated rivers of the Midwestern USA. Samples were collected monthly (March to December 1999) from impoundments of the Ohio (McAlpine Pool), Cumberland (Lake Barkley), and Tennessee (Kentucky Lake) Rivers. Bacterial production was tightly coupled to phytoplankton production at each site (r ² = 0.63–0.85). Ratios of phytoplankton production to bacterial production ranged from <1 to 15 in the Tennessee and Cumberland Rivers and 2 to 90 in the Ohio River. The ratio of primary production to community respiration (P:R) ranged from 0.03 to 2.76 across all sites, with average P:R values lower in the Ohio River (0.14) than the Tennessee River (0.39) and the Cumberland River (1.10). P:R values above unity (P > R) were observed only in the Tennessee and Cumberland Rivers during seasonal (April–July) spikes in primary production. We estimate that 3, 6, and 20% of annual bacterial carbon requirements were met by exudates from in situ phytoplankton in the Ohio River, Tennessee River, and Cumberland River, respectively. Our findings indicate that heterotrophic bacteria were largely dependant upon allochthonous carbon. Autochthonous sources provided supplemental organic matter (up to 40% of bacterial carbon demand) during summer low flow. Handling editor: J. Padisak     

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.     

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.     

The influence of cyanobacteria blooms on the attenuation of nitrogen throughputs in a Baltic coastal lagoon

We combined a mass balance approach with measurements of air–water and sediment–water nitrogen (N) exchange to better understand the mechanisms attenuating N throughputs in a eutrophic coastal lagoon. We were particularly interested in how seasonal shifts in external versus internal N fluxes and the transition from diatom- to cyanobacteria- dominated phytoplankton communities influence N storage and loss to the atmosphere. We found that on an annual basis almost all of the N removed by the lagoon was due to sediment storage following the spring diatom bloom. This period was characterized by high riverine inputs of dissolved inorganic nitrogen, high rates of assimilatory conversion to particulate nitrogen (PN), and net accrual of N in sediments. By contrast, the larger summer bloom was associated with low sediment N storage, which we attribute in part to the presence of positively-buoyant cyanobacteria. Low settling rates during cyanobacteria blooms favored export of PN to the Baltic Sea over sediment accrual in the lagoon. In addition, summer dinitrogen (N₂) fixation by cyanobacteria largely offset annual N₂ losses via denitrification. These findings show that cyanobacteria blooms diminish N attenuation within the lagoon by altering the balance of N exchange with the atmosphere and by promoting export of particulate N over sediment burial.     

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.     

Phytoplankton growth in the Ohio, Cumberland and Tennessee Rivers, USA: inter-site differences in light and nutrient limitation

Seasonal patterns in resource limitation of phytoplankton growth were assessed monthly within three large rivers with differing extents of water regulation. The Ohio River is regulated by low dams that do not substantially modify discharge, while the Cumberland and Tennessee Rivers are impounded by a series of high dams to enhance water storage for downstream flood control. Laboratory dilution assays with light and nutrient manipulations indicated that light was the main factor limiting phytoplankton growth at irradiances below 7 E m^–2 d^–1. Light limited growth was frequent in the turbid, higher discharge of the Ohio River, but was rare in the heavily regulated Tennessee and Cumberland Rivers. When irradiance exceeded 7 E m^–2 d^–1, phytoplankton were either P-limited (Cumberland River), co-limited by P and N (Tennessee River), or Si limited (Ohio River). Site-specific differences in nutrient limitation were consistent with differences in ambient nutrient levels, with the Tennessee and Cumberland Rivers characterized by lower N and P concentrations, and the Ohio River by lower Si. Downstream nutrient depletion was evident in the Ohio River through comparison of an upstream and a downstream site, with nutrient limitation (Si) occurring more frequently downstream. Phytoplankton growth rates at ambient light and nutrient levels ranged from 0.1 to 1.5 d^–1 in the Ohio River and 0.2 to 0.6 d^–1 in the Tennessee and Cumberland Rivers. Growth rates were greatest at the onset of the summer base pool, as light intensities increased and nutrient levels were maximal. Our findings indicate that multiple factors regulate phytoplankton growth in regulated rivers and that spatial complexity may arise from differences in discharge and water aging.     

Effects of Prescribed Fire on Nutrient Pools and Losses from Glades Occurring Within Oak-Hickory Forests of Central Kentucky

Forest openings, also known as glades, arise through a variety of mechanisms including disturbance (fire and blow downs) and local variation in soil or bedrock geology. They are common in many forest types and are often dominated by locally rare herbaceous species. Prescribed burning is increasingly used as a management approach for maintaining glades although little is known about the effects of fire on these habitats. Of particular concern is the potential for nutrient loss during and after fire because glades are often characterized by nutrient‐poor soils. We quantitated nutrient losses through combustion and leaching for glade and adjacent forest habitats subjected to a prescribed burn. Our findings suggest that spring burns do not result in appreciable loss of nutrient capital from glades in comparison with those observed in the surrounding forest. Fire resulted in a substantial loss of litter mass (37%) in the forest but no measurable loss in the glade. Nitrogen losses through combustion were significant in the forest and were equivalent to 4.5 years of atmospheric inputs. Fire significantly increased soil nitrate pools in forest but not in glade plots. No detectable increases in nitrogen, phosphorus, or base cation leaching were observed in either forest or glade habitats within 4 months after the burn. These findings suggest that plant and microbial nutrient uptake rapidly reestablish control over leaching losses when burns are conducted at the start of the growing season. Biotic retention minimizes fire impacts on nutrient loss from the ecosystem.     

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.