Acidification Of Little Rock Lake Research Articles

Asymmetrical food web responses in trophic-level richness, biomass, and function following lake acidification

We tested for disproportional changes in annual and seasonal species richness and biomass among five trophic levels (phytoplankton, herbivorous, omnivorous, and carnivorous zooplankton, and fish) as well as altered trophic structure and ecosystem function following the 5-year experimental acidification of Little Rock Lake (Wisconsin, USA) from pH 6.1 to 4.7. Abiotic and biotic controls of trophic level response during acidification were also identified. Asymmetric reductions of species richness among trophic levels, separated by life stage and feeding type, were evident and changes in trophic structure were most pronounced by the end of the acidification period. Relative declines in richness of fish and zooplankton were greater than phytoplankton, which were generally unaffected, leading to a reduction of upper trophic level diversity. Each of the lower four trophic levels responded to a distinct combination of abiotic and biotic variables during acidification. pH was identified as a direct driver of change for only carnivorous zooplankton, while all other trophic levels were affected more by indirect interactions caused by acidification. Fluctuations in ecosystem function (zooplankton biomass and primary production) were also evident, with losses at all trophic levels only detected during the last year of acidification. The acidified basin displayed a tendency for greater variation in biomass for upper trophic levels relative to reference conditions implying greater unpredictability in ecosystem function. Together, these results suggest that trophic asymmetry may be an important and recurring feature of ecosystem response to anthropogenic stress.

Trajectories Of Zooplankton Recovery In The Little Rock Lake Whole-Lake Acidification Experiment

Understanding the factors that affect biological recovery from environmental stressors such as acidification is an important challenge in ecology. Here we report on zooplankton community recovery following the experimental acidification of Little Rock Lake, Wisconsin, USA. One decade following cessation of acid additions to the northern basin of Little Rock Lake (LRL), recovery of the zooplankton community was complete. Approximately 40% of zooplankton species in the lake exhibited a recovery lag in which biological recovery to reference basin levels was delayed by 1-6 yr after pH recovered to the level at which the species originally responded. Delays in recovery such as those we observed in LRL may be attributable to "biological resistance" wherein establishment of viable populations of key acid-sensitive species following water quality improvements is prevented by other components of the community that thrived during acidification. Indeed, we observed that the recovery of species that thrived during acidification tended to precede recovery of species that declined during acidification. In addition, correspondence analysis indicated that the zooplankton community followed different pathways during acidification and recovery, suggesting that there is substantial hysteresis in zooplankton recovery from acidification. By providing an example of a relatively rapid recovery from short-term acidification, zooplankton community recovery from experimental acidification in LRL generally reinforces the positive outlook for recovery reported for other acidified lakes.

RESPONSES OF NUTRIENTS TO EXPERIMENTAL ACIDIFICATION AND RECOVERY IN LITTLE ROCK LAKE, U.S.A.

Nutrient concentrations and cycling processes were studied in Little Rock Lake, Wisconsin, U.S.A., during experimental acidification from pH 6.1 to target pH values of 5.6, 5.1 and 4.7 and during the first four years of recovery. Surface waterconcentrations of ammonium, nitrate, soluble reactive phosphate,and total phosphate were not affected, but total nitrogen was lower from year 2 at pH 5.6 until the end of recovery year 3 (pH 5.3). The decrease was attributed to lower dissolved organicN. Annual maximum concentrations of N and P forms at 9 m depth in the hypolimnion increased with decreasing pH, but most of theincrease resulted from a drought-induced decline in lake levelduring acidification that brought the 9 m sampling location closer to the sediment. In contrast to findings elsewhere, acidification of Little Rock Lake had no effect on rates of nitrification during winter ice-cover. Dissolved silica concentrations increased slightly at pH 4.7, probably because of a pH-induced decrease in the diatom population, but hydrologicfactors were more important in controlling silica levels than waspH. Most of the predictive hypotheses made about the effects ofacidification on nutrient forms and processes or their recoveryin the lake were qualitatively correct.

COMPENSATORY DYNAMICS IN ZOOPLANKTON COMMUNITY RESPONSES TO ACIDIFICATION: MEASUREMENT AND MECHANISMS

Previous studies indicate substantial variation in ecological responses to perturbation. In some cases, ecosystems are resilient to perturbation due to compensatory dynamics in which losses of sensitive species are offset by population increases of species that perform similar ecological functions. Here, we report a detailed evaluation of compensatory dynamics in zooplankton community responses to the experimental acidification of Little Rock Lake, Wisconsin, USA. We used a variance ratio to quantify compensatory dynamics in functional groups of zooplankton containing species that use similar resources and are vulnerable to the same predators. We also used first-order autoregressive models to explore mechanisms driving the dynamics of each functional group. Our results indicate that responses of functional groups to acidification can be highly variable. Herbivorous copepods and medium-sized herbivorous cladocerans exhibited significant compensatory dynamics in response to acidification, whereas other functional groups exhibited independent or synchronous dynamics. First-order autoregressive models indicated that groups exhibiting compensatory dynamics contained both acid-tolerant and acid-sensitive species that competed. In contrast, groups that contained only acid-sensitive or acid-tolerant species exhibited more independent or synchronous dynamics. Overall, our study highlights the combined roles of sensitivity to environmental perturbation and species interactions in determining the extent of compensatory dynamics in zooplankton functional group responses to acidification.

Multiple stresses from a single agent: Diverse responses to the experimental acidification of Little Rock Lake, Wisconsin

A single stress, acidification with sulfuric acid, was applied to Little Rock Lake in a whole‐ecosystem manipulation. We documented a wide range of responses to the acidification, including increases in the concentrations of various chemicals, shifts in microbial processes and a major increase in water clarity to UV‐B radiation. Each of these changes could in itself be considered as a separate ecosystem stress that is distinct from the intended manipulation. Acidification in Little Rock Lake was accompanied by a number of substantial changes in the occurrence of organisms. A series of detailed investigations indicates that the mechanisms underlying these organismal changes are varied but cannot usually be tied to the direct effects of acidification. Overall, our results demonstrate how multiple stresses can arise from a single agent operating on an ecosystem and suggest that singly operating stresses may actually be quite rare.

Zooplankton Community Responses during Recovery from Acidification in Little Rock Lake, Wisconsin

Follow‐up studies after whole‐ecosystem‐stress experiments can provide important insights into the recovery process itself and into basic ecosystem properties. We report here on zooplankton community recovery during the first 5 years following the experimental acidification of Little Rock Lake, Wisconsin, U.S.A. Acidity in the lake’s treatment basin returned quickly to near pre‐manipulation levels. Zooplankton population shifts, however, did not support our hypothesis that species that had increased in abundance with acidification would persist and resist the return of the pre‐manipulation community. The three species that had proliferated most dramatically under low pH conditions—Daphnia catawba, Tropocyclops extensus, and Keratella taurocephala, returned close to their originally low, pre‐acidification population levels during the early stages of acid recovery. Some species that had been reduced during low pH conditions, such as Diaptomus minutus and Daphnia dubia, did not recover to pre‐manipulation levels. Overall, the zooplankton community in the treatment basin exhibits little similarity to that in the reference basin, a condition quite different from that which had occurred prior to the imposition of acid stress.

Rotifer responses to increased acidity: long-term patterns during the experimental manipulation of Little Rock Lake

Little Rock Lake, Wisconsin, U.S.A. has been the site of a whole-ecosystem experiment since 1983. It was divided into a treatment basin that was acidified in three, two-year stages and a reference basin. The rotifer community in the treatment basin exhibited a variety of responses to the manipulation. Many species decreased in abundance under reduced pH conditions but other rotifers increased at the same time such that there were ultimately increases with acidification in total rotifer biomass, and quite conspicuously, in the proportion that rotifers comprised of total zooplankton biomass. Ten rotifer species decreased at some stage during the acidification (e.g., Kellicottia longispina, Asplanchna priodonta and Keratella cochlearis) while four species increased dramatically (e.g., Synchaeta sp. and Keratella taurocephala ). Similarity indices and total rotifer biomass differences measured between the two basins exhibited very different temporal patterns of response to acidification. Similarity decreased regularly beginning with the earliest stages of acid additions while biomass was nearly the same between the basins until the late stages of the experiment. Comparisons with other nearby lakes indicate, however, that acid conditions are not the only factors generating among-lake differences in rotifer community characteristics. Changes observed with acidification in Little Rock Lake were such that its total rotifer biomass grew more similar to that in a nearby acidic-bog lake and different from that in a near-neutral-pH lake. At the same time, abundance patterns for individual rotifer species in Little Rock Lake were not particularly similar to those in the other lakes. It appears that, although they are important, acid conditions alone can not account for all observed rotifer community differences among lakes. Higher proportions of rotifer biomass and high populations of K. taurocephala do seem to be common features of many low pH habitats.

Ultraviolet radiation in North American lakes: Attenuation estimates from DOC measurements and implications for plankton communities

Climate warming in North America is likely to be accompanied by changes in other environmental stresses such as UV‐B radiation. We apply an empirical model to available DOC (dissolved organic C) data to estimate the depths to which 1% of surface UV‐B and UV‐A radiation penetrate for several major regions of North America. UV attenuation depths are also estimated from DOC data collected from treatment and reference basins during the experimental acidification of Little Rock Lake, Wisconsin.In some regions of North America 25% of the lakes have 1% attenuation depths for UV‐B radiation on the order of 4 m or more (western and northwestern U.S., Newfoundland). In other regions, 75% of the lakes have 1% attenuation depths for UV‐B shallower than 0.5 m (Florida, upper midwestern U.S., northwestern Ontario, Quebec, and Nova Scotia). Attenuation depths for UV‐A radiation are ∼2.5 times as deep as those for UV‐B. Experimental acidification approximately doubled the estimated 1% attenuation depths for UV radiation in Little Rock Lake.The strong dependence of 1% attenuation depth on DOC below the 1–2 mg liter‒1 DOC range suggests that UV attenuation in low DOC lakes is highly sensitive to even very small changes in DOC. We conclude that changes in climate, lake hydrology, acid deposition, and other environmental factors that alter DOC concentrations in lakes may be more important than stratospheric ozone depletion in controlling future UV environments in lakes.

Experimental acidification of Little Rock Lake, Wisconsin: The first four years of chemical and biological recovery

Little Rock Lake was experimentally acidified in 1984-1990 during which sulfuric acid was added to one basin, decreasing pH from 6.1 to 5.6, 5.1 and 4.7. The lake has been allowed to recover without manipulation since autumn 1990. By the third year of recovery, ∼40% of the change necessary to return to pre-acidification values of pH, acid neutralizing capacity (ANC), sulfate (SO 4 2- ) and calcium (Ca 2+ ) had occurred. During recovery years 1-2, ANC was closely predicted by models based on acidification phase observations, but recovery during years 3-4 was slower than predicted. A possible explanation for the slowed recovery is acidification of the upper 0-5 cm of sediment, which acts as a sink for the ANC generated via SO 4 2- reduction, the primary recovery mechanism. Trends for zooplankton did not follow pH recovery very closely. Species diminished by acidification (e.g. Keratella cochlearis, Daphnia dubia) have not recovered, but species that dominated the community at pH 4.7 (e.g. K. taurocephala, D. catawba) have not maintained high populations. The time required for the zooplankton community to recover to pre-manipulation conditions is uncertain. Delays also have been observed for the mayfly species Caenis, which had disappeared at pH 4.7. In contrast, reproductive success of largemouth bass (Micropterus salmonides) mirrored that observed during acidification ; egg hatch and survival of young-of-the-year to autumn recurred when pH exceeded response levels documented during acidification. Overall, recovery has not closely followed the pattern predicted by acidification responses.

Experimental Acidification of Little Rock Lake, Wisconsin: Chemical and Biological Changes over the pH Range 6.1 to 4.7

The two basins of this seepage lake were separated by a vinyl curtain in August 1984 after a year of background studies, and acidification of one basin with H2SO4 began at ice-out in 1985. Chemical and biological responses measured during successive 2-yr periods at pH ~5.6, 5.1, and 4.7 verified some but not all impacts predicted at the outset. Changes in major, minor, and trace ions generally agreed with predictions. Internal alkalinity generation (IAG) increased at lower pH, and sulfate reduction eliminated ~50% of added H2SO4. Sediment cation exchange was important in IAG and acidified surface sediments, possibly diminishing the lake's ability to counteract further H+ inputs. Mass loss of oak leaves was reduced at pH 5.1 (birch leaves at pH 4.7). Population parameters were more sensitive than community measures for plankton. Species composition changed at each pH, especially at pH 4.7. Many changes in zoopiankton and benthos were indirect responses to an algal mat that developed at lower pH or to food web interactions; these were not predicted accurately. Sensitivity of major fishes to lower pH was Ambloplites rupestris > Micropterus salmoides > Pomoxis nigromaculatus > Perca flavescens. Fish production was reduced at pH's above those resulting in population decreases.

Response of predatory zooplankton populations to the experimental acidification of Little Rock Lake, Wisconsin

To assess the effects of lake acidification on large predatory zooplankton, we monitored population levels of four limnetic taxa for 6 years in a lake with two basins, one of which was experimentally acidified (2 years at each of three levels: pH 5.6, 5.2 and 4.7). Concentrations of phantom midge ( Chaoborus spp.), the most abundant large predator, remained similar in the treatment and reference basins until the fourth year (pH 5.2) when they increased in the treatment basin. In contrast, Epischuru lacustris and Leptodora kindtii disappeared from limnetic samples, and water mites declined to near zero upon acidification. Treatment basin populations of E.Iacustris declined sharply during the second year of acidification. The nature of the decline suggested sensitivity of an early life stage during the first year at pH 5.6. Leptodora kindtii showed no population response at pH 5.6, but declined to essentially zero at pH 5.2. Treatment basin populations of water mites fluctuated until declining in the fifth and sixth years (pH 4.7). These changes indicate a variety of direct and indirect responses to lake acidification.

Complex biological responses to the experimental acidification of Little Rock Lake, Wisconsin, USA

Acidification can affect aquatic organisms directly through hydrogen ion toxicity, and indirectly through disrupted food web dynamics and altered abiotic conditions. Field populations from selected taxa were studied during the Little Rock Lake whole-basin acidification experiment to illustrate patterns whose timing suggests direct (i.e. immediate) or indirect (i.e. delayed or non-uniform) responses to pH change. As the treatment basin was acidified to pH 5·6, 5·2 and 4·7, immediate changes consistent with a direct pH response were observed for species representing several trophic levels. For other taxa (e.g. littoral invertebrates associated with filamentous algal mats, several species of pelagic zooplankton), indirect mechanisms induced by food web changes were more likely explanations for abundance patterns. The results presented here suggest that the responses of aquatic ecosystems to acidification involve a complex interplay between direct pH effects and subsequent indirect interactions.

Changes in trace metal concentrations in lake water and biota during experimental acidification of Little Rock Lake, Wisconsin, USA

Little Rock Lake, a small (18 ha), low-alkalinity (25 μeq litre −1, pH 6·1) seepage lake in northern Wisconsin, was divided into two basins by a flexible, inert barrier and, beginning in spring 1985, the north basin was acidified in three 2-year steps to pH 5·6, 5·1 and 4·7. The annual average pH of the reference basin remained near 6·1. As part of a comprehensive programme to determine the chemical and biological responses to acidification, minor metals (Al, Fe, Mn) and trace metals (Cd, Cu, Pb, Zn) in lake water (0·4 μm pore filtered samples), periphyton, zooplankton, and yellow perch ( Perca flavescens) were measured. At pH 5·6, dissolved Mn and Fe increased in the acidified basin. At pH 5·1 and 4·7, dissolved Al, Fe, Mn, Cd and Zn were elevated in the acidified basin. At pH 4·7, dissolved Pb in the acidified basin became elevated over reference basin levels. Dissolved Cu remained similar in both basins down to pH 4·7. Cd burdens in periphyton collected on artificial substrates were lower in the treatment basin at pH 5·1 (1·8 μg g −1 dry wt.) than in the reference basin at pH 6·1 (7·5 μg g −1 dry wt.), but Al and Fe burdens in periphyton were similar in both basins. Likewise, Cd levels in muscle tissue of perch from the treatment basin at pH 4·7 were lower (26 ng g −1 dry wt.) than in the reference basin at pH 6·1 (36 ng g −1 dry wt.); Al and Fe burdens were similar in perch muscle tissue from both basins. Levels of Cd and Fe in zooplankton from the acidified basin at pH 4·7 were ≊2× higher than in animals from the reference basin. In both basins of the lake, Al and Cd levels in lake biota decreased with increasing trophic level, demonstrating that food chain biomagnification does not occur for these metals.

Experimental acidification of Little Rock Lake (Wisconsin): fish research approach and early responses.

One goal of research at Little Rock Lake, Wisconsin, is to enhance understanding of lake acidification effects on warm- and cool-water fishery resources. The Little Rock Lake fish assemblage is characteristic of many acid sensitive waters in North America and is dominated by yellow perch (Percidae) and sunfishes (Centrarchidae). Analyses of reproduction, early survival and growth rates in the field were designed around the differing reproductive modes of these taxa. Complementary laboratory research on early life stages was conducted to assist in isolating direct effect mechanisms and to determine the reliability of laboratory results in predicting field response. Preliminary findings suggest that lake acidification to pH 5.6 has not influenced reproductive activity of the four most abundant fish species. However, the field results suggest that year-class failure of rock bass (Ambloplites rupestris) may be occurring due to reduced survival of early life stages. Reduced growth and food conversion efficiency of Age 0 largemouth bass (Micropterus salmoides) is also suggested. The laboratory bioassays indicate rock bass is the most acid-sensitive Little Rock Lake species tested. However, rock bass fry survival was not significantly affected until pH was reduced from 5.6 to 5.0.

Early zooplankton response to experimental acidification in Little Rock Lake, Wisconsin, USA

Early zooplankton response to experimental acidification in Little Rock Lake, Wisconsin, USA