Richard C. Lathrop

Phosphorus Loads to Surface Waters: A Simple Model to Account for Spatial Pattern of Land Use

Modeling nonpoint-source phosphorus (P) loading from land to surface wa- ters can be both complex and data intensive. Our goal was to develop a simple model that would account for spatial pattern in topography and land use using geographic information system (GIS) databases. We estimated areas of the watershed that strongly contributed to P loading by approximating overland flow, and modeled annual P loading by fitting three parameters to data obtained by stream monitoring. We calibrated the model using P loading data from two years of contrasting annual precipitation for Lake Mendota, a Wisconsin eutrophic lake in a watershed dominated by agriculture and urban lands. Land-use scenarios were developed to estimate annual P loading from pre-settlement and future land uses. As much as half of the Lake Mendota watershed did not contribute significantly to annual P loading. The greatest contribution to loading came from a heterogeneous riparian corridor that varied in width from 0.1 km to :6 km depending on topography and runoff conditions. We estimate that loading from pre-settlement land use was one-sixth of the loading from present land use. A future scenario, representing an 80% increase in existing urban land (from 9 to 16% of total watershed area, which would be reached in 30 yr with current land- use trends), showed only modest increases in annual P loading but possible significant effects on water quality. If the watershed were to become entirely urbanized, P loading to the lake would double and potential effects on water quality would be severe. Changes in P loading were strongest with conversions of undisturbed vegetated lands, especially ri- parian areas, to either urban or agricultural uses. Variability in total annual rainfall leads to variability in the riparian area that affects P loading, with implications for policies intended to control nonpoint nutrient inputs.

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.

The Rise and Fall of a Dominant Planktivore: Direct and Indirect Effects on Zooplankton

We analyzed a 14-yr time series (1976-1989) of planktivorous fish and zooplankton from Lake Mendota, Wisconsin. Planktivory rates changed by an order of magnitude during this time period, primarily due to the rise and fall of the 1977 year class of cisco (Coregonus artedi) that dominated planktivory rates for a period of 10 yr. Plank- tivory increased between 1977 and 1978 due to an increase in biomass of that year class and decreased in August 1987 after a summer kill of cisco in the lake. Yellow perch (Perca flavescens) and other cisco year classes contributed <25% of the total planktivory during 1978 to 1987. Time series analysis revealed that this 10-yr pulse in planktivory rates was associated with changes in the Daphnia species and biomass. In years with low planktivory rates, higher biomass of daphnids dominated by Daphnia pulicaria developed earlier in the spring and lasted longer into the summer. This was also associated with an earlier and longer spring clear-water phase. In years with high planktivory rates, daphnid biomass was generally lower and dominated by the smaller Daphnia galeata mendotae. However, there was no significant effect of planktivory on the early summer peak in Daphnia biomass that is associated with a clear-water phase. The dynamics of this Daphnia peak are apparently regulated by Daphnia-algae interactions and not by planktivory rates. The seasonal and interyear changes in Daphnia species composition can be explained by the combined effects of planktivory, dynamics of food resources, and the physiological ecology of the two Daphnia species. There was no significant effect of increased planktivory on total zoo- plankton biomass due to a compensatory increase in cyclopoid copepods and no response by calanoid copepods. The recent history of Lake Mendota illustrates a 10-yr period of predation imposed by a single year class of a large, long-lived, obligate planktivore. It is an example of variability in a lake ecosystem scaled to the life-span of a dominant fish species.

Understanding Regional Change: A Comparison of Two Lake Districts

ABSTRACT We compared long-term change in two lake districts, one in a forested rural setting and the other in an urbanizing agricultural region, using lakes as sentinel ecosystems. Human population growth and land-use change are important drivers of ecosystem change in both regions. Biotic changes such as habitat loss, species invasions, and poorer fishing were prevalent in the rural region, and lake hydrology and biogeochemistry responded to climate trends and landscape position. Similar biotic changes occurred in the urbanizing agricultural region, where human-caused changes in hydrology and biogeochemistry had conspicuous effects. Feedbacks among ecosystem dynamics, human uses, economics, social dynamics, and policy and practice are fundamental to understanding change in these lake districts. Sustained support for interdisciplinary collaboration is essential to build understanding of regional change.

Phosphorus Flow in a Watershed-Lake Ecosystem

Cultural eutrophication of lakes caused by excess phosphorus (P) loading from agricultural areas is a persistent and serious environmental problem. We quantified P flows in a watershed-lake ecosystem using a simple mathematical model that coupled in-lake and upland processes to assess and compare the long-term impacts of various management strategies. Our model compares abatement by in-lake strategies (such as increasing the flux of P from algae to consumers and alum application) with riparian management to decrease P flow and with balancing P budgets at the watershed scale. All of these strategies are effective to some extent. However, only reducing the amount of fertilizer P imported to the watershed will decrease the total P in the system at steady state. Soil P—a large reservoir with slow turnover rate—governs long-term flux to the lake and must be decreased in size to maintain long-term control of eutrophication.

Perspectives on the eutrophication of the Yahara lakes

Abstract Eutrophication of the four Yahara lakes—Mendota, Monona, Waubesa, and Kegonsa—near Madison, Wisconsin, has been dramatic since the mid-1800s. For Lake Mendota, the erosion of sediments from higher water levels established by the damming of the lakes outlet, plus the agricultural expansion of its watershed, resulted in blue-green algal growths. These impacts, however, were dwarfed by water quality problems stemming from Madisons wastewater inputs that directly entered Lake Monona from the late 1800s through 1936, and then Lake Waubesa until 1958. Blue-green algal blooms were so bad in the lower Yahara lakes that the Madison Public Health Department conducted major copper sulfate treatments during 1925–1954. During the wastewater input years, inorganic nitrogen (N) and especially dissolved reactive phosphorus (DRP) concentrations in the surface waters were very high (particularly in Waubesa and Kegonsa), indicating neither nutrient was limiting algal growth. No P legacy from the wastewater inputs was found in Waubesa and Kegonsas sediments; minimal P-binding potential due to low iron (Fe) availability is the hypothesized reason. Mendotas algal blooms were not a problem until the mid-1940s when wastewater inputs from upstream communities increased as well as the agricultural use of N and P fertilizers. This increase in eutrophication symptoms coincided with an increase in indices of DRP and inorganic N concentrations in the lake. After wastewater diversion in 1971, blue-green algal blooms persisted in Lake Mendota, and the onus of the problem shifted to agricultural and urban nonpoint source pollution. While much progress has been made in recent years to control these pollution sources to Mendota, manure runoff during late winter continues as a management problem. As evidence, P loadings during January to March constituted 48% of total loadings measured for 1990–2006 in the Yahara River subwatershed. Much of this runoff P was dissolved and not associated with high sediment loads, whereas during other months, more of the runoff P was bound to sediments that could settle out in lower stream reaches prior to entering the lake. However, low P-binding potential of recently deposited sediments in Mendota along with signs of water quality improvements following periods of drought indicate the lake could respond rapidly to nutrient input reductions. Finally, DRP and inorganic N concentrations since 1980 have indicated that algal growth in the Yahara lakes during July–August may have been limited by not only P, but N (especially in the lower Yahara lakes). Aggressive programs to reduce inputs from both nutrients will be important to prevent scum-forming blue-green algal blooms and filamentous algal growths that could become problematic once zebra mussels become established in the Yahara lakes.

Lake restoration: capabilities and needs

Lake degradation results from excessive nutrient inputs, toxic substances, habitat loss, overfishing, species invasions and extirpations. The scientific basis of lake degradation is generally well understood, although each restoration project requires some level of new site-specific research. Remediation may require management actions which are difficult to implement for social or institutional reasons. Even where large-scale remediations are attempted, it is difficult to sustain scientific assessments for long enough to evaluate success. Collaborations of scientists and managers have sometimes succeeded in overcoming limitations to lake restoration, and produced important advances in our capability to restore lakes.

Cultural impacts on macrophytes in the Yahara lakes since the late 1800s

Abstract Vegetation changes in the Yahara lakes around Madison, Wisconsin, USA, are evaluated using historical data. Present vegetation is less diverse and less extensive, produces less biomass and is composed of more disturbance tolerant species than it was 80–100 years ago. Many changes are related to human impacts that began about 150 years ago which directly altered plant habitat, decreased water clarity, were toxic to plants, removed plant biomass or reproductive structures, or resulted from exotic invaders. The impacts are interrelated and overlap in time and space so change did not occur from simple cause and effect relationships. However, cumulative effects of the impacts are very evident. Management recommendations are made that are applicable to North American lakes with a similar history.

Patterns of vegetation change in Lake Wingra following a Myriophyllum spicatum decline

Abstract The invading aquatic plant, Myriophyllum spicatum L. is a management concern in many North American lakes because it replaces native species and because its dense growth can be a nuisance to lake users. It is common for M. spicatum to expand quickly upon reaching a lake, remain the most abundant littoral plant for a number of years, and then decline rather rapidly. This pattern held true for Lake Wingra, Dnne County, WI, where M. spicatum dominated the littoral vegetation during the late 1960s, but abruptly declined during the 1970s. In this paper, we explore the changes in the Lake Wingra plant community that have occurred in the wake of the M. spicatum decline. We present results of 1991 and 1992 vegetation surveys indicating that M. spicatum , while no longer the dominant macrophyte, remains an important member of the Lake Wingra plant community. It and Ceratophyllum demersum L. now make up roughly equal parts of the littoral vegetation, and native species, rare or absent during the 1960s, are growing well. By comparing current plant distributions with those found earlier, we examined probable causes for the M. spicatum decline; no single factor seemed to be responsible. The milfoil decline in Lake Wingra has been sustained over roughly two decades while the native vegetation has expanded.

Phytoplankton and Their Relationship to Nutrients

The phytoplankton that commonly occur in Lake Mendota are the central focus of the food web research detailed in this volume (Figure 7.1). The role of nutrients, particularly phosphorus, in regulating algal biomass and stimulating blooms is well established (Vollenweider 1968; Schindler 1977, 1988). Herbivory by zooplankton has also been long recognized as an important influence on phytoplankton abundance and species composition (Hrbacek 1962; Brooks and Dodson 1965; Shapiro et al. 1975; Shapiro and Wright 1984; Carpenter and Kitchell 1988). While phosphorus reductions cause declines in blue-green algal densities (Schindler 1988), the effects of herbivory are more variable and complicated (Sterner 1989; Carpenter, Ch. 23). For example, increased herbivory has been both stimulated (Lynch 1980; Anderson and Cronberg 1984) and suppressed (Shapiro and Wright 1984; Carpenter et al. 1987; Vanni et al. 1990) blue-green algae.