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The Lake Landscape Position Project:

Contrasting Geographic Characters and Water Chemistry as
Determinants of Biodiversity and Biotic Community Structure

Introduction

The importance of the position of a lake in the landscape relative to hydrologic flow has been a focus of the Long-Term Ecological Research Project, based at the UW-Madison Center for Limnology, for almost two decades. We have found that there are strong patterns in water chemistry, primary productivity, and morphology driven by ground and surface water inputs (Magnuson et. al. 1990, Kratz et al. 1997, Riera et al. in press). Also, lake size and physico-chemical conditions within a lake have been shown to influence community structure in fishes (Tonn and Magnuson 1982, Tonn et al.1990). However, we know much less about how landscape level traits, such as surface water connections, influence biotic diversity and community structure within lakes and whether the species pool changes across watersheds of North-Central Wisconsin (Hrabik and Magnuson 1999). The primary goal of our project is to conduct field experiments that contrast lake chemical and geographical properties and explore the role of those properties in shaping biotic diversity and community structure.

Specifically, we will be addressing two broad ecological questions:

1. What is the relative importance of water chemistry and stream connections in shaping biotic diversity and community structure in lakes?
2. Are there significant differences in biotic communities between connected lakes within the same drainage area, or between lakes located in separate watersheds?

Methods

Fish Sampling

Fish sampling was conducted on each lake at least one month after thermal stratification had taken place, beginning on the 3rd week in June and running through the 3rd week in July. This was done to minimize the effects of winter stress and spawning on fish weight given their length. Several gears were employed to estimate fish diversity in each lake, each being effective at catching a different set of fishes.

Vertical gillnets were employed to sample pelagic fishes. A spectrum of mesh sizes (19, 32, 51, 64, 89-mm stretch mesh) were used, with each mesh size effectively catching a different size range of fish. The nets were fished in the deep basin of each lake for one diel cycle.

Fyke nets were employed to sample fishes in the shallow near shore areas. Mini fyke nets with a mouth opening 0.75-m high by 1.25-m wide constructed with 4-mm delta mesh, with a 1-m by 5-m single lead were set so as the lead ran perpendicular from shore and that the mouth sat in approximately 1-m of water. There were 3 nets set at differing locations defined by substrate type (muck, sand and cobble) for one diel cycle. Three crayfish traps were set along side each of the fyke nets so as to sample the same habitat type sampled by each fyke net.

Electrofishing occurred in the near shore area between 0.3 and 1.5-m in depth. Two 30 minute transects were performed such that a variety of substrate types were sampled. The dipnets used to net fish during electrofishing consisted of 4-mm delta mesh and were capable of retaining small fishes (down to 20-mm). Our goal was to capture and identify as many game and non-game fish species as possible.

Fish caught in each gear type were processed by measuring mass and total length of all fish of each species; however, a subset of each species was measured when the catch rate was high. Two fish in each 5-mm size class for each species were weighed and length measurements were taken so as to collect weight measurements for a wide size range of each species. If the catch of a given species in a given size class (small, medium or large) within a particular set or electro-shocking run exceeded 30 fish, 30 were measured for each species. Those not measured for length in each size class were counted and recorded so as to associate them with those that were measured to allow length frequency distributions to be generated while expediting our processing and avoiding redundant weight and length measuring. Each fish was identified to genus and species using the taxonomic key in Becker (1983). Any game fish killed were turned over to the appropriate Department of Natural Resource Game Warden.

Relative body condition of walleye, smallmouth bass, bluegill and yellow perch were assessed using a second order linear weight vs. total length regression for each species. The linear model was applied to log transformed body mass and total length data pooled according to species from fish surveys that took place in each lake in 1998. Fish <75-mm (~3") were excluded from the analysis because of error associated with measuring their mass in the field. The mean residual error relative to the least squares regression line for each lake represented the relative condition of fish >75mm (~3") of each species in each respective lake.

Macrophyte surveys were conducted in mid-July when macrophytes began reaching their greatest summer biomass. Macrophytes were surveyed along a transect running perpendicular to shore in sand and muck substrates. With the exception of species taken for identification purposes, sampling was non-destructive and conducted entirely by scuba divers. Divers enumerated species at one meter intervals along each transect, noting depth, distance along transect, and sediment type. Abundance of each species was determined from haphazardly placed quadrats in which stem density and percent cover for each specieswere determined; the quadrats were placed at depths of one, two, three and four meters.

Each study lake was sampled for the following water chemistry parameters: dissolved oxygen at depth (one meter intervals), temperature at depth (one meter intervals), vertically integrated chlorophyll, alkalinity, pH, conductivity, total nitrogen, total phosphorus, dissolved organic and inorganic carbon, total organic and inorganic carbon, color, and secchi depth. In addition, zooplankton and phytoplankton were sampled for species richness and abundance.

All water chemistry parameters except chlorophyll, temperature, dissolved oxygen and secchi depth were estimated at 1 m below the surface and 2 m above the bottom, with the bottom sample being collected only when the study lake was stratified. Water from each discrete depth was collected using a peristaltic pump and flexible tygon tubing. The tubing was lowered to each depth and a sample was obtained for each parameter. DOC/DIC, color, and samples for total ion concentration required filtering, and the water for those samples was filtered using Nucleopore filters capable of retaining particles > 0.44 micrometers in diameter.

Chlorophyll samples were collected by pumping water from each thermal strata (epilimnion, metalimnion and hypolimnion) through a glass fiber filter. The location of the epi- meta- and hypolimnion was determined from the temperature profile of each lake and each chlorophyll sample was collected so as to obtain an integrated sample. Specifically, within each thermal zone, the sampling tubing was raised and lowered at a constant rate until sufficient sample has been obtained from each depth strata.

Temperature and oxygen profiles were performed using a YSI model 58 temperature/oxygen meter or a Sonde 600xl field meter. Secchi transparency was estimated using a black and white disk 20-cm in diameter.

Zooplankton were sampled by performing replicate tows with a 80-um mesh Wisconsin net from the deepest point to the surface of each lake. An integrated phytoplankton sample was collected using a peristaltic pump and tygon tubing. While the pump is running, the tubing was systematically raised and lowered within the photic zone so as to collect a 250-ml sample representing the phytoplankton community within the photic zone.

Our sampling schedule entailed samples being collected on each of 20 lakes for water chemistry and plankton on a minimum of two sampling dates. Fish, benthos, and macrophytes were sampled only once during the summer owing to time constraints and the large number of lakes in the study. Any data collected on our study lakes will be available to interested lake associations upon request. Also, any reports or theses containing the data will also be furnished upon request with the condition that we have first opportunity to publish our findings

Becker, G. C. 1983. Fishes of Wisconsin. University of Wisconsin Press. Madison, Wisconsin.

Hrabik, T. R., and J. J. Magnuson. 1999. Simulated Dispersal of Exotic Rainbow Smelt in a Northern Wisconsin Lake District and Implications for Management. Canadian Journal of Fisheries and Aquatic Sciences. 00:00-00. In Press.

Kratz, T. K., K. E. Webster, C. J. Bowser, J. J. Magnuson, and B. J. Benson. 1997. The influence of landscape position on lakes in northern Wisconsin. Freshwater Biology 37:290-317

Magnuson, J. J., B. J. Benson and T. K. Kratz. 1990. Temporal coherence in the limnology of a suite of lakes in Wisconsin, U.S.A. Freshwater Biology 23:145-59.

Riera, J. L., J. J. Magnuson, T. K. Kratz, and K. E. Webster. (in press). A geomorphic template for the analysis of lake districts applied to the Northern Highland Lake District, Wisconsin, USA. Freshwater Biology.

Tonn W. M. and J. J. Magnuson. 1982. Patterns in the species composition and richness of fish assemblages in northern Wisconsin lakes. Ecology 63: 1149-1166

Tonn, W. M.,J. J. Magnuson, M. Rask, and J. Toivonen. 1990. Intercontinental comparison of small-lake fish assemblages: the balance between local and regional processes. American Naturalist 136:345-75.