site. Below, we highlight what we feel are the most significant scientific contributions from the

Transcription

1 NTL LTER Major Findings This past year has been a productive one for the North Temperate Lakes (NTL) LTER site. Below, we highlight what we feel are the most significant scientific contributions from the past year, drawing on findings from a wide spectrum of topics. These papers are from both published and in press sources, as well as draft manuscripts we are currently preparing for submission. As is evident from our publication list (which includes publications from 1999 to the present), these five research nuggets represent only a small portion of a much larger body of work from our site. For a much more inclusive description of research findings and publications, please refer to the extensive listing on our website at

2 Landscape position and the taxonomic richness of lakes Understanding the factors influencing the taxonomic diversity of an area has been a longstanding goal of ecology. The theory of island biogeography predicts that the taxonomic richness of a group of organisms is a dynamic equilibrium between rates of immigration and extinction (MacArthur and Wilson 1967, MacArthur 1972). Considering lakes as islands in a terrestrial sea, we evaluated factors influencing taxonomic richness of macrophytes, benthic invertebrates, snails and fish in a series of north-temperate lakes. Lake area, the presence and order of stream inlets and outlets, and aspects of water chemistry such as ionic strength are all likely to influence taxonomic richness of lakes through their effect on local immigration and extinction rates. However, because these factors tend to covary in lakes (Kratz et al. 1997, Magnuson and Kratz 2000, Riera et al. 2000), it has been difficult to separate the relative importance of these features independently (Tonn et al. 1990, Magnuson et al. 1998). By carefully selecting study lakes with contrasting stream connectivity and ionic strength we were able to separate the effects of these variables on taxonomic diversity while controlling for effects of lake size (Hrabik et al., in preparation). Linear regression and classification and regression tree analyses showed that richness within each of the four taxonomic groups was positively related to ionic strength (as measured by specific conductance). After variance owing to ionic strength was removed, the presence of stream connections was positively related to increased richness in fishes, macrophytes, and snails (Figure 1). Our results are consistent with the notion that the higher ionic strength of water in lakes occurring lower in the hydrologic flow system reduces extinction rates and that stream connectivity to lakes enhances immigration rates. These findings link processes of immigration

3 and extinction of four groups of organisms to landscape level hydrologic characteristics left as the result of the glacial history of the region Fish Richness Snail Richness Isolated Connected 0 Isolated Connected Richness30 Invertebrate Plant Richness Isolated Connected 0 Isolated Connected Low Conductivity High Conductivity Figure 1. Boxplots of species richness for four groups of organisms segregated by levels of lake specific conductance and the presence or absence of stream connections to other lakes. Filled boxes indicate values for low conductance lakes, open boxes indicate values for high conductance lakes. Literature Cited Hrabik, T.R., B.K. Greenfield, D.B. Lewis, A.I. Pollard, K.A. Wilson and T.K. Kratz. In preparation. Landscape scale variation in taxonomic diversity in four groups of aquatic organisms: the influence of physical, chemical, and biological properties. Kratz T.K., K.E. Webster, C.J. Bowser, J.J. Magnuson, and B.J. Benson The influence of landscape position on lakes in northern Wisconsin. Freshw. Biol. 37: MacArthur R.H., and E.O. Wilson The theory of island biogeography. Monographs in population biology. Princeton University Press, Princeton, N.J. MacArthur R.H Geographical ecology. Patterns in the distribution of species. Harper Row, New York. Magnuson J.J., and T.K. Kratz Lakes in the landscape: approaches to regional limnology. Verh. Int. Ver. Limnol. 27:1-14.

4 Magnuson, J.J., W. M. Tonn, A. Banerjee, J. Toivonen, O. Sanchez and M. Rask Isolation vs. extinction in the assembly of fishes in small northern lakes. Ecology 79(8): Riera J. L., J.J. Magnuson, T.K. Kratz, and K.E. Webster A geomorphic template for the analysis of lake districts applied to the Northern Highland Lake District, Wisconsin, USA. Freshw. Biol. 43: Tonn, W. M., J.J. Magnuson, M. Rask and J. Toivonen Intercontinental comparison of small-lake fish assemblages: the balance between local and regional processes. Am. Nat. 136:

5 Spatial, temporal and taxonomic patterns of zooplankton variability Aquatic scientists must consider the variability in populations and communities to understand the relative influence of spatial and temporal processes, detect effects of anthropogenic disturbances or experimental treatments, and develop or test ecological theory (McKnight et al. 1996, Cottingham et al. 2000). For example, without an appreciation of spatial variation at regional, lake district, and individual lake scales and temporal variation at decadal and annual scales, we will have problems extrapolating the results of whole-lake experiments or implementing ecological restoration, monitoring, and management efforts. Although the need is evident, long-term data from multiple study sites covering broad regions have seldom been available. Such data have now been compiled for the pelagic zooplankton community in lakes throughout the Upper Great Lakes area. Pelagic zooplankton are a critical link in the aquatic food web and a group of organisms increasingly being used as ecological indicators of environmental stress and recovery (Attayde and Bozelli 1998, Keller and Yan 1998). Indicator species should have sensitive and specific responses to particular stressors as well as low baseline variability. Although a great deal is known about how individual zooplankton taxa respond to a variety of stressors, comparisons of baseline variability among taxa are lacking. We addressed these deficiencies by comparing relative and absolute variability of common zooplankton taxa in space and time (Rusak et al. 2002). We contrasted variability among regions, lakes, and years in pelagic crustacean zooplankton abundance in unmanipulated lakes. The dataset included 261 lake-years ( ) of ice-free density estimates from 22 lakes in three lake-rich regions in the Upper Great Lakes area of North America (the NTL-LTER site in northern Wisconsin, the Experimental Lakes Area in north-western Ontario, and the Dorset Research Centre lakes in south-central

6 Ontario). Our first objective was to characterize relative variation at these spatial and temporal scales, hypothesizing that variability among regions would be greater than that found among lakes within a region, which would in turn be greater than interannual variation. This hierarchy emerged from our belief that geology, climate and glacial history influence zooplankton population size, and differences in these factors increase with increasing lake separation. Secondly, we examined how relative and total explained variance differ among taxa and change with aggregation, thus informing our choice of indicator species as well as our understanding of the relative temporal and spatial controls of zooplankton population dynamics. small Daphnia SDA 1.60 small Cladocera SCL 0.49 small Calanoida SCA 0.68 nauplii N 0.16 large Daphnia LDA 1.68 small Cyclopoida ICY 0.93 Holopedium HOL 1.38 chydorids CHY 1.54 large Cyclopoida BCY 0.55 large Calanoida BCA 1.29 YEAR REGION LAKE YEAR*REGION YEAR*LAKE Spatial sources of variation dominated the analysis, but regional variation was half that found among lakes and a large interaction Figure 1. Bar plot of relative spatial and temporal variability among zooplankton functional groups. The numeric column to the left of the graph reports total variance. between lakes and years indicated that time cannot be ignored (Figure 1). These results indicate that monitoring programs employing single sentinel sites, even with long-term data, will not be able to extrapolate their results to larger scales with much confidence. Relative variance components differed widely among functional groups, indicating that species strongly differ in their response to environmental controls and sensitivity to perturbation. Total explained variation also differed widely among

7 zooplankton and decreased with increasing aggregation of taxa (Figure 2) indicating that larger aggregates of zooplankton taxa, given an acceptable sensitivity to a particular stress, will make the best indicator groups. Whether choosing ecological indicators, or designing experiments and monitoring programs, these results clearly SMCL small Cladocera LGCL large Cladocera CYC Cyclopoida CAL Calanoida YEAR REGION LAKE YEAR*REGION YEAR*LAKE show that large-scale temporal and spatial variability will be an 0% 50% 100% Figure 2. Bar plot of relative spatial and temporal variability among larger taxonomic aggregates. The numeric column to the left of the graph reports total variance. important consideration. Literature Cited Attayde, J.L. and R.L. Bozelli Assessing the indicator properties of zooplankton assemblages to disturbance gradients by canonical correspondence analysis. Can. J. Fish. Aquat. Sci. 55: Cottingham, K.L., J.A.Rusak, and P.R. Leavitt Increased ecosystem variability and reduced predictability following fertilisation: evidence from paleolimnology. Ecol. Lett. 3: Keller, W. and N.D. Yan Biological recovery from lake acidification: zooplankton communities as a model of patterns and processes. Restoration Ecology 6: McKnight, D.M., E.R. Blood, and C.R. O'Melia Fundamental research questions in inland aquatic ecosystem science. In Freshwater Ecosystems: Revitalizing Educational Programs in Limnology. Edited by National Research Council Committee on Inland Aquatic Ecosystems. National Academy Press, Washington, USA pp Rusak, J.A., N.D. Yan, K.M. Somers, K.L. Cottingham, F. Micheli, S.R. Carpenter, T.M. Frost, M.J. Paterson, and D.J. McQueen Temporal, spatial, and taxonomic patterns of crustacean zooplankton variability in unmanipulated north-temperate lakes. Limnol. Oceanogr. 47:

8 Autotrophy and heterotrophy in aquatic ecosystems Recent studies of lake metabolism (del Giorgio and Peters 1994; Carignan et al. 2000; Prairie et al. 2002) have come to different conclusions as to whether lakes are net autotrophic, gross primary production GPP exceeds respiration (R), or net heterotrophic (GPP<R). The debate over lake trophic classification has largely resulted from differences in methods and lake types among different studies. The method typically used for measuring metabolism is that of Fee (1990), in which one collects field samples and conducts bottle experiments in the laboratory to determine GPP and R. Additionally, the range of lakes for which metabolism has been measured is typically narrow, and the lakes are usually oligotrophic. In this study, we used in situ measurements of diel dynamics of dissolved CO2 and O2 to estimate GPP and R in a suite of lakes that spanned broad ranges of chlorophyll and dissolved organic carbon (DOC) concentrations (Hanson et al. In preparation). We compared the diel dynamics of dissolved CO 2 with those of O 2 and determined that both gases showed similar, but opposite patterns (Figure 1). Estimates of GPP and R made from the two gases were closely related in lakes with neutral to low ph. In high ph lakes, GPP and R estimates based on CO 2 measurements Figure 1. Changes in dissolved CO2 mirror changes in dissolved O2 on a near 1:1 molar ratio in West Long Lake, MI.

9 were less than those based on O 2, which suggests that measuring O 2 is a more reliable method over a broad range of lake types. We also graphically evaluated the relationship between lake trophic status, DOC concentration, and lake metabolism (Figure 2). Multiple linear regression analyses revealed that both NEP* (mgc m -3 d -1 ) DOC (mg l -1 ) chlorophyll and DOC concentration are important predictors of lake metabolism - as DOC increased, lakes tended to become more heterotrophic. Thus, the recent controversy Figure 2. NEP values normalized for chlorophyll (NEP*) plotted against DOC from three studies, del Giorgio and Peters ( open squares), Carignan et al. ( open triangles), and this study (filled circles). over lake metabolism may be resolved by more explicit consideration of methodology (bottle measurements represent only pelagic plankton in oligotrophic lakes, whereas the sonde method represents pelagic and littoral components) and lake DOC concentrations. This project also represents a pioneering application of a new technology that uses instrumented lake buoys to collect and transmit detailed limnological data with spread spectrum radios for real-time availability. Literature Cited Carignan, R., P. Dolors, and C. Vis Planktonic production and respiration in oligotrophic Shield lakes. Limnol. Oceonogr. 45: del Giorgio, P.A., and R.H. Peters Patterns in planktonic P:R ratios in lakes: Influence of lake trophy and dissolved organic carbon. Limnol. Oceanogr. 39: Fee, E.J Computer programs for calculating in situ phytoplankton photosynthesis. Can.

10 Tech. Rep. Fish. Aq. Sci Hanson, P.C., D.L. Bade, S.R. Carpenter, and T.K. Kratz. In preparation. Lake metabolism: Relationships with dissolved organic carbon and chlorophyll. Prairie, Y.T., D. F. Bird, and J.J. Cole The summer metabolic balance in the epilimnion of southeastern Quebec lakes. Limnol. Oceanogr. 47:

11 Lake management using a long-term experimental approach Long-term data has allowed North Temperate Lakes LTER researchers, in collaboration with scientists from the state management agency (WDNR), to interpret a massive long-term ecosystem experiment aimed at improving water clarity of Lake Mendota. Lake Mendota is a eutrophic lake suffering from high algal productivity, which leads to decreased water clarity, fish kills, and extensive economic losses each year. In 1986, University and WDNR researchers initiated a biomanipulation project to improve the lake s water quality and sport fishing opportunities. Researchers stocked Lake Mendota with walleye and northern pike fingerlings from 1987 to Walleye and pike are piscivorous fish that predate heavily on planktivorous fishes (e.g., cisco and yellow perch). The planktivores feed upon the large zooplankton (Daphnia), which efficiently graze algae. Therefore, by increasing the walleye and pike populations, the resultant trophic cascade predicts that the planktivore population will decrease and the zooplankton population will increase, enabling large zooplankton to consume the algae impairing water clarity (Carpenter et al. 1985). Although successful biomanipulation projects have been conducted in shallow lakes where nutrient levels are not excessive (Benndorf 1990, Meijer et al. 1999), uncertainty exists as to whether this management technique will work in relatively large eutrophic lakes like Lake Mendota (McQueen et al. 1986). The biomass of both piscivore species substantially increased in the lake as a result of the stocking and harvest regulations. Concurrently, planktivore populations did decline (Figure 1) and water clarity increased (Figure 2) suggesting that piscivory could be controlling planktivore populations with these interactions ultimately cascading through the zooplankton to control algal biomass. However, the large increase in yellow perch in 1999 indicated that a strong year class

12 of planktivores can develop even with the relatively high piscivore biomasses attained in the lake. Large variations in phosphorus concentrations and a natural cisco die-off during the experiment also Figure 1. Planktivore biomass estimates (kg ha -1 ) for cisco, yellow perch and white bass for Lake Mendota, appear to have complicated algal responses to biomanipulation. In summary, the Lake Mendota biomanipulation project has been a success given that high densities of the large-bodied Daphnia have continued to dominate for over a decade, and the diversity of fishing Figure 2. Secchi disk readings as measure of water clarity and algal densities for Lake Mendota, opportunities has improved for walleye and northern pike, and more recently yellow perch. In addition, scientists and managers have learned just how far a large eutrophic urban lake like Mendota can be pushed by biomanipulation. Synergy between biomanipulation and nonpoint pollution control may be an important topic of future research and management initiatives.

13 Literature Cited Benndorf, J Conditions for effective biomanipulation; conclusions derived from wholelake experiments in Europe. Hydrobiologia 200/201: Carpenter S.R., J.F. Kitchell and J.R. Hodgson Cascading trophic interactions and lake productivity. BioScience 35: Lathrop, R.C., B.M. Johnson, T.B. Johnson, M.T. Vogelsang, S.R. Carpenter, T.R. Hrabik, J.F. Kitchell, J.J. Magnuson, L.G. Rudstam, and R.S. Stewart. In press. Stocking piscivores to improve fishing and water clarity: a synthesis of the Lake Mendota Biomanipulation Project. Freshw. Biol. McQueen D.J., J.R.Post, and E.L.Mills Trophic relationships in freshwater ecosystems. Can. J. Fish. Aquat. Sci. 43: Meijer M.-L., I. de Boois, M. Scheffer, R. Portielje, and H. Hosper Biomanipulation in shallow lakes in The Netherlands: an evaluation of 18 case studies. Hydrobiologia 408/409:13-30.

14 Web-based NTL database query and retrieval One of the goals of the NTL-LTER program has been to make our comprehensive, longterm data available to the scientific community and the general public. In the past, we have made entire datasets available as text files via the worldwide web. In a major advance in our information management capability, we now make it possible to extract only the data of interest from large datasets and obtain these data in formats easily transported to popular data analysis software. We have created a Java application that provides dynamic database access to the NTL Oracle database from the NTL website. The NTL website is the primary means of external data distribution. This addition of query functionality to the website permits users to select dataset attributes of interest, to sort by selected attributes, and to construct conditional filters that allow subsetting of the dataset by attributes such as lake or species or year. The user also has a choice of different output formats including display to the screen, comma-delimited text, MS Excel, and XML. Figure 1. A schematic plot of the NTL-LTER dynamic database access and retrieval application (after Smith et al., 2002). Thus, the application significantly enhances access to NTL data for researchers outside the NTL group and the public as well as providing another access method for NTL researchers.

15 The design of this query system (Smith et al. 2002) provides important database extensibility. The user interface is constructed by a query engine driven by metadata stored in the Oracle database. Extending the interface for a new data set simply involves adding the metadata for the dataset to the database; no new programming is necessary. The content standard for the metadata was a beta version of EML (Ecological Metadata Language, the metadata standard adopted by LTER) augmented by metadata needed to control display and filtering. Upon release of version 2 of EML, we will update the program to be in compliance with this version of the standard. The program will then supply EML2 metadata upon demand. The interface also gathers information about users who access NTL data and stores this information in the database. Literature Cited Smith, D.J., B.J. Benson, and D.F. Balsiger Designing web database applications for ecological research. In Proceedings of the 6th World Multiconference on Systemics, Cybernetics and Informatics. Vol VII: Informatics Systems Development II, Orlando, FL.