Linkagesbetween chemical Section 3 and physicalresponses and biologicalpopulations Limnol. Oceanogr., 41(5), 1996, 1041-1051 0 1996, by the American Society of Limnology and Oceanography, Inc. Annual cycle of autotrophic and heterotrophic production in a small, monomictic Piedmont lake (Lake Oglethorpe): Analog for the effects of climatic warming on dimictic lakes Karen G. Porter and Patricia A. Saunders Institute of Ecology, University of Georgia, Athens 30602 Kurt A. Haberyan Department of Biology, Northwest Missouri State University, Maryville 64468 Alexander E. Macubbin Department of Experimental Therapeutics, Roswell Park Cancer Institute, Buffalo, New York 14263 Timothy R. Jacobsen Cumberland County College, Vinelands, New Jersey 08360 Robert E. Hodson School of Marine Sciences, University of Georgia, Athens 30602 Abstract Lake Oglethorpe, Georgia, is a small, monomictic impoundment in the southeastern U.S. Piedmont region. Seasonal differences in ecological processes correspond to the two major stratification patterns and result in seasonal differences in pelagic community structure,. The winter mixed period (November-March) provides light and temperatures suitable for active nutrient regeneration and uptake, population growth of edible phytoplankton and crustacean zooplankton, and low bacterial abundance and production. Monthly estimates of daily primary productivity are in the mesoeutrophic range (40-l ,420 mg C m-2 d-l); however, mean annual primary productivity is in the eutrophic range (500 mg C m-2 yr-I) because of greater activity in winter and an extended “growing season.” The stratified period (April-October) is characterized by rapid development of hypolimnetic anoxia, high bacterial production, cyanobacteria and large algae, heterotrophic flagellates, ciliates, rotifers, small cyclopoid copepods, and Chaoborus. Heterotrophic activity, HPLC chlo- rophyll a, and grazer activity were highest at the epilimnetic-metalimnetic boundary. Bacterial abundances (1.5-32 x 1O6 cells ml- I) increased in the hypolimnion during the stratified period. Long-term (1978-l 994) warming within the water column (0.06 1-O. 10°C yr-I) follows midwinter (January) air temperature trends. Using Lake Oglethorpe as an analog, we predict as consequences of climatic warming in northern dimictic lakes: increases in nutrient cycling rates, winter primary and secondary productivity, summer heterotrophic production, and microbial components of the food web, . Acknowledgments In addition to the authors, Adrieen Debiase, Yvette Feig, Hal King, Doug Leeper, Bob McDonough, Judy Meyer, Michael Pace, John Orcutt, Amy Parker, Bob Sanders, and Susan Bennett Wilde provided major assistance in data collection, analysis and interpretation. John Chick and Phil Dixon aided in statistical analysis of the long-term thermal data, and Rob Reinert, David Bruce, and Mike Van Den Avyle provided insights about fish assemblages.Thanks to the Helfmeyers for accessto Lake Oglethorpe from their property. This research was supported indirectly by several grants from the National Science Foundation. Contribution 49 of the Lake Oglethorpe Limnological Association. 1041 1042 Porter et al. Most small natural lakes in North America are glacial driven ecological patterns to predict the effects of climatic in origin and are classified as temperate and dimictic warming on dimictic lakes in north-temperate regions. (Hutchinson 1957). Studies of these north-temperate lakes are the basis for many limnological paradigms. Compa- rable small water bodies below the southern extent of Methods dimixis (36”N in the Midwest, 40”N in the Northwest, and 38”N in the Southeast, Hutchinson 1957) are gen- Study site-Lake Oglethorpe is in Oglethorpe County erally warm monomictic impoundments and have re- (33”52’12”N, 83’13’49”W) and has been a study site for ceived relatively little study. researchers at the nearby University of Georgia, Athens, Limnological comparisons have been made among since 1978. This small (AO = 30 ha) impoundment was tropical and temperate lakes but rarely include those at created in 197 1. It has a maximum depth of 8.7 m, a intermediate latitudes (e.g. Lewis 1987). Most reviews mean depth of 2.3 m, and a mean hydraulic residence considering lentic systems of southeastern North America time of 80 d (Raschke 1993). Compared with other small focus on natural lakes, which occur primarily on the impoundments in the southeastern Piedmont, Lake Ogle- Coastal Plain (Frey 1963; Crisman 1992), farm ponds thorpe is characterized as mesotrophic by the EPA be- (Menzel and Cooper 1992), and large reservoirs (Soballe cause of moderate epilimnetic nutrient (24 pg liter-* mean et al. 1992). A recent EPA study of small (~648 ha), total P) and chlorophyll a concentrations (10 pg liter-l stratifying impoundments in the Piedmont of North Car- mean Chl a) and a Carlson trophic state index of 53 during olina, South Carolina, and Georgia provides the first com- summer stratified period (Raschke 1994, pers. comm.). parative survey of these systems (Raschke 1993, 1994). The watershed consists of forests, farmlands, and private Lake Oglethorpe, our study site, was included in the EPA residences. The red-brown color associated with clay survey and is considered typical of small impoundments loading has been observed in the lake, but in recent years in the region with lakelike characteristics, being deep these events have been rare and have apparently declined enough to stratify and having a relatively long water res- as watershed development-a major cause of clay loading idence time. We define a small lake as any standing body in the Piedmont (Mulholland and Lenat 1992)-has lev- of water, natural or anthropogenic, that is lo-500 ha in eled off. The lake is used primarily for recreation, partic- surface area, deep enough to stratify, and with a long mean ularly fishing and swimming. The fish assemblage in both water residence time (> 2 months). 1977 and 1993 included bluegill (Lepomis machrochirus), Small monomictic impoundments are the appropriate green sunfish (Lepomis cyanellus), redbreast (Lepomis au- systems to compare and contrast with small dimictic lakes ritus), black crappie (Pomoxis nigromaculatus), golden of the north-temperate zone. Monomictic lakes may also shiner (Notemigonis crysoleucus), and largemouth bass be good analogs for the changes that could occur in dim- (Micropterus salmoides; Orcutt 1982; D. Bruce unpubl. ictic lakes due to climatic warming. In Canadian Shield data). lakes, regional warming trends have resulted in an in- crease in ice-free period and winter phytoplankton bio- Sampling and analyses -In 1979, monthly midday mass (Schindler et al. 1990). If warming trends continue, sampling was done at a permanent deep-water, foredam the thermal regimes of many small dimictic lakes may station at l-m depth intervals (down to 7 m). Temper- become more like those in warm monomictic systems ature and dissolved oxygen profiles were measured with (i.e. lacking a winter stratification period and with warmer an in situ thermistor and dissolved oxygen electrode (YSI water temperatures year-round). A better understanding Model 57). To look for possible long-term trends, we of limnological processes in warm monomictic lakes such examined temperature profiles collected at irregular in- as Lake Oglethorpe may improve predictions of the effects tervals over a 17-yr period (1978-l 994). Although a large of climatic warming in northern lakes. data set has been accumulated (N = 303 dates with tem- Our purpose is to provide a conceptual model of a perature profiles), profiles were not obtained at consistent monomictic lake in the southeastern Piedmont by de- time intervals. Therefore, we calculated detrended scribing ecological processes and associations over an an- monthly means (N = 108) for measurements of temper- nual cycle in Lake Oglethorpc, Georgia. In order to char- ature from each discrete depth (l-m intervals to 7-m acterize seasonal differences, we present a 13-month study depth) and the water-column average. Data were entered of physical and chemical patterns and of autotrophic and into general linear models procedure (SAS, version 6) heterotrophic processes during 1978-l 979. Our discus- using month and year as independent variables and month sion of physical and chemical patterns includes exami- as a class variable. Patterns in air temperature and rainfall nation of temperature and dissolved oxygen measure- data for the area (NCDC 1978-1994) were similarly as- ments spanning a 16.5-yr period. An annual cycle of or- sessed (SAS, version 6). Light profiles were measured at ganic sedimentation is provided for 1983-l 984. We sum- 0.25-m intervals with a LiCor LI-188B PAR irradiance marize studies of specific aspects of microbial production meter. Extinction coefficients were calculated according and consumption. These provide a general description of to Wetzel and Likens (1979). seasonal shifts in the relative importance of autotrophic Whole-water samples were collected at l-m depth in- and heterotrophic production to food-web structure that tervals with an g-liter PVC Van Dorn bottle. Subsamples correspond to the two major patterns of thermal regime: were processed and analyzed for concentrations of soluble mixis and stratification. Finally, we use these thermally reactive P (SRP) and total P (TP) (Menzel and Corwin Monomictic Lake Oglethorpe 1043 1965; Murphy and Riley 1962), ammonia N (Solorzano Temperature (“Cl 1969), and nitrate and nitrite (Strickland and Parsons 197