-- = no addition +P = added Phosphorus +N = added Nitrogen +PN = added P and N -- +P +N +PN Supplements of DOC made on Days 0 and 7 (after sampling) resulted in a gradient of DOC concentration across the control and experimental treatments (Fig. 2A). The DOC fluorescence signal (F450:F500) in the control mesocosm was approximately 1.8 throughout the experiment, indicating DOC of primarily algal origin. The corresponding F450:F500 ratio of <1.4 in the DOC treatment mesocosms indicated primarily terrestrial DOC. Bacterial production (BP) increased after DOC additions, although not always directly corresponding with the amount of DOC supplementation (Fig. 2B). This is reflected in the significant relationship of BP with DOC over Day 1 through Day 4, but a non-significant relationship when compared over the whole study period (Table 1). HNAN abundances had similar trends in the control and all treatment mesocosms throughout the experiment (Fig. 2D), although there was no significant relationship between DOC concentration and HNAN abundance. Contrary to expectations, increased BP did not always lead to increased HNAN abundance on the following day (Fig. 2B, 2D). Primary production (PP) was greatest 7 days after the initial supplement in the high DOC treatments, and remained elevated relative to controls throughout the experiment in the two highest DOC treatments (Fig. 2C). The highest PP rates (Day 7) in all treatments preceded increased abundances of PNAN as well as a peak in the maximum biovolume observed for edible net phytoplankton (>5 to 40µm) by Day 9 (Fig. 2E, 2G). There were significant main effects of DOC and time on the edible net phytoplankton, and a significant interaction (Table 1). The biovolume of inedible net phytoplankton (mucilaginous and/or >40µm) remained low during the first half of the experiment, but exceeded that of edible phytoplankton by the end of the experiment, especially in the higher DOC treatments (Fig. 2I). A bioassay experiment with additions of P, N, or P+N without DOC added to the lakewater indicated that the phytoplankton in Lake Giles were co-limited by nitrogen and phosphorus (Fig. 3A). In the presence of DOC, phytoplankton biomass increased by Day 4 when P was added, but not when N was added (Fig. 3B). This indicated that the DOC had a biologically available nitrogen content. Ciliate abundance (Fig. 2F) peaked at the same time as as edible phytoplankton biomass (Day 9), and generally reached greater abundances in the higher DOC additions, although the trend with DOC as a main effect was not significant. RM-ANOVA did indicate a significant interaction of DOC x Time (Table 1). Heliozoans (data not shown) followed the same trends with abundances approximately 50% of ciliates. Both groups ingest nanoplankton, and the decline in edible phytoplankton biomass and HNAN (but not PNAN) by Day 10 suggest a top-down effect of ciliates and heliozoans in these experiments. There were significant effects of DOC, time and DOC x Time (Table 1) for both Daphnia (Fig 2H) and calanoid copepods (Fig 2J). The initial effects were negative and the reduction of crustacean zooplankton populations were inversely related to DOC concentration. There was some recovery during the last week of the experiment in the lower DOC concentrations, but not at the highest concentration. These results suggest some toxic effect of the natural DOC on the crustacean zooplankton in Lake Giles. Table 1. Repeated Means ANOVA Results (p-values) Response VariableDOCDOC x TimeTime Bacterial Production Days ns0.010 Bacterial Production All Days ns <0.001 Heterotrophic Nanoflagellates ns <0.001 Phototrophic Nanoflagellates ns <0.001 Ciliatesns0.043<0.001 Edible Net Phytoplankton Biomass <0.001 Inedible Net Phytoplankton Biomass ns Daphnia sp ns Calanoid Copepods0.004<0.001 Introduction Results Methods Twelve mesocosms constructed of UV-transparent Aclar were suspended from a floating dock in the central basin of Lake Giles in the Pocono Mountains, Pennsylvania, USA (Fig. 1). Approximately 3,500 liters of water from the lake was added to each mesocosm, after which aliquots of CDOM were mixed into three replicated treatments and an equivalent volume of lakewater mixed into the triplicate control mesocosms. CDOM from water collected at a nearby high-DOC lake was concentrated using reverse osmosis. Two additions were made to each experimental mesocosm one week apart to raise the CDOM from a background of 1.4 ppm (Control) to 2.5, 3.2 and 4.4 ppm (Low, Medium, High). The highest concentration was equivalent to the [DOC] in a nearby mesotrophic lake. Samples for all organisms except zooplankton were at collected from the surface mixed layer at several depths using a peristaltic pump and integrated in darkened containers. Zooplankton were collected at the end of each week by vertical net tows in the center of each mesocosm. Zooplankton abundances were estimated from formalin-fixed samples. Abundances for ciliates, radiolarians and net phytoplankton were determined microscopically from Lugol’s-fixed samples. “Inedible Net Phytoplankton” include species that were > 40 µm in any dimension and/or with gelatinous coats; “Edible Net Phytoplankton” included all other phytoplankton > 5µm. Phytoplankton < 5 µm were determined separately as phototrophic nanoflagellates. Phototrophic (PNAN) and heterotrophic (HNAN) nanoplankton were enumerated with epifluorecence microscopy and distinguished by presence/absence of chlorophyll fluorescence. Bacterial production was determined by the 3 H-leucine uptake method. Changes in global climate, land use and hydrology are all factors that alter the amount and quality of dissolved organic matter (DOM) that enters lacustrine systems. In turn, DOM influences the primary productivity of these systems by: 1) altering light environments, including both photosynthtically active (PAR) and ultraviolet radiation (UVR) levels; 2) modifying nutrient concentrations and ratios; and 3) via direct and indirect effects on members of the planktonic food web from bacteria to zooplankton. The objective of this study was to determine how an influx of colored dissolved organic matter (CDOM) into a low-DOC lake would modify the plankton community. The general approach was to add CDOM concentrated from a nearby high-DOC lake into replicated mesocosms at three levels above background, and to follow the population responses of bacteria, heterotrophic protists, phytoplankton and zooplankton over a period of three weeks. Summary & Conclusions Figure 1. Diagram (A) and photograph (B) of the mesocosm setup in Lake Giles. 1.Loading of concentrated naturally-derived DOC into oligotrophic water led to rapid increases in both phototrophic and heterotrophic organisms. 2.The natural DOC from a high DOC lake appeared to have some toxicity for crustacean zooplankton when added to oligotrophic lakewater. 3.Bacteria, HNAN and small, edible phytoplankton responded rapidly to each DOC addition. An expectation of DOC addition to stimulate heterotrophic microorganisms at the expense of autotrophic groups was not observed. 4.The chemical nature of “DOC” is important to its effects when it enters an oligotrophic system. Using DOC with little associated nutrients (e.g., glucose) tends to increase competitive abilities of heterotrophic vs autotrophic microorganisms for inorganic nutrients. 1,2 But, if nutrients are associated with DOC, autotrophs may be competitive with bacteria either by direct hydrolysis to access DOC- associated N and/or P, or by uptake of nutrients after rapid remineralization by heterotrophic protists. Dissolved Organic Carbon (DOC) Forcing of Plankton Dynamics in a Lake Ecosystem R.W. Sanders 1, S.L. Cooke 2, J.M. Fischer 3, W.H. Jeffrey 4, A.L. Macaluso 1, R.E. Moeller 5, D.P. Morris 6, J.D. Pakulski 4, J.A. Porter 7, D.M. Schoener 8, C.E. Williamson 5 1 Temple University, 2 Duke University, 3 Franklin & Marshall College, 4 University of West Florida, 5 Miami University, 6 Lehigh University, 7 University of the Sciences in Philadelphia, 8 University of Connecticut 4.4 ppm3.2 ppm2.5 ppmNo Addition DOCK HighMediumLowControl A.B. Figure 2. Response of planktonic community to DOC additions (A). A. B. C. E. G. I. D. F. H. J. Supported by NSF grant # IRCEB DEB Fig. 3. DOC Algal Nutrient Assay (P, N, P+N) for Lake Giles