1. Effects of increased dissolved organic matter on zooplankton vertical distribution in a UV-transparent lake: Results from large-scale mesocosms Sandra L. Cooke 1,4, Janet M. Fischer 2, Craig E. Williamson 3, and Donald P. Morris 4 1 Biology Dept., Duke University, Durham, NC (corresponding author: s.cooke@duke.edu); 2 Dept. of Biology, Franklin & Marshall University, Lancaster, PA; 3 Dept. of Zoology, Miami University, Oxford, OH; 4 Dept. Earth & Environmental Sciences, Lehigh University, Bethlehem, PA Acknowledgements & References Thanks to all students, postdocs, and principle investigators involved with the “UV-Lakes Project” for field and lab assistance and advice. This work was supported by NSF IRCEB grant DEB-0210972 1 Evans, C. D., Chapman, P. J., Clark, J. M., et al. Glob. Change Biol. 12, 2044–2053 (2006) 2 Leech, D. M., & Williamson, C. E. Limnol. Oceanogr. 46, 426-420 (2001) 3 Fischer, J. M., Nicholai, J. L., Williamson, C. E., et al. Hydrobiol. 563: 217-224 (2006) 4 http://www.cnas.missouristate.edu/zooplankton/diaptomus1.htm 5 De Stasio, Jr., B. T. Bull. Mar. Sci. 53, 44-64 (1993) Introduction Recent work suggests that climate change combined with decreases in anthropogenic sulfate deposition are increasing dissolved organic matter (DOM) export to lakes 1. Such DOM increases may alter lake ecology through multiple mechanisms, such as reducing the amount of ultraviolet radiation (UV) reaching organisms and increasing food availability. Our purpose was to determine if DOM alters the day- and nighttime vertical distribution of Leptodiaptomus minutus, a calanoid copepod (Fig. 1). Some studies indicate that L. minutus migrates deeper in the water column in response to UV 2 but other work suggests that this daytime migration is not due to UV 3. We hypothesize that L. minutus vertical distribution will be altered by DOM during both the daytime and nighttime due to possible optical and food availability effects. Methods Study site: Lake Giles, a transparent lake (dissolved organic carbon (DOC) = 1.6 mg L -1 ) in northeastern Pennsylvania, USA (pictured by title). In situ mesocosm experiment: 12 mesocosms (8 m-long, 0.75 m diameter, 3440 L) with DOM addition treatments. To attain the DOC concentrations listed below, reverse-osmosis concentrated DOC was added on the first day of the experiment (June 7, 2004) and then 1 week later (June 14). Zooplankton were pumped with a tube sampler at 1 meter intervals from 0.5 to 7.5 m during midday and midnight on June 9 and June 23 (Fig. 2) control (DOC = 1.6 mg L -1 ; n=3) low DOM (DOC = 2.5 mg L -1 ; n=3) medium DOM (DOC = 3.0 mg L -1 ; n=3) high DOM (DOC = 4.3 mg L -1 ; n=2 – lost one) Results During the day nauplii were shallower in the control, low, and medium after the 1 st DOM addition (Fig. 3a; p=0.000) but were shallower in the control after the 2nd addition (Fig. 3c; p=0.001). At night nauplii were shallower in the control on both dates (Fig. 3b,d; p=0.005 for 1st date; p=0.012 for 2nd date). During the day adult & copepodid (AD & CP) distribution was shallower in the control, and deeper in the high (Fig. 4a; p=0.034) after the 1st addition, but only 5.4 to 21.6% of the population was present in daytime samples according to the diurnal deficits. At night AD & CP distribution was not significant overall after the 1st addition, but there were higher percentages at 2.5 m in the low and medium (Fig. 4b). At night after the 2nd addition, AD&CP were shallower with decreasing DOM (Fig. 4d, p=0.000). AD & CP diurnal deficits decreased with DOC after the first addition (Fig. 5), and also decreased with a 320 and increased with z 1% 320 (not shown). Conclusions Figure 3. Nauplii distribution after the 1 st (a,b) and 2 nd (c,d) DOM additions. Only + standard error bars are shown for clarity. Figure 4. L. minutus distribution after the 1 st (a,b) and 2 nd (c,d) DOM additions. Only + standard error bars are shown for clarity. Figure 5. L. minutus diurnal deficit was significantly correlated to several factors, including DOC concentration DOM may alter the distribution of all L.minutus life stages through mechanisms other than UV-attenuation (inferred from nighttime results and shallow daytime distribution of nauplii in the control). Adult and copepodid diurnal deficits decreased with increasing DOC (Fig. 5). A mechanism for this is not clear, but it is possible that greater proportions of L. minutus were near the mesocosm bottom with decreasing DOC and increasing transparency. This pattern was only significant after the first DOM addition. Nauplii distribution differs from that of adults and copepodids, especially during the day. Also, nauplii diurnal deficits were smaller than that of adults and copepodids (not shown). 0.5 m 1.5 m 2.5 m 3.5 m 4.5 m 5.5 m 6.5 m 7.5 m Hand pump Data analysis: Adults, copepodids, and nauplii were counted without sub-sampling, and effects of DOM on vertical distribution were determined using ANOVA with depth and DOM treatment as the independent variables, and % of total at each depth as the dependent variable. Separate analyses were done for each date and time. Also, for each mesocosm we calculated an index of diurnal deficit 5 : 100 x (day density – night density) / night density Figure 2. Mesocosms were sampled by lowering a 3 cm diameter tube to discrete depths and hand- pumping 8-10 L of water (exact volume was noted). Figure 1. L. minutus 4 (a) (b) (c) (d) (a) (b) (c)(d)