Temporal and Spatial Patterns in Chlorophyll Concentration of Transported Organic Matter in the Upper Mississippi Emily Forde & Michael Delong Large River Studies Center, Biology Department, Winona State University, Winona, MN Abstract Conclusions Introduction Methods Acknowledgements Study Site Samples were collected from the Upper Mississippi River, River Km , in side channel, main channel, and backwater habitats. Samples were collected monthly, April – September Sample Methods Water samples were collected using a pump and drawing water from the depth of 1-m Main channel and side channel samples were collected at 4 points along a transect and pooled to form a single site. A backwater site consisted of 2 combined sample points. Transport Organic Matter (TOM) was separated into ultrafine (UTOM;  m) and fine (FTOM; 100 – 1000  m) fractions. UTOM and FTOM samples were separated into algal and detrital fractions using colloidal silica separation (Hamilton and Lewis 1992). Chlorophyll concentration was determined using the monochromatic method (Wetzel and Likens 1994). Objectives This study examined transported organic matter from main channel, side channel, and backwater habitats of the Upper Mississippi River to: Compare chlorophyll concentrations of the algal and detrital fractions of transported organic matter as a function of habitat type. Describe temporal changes in chlorophyll concentration relative to hydrologic patterns. Examine temporal relationships algal and detrital fractions of transported organic matter in the main channel. Recent studies have shown algal transport organic matter (TOM) to be an important influence on food web dynamics in large rivers. Unfortunately, research has been limited to main channel habitats and has addressed limited spatial scales. This study examined how the abundance of algal TOM changed temporally and spatially in the Upper Mississippi River. This was addressed by sampling different particle sizes of TOM from April to September 2004 in side channel, backwater and main channel habitats of the Upper Mississippi River near Winona, MN. Samples were drawn monthly from four point transects across the channel at a depth of 1 meter and divided into fine (FTOM; mm) and ultra-fine (UTOM; mm). TOM samples were then split into algal and detrital fractions by centrifuging with 76% colloidal silica solution. Chlorophyll concentrations were determined spectrophotometrically, using the monochromatic method. Similar temporal trends were evident for both UTOM and FTOM. Chlorophyll concentrations increased between April and May, which correspond to a decrease in discharge. Concentrations declined from May to June as discharge increased to its spring maxima. Both UTOM and FTOM increased to highest observed concentrations in August during baseflow conditions. Only small differences in chlorophyll concentrations were evident between habitats, with all three exhibiting similar temporal patterns. Overall, however, concentrations were slightly highest in side channels. This study indicated that an inverse relationship exists between chlorophyll concentrations of TOM and discharge. Small differences observed between habitats demonstrate the productivity of lotic habitats within the river. This project was supported by the Thomas Wayne Schultz Memorial Scholarship (Forde) and by WSU through Professional Improvement Funds (Delong). I would also like to thank everyone involved with the LRSC: Laura Luger, Rebecca Bown, Cheng Xiong, Paul Bates, Steve Edson, and Holley Schmidlapp. This study was performed as part of an undergraduate capstone research project at Winona State University. Chlorophyll concentrations were lowest during periods of high discharge (e.g., April and June) and highest during periods of prolonged low discharge (May, June-July). Comparisons between habitats of the Upper Mississippi River indicate temporal changes of chlorophyll concentrations are more significant than spatial differences (Fig 1-7). Backwater habitats consistently possess lower chlorophyll concentrations than side channels due to increased shading from shore vegetation, abiotic turbidity, and nutrient depletion caused from disconnection to the main channel nutrient pool (Fig 2-5). Seasonal variation of mean chlorophyll concentrations can be attributed to nutrient availability, shading, turbidity (organic and inorganic), and flow patterns. Detrital UTOM and FTOM in the main channel exhibited very little variation over the sample period (Fig 6-7). Algal UTOM and FTOM fractions were also similar with both increasing Jun through Aug with a sharp decrease when base flow period ends (Fig 6-7). Hydrological processes appear to function as a major determinant of chlorophyll concentrations in the main channel, with increasing retention time leading to greater chlorophyll concentrations. Literature Cited Floodplain rivers have a unique ability to connect and disconnect themselves to adjacent areas in times of flood and low flow, which makes them the most dynamic habitat on Earth (Power 1995). River ecosystems are spatially and temporally complex, and are characterized by complex food webs (Thorp and Delong 2002). Currently, there is a limitation to our understanding of the spatial and temporal dynamics of phytoplankton and other forms of organic matter carried in the water column. A better understanding of the spatial and temporal dynamics of phytoplankton and other transported organic matter is essential considering the increasing recognition of the importance of autochthonous production to riverine food webs (Thorp and Delong 2002). Phytoplankton abundance can be influenced by hydrological (discharge, water residence time), chemical (nutrient concentrations), physical (light conditions), and biotic (grazing, competition) attributes (Basu and Pick 1997). Water age, which is a measure of hydrological retention in rivers, has been to shown to affect phytoplankton abundance in lowland rivers. These studies demonstrate that the abundance and composition of phytoplankton should change in response to intra-annual variability in hydrological processes within large rivers. Furthermore, differences in both intra- and interannual variation in the hydrology of different habitats within a river-floodplain complex should also stimulate differences in phytoplankton composition and abundance, with implications toward system productivity. Basu, B.K. and F.R. Pick Phytoplankton and zooplankton development in a lowland temperate river. Journal of Plankton Research 19: Hamilton SK., WM. 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