NITROGEN RECYCLING in the CHOWAN RIVER 1 Donald W
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# NITROGEN RECYCLING IN THE CHOWAN RIVER Donald W. Stanley 1 , Research Associate, Zoology John E. Hobbie 2 , Professor of Zoology Department of Zoology N.C. State University Raleigh, N. C. 27607 I Present address: Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, North Carolina 27514. 3 LPresent address: The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543. The work upon which this publication is based was supported by funds provided by the Office of Water Research and Technology, U.S. Department of the Interior, through The University of North Carolina Water Resources Research Institute, as authorized under the Water Resources Research Act of 1964, as amended. Project No. B-077-NC Agreement No. 14-31-0001-5097 December, 1976 TABLE OF CONTENTS Page ACKNOWLEDGMENTS .................... iv ABSTRACT ........................ v LISTOFFIGURES .................... vi LIST OF TABLES .....................viii SUMMARY AND CONCLUSIONS ................ ix RECOMMENDATIONS ....................xiii INTRODUCTION ................a*.... 1 A.Background. ....................., 1 B . Objectives and Research Plan ............ 2 C . The Chowan River and Its Drainage Basin ...... 3 METHODS ........................ 9 A.Sampling ...................... 9 B . Carbon andt Nitrogen Uptake Experiments ....... 9 C . Nutrient Analyses and Algal Biomass ........ 12 RESULTS AND DISCUSSION ................. 14 A . Seasonal Patterns ................. 14 1. Nitrogen Concentrations ............ 14 2 . Phytoplankton Composition and Biomass ..... 23 3 . Photosynthesis and Inorganic Nitrogen Uptake . 26 B . Kinetics of Photosynthesis and Nitrogen Uptake . 30 1. Temperature Effects .............. 30 2 . Light Effects and Die1 Uptake ......... 30 3 . Nitrogen Concentration Effects ......... 35 a . Enrichment Experiments ........... 35 b . Nitrate-Ammonia Interaction ........ 39 C . Annual Input and Output of Nitrogen ........ 40 D . Annual Carbon and Nitrogen Uptake ......... 46 E . Nitrogen Recycling ................. 52 REFERENCES ................... 56 iii ACKNOWLEDGMENTS This study was made possible by a grant from the North Carolina Water Resources Research Institute to J. E. Hobbie, Zoology Department, North Carolina State University, Raleigh. Several people were associated with the project in various ways. Bruce Dornseif, William Bowden, Charles Balducci, and Adrianne Zlotowitz provided help with the sampling, field experiments, and laboratory analyses. Mr. Grover Cook, co- ordinator of The ChowantRiver Study, cooperated in providing work space for the project in a portable field laboratory located near Winton, N.C. Dr. Jay Langfelder of the Center for Coastal and Marine Studies at N.C. State University made available to us other field laboratory and sleeping facilities. Dr. A.M. Witherspoon's laboratory provided estimates of algal species composition and abundance in samples collected every other week from the four sampling sites. Dr. Charles Daniels of the U.S. Geological Survey provided river flow data. Total Kjeldahl nitrogen analyses were performed in the laboratory of Dr. Michael Overcash of the Biological and Agricultural Engineering Department at N.C. State University. We aref especially grateful to Dr. Richard Volk of N.C. State Univer- sity, in whose laboratory all the samples were analyzed by mass spectrometry. Dr. John Miller served as principal investigator during the last six months of the project, following J. E. Hobbie's resignation from the University to accept a position with the Marine Biological Laboratory, Woods Hole, Massachusetts. ABSTRACT The repeated occurrence of nuisance algal blooms in the Chowan River during the past few summers may have been caused by increased nitrogen loading in the river. That possibility prompted this study of the relationship between nitrogen and algal growth in the river. The lower Chowan River, located in northeastern North Carolina, is actually a freshwater tidal estuary emptying into Albemarle Sound. As is typical for this region, dissolved inorganic nitrogen concentrations in the Chowan are high in winter and low in summer. This pattern results from a combi- nation of high rates of input from land runoff in the winter and high rates of removal by rapidly growing algae in the sum- mer. Dissolved organic nitrogen is the most abundant form of nitrogen in the river, and the concentrations decrease down- river, suggesting that it is transformed to other forms within the river. Annual algal production in the river was around 100 g over 90% of which occurred between May and October, a period when blue-green, dinoflagellate and green algae made up most of the algal biomass. Annual inorganic nitrogen uptake, mea- sured by ls~isotope techniques, was 33 g NHq.-Nmm-2 and 12 g -2 NO 3- Ngm . Carbon-nitrogen ratios calculated from these data are low, probably because of nitrogen assimilation by bacteria in the samples and because of luxury uptake of nitrogen by the phytoplankton. During winter rapid flushing rates, low light intensities and low temperatures are the most important factors limiting algal photosynthesis and nitrogen uptake in the river. During summer inorganic nitrogen became limiting as nitrate and ammonia levels fell below 50 pg N liter-', the concentration found necessary for maximum uptake. However, rapid regeneration of ammonia permitted rapid algal growth throughout the summer despite the low concentrations. LIST OF FIGURES Page The Chowan River drainage basin in southeastern Virginia and northeastern North Carolina. Sampling stations are Edenhouse (E), Colerain (C) , Harrells- ville (H), andWinton(~) .............. 4 Discharge for the Chowan River at Winton, N.C. The average rates for each day are plotted ....... 6 Chowan surface water temperatures and depth of pene- tration of 1% of the surface light, at the Winton station. ...................... 8 Nitrate and ammonia concentrations at the Winton and Harrellsville stations ........,.... 15 Nitrate and ammonia concentrations at the Colerain and Edenhouse stations, ........16 Particulate nitrogen concentrations at the Winton and Harrellsville stations .........19 Particulate nitrogen concentrations at the Colerain and Edenhouse stations. ............... 20 Dissolved organic nitrogen concentrations at the Winton and Harrellsville stations .......... 21 Dissolved organic nitrogen concentrations at the ColerainandEdenhousestations. .......22 Seasonal variation in algal wet weight biomass and species composition at the Winton and Harrellsville stations.... ................ 24 Seasonal variation in algal wet weight biomass and species composition at the Colerain and Edenhouse stations.........,............. 25 Midday rates of algal photosynthesis and nitrate and ammonia uptake for surface samples from the Winton and Harrellsville stations .......27 ~iddairates of algal photosynthesis and nitrate and ammonia uptake for surface samples from the Colerain and Edenhouse stations ........... 28 Effect of increasing temperature on rates of algal photosynthesis and nitrate and ammonia uptake at three Chowan River stations: Edenhouse (E), Colerain (C), and Harrellsville (H). .............31 Page 15. Effect of increasing light intensity on photosyn- thesis and nitrate and ammonia uptake at the Colerain station on 15 June 1975. .......... 32 16, Die1 pattern of photosynthesis and nitrate and ammo~iauptake at the Winton station on 31 August- 1 September 1975. .................. 36 17. Effect of increasing nitrate and ammonia concen- tration on the uptake rates of these nutrients at three Chovian stations ................ 37 18. Effect of increasing ammonia cor~centrationon the fraction of DIN uptake that is nitrate. ....... 41 19. Inflow, uptake, and outflcw of nitrate and ammonia in the lotlTer Chowan River between Nove,mber 1974 and November 1975. Size of the blocks is proportional to the quantity of nitrogen. ............ 53 20. Total annual input, assimilation, and output of nitrogen for the lower Chowan River between 1 November 1974 and 31 October 1975 .......... 55 vii .- .. .- .. SUMMARY AND CONCLUSIONS This project studied the utilization and recycling of nitrogen in the lower Chowan River, North Carolina. The Chowan River Basin drains approximately 12,600 km2 in south- eastern Virginia and northeastern North Carolina. One objec- tive was to determine how significant recycling of the nitrogen nutrients is for algal growth in the river. Second, we wished to determine what factors limit algal growth in the Chowan at different times of the year. A final objective was to gen- erate information that could be used in models that would sim- ulate biological processes (e.g., algal growth) in the river. Specific findings were: 1. In the lower Chowan (the section of river included in this study between Winton and the mouth at Edenhouse) flow often exceeds 15,000 cfs in winter but is usually less than 3,000 cfs in summer when tributary inflow is lowest. Thus, flushing times for the lower Chowan are typically 50 days or more in the summer, compared to 10 days or less in winter. Water temperatures range from around 5'~in January to near 30'~ in August. Light penetration in the river water is so limited by high turbidity that the depth of penetration of 1%of the surface light seldom exceeds 2 m. 2. At the four stations sampled, nitrogen levels were typical of estuarine systems in this region. Nitrate concentrations were high in winter and low during the summer. This seasonal pattern results