Nitrogen Recycling in the Chowan River

The repeated occurrence of nuisance a l g a l blooms i n the Chowan River during the pas t few summers may have been caused by increased ni trogen loading i n the r i v e r . That p o s s i b i l i t y prompted t h i s study of the r e l a t i onsh ip between ni t rogen and a l g a l growth i n the r i v e r . The lower Chowan River, located i n nor theas tern North Carolina, is ac tua l ly a freshwater t i d a l es tuary emptying i n t o Albemarle Sound. A s is typ i ca l f o r t h i s region, dissolved inorganic ni trogen concentrations i n the Chowan a re high i n winter and l o w i n summer. This pa t t e rn r e s u l t s from a combinat ion of high r a t e s of input from land runoff i n the winter and high r a t e s of removal by rapidly growing algae i n the summer. Dissolved organic n i t rogen is the most abundant form of n i t rogen i n the r i v e r , and the concentrations decrease downr i v e r , suggesting t h a t it is transformed t o o ther forms within the r i v e r . Annual a l g a l production i n the r i v e r w a s around 100 g over 90% of which occurred between May and October, a period when blue-green, d inof lage l la te and green algae made up most of the a l g a l biomass. Annual inorganic n i t rogen uptake, measured by l s ~ isotope techniques, w a s 33 g NHq. -Nmm-2 and 12 g -2 NO N g m . Carbon-nitrogen r a t i o s ca lcula ted from these data 3a r e low, probably because of n i t rogen ass imi la t ion by bac t e r i a i n the samples and because of luxury uptake of n i t rogen by the phytoplankton. During winter rapid f lush ing r a t e s , low l i g h t i n t e n s i t i e s and low temperatures a r e the most important f a c t o r s l im i t i ng a l g a l photosynthesis and ni t rogen uptake i n the r i v e r . During summer inorganic ni trogen became l imi t i ng a s n i t r a t e and ammonia l e v e l s f e l l below 50 p g N l i t e r ' , the concentration found necessary f o r maximum uptake. However, rap id regenerat ion of ammonia permitted rap id a l g a l growth throughout the summer despi te 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) , Harrellsville (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 penetration 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 C o l e r a i n a n d E d e n h o u s e s t a t i o n s . 22 Seasonal variation in algal wet weight biomass and species composition at the Winton and Harrellsville s t a t i o n s . 24 Seasonal variation in algal wet weight biomass and species composition at the Colerain and Edenhouse s t a t i o n s . , . 25 Midday rates of algal photosynthesis and nitrate and ammonia uptake for surface samples from the Winton and Harrellsville stations . 27 ~iddai rates 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