Seasonal variation in nitrification and denitrification in estuarine sediment colonized by benthic microalgae and bioturbating infauna

Abstract

Measurements of seasonal vanationIn oxygen fluxes nutiient fluxes and denltrif~catlon were obtalned in a n estuarine sedlment lnhablted by benthlc microalgae and bloturbating infauna Oxygen dynamics in the upper sediment strata were found to be controlled by the ln~croalgae and there was a net flux of O2 out of the sediment during spring and autumn Hlgh assimllatlon by the microalgae reduced the efflux of NH,and PO,' from the sediment to the water column d u n n g daytime Denltrification based on NO-, from the water column (D,,] only occurred in wlnter and spnng, when NO, was present in the water column and act~vity was proportional to the water column NO, concentratlon The rate of D,, was reduced during d a y t ~m e when the upper oxic zone of the sediment increased due to 0, production by benthic m~croalgae Coupled nitrif~catlon-denltrlfication (D,) In the sedlment was stimulated by the O2 production durlng w i n t e ~ and spnng at which times NO? and NH,' were present in the wciter column In hlgh concentrations In contrast, d u r ~n g summer when the concentration of NO; and NH,-In the water column was low, benthic microalgae Inhibited D, by competing with nltr~fying bacterla for NH,' D,, accounted for 80'Yi of the total denltrificatlon during winter, while on an annual basis.D, and D, edch accounted for 50% of the total denitnficatlon activity Benthic ~n f a u n a , such as Corophlurn spp Hydrob~a spp , and N e r e ~s spp , occurred in densities of up to several thousand ind in from May to October Oxygen consumption, D,, and D, were llneaily correlated with the d e n s ~t y of the amphlpod Coroph~um spp , all the processes studied belng stimulated by the pumping of 02-and NO3 -nch water through the burrows in the upper 2 to 6 cm of the sediment Dunng summer, the D, activity was, therefore, the net result of the inhibitory effect by benthic microalgae and the stimulatory effect of the benthlc infauna However, as the concentration of lnorganlc nitrogen in the overlying water and the sediment nitrification potent~al are both low in shallow coastal waters during summer, when b e n t h ~c infauna d e n s ~t y is high we conclude that the stlmulatory effect of bioturbatlng infauna on both D, and D, IS of mlnor importance to the annual denltnf~cation budget