Benthic nutrient fluxes in euphotic sediments along shallow sub-tropical estuaries, northern New South Wales, Australia

Abstract

Diurnal benthic fluxes of dissolved inorganic and organic nitrogen (DIN and DON) and dinitrogen gas (N 2 ) were measured in euphotic sediments of 3 shallow sub-tropical Australian estuaries during 4 seasons. The estuaries included 2 impacted by sewage effluent (Brunswick and Simpsons estuaries) and 1 relatively pristine system (Sandon estuary). Sediments acted predominantly as net sinks of DIN throughout the year, except in the nutrient-enriched upper reaches of the Brunswick estuary, where large effluxes of NH 4 + occurred during the summer wet season. Distinct light/dark variations in NH 4 + fluxes with reduced effluxes or reversal to uptakes occurred during the light in productive sediments. NO 3 -was predominantly taken up by sediments at rates proportional to ambient concentrations in the water column. DON commonly comprised a major fraction of fluxes and was controlled primarily by heterotrophic processes in the upper to middle estuaries and autotrophic processes in the lower estuaries. Large deficits in the amount of remineralised DIN (assuming the breakdown of 'Redfield' algae) indicated that a significant amount of nitrogen is either denitrified or immobilised in biomass in the sediments. Benthic fluxes of N 2 suggest that denitrification accounts for a relatively small fraction of the missing nitrogen and immobilisation in biomass (and flow up the food chain) is a potentially major pathway of nitrogen removal in these estuaries. Significant rates of benthic productivity also stimulated secondary heterotrophic production, promoting competition for nutrient resources in the sediments. The highest rates of DIN uptake coincided with maximum metabolic rates in sediments where the diurnal p/r (gross productivity/respiration) was between 0.5 and 1, suggesting a peak in competition at this metabolic state. Denitrification rates were lowest at p/r 0.5 to 1 and strongly related to uptake of DIN from the water column, suggesting that competition may cause NO 3 -limitation in these euphotic sediments. Dissimilatory NO 3 -reduction to ammonium may become relatively more important as water column oxygen saturation drops below 40%, a condition that occurs regularly in the upper Brunswick estuary. NH 4 + was only effluxed in net heterotrophic sediments (p/r < 1), while fluxes tended towards zero in net autotrophic sediments (p/r > 1). A conceptual model for benthic nutrient cycling in shallow sub-tropical estuaries is proposed whereby benthic productivity favours to the immobilisation of nitrogen in biomass at the expense of denitrification and recycling back to the water column.

Effects of green macroalgal blooms on intertidal sediments: net metabolism and carbon and nitrogen contents

“…Nutrient concentrations in the extracted interstitial water were measured using a TRAACS-800 autoanalyser. This method extracts both pore water nutrients and intracellular nutrients from the sediment microbiota (Lomstein et al 1990, Sayama 2001, Ferguson et al 2004.…”

“…The freeze-lysable inorganic nutrient fraction measured in the sediment contains both the nutrients dissolved in the pore water and the intracellular nutrients released due to cell breakage (Lomstein et al 1990, Sayama 2001, Ferguson et al 2004). The intracellular pool has been shown to represent most of the freezelysable fraction in the upper layer of the sediment (0 to 2 mm), declining sharply with depth (Lomstein et al 1990, Sayama 2001.…”

Benthic mineralization and nutrient exchange over the inner continental shelf of western India

“…The aerobic mineralization of planktonic organic matter can be expressed in the following equations: The flux of biogenic elements is expected to occur in the Redfield ratio, but interplay of several processes such as nitrification, denitrification, adsorption, desorption, and macrobenthic activities may cause deviation from the expected ratio (Koop et al, 1990;Cowan and Boynton, 1996;Giblin et al, 1997;Ferguson et al, 2004;Faganeli and Ogrinc, 2009) in coupled nitrification-denitrification (Jenkins and Kemp, 1984) and NH + 4 adsorption onto clay minerals (Rosenfeld, 1979;Mackin and Aller, 1984). Decrease in the NH 4 flux ratio during the October incubations (0.55-0.78) apparently would favor the growth of non-siliceous phytoplankton during the late monsoon.…”

Carbon and nitrogen cycling on intertidal mudflats of a temperate Australian estuary. IV. Inverse model analysis and synthesis

“…Previous studies have shown that the production to respiration (P:R) ratio of the sediment is a useful metric of MPB activity and has a strong influence on the nutrient demand of the sediments (Rizzo et al 1996, Ferguson et al 2004b, Engelsen et al 2008. At P:R > 1, sediments tend to be a sink for nutrients as a consequence of MPB nutrient requirements exceeding the capacity of the sediments to supply nutrients, making MPB most likely nutrient-limited under these conditions.…”

“…At P:R < 1, sediments will be a source for nutrients, with the capacity of the sediments to supply nutrients exceeding the assimilative requirements of MPB, so that MPB are highly unlikely to become nutrient-limited. Furthermore, the sediment P:R ratio has also been shown to have a strong influence over benthic denitrification rates and the relative importance of organic nutrient fluxes, highlighting its strong influence over a range of microbial processes (Risgaard-Petersen 2003, Ferguson et al 2004b, Engelsen et al 2008.…”