Diatoms control nutrient cycles in a temperate, wave‐dominated estuary (southeast Australia)

Abstract

Diatoms are important primary producers within pelagic, benthic, and epiphytic communities, and their siliceous frustules sink rapidly to the sediment. We measured benthic nutrient and gas fluxes and water column and sediment properties in an estuary of southeastern Australia to identify control mechanisms coupling benthic and pelagic processes, in particular, how nutrients become fractionated by processes affecting benthic nutrient fluxes. During late spring, the water column of St. Georges Basin was oligotrophic, primary production was likely phosphorus (P) limited, and the phytoplankton community was dominated by cyanophytes. Molar ratios of dissolved inorganic carbon (TCO 2 ) to silica (Si) benthic fluxes, however, were equal to the molar composition of diatoms, indicating that diatoms preferentially sink and deliver the most labile organic matter fraction to the sediment. The congruent release of Si and carbon (C) implies a coupling of processes mobilizing Si and C. Extracellular polymeric substances surrounding the siliceous frustule are the primary labile organic matter fraction, and their rate of mineralization limits the dissolution of the siliceous frustule. Because decomposing biomass in sediments leads to net di-nitrogen (N 2 ) production and very efficient burial of P, the fate of diatoms significantly contributes to the removal of bioavailable nutrients. High dissolved inorganic nitrogen to dissolved inorganic P benthic flux ratios of 290 to 900 promote P limitation, particularly in shallow waterbodies with long water residence times.

Biogeochemical processes at the sediment–water interface, Bombah Broadwater, Myall Lakes

“…These differences between the muddy and sandy sediments we speculate may result from differences in anaerobic OM oxidation rates where those in the muds were more important in oxidising OM than those in the sands (Table 4). The denitrification efficiencies in the muddy sediments of BB were similar to those measured in St Georges Basin a semi-enclosed body of water with restricted circulation (Haese et al, 2007), and those muddy- ML-3 (sand) 11.0 ± 4.3 3.0 ± 3.9 86 ± 6.3 SRR = sulphate reduction rate; DE = denitrification efficiency sediment sites within Wilson Inlet (WA) where denitrification rates were *50-80% (Geoscience Australia unpublished). These rates however were low compared to the central basin of Port Philip Bay where denitrification efficiencies were *60-100% (Berelson et al, 1998;Harris et al, 1996;Heggie et al, 1999a, b).…”

“…The nitrate may be derived either from the water column (via diffusion into the sediments) or from nitrate produced during the oxidation of ammonia (nitrification) generated within the sediments (sedimentary coupled nitrification-denitrification). This process was important in Port Phillip Bay (Harris et al, 1996;Berelson et al, 1998;Heggie et al, 1999a, b) and in St Georges Basin (Haese et al, 2007).…”

“…Nitrogen is the limiting nutrient in most estuarine systems (Smith, 1984;Howarth & Marino, 2006), including Australia (Harris, 2001), although limitation by other nutrients may be possible (Smith & Atkinson, 1983;Thompson, 1998; Haese et al, 2007). Sediment denitrification is the microbially mediated reduction of NO x -to N 2 under sub-oxic to anoxic conditions.…”

Effect of nutrient loading on biogeochemical processes in tropical tidal creeks

“…, fNO x , fPO 4 3-, fSiO 4 ) and dissolved gas (fDO, fDIC, fN 2 ). Four manually operated benthic chambers (two transparent, two opaque), as described by Haese et al (2007), were deployed at each site. Self-logging probes (YSI-600XL) continuously measured temperature, salinity and oxygen concentration inside and outside of the chambers.…”

“…Chl a was extracted from sediment and water column filters prior to analysis by high performance liquid chromatography following the procedures outlined in Cook et al (2004) and Haese et al (2007), respectively. Sediment samples for stable isotope analysis (d 13 C and d 15 N) were processed and analyzed as described in Cook et al (2004).…”