Silicate, Diatoms, Organic Matter Degradation and Nutreint Cycling in Moreton Bay  

Authors / CoAuthors

Heggie, D.T. | Skyring, G.W. | Berelson, W.M. | Radke, L.A. | Revill, A.

Abstract

Moreton Bay (MB) is a large (~1800 square km), stressed (with recent outbreaks of the cyanobacteria Lyngbia majscula), sub-tropical estuary which receives urban and rural runoff from a large catchment. Silicon is an essential nutrient for diatomaceous phytoplankton growth in coastal ecosystems. BSi (biogenic silicon) in surface sediments, pore water DSi (dissolved silicate, SiO4--) and benthic DSi fluxes were used as tracers of the formation and degradation of organic matter (OM) in MB. This work has implications for N & P cycling, water quality and eutrophication. BSi, TOC (both up to 2 wt%), TN & TP and diatom sterol biomarkers were all highest in the muddy sediments of western MB that is ~65% of the bay's area. We found that diatoms dominated OM cycling in western MB, and the benthic DSi flux accounted for ~80% of the pelagic productivity. Our conceptual model is that diatoms being heavy (because of their Si content) sink rapidly to the sediments where their biomass-N (OM-N) was denitrified to N2 and lost to the atmosphere with an efficiency of about 50%. Approximately 60% of OM-P, subsequent to degradation, remained trapped within the sediment. Diatoms therefore are an important vector to repeatedly deliver river-borne N & P to their respective sinks. However, diatomaceous OM contributed only about 20% of the OM input to the marine sands of eastern MB, about 34% of the bay's area. The principal OM input to the sandy sediments was attributed to benthic photosynthesis and N-fixation with rates of N-fixation (estimated from pore water DIN gradients) at 1.5 - 3.5 mmol m-2d-1. OM was rapidly and efficiently degraded (principally by O2), with little net accumulation and burial in sediments. N was denitrified efficiently (~100%). DIP must have been recycled rapidly in the top few cm's of the sandy sediments to support N-fixation. A whole-bay silicate budget indicated that: 1. DSi fluxes through the western margin of MB were about 4- fold those in eastern MB. 2. Pelagic diatom productivity was supported (approximately) by the benthic fluxes of DSi. 3. The DSi inventory was recycled through diatomaceous phytoplankton in about 15 days. 4. The export of DSi to the sea was about the same as the combined terrestrial and small marine inputs.

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nonGeographicDataset

eCat Id

65241

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• External PublicationScientific Journal Paper

( Theme )

• environmental

( Theme )

• coasts

( Theme )

• geochemistry

( Theme )

• geoscience education

( Theme )

• marine

Australian and New Zealand Standard Research Classification (ANZSRC)

• Earth Sciences

• Published_Internal

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2007-05-25T00:00:00

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