Sources of nutrients driving production in the Gulf of Carpentaria, Australia – a shallow tropical shelf system
Michele A. Burforda*, Peter C. Rothlisbergb, Andy T. Revillc a
Australian Rivers Institute, Griffith University, Nathan, QLD 4111, Australia CSIRO Marine & Atmospheric Research, PO Box 120, Cleveland, QLD 4163, Australia c CSIRO Marine & Atmospheric Research, GPO Box 1538, Hobart, TAS 7001, Australia b
Running header: Nitrogen processes in a tropical shelf system
*
Corresponding author: Ph: +61 7 3735 6723 Fax: +61 7 3735 7615 Email:
[email protected] Address: Australian Rivers Institute, Griffith University, Nathan QLD 4111, Australia
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Abstract
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The tropical Gulf of Carpentaria, Australia has recently been identified as one of the
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world’s least impacted marine areas presenting a unique opportunity to understand the
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nutrient drivers of productivity. This study examined the nitrogen (N) sources and
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transformations in this pristine area, and its’ role in fuelling primary productivity
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principally based on summer data. The nitrogen budget estimates on a whole-of-Gulf
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basis suggest that river nitrogen inputs are unlikely to be major contributors to
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primary productivity. In the deeper waters of the Gulf, beyond the coastal boundary
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current, the main source of nitrogen is estimated to be nitrogen fixation by
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cyanobacteria, principally the abundant genus, Trichodesmium. Our study measured
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high nitrogen fixation rates and depleted δ15N-nitrogen ratios in the particulate matter
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in the water column during a summer bloom. During summer, bottom nitrogen
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concentrations increased and δ15N-nitrogen ratios were depleted, suggesting that
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benthic mineralization is occurring. It is therefore likely that detrital material from
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nitrogen-rich Trichodesmium is an important contributor to benthic processes. During
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winter, wind-driven mixing results in nitrogen from bottom waters reaching the
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euphotic zone, and fuelling primary productivity. Therefore, Trichodesmium has an
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important direct and indirect role in contributing to primary productivity in this
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pristine tropical ecosystem.
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Additional keywords: nitrogen fixation, Trichodesmium, nitrogen budget
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Introduction
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Tropical coastal waters are characterized by high water temperatures and large
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summer rainfall events resulting in significant river inflows and associated inputs of
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nutrients from adjacent rivers. This contributes to high year-round primary
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production (Longhurst and Pauly 1987). Development of catchments for agriculture
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and other human activities has typically reduced or regulated river flows in many
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rivers, and increased nutrient loads to coastal waters (Carpenter et al. 1998; Beman et
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al. 2005). The increase in algal blooms worldwide, including harmful species, and
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development of dead zones in coastal waters has been attributed to increasing nutrient
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inputs from catchments (Ferber 2001; Anderson et al. 2002; Chen et al. 2007).
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Further offshore, many areas of the tropics have low dissolved inorganic nitrogen (N)
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concentrations and low N:P ratios, resulting in N limitation. In these areas,
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atmospheric N inputs from cyanobacteria can be an important source of N. The most
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studied species is the filamentous diazotroph Trichodesmium which can form large
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surface rafts stretching hundreds of kilometres, adding N to fuel productivity
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(Westberry and Siegal 2006). More recently, the importance of other symbiotic and
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small unicellular cyanobacteria in providing N to tropical food webs has also been
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established (Carpenter et al. 1999; Zehr et al. 2001; Montoya et al. 2004; Capone et
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al. 2006).
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The Gulf of Carpentaria in tropical Australia is a large coastal sea (330,000 km2)
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surrounded by many large rivers seasonally providing 92,000 GL of water to the Gulf
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each year during the NW monsoonal wet season (January to March). The shallow
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depth of the Gulf (
