NP Budget of the Central Great Barrier Reef, Australia

S. V. Smith

7 October 1996

The Great Barrier Reef province of Queensland, Australia, covers an area of approximately 200,000 km² between the latitudes 10 and 24.5 ° S. Although this region is often referred to as the largest coral reef in the world, it is not actually a reef, but rather a continental shelf area which is characterized by the abundance of reefs rising from the shelf platform.

Furnas and Mitchell (see citations in 1996) have published a series of important papers describing various aspects of the hydrography of the central Great Barrier Reef region (16.5-18.5 ° S; 145.5-146.5 ° E) , an area of 14,000 km² and with an average depth of 37 m. Within this region, reefs only occupy a total area of about 5% (based on percent of the area shallower than 10 m). Their 1996 paper is an assessment of nutrient inputs which, together with some simple assumptions about outputs, can be used to develop a nutrient budget for this portion of the Great Barrier Reef. Those authors divide the central Great Barrier Reef into two "boxes": the Cairns box and the Tully box. For the present analysis, I have grouped those back into one.

Those authors conclude that there are four significant sources of nutrient delivery from outside the system: sewage from both the continental mainland and island resorts within the region, rivers (presumably mostly drainage from extensive agricultural areas in the hinterlands), rainfall, episodic upwelling of "Subtropical Lower Water" (SLW) which intrudes onto the shelf from the Coral Sea.

In addition to these external N inputs, these authors observe that there are very uncertain estimates for nitrogen fixation by both reef organisms and the planktonic cyanobacteria, Trichodesmium. For the nitrogen fixation, they estimate a total N input of 179 x 10⁶ mol/yr. Per unit area of reef and on a daily basis, this is equivalent to about 0.7 mmol N m^-2 d^-1—a modest estimation of N fixation on reefs. The input from Trichodesmium is both far larger and far more uncertain: 328-15,200 x 10⁶ mol/yr. Across the entire region, this is equivalent to a daily rate of 0.06-3.0 mmol m^-2 d^-1. Therefore these internal sources appear to account for 1-4 mmol N m^-2 d^-1 across the entire region. Note that these internal sources are not directly used in the budget but become useful below in the interpretation of the budget.

Table 1 is a summary of the estimated terrigenous, atmospheric, and hydrographic inputs of N. Furnas and Mitchell further estimate that essentially all of the P input is as DIP and the N input is about equally divided between NO₃ and DON. In addition to these inputs, there must be a hydrographic output. The volume of upwelling water intruding onto the shelf can be up to about 20% of the total shelf volume, an amount which seems far too large to be temporarily "stored" as elevated sea level. It must, therefore, displace roughly an equivalent volume of water off the shelf. Salinity data are both insufficient and too complex to be readily resolved into a salt budget of water exchange, so we use a simple consideration of water exchange. We assume, from the authors’ analysis of nutrient concentration versus temperature, that the displaced water is lagoon water (LW), and that this water has nutrient concentrations which are of the order of 10% of the upwelled SLW. That is, the displaced water is assumed to remove an amount of nutrients equal to roughly 10% of the upwelled water. This assumption allows us to complete the budget.

Table 1. Estimated external inputs and outputs of dissolved N and P with respect to the central Great Barrier Reef (10⁶ mol/yr).

The "uncertainty" in the upwelling (hence, in the outward displacement) represents interannual variation seen between 1989 and 1992. The numbers in parentheses for both upwelling and outward displacement are used here as estimates to arrive the mean system performance. These inputs must be matched by outputs of organic matter, as recognized (but not directly quantified) by Furnas and Mitchell. The implied outputs become D P and D N (i.e., the nonconservative fluxes of dissolved N and P) in the terminology of Gordon et al. (1996) and are equivalent to D P (mostly as DIP) = -0.012; D N = -0.174 mmol m^-2 d^-1.

If we assume that the major nonconservative processes affecting P and N are biotic, then we can attribute D DIP and some amount of D DIN to the difference between primary production (p, which takes up nutrients) and respiration (r, which releases nutrients). Furnas and Mitchell (1987) estimate that p for the central portion of the Great Barrier Reef is about 290 g C m^-2 yr^-1 (or 66 mmol m^-2 d^-1), of which 56% is plankton and the rest is reef production. For stoichiometric scaling, a material made up of a composition intermediate between plankton (C:N:P = 106:16:1) and benthic plants (550:30:1) is assumed (300: 20:1 adopted here) is used.

(1)

(p-r) is estimated according to equation (1) to be approximately 4 mmol m^-2 d^-1. If p = 66 and (p-r) = 4, then r = 62 mmol C m^-2 d^-1. In this shelf system p exceeds r by approximately 7%.

The difference between D N predicted to match the D P and the observed D N is attributed to the difference between nitrogen fixation and denitrification (nfix-denit):

(2)

According to this equation, (nfix-denit) = +0.1 mmol m^-2 d^-1. That is, nitrogen fixation and denitrification appear almost exactly in balance. Recall (above) that nitrogen fixation itself has been estimated to be 0.1-4 mmol m^-2 d^-1. The implication of a near balance between these two processes is that—whatever value is chosen to be the best estimate of nitrogen fixation, denitrification is about equal to this value. In this shelf system, the net process of (nfix-denit) appears to be neither a net source nor a net sink of N.

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