Carbon, nitrogen, and phosphorus budgets for a shallow subtropical coastal embayment (Moreton Bay, Australia)

Average annual carbon, nitrogen, and phosphorus budgets were constructed for Moreton Bay. Primary production was the dominant source of carbon (by two orders of magnitude), N fixation was the dominant source of nitrogen, and point sources were the dominant source of phosphorus to the bay. About 41% of the nitrogen and 70% of the phosphorus entering Moreton Bay was exported to the ocean, and about 56% of the nitrogen was lost to denitrification. The high percentage loss of phosphorus to the ocean was directly related to the short residence time of the bay (46 d), which was consistent with other shallow coastal ecosystems. In contrast, the percentage loss of nitrogen to the ocean was low compared to other coastal systems due to the high percentage loss through denitrification associated with autotrophic sediments in the bay that enhance denitrification. Because most denitrification studies have been carried out using only dark incubations, the importance of denitrification to the nitrogen budgets of coastal systems in general may be underestimated. Carbon loss from Moreton Bay was dominated (by two orders of magnitude) by atmospheric exchange of CO2 associated with benthic and pelagic respiration. The distinct difference between Moreton Bay (subtropical) and temperate systems was the dominance of biological (microbial: N fixation and denitrification) over physical inputs and losses of nitrogen. High N fixation in turn fuels a positive annual mean net ecosystem metabolism (NEM) of 21 g C m22 yr21 and suggests that primary production in the bay is phosphorus limited at the whole ecosystem scale. The metabolism of temperate coastal ecosystems is closely linked to the magnitude of terrestrially derived nutrient (C, N, P) loads (Nixon et al. 1986; Borum 1996; Kemp et al. 1997). However, it remains difficult to predict the impact of increased nutrient loading on different coastal ecosystems, particularly tropical systems, because of confounding physical and biogeochemical factors (Pennock et al. 1994; Eyre and Balls 1999; Cloern 2001) and the influence of other nutrient sources and sinks such as the atmosphere and adjacent ocean (Smith 1984; Paerl et al. 1994; Eyre and France 1997; Smith and Hollibaugh 1997). Construction of nutrient budgets is a way of constraining these compounding factors, which then allows an evaluation of the pathways of nutrient flow and the fate and effect of nutrient loads on the metabolism of coastal ecosystems. Nutrient budgets also provide a means of quantitatively comparing different coastal systems. There are many published nutrient budgets for temperate shallow coastal ecosystems (e.g., Boynton et al. 1995; Nixon et al. 1995; Kemp et al. 1997; Smith and Hollibaugh 1997). However, whole ecosystem scale nutrient mass balance studies in tropical and subtropical coastal systems are rare (e.g., Smith 1984; Eyre 1995; Eyre et al. 1999; McKee et al. 2000a), and these previous studies were restricted to only physical boundary fluxes of nitrogen and phosphorus and internal biological fluxes were only inferred. There have been no complete (i.e., physical boundary and internal bio1 Corresponding author ([email protected]). 2 Current address: San Francisco Estuary Institute, 180 Richmond Field Station, 1325 South 46th Street, Richmond, California 94804.