Benthic processes across mixed terrigenous-carbonate sedimentary facies on the central Great Barrier Reef continental shelf

Abstract

The shallow continental shelf of the central Great Barrier Reef province is characterized by an across-shelf gradient of terrigenous to carbonate sedimentary facies, and is a region subjected to physical disturbances from commercial prawn trawling and, less frequently, from summer cyclones and storms. The benthic regime was examined during austral spring and summer to delimit changes in relation to the sedimentary transition. Standing stocks of bacteria ( x = 1.2;range= 0.4-2.7 × 10 10cells g −1 dry wt), protozoa ( x = 1.6;range= 1.0–2.1 × 10 6cells m -2) and meiofauna (abundances: x = 1955;range= 810–3255ind.10 cm −2; biomass: x = 2028;range= 694–3130μg dry wt 10 cm −2) generally did not vary significantly across the shelf and are very abundant compared to their counterparts on other shelves. Bacterial productivity ( 3H thymidine incorporation into DNA) and specific growth rates (μ) ranged from 0.4 to 2.6 g C m −2 d −1 ( x = 1.5) and from 0.1 to 0.8 d −1 ( x = 0.3), respectively, in surface sediments and generally did not decline significantly with sediment depth. Standing amounts of chlorophyll a ( x = 0.6;range= 0.1–2.0 μg g −1dry wt) and phaeopigments ( x = 3.9;range= 1.0–1.6 μ g −1dry wt) were low compared with pigment levels in adjacent slope and bathyal trough sediments, indicating low deposition rates of phytodetritus on the shelf proper. Macroinfaunal densities ( x = 3056;range = 2060–5406 ind. m −2) were moderate to high, but biomass ( x = 1.96;range = 0.94-3.37 g ash-free dry wt m −2) was low and decreased significantly across the shelf. Small, tube-building deposit and suspension-feeding polychaetes and amphipods dominated the infauna with a notable absence of bivalve molluscs. X-radiography revealed sedimentary profiles characteristic of oligotrophic seafloors experiencing low and variable rates of sedimentation and infaunal bioturbation, but subjected to episodes of physical reworking. The sediment fabric was extensively bioturbated mainly in the upper 2 cm with compact (<40% water) sediments devoid of large equilibrium infauna below this depth. Levels of organic carbon, total nitrogen, total phosphorus and porewater (NH 4 +, NO 2 − + NO 3 − Si(OH) 4, PO 4 3−) nutrients were low compared to other shelf sediments and generally varied little with sediment depth indicating some vertical mixing, but little burial of reactive organic matter. Dissolved inorganic nutrient fluxes measured in incubated ☐corers were low (ΣN = 0–2591; PO 4 3−; = 0–142; Si(OH) 4 = 1100–5790 μmol m −2 d −1) compared to other benthic environments suggesting low rates of detrital input. Oxygen consumption rates ranged from 13 to 40 mmol O 2 m −2 d −1. Nutrient fluxes were not directly related to surface respiration, bacterial growth or temperature. The average stoichiometric ratios of solid-phase (atomic C:N:P = 23:5:1) nutrients and measured fluxes (Si:P = 66:1; N:P = 9:1; O:N = 39:1) differed substantially from the Redfield ratio. Departure from the predicted ratios was attributed to denitrification and preferential retention of P due to binding with Fe and Mn oxyhydroxides and CaCO 3. Benthic standing crops and processes on the central shelf appear to be regulated by a variety of factors, including low and intermittent inputs of detritus, continually warm temperatures and physical disturbances caused by occasional cyclones and by frequent prawn trawling. Such disturbances and oligotrophic conditions perpetuate the dominance of a pioneering benthic regime as well as the cycle of low plankton production and sedimentation to the seafloor. Nutrient release from the benthos contributes, on average, 13 and 24% of the annual N and P requirements of the shelf phytoplankton, suggesting that a moderate coupling exists between benthic and pelagic regimes on this shallow tropical shelf.