Reef framework diagenesis across wave-flushed oxic-suboxic-anoxic transition zones

Abstract

The temporal and spatial variability of inorganic nutrient concentrations in overlying- and interstitial-seawater in Checker Reef, Oahu was examined for response to incident wave magnitude and direction. Well-point samplers were used to profile interstitial nutrient concentrations across oxic-suboxic-anoxic transition zones in the upper meter of the reef framework at four sites aligned across the patch reef. Samples were acquired over February, 1992, during which time dominant E-NE trade winds directed waves across the reef from the fore-reef to back-reef. However, W-SW “Kona” winds periodically interrupted this pattern and directed waves in the reverse direction. The interstitial microbial habitats of fore- and back-reef framework were distinct from those within the mid-reef framework. Maximum concentrations of PO4, Si, and NH4 in interstitial waters occurred at framework depths of 1–2 m, with the highest concentrations occuring within the mid-reef framework. Maximum concentrations of NO3 and NO2, which were used to delineate the core of the suboxic zone, occurred at framework depths of 5–10 cm at all stations and attained 2–4 fold higher peak concentrations within the mid-reef and back-reef than within the fore-reef. Variability in interstitial nutrient concentrations was greatest within the back-reef and is consistent with reversals of wave-direction, with the resultant increases in mixing between interstitial and overlying seawater due to flushing caused by the S-SW Kona wind events. The ratio of molar concentrations of total inorganic nitrogen to phosphate (TIN : PO4) for the fore-reef was 5 : 1; while ratios for the mid- and back-reef were 13–15 : 1, reflecting that the dominant source of particulate organic matter to the fore-reef framework is plankton, while that of the mid- and back-reef is benthic reef plants.