Seasonal coupling and de‐coupling of net calcification rates from coral reef metabolism and carbonate chemistry at Ningaloo Reef, Western Australia

Abstract

Rates of net production, net calcification, and nutrient uptake were measured in a coral‐dominated reef flat community on Ningaloo Reef in northwestern Australia under seasonally minimum and maximum light levels. Daily integrated light decreased twofold while water temperatures remained relatively constant increasing by only 1°C on average from summer to winter. Rates of daily community gross primary production ( GPP ) were only 33% ± 9% higher in summer than in winter (1400 ± 70 versus 1050 ± 60 mmol C m −2 d −1 ), far less than the twofold seasonal changes reported for most shallow reef communities. Rates of daily community net calcification ( G net ) were not significantly different between seasons (190 ± 40 mmol CaCO 3 m −2 d −1 in summer versus 200 ± 10 mmol CaCO 3 m −2 d −1 in winter). The average rate of total nitrogen uptake (dissolved + particulate) was also not significantly different between summer and winter (8.3 ± 3.8 versus 6.6 ± 3.4 mmol N m −2 d −1 , respectively), despite evidence of sporadically high nitrate uptake in both seasons. In summer, rates of hourly net calcification ( g net ) were linearly correlated with diurnal changes in net production, pH, and aragonite saturation state (Ω ar ); and were mostly correlated with light except at mid‐day under heavy cloud cover. However, in winter, g net was independent of diurnal changes in light, net production, pH, and Ω ar indicating that the reef flat community had possibly reached a threshold above which rates of net calcification were insensitive to diurnal changes in their environment.