Seasonal net calcification by secondary calcifiers in coral reefs of the Eastern Tropical Pacific Ocean

Abstract

This study assesses whether secondary calcification is driven by a contrasting seasonal pattern (rainy vs dry) that occurs in the Eastern Tropical Pacific (ETP). Secondary calcifiers net calcification rates and coverage were measured in two reefs: the semi-enclosed Bahía Tiburón reef (BT [21°52′30 "N, 105°54/54 "W]) and the open Las Monas fringing reef (LM [21°51ʹ00ʹʹN, 105°52ʹ45ʹʹW]). Measurements were made from 2013 to 2016 using Calcification Accretion Units (CAUs). Seawater temperature, illuminance, pCO₂, pH, ΩCa, and ΩAr were also measured. Low means of pCO₂, and high means of ΩCa and ΩAr, were measured during the rainy season. At Las Monas, the composition of the calcifier community differed between seasons. A seasonal effect on net calcification was recorded in the semi-enclosed reef and in the exposed microhabitat of both reefs. Overall, net calcification (mean ± SD) was 1.17 ± 1.13 g·CaCO₃·m⁻²·day⁻¹. Calcification in the open fringing reef (1.51 ± 1.32 g·CaCO₃·m⁻²·day⁻¹) was almost double that in the semi-enclosed reef (0.83 ± 0.78 g·CaCO₃·m⁻²·day⁻¹). Calcification also decreased dramatically between 2014 (1.57 g·CaCO₃·m⁻²·day⁻¹) and 2016 (0.99 g·CaCO₃·m⁻²·day⁻¹). The ENSO event of 2015 raised the water temperature almost 1 °C above the decadal average, which led to a mass coral bleaching in both reefs. That thermal stress might explain the calcification decline in 2015–2016, but probably also obscured a clearer seasonal pattern in net calcification. This study is the first to show that anomalous and persistent high seawater temperatures can affect carbonate production by secondary calcifiers.