Nutrient pollution disrupts key ecosystem functions on coral reefs

Abstract

There is a long history of examining the impacts of nutrient pollution and pH on coral reefs. However, little is known about how these two stressors interact and influence coral reef ecosystem functioning. Using a six-week nutrient addition experiment, we measured the impact of elevated nitrate (NO^-₃) and phosphate (PO^3-₄) on net community calcification (NCC) and net community production (NCP) rates of individual taxa and combined reef communities. Our study had four major outcomes: (i) NCC rates declined in response to nutrient addition in all substrate types, (ii) the mixed community switched from net calcification to net dissolution under medium and high nutrient conditions, (iii) nutrients augmented pH variability through modified photosynthesis and respiration rates, and (iv) nutrients disrupted the relationship between NCC and aragonite saturation state documented in ambient conditions. These results indicate that the negative effect of NO^-₃ and PO^3-₄ addition on reef calcification is likely both a direct physiological response to nutrients and also an indirect response to a shifting pH environment from altered NCP rates. Here, we show that nutrient pollution could make reefs more vulnerable to global changes associated with ocean acidification and accelerate the predicted shift from net accretion to net erosion.

Keywords: biological feedbacks; coral reefs; nutrient pollution; pH.

Figure 1.

(a) Conceptual diagram highlighting a biological feedback loop on coral reefs in response to nutrient addition, and (b) NO⁻₃ + NO⁻₂ and (c) PO³⁻₄ concentrations used in this experiment. Reef constituents photosynthesize and respire, modifying the local pH environment through uptake and release of CO₂. Changes in pH can in turn influence calcification and dissolution rates on coral reefs, where calcification decreases and dissolution increases with ocean acidity. Nutrient pollution can alter this biological feedback loop by enhancing or depressing photosynthesis and respiration. Subsets (b) and (c) show the means ± s.e. (n = 14) for NO⁻₃ + NO⁻₂ and PO³⁻₄, respectively, from the header tanks over the two 24 h water sampling events. NO⁻₃ and PO³⁻₄ were elevated in concert in each of the three treatments. Drawings of reef organisms are Courtesy of the Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/symbols/).

Figure 2.

(a–e) Net community calcification rates and (f–j) net community production rates (μmol g⁻¹ h⁻¹) for coral, algae, rubble, sand and the mixed community. Values are means ± s.e. for samples collected during the day (open triangles), night (closed triangles) and over the 24 h sampling period (closed circles). Significant relationships between nutrients and NCC or NCP (p < 0.05) for daytime, nighttime and net rates are represented with asterisks. Model results for NCC and NCP are in electronic supplementary material, tables S1 and S2, respectively. Dotted line shows when NCC and NCP are equal to zero.

Figure 3.

Aragonite saturation state (Ω_arag) versus net community calcification (μmol g⁻¹ h⁻¹) for (a) coral, (b) algae, (c) rubble, (d) sand and (e) the mixed community. Lines represent significant relationships between NCC and Ω_arag and model results are presented in electronic supplementary material, tables S4–S8.