High summer temperatures amplify functional differences between coral- and algae-dominated reef communities.

Florian Roth,Burton H Jones,Vincent Saderne,Benjamin Kürten,Christian Wild,Maria Ll Calleja,Luis Silva,Christian R Voolstra,Andrea Anton,Carlos M Duarte,Nils Radecker,Xosé Anxelu G Morán,Susana Carvalho

doi:10.1002/ecy.3226

Abstract

Shifts from coral to algal dominance are expected to increase in tropical coral reefs as a result of anthropogenic disturbances. The consequences for key ecosystem functions such as primary productivity, calcification, and nutrient recycling are poorly understood, particularly under changing environmental conditions. We used a novel in situ incubation approach to compare functions of coral‐ and algae‐dominated communities in the central Red Sea bimonthly over an entire year. In situ gross and net community primary productivity, calcification, dissolved organic carbon fluxes, dissolved inorganic nitrogen fluxes, and their respective activation energies were quantified to describe the effects of seasonal changes. Overall, coral‐dominated communities exhibited 30% lower net productivity and 10 times higher calcification than algae‐dominated communities. Estimated activation energies indicated a higher thermal sensitivity of coral‐dominated communities. In these communities, net productivity and calcification were negatively correlated with temperature (>40% and >65% reduction, respectively, with +5°C increase from winter to summer), whereas carbon losses via respiration and dissolved organic carbon release more than doubled at higher temperatures. In contrast, algae‐dominated communities doubled net productivity in summer, while calcification and dissolved organic carbon fluxes were unaffected. These results suggest pronounced changes in community functioning associated with coral‐algal phase shifts. Algae‐dominated communities may outcompete coral‐dominated communities because of their higher productivity and carbon retention to support fast biomass accumulation while compromising the formation of important reef framework structures. Higher temperatures likely amplify these functional differences, indicating a high vulnerability of ecosystem functions of coral‐dominated communities to temperatures even below coral bleaching thresholds. Our results suggest that ocean warming may not only cause but also amplify coral–algal phase shifts in coral reefs.

Highlights

Community shifts and the ongoing loss of biodiversity (Brondizio et al 2019) are altering the productivity and biogeochemistry of many ecosystems globallyManuscript received 17 March 2020; revised 6 July 2020; accepted 24 August 2020
Light readings were converted from lux to photosynthetically active radiation (PAR; μmol quantaÁm−2Ás−1; 400–700 nm wavelengths) by intercalibration and conversion as outlined in Roth et al (2018), and values are presented as daytime means
Discrete water samples for dissolved inorganic carbon (DIC), total alkalinity (TA), dissolved organic carbon (DOC), and dissolved inorganic nitrogen (DIN) were withdrawn from the sampling ports with acid-washed syringes at the beginning and the end of each incubation

Summary

INTRODUCTION

Community shifts and the ongoing loss of biodiversity (Brondizio et al 2019) are altering the productivity and biogeochemistry of many ecosystems globally. Laboratory experiments can only provide a glimpse of the complex environmental dynamics (e.g., seasonality) that shape the ecological processes of reef communities (Damgaard 2019) To overcome these experimental constraints, we used a novel in situ approach that allowed the quantification of major metabolic and biogeochemical pathways (Roth et al 2019) of co-occurring natural coral- and algaedominated reef communities in the Red Sea. With a total of 112 light and dark in situ incubations, we measured rates of community production (i.e., net community production [NCP], community respiration [CR], and gross primary production [GPP]), net community calcification (NCC), net dissolved organic carbon (DOC), and dissolved inorganic nitrogen (DIN) fluxes bimonthly for an entire year. We (1) directly compare the magnitudes and directions of key functions of coral-dominated and phase-shifted algae-dominated reef communities, (2) derive their functional responses to environmental changes induced by seasonality, and (3) describe their thermal sensitivity to seasonally variable temperature changes

MATERIALS AND METHODS

RESULTS

DISCUSSION